content: /*
   * Copyright (C) 2014 The Android Open Source Project
   * Copyright (c) 1997, 2014, Oracle and/or its affiliates. All rights reserved.
   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   *
   * This code is free software; you can redistribute it and/or modify it
   * under the terms of the GNU General Public License version 2 only, as
   * published by the Free Software Foundation.  Oracle designates this
   * particular file as subject to the "Classpath" exception as provided
   * by Oracle in the LICENSE file that accompanied this code.
   *
   * This code is distributed in the hope that it will be useful, but WITHOUT
   * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
   * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
   * version 2 for more details (a copy is included in the LICENSE file that
   * accompanied this code).
   *
   * You should have received a copy of the GNU General Public License version
   * 2 along with this work; if not, write to the Free Software Foundation,
   * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
   *
   * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
   * or visit www.oracle.com if you need additional information or have any
   * questions.
   */
  
  package java.util;
  import java.io.Serializable;
  import java.io.ObjectOutputStream;
  import java.io.IOException;
  import java.io.ObjectOutputStream;
  import java.io.Serializable;
  import java.lang.reflect.Array;
  import java.util.function.BiConsumer;
  import java.util.function.BiFunction;
  import java.util.function.Consumer;
  import java.util.function.Function;
  import java.util.function.Predicate;
  import java.util.function.UnaryOperator;
  import java.util.stream.IntStream;
  import java.util.stream.Stream;
  import java.util.stream.StreamSupport;
  
  import dalvik.system.VMRuntime;
  
  /**
   * This class consists exclusively of static methods that operate on or return
   * collections.  It contains polymorphic algorithms that operate on
   * collections, "wrappers", which return a new collection backed by a
   * specified collection, and a few other odds and ends.
   *
   * <p>The methods of this class all throw a <tt>NullPointerException</tt>
   * if the collections or class objects provided to them are null.
   *
   * <p>The documentation for the polymorphic algorithms contained in this class
   * generally includes a brief description of the <i>implementation</i>.  Such
   * descriptions should be regarded as <i>implementation notes</i>, rather than
   * parts of the <i>specification</i>.  Implementors should feel free to
   * substitute other algorithms, so long as the specification itself is adhered
   * to.  (For example, the algorithm used by <tt>sort</tt> does not have to be
   * a mergesort, but it does have to be <i>stable</i>.)
   *
   * <p>The "destructive" algorithms contained in this class, that is, the
   * algorithms that modify the collection on which they operate, are specified
   * to throw <tt>UnsupportedOperationException</tt> if the collection does not
   * support the appropriate mutation primitive(s), such as the <tt>set</tt>
   * method.  These algorithms may, but are not required to, throw this
   * exception if an invocation would have no effect on the collection.  For
   * example, invoking the <tt>sort</tt> method on an unmodifiable list that is
   * already sorted may or may not throw <tt>UnsupportedOperationException</tt>.
   *
   * <p>This class is a member of the
   * <a href="{@docRoot}/../technotes/guides/collections/index.html">
   * Java Collections Framework</a>.
   *
   * @author  Josh Bloch
   * @author  Neal Gafter
   * @see     Collection
   * @see     Set
   * @see     List
   * @see     Map
   * @since   1.2
   */
  
  public class Collections {
      // Suppresses default constructor, ensuring non-instantiability.
      private Collections() {
      }
  
      // Algorithms
  
      /*
       * Tuning parameters for algorithms - Many of the List algorithms have
       * two implementations, one of which is appropriate for RandomAccess
       * lists, the other for "sequential."  Often, the random access variant
       * yields better performance on small sequential access lists.  The
       * tuning parameters below determine the cutoff point for what constitutes
       * a "small" sequential access list for each algorithm.  The values below
       * were empirically determined to work well for LinkedList. Hopefully
       * they should be reasonable for other sequential access List
       * implementations.  Those doing performance work on this code would
       * do well to validate the values of these parameters from time to time.
       * (The first word of each tuning parameter name is the algorithm to which
       * it applies.)
       */
      private static final int BINARYSEARCH_THRESHOLD   = 5000;
      private static final int REVERSE_THRESHOLD        =   18;
      private static final int SHUFFLE_THRESHOLD        =    5;
      private static final int FILL_THRESHOLD           =   25;
      private static final int ROTATE_THRESHOLD         =  100;
      private static final int COPY_THRESHOLD           =   10;
      private static final int REPLACEALL_THRESHOLD     =   11;
      private static final int INDEXOFSUBLIST_THRESHOLD =   35;
  
      // Android-added: List.sort() vs. Collections.sort() app compat.
      // Added a warning in the documentation.
      // Collections.sort() calls List.sort() for apps targeting API version >= 26
      // (Android Oreo) but the other way around for app targeting <= 25 (Nougat).
      /**
       * Sorts the specified list into ascending order, according to the
       * {@linkplain Comparable natural ordering} of its elements.
       * All elements in the list must implement the {@link Comparable}
       * interface.  Furthermore, all elements in the list must be
       * <i>mutually comparable</i> (that is, {@code e1.compareTo(e2)}
       * must not throw a {@code ClassCastException} for any elements
       * {@code e1} and {@code e2} in the list).
       *
       * <p>This sort is guaranteed to be <i>stable</i>:  equal elements will
       * not be reordered as a result of the sort.
       *
       * <p>The specified list must be modifiable, but need not be resizable.
       *
       * @implNote
       * This implementation defers to the {@link List#sort(Comparator)}
       * method using the specified list and a {@code null} comparator.
       * Do not call this method from {@code List.sort()} since that can lead
       * to infinite recursion. Apps targeting APIs {@code <= 25} observe
       * backwards compatibility behavior where this method was implemented
       * on top of {@link List#toArray()}, {@link ListIterator#next()} and
       * {@link ListIterator#set(Object)}.
       *
       * @param  <T> the class of the objects in the list
       * @param  list the list to be sorted.
       * @throws ClassCastException if the list contains elements that are not
       *         <i>mutually comparable</i> (for example, strings and integers).
       * @throws UnsupportedOperationException if the specified list's
       *         list-iterator does not support the {@code set} operation.
       * @throws IllegalArgumentException (optional) if the implementation
       *         detects that the natural ordering of the list elements is
       *         found to violate the {@link Comparable} contract
       * @see List#sort(Comparator)
       */
      @SuppressWarnings("unchecked")
      public static <T extends Comparable<? super T>> void sort(List<T> list) {
          // Android-changed: List.sort() vs. Collections.sort() app compat.
          // Call sort(list, null) here to be consistent with that method's
          // (changed on Android) behavior.
          // list.sort(null);
          sort(list, null);
      }
  
      // Android-added: List.sort() vs. Collections.sort() app compat.
      // Added a warning in the documentation.
      // Collections.sort() calls List.sort() for apps targeting API version >= 26
      // (Android Oreo) but the other way around for app targeting <= 25 (Nougat).
      /**
       * Sorts the specified list according to the order induced by the
       * specified comparator.  All elements in the list must be <i>mutually
       * comparable</i> using the specified comparator (that is,
       * {@code c.compare(e1, e2)} must not throw a {@code ClassCastException}
       * for any elements {@code e1} and {@code e2} in the list).
       *
       * <p>This sort is guaranteed to be <i>stable</i>:  equal elements will
       * not be reordered as a result of the sort.
       *
       * <p>The specified list must be modifiable, but need not be resizable.
       *
       * @implNote
       * This implementation defers to the {@link List#sort(Comparator)}
       * method using the specified list and comparator.
       * Do not call this method from {@code List.sort()} since that can lead
       * to infinite recursion. Apps targeting APIs {@code <= 25} observe
       * backwards compatibility behavior where this method was implemented
       * on top of {@link List#toArray()}, {@link ListIterator#next()} and
       * {@link ListIterator#set(Object)}.
       *
       * @param  <T> the class of the objects in the list
       * @param  list the list to be sorted.
       * @param  c the comparator to determine the order of the list.  A
       *        {@code null} value indicates that the elements' <i>natural
       *        ordering</i> should be used.
       * @throws ClassCastException if the list contains elements that are not
       *         <i>mutually comparable</i> using the specified comparator.
       * @throws UnsupportedOperationException if the specified list's
       *         list-iterator does not support the {@code set} operation.
       * @throws IllegalArgumentException (optional) if the comparator is
       *         found to violate the {@link Comparator} contract
       * @see List#sort(Comparator)
       */
      @SuppressWarnings({"unchecked", "rawtypes"})
      public static <T> void sort(List<T> list, Comparator<? super T> c) {
          // BEGIN Android-changed: List.sort() vs. Collections.sort() app compat.
          // list.sort(c);
          int targetSdkVersion = VMRuntime.getRuntime().getTargetSdkVersion();
          if (targetSdkVersion > 25) {
              list.sort(c);
          } else {
              // Compatibility behavior for API <= 25. http://b/33482884
              if (list.getClass() == ArrayList.class) {
                  Arrays.sort((T[]) ((ArrayList) list).elementData, 0, list.size(), c);
                  return;
              }
  
              Object[] a = list.toArray();
              Arrays.sort(a, (Comparator) c);
              ListIterator<T> i = list.listIterator();
              for (int j = 0; j < a.length; j++) {
                  i.next();
                  i.set((T) a[j]);
              }
          }
          // END Android-changed: List.sort() vs. Collections.sort() app compat.
      }
  
  
      /**
       * Searches the specified list for the specified object using the binary
       * search algorithm.  The list must be sorted into ascending order
       * according to the {@linkplain Comparable natural ordering} of its
       * elements (as by the {@link #sort(List)} method) prior to making this
       * call.  If it is not sorted, the results are undefined.  If the list
       * contains multiple elements equal to the specified object, there is no
       * guarantee which one will be found.
       *
       * <p>This method runs in log(n) time for a "random access" list (which
       * provides near-constant-time positional access).  If the specified list
       * does not implement the {@link RandomAccess} interface and is large,
       * this method will do an iterator-based binary search that performs
       * O(n) link traversals and O(log n) element comparisons.
       *
       * @param  <T> the class of the objects in the list
       * @param  list the list to be searched.
       * @param  key the key to be searched for.
       * @return the index of the search key, if it is contained in the list;
       *         otherwise, <tt>(-(<i>insertion point</i>) - 1)</tt>.  The
       *         <i>insertion point</i> is defined as the point at which the
       *         key would be inserted into the list: the index of the first
       *         element greater than the key, or <tt>list.size()</tt> if all
       *         elements in the list are less than the specified key.  Note
       *         that this guarantees that the return value will be &gt;= 0 if
       *         and only if the key is found.
       * @throws ClassCastException if the list contains elements that are not
       *         <i>mutually comparable</i> (for example, strings and
       *         integers), or the search key is not mutually comparable
       *         with the elements of the list.
       */
      public static <T>
      int binarySearch(List<? extends Comparable<? super T>> list, T key) {
          if (list instanceof RandomAccess || list.size()<BINARYSEARCH_THRESHOLD)
              return Collections.indexedBinarySearch(list, key);
          else
              return Collections.iteratorBinarySearch(list, key);
      }
  
      private static <T>
      int indexedBinarySearch(List<? extends Comparable<? super T>> list, T key) {
          int low = 0;
          int high = list.size()-1;
  
          while (low <= high) {
              int mid = (low + high) >>> 1;
              Comparable<? super T> midVal = list.get(mid);
              int cmp = midVal.compareTo(key);
  
              if (cmp < 0)
                  low = mid + 1;
              else if (cmp > 0)
                  high = mid - 1;
              else
                  return mid; // key found
          }
          return -(low + 1);  // key not found
      }
  
      private static <T>
      int iteratorBinarySearch(List<? extends Comparable<? super T>> list, T key)
      {
          int low = 0;
          int high = list.size()-1;
          ListIterator<? extends Comparable<? super T>> i = list.listIterator();
  
          while (low <= high) {
              int mid = (low + high) >>> 1;
              Comparable<? super T> midVal = get(i, mid);
              int cmp = midVal.compareTo(key);
  
              if (cmp < 0)
                  low = mid + 1;
              else if (cmp > 0)
                  high = mid - 1;
              else
                  return mid; // key found
          }
          return -(low + 1);  // key not found
      }
  
      /**
       * Gets the ith element from the given list by repositioning the specified
       * list listIterator.
       */
      private static <T> T get(ListIterator<? extends T> i, int index) {
          T obj = null;
          int pos = i.nextIndex();
          if (pos <= index) {
              do {
                  obj = i.next();
              } while (pos++ < index);
          } else {
              do {
                  obj = i.previous();
              } while (--pos > index);
          }
          return obj;
      }
  
      /**
       * Searches the specified list for the specified object using the binary
       * search algorithm.  The list must be sorted into ascending order
       * according to the specified comparator (as by the
       * {@link #sort(List, Comparator) sort(List, Comparator)}
       * method), prior to making this call.  If it is
       * not sorted, the results are undefined.  If the list contains multiple
       * elements equal to the specified object, there is no guarantee which one
       * will be found.
       *
       * <p>This method runs in log(n) time for a "random access" list (which
       * provides near-constant-time positional access).  If the specified list
       * does not implement the {@link RandomAccess} interface and is large,
       * this method will do an iterator-based binary search that performs
       * O(n) link traversals and O(log n) element comparisons.
       *
       * @param  <T> the class of the objects in the list
       * @param  list the list to be searched.
       * @param  key the key to be searched for.
       * @param  c the comparator by which the list is ordered.
       *         A <tt>null</tt> value indicates that the elements'
       *         {@linkplain Comparable natural ordering} should be used.
       * @return the index of the search key, if it is contained in the list;
       *         otherwise, <tt>(-(<i>insertion point</i>) - 1)</tt>.  The
       *         <i>insertion point</i> is defined as the point at which the
       *         key would be inserted into the list: the index of the first
       *         element greater than the key, or <tt>list.size()</tt> if all
       *         elements in the list are less than the specified key.  Note
       *         that this guarantees that the return value will be &gt;= 0 if
       *         and only if the key is found.
       * @throws ClassCastException if the list contains elements that are not
       *         <i>mutually comparable</i> using the specified comparator,
       *         or the search key is not mutually comparable with the
       *         elements of the list using this comparator.
       */
      @SuppressWarnings("unchecked")
      public static <T> int binarySearch(List<? extends T> list, T key, Comparator<? super T> c) {
          if (c==null)
              return binarySearch((List<? extends Comparable<? super T>>) list, key);
  
          if (list instanceof RandomAccess || list.size()<BINARYSEARCH_THRESHOLD)
              return Collections.indexedBinarySearch(list, key, c);
          else
              return Collections.iteratorBinarySearch(list, key, c);
      }
  
      private static <T> int indexedBinarySearch(List<? extends T> l, T key, Comparator<? super T> c) {
          int low = 0;
          int high = l.size()-1;
  
          while (low <= high) {
              int mid = (low + high) >>> 1;
              T midVal = l.get(mid);
              int cmp = c.compare(midVal, key);
  
              if (cmp < 0)
                  low = mid + 1;
              else if (cmp > 0)
                  high = mid - 1;
              else
                  return mid; // key found
          }
          return -(low + 1);  // key not found
      }
  
      private static <T> int iteratorBinarySearch(List<? extends T> l, T key, Comparator<? super T> c) {
          int low = 0;
          int high = l.size()-1;
          ListIterator<? extends T> i = l.listIterator();
  
          while (low <= high) {
              int mid = (low + high) >>> 1;
              T midVal = get(i, mid);
              int cmp = c.compare(midVal, key);
  
              if (cmp < 0)
                  low = mid + 1;
              else if (cmp > 0)
                  high = mid - 1;
              else
                  return mid; // key found
          }
          return -(low + 1);  // key not found
      }
  
      /**
       * Reverses the order of the elements in the specified list.<p>
       *
       * This method runs in linear time.
       *
       * @param  list the list whose elements are to be reversed.
       * @throws UnsupportedOperationException if the specified list or
       *         its list-iterator does not support the <tt>set</tt> operation.
       */
      @SuppressWarnings({"rawtypes", "unchecked"})
      public static void reverse(List<?> list) {
          int size = list.size();
          if (size < REVERSE_THRESHOLD || list instanceof RandomAccess) {
              for (int i=0, mid=size>>1, j=size-1; i<mid; i++, j--)
                  swap(list, i, j);
          } else {
              // instead of using a raw type here, it's possible to capture
              // the wildcard but it will require a call to a supplementary
              // private method
              ListIterator fwd = list.listIterator();
              ListIterator rev = list.listIterator(size);
              for (int i=0, mid=list.size()>>1; i<mid; i++) {
                  Object tmp = fwd.next();
                  fwd.set(rev.previous());
                  rev.set(tmp);
              }
          }
      }
  
      /**
       * Randomly permutes the specified list using a default source of
       * randomness.  All permutations occur with approximately equal
       * likelihood.
       *
       * <p>The hedge "approximately" is used in the foregoing description because
       * default source of randomness is only approximately an unbiased source
       * of independently chosen bits. If it were a perfect source of randomly
       * chosen bits, then the algorithm would choose permutations with perfect
       * uniformity.
       *
       * <p>This implementation traverses the list backwards, from the last
       * element up to the second, repeatedly swapping a randomly selected element
       * into the "current position".  Elements are randomly selected from the
       * portion of the list that runs from the first element to the current
       * position, inclusive.
       *
       * <p>This method runs in linear time.  If the specified list does not
       * implement the {@link RandomAccess} interface and is large, this
       * implementation dumps the specified list into an array before shuffling
       * it, and dumps the shuffled array back into the list.  This avoids the
       * quadratic behavior that would result from shuffling a "sequential
       * access" list in place.
       *
       * @param  list the list to be shuffled.
       * @throws UnsupportedOperationException if the specified list or
       *         its list-iterator does not support the <tt>set</tt> operation.
       */
      public static void shuffle(List<?> list) {
          Random rnd = r;
          if (rnd == null)
              r = rnd = new Random(); // harmless race.
          shuffle(list, rnd);
      }
  
      private static Random r;
  
      /**
       * Randomly permute the specified list using the specified source of
       * randomness.  All permutations occur with equal likelihood
       * assuming that the source of randomness is fair.<p>
       *
       * This implementation traverses the list backwards, from the last element
       * up to the second, repeatedly swapping a randomly selected element into
       * the "current position".  Elements are randomly selected from the
       * portion of the list that runs from the first element to the current
       * position, inclusive.<p>
       *
       * This method runs in linear time.  If the specified list does not
       * implement the {@link RandomAccess} interface and is large, this
       * implementation dumps the specified list into an array before shuffling
       * it, and dumps the shuffled array back into the list.  This avoids the
       * quadratic behavior that would result from shuffling a "sequential
       * access" list in place.
       *
       * @param  list the list to be shuffled.
       * @param  rnd the source of randomness to use to shuffle the list.
       * @throws UnsupportedOperationException if the specified list or its
       *         list-iterator does not support the <tt>set</tt> operation.
       */
      @SuppressWarnings({"rawtypes", "unchecked"})
      public static void shuffle(List<?> list, Random rnd) {
          int size = list.size();
          if (size < SHUFFLE_THRESHOLD || list instanceof RandomAccess) {
              for (int i=size; i>1; i--)
                  swap(list, i-1, rnd.nextInt(i));
          } else {
              Object arr[] = list.toArray();
  
              // Shuffle array
              for (int i=size; i>1; i--)
                  swap(arr, i-1, rnd.nextInt(i));
  
              // Dump array back into list
              // instead of using a raw type here, it's possible to capture
              // the wildcard but it will require a call to a supplementary
              // private method
              ListIterator it = list.listIterator();
              for (int i=0; i<arr.length; i++) {
                  it.next();
                  it.set(arr[i]);
              }
          }
      }
  
      /**
       * Swaps the elements at the specified positions in the specified list.
       * (If the specified positions are equal, invoking this method leaves
       * the list unchanged.)
       *
       * @param list The list in which to swap elements.
       * @param i the index of one element to be swapped.
       * @param j the index of the other element to be swapped.
       * @throws IndexOutOfBoundsException if either <tt>i</tt> or <tt>j</tt>
       *         is out of range (i &lt; 0 || i &gt;= list.size()
       *         || j &lt; 0 || j &gt;= list.size()).
       * @since 1.4
       */
      @SuppressWarnings({"rawtypes", "unchecked"})
      public static void swap(List<?> list, int i, int j) {
          // instead of using a raw type here, it's possible to capture
          // the wildcard but it will require a call to a supplementary
          // private method
          final List l = list;
          l.set(i, l.set(j, l.get(i)));
      }
  
      /**
       * Swaps the two specified elements in the specified array.
       */
      private static void swap(Object[] arr, int i, int j) {
          Object tmp = arr[i];
          arr[i] = arr[j];
          arr[j] = tmp;
      }
  
      /**
       * Replaces all of the elements of the specified list with the specified
       * element. <p>
       *
       * This method runs in linear time.
       *
       * @param  <T> the class of the objects in the list
       * @param  list the list to be filled with the specified element.
       * @param  obj The element with which to fill the specified list.
       * @throws UnsupportedOperationException if the specified list or its
       *         list-iterator does not support the <tt>set</tt> operation.
       */
      public static <T> void fill(List<? super T> list, T obj) {
          int size = list.size();
  
          if (size < FILL_THRESHOLD || list instanceof RandomAccess) {
              for (int i=0; i<size; i++)
                  list.set(i, obj);
          } else {
              ListIterator<? super T> itr = list.listIterator();
              for (int i=0; i<size; i++) {
                  itr.next();
                  itr.set(obj);
              }
          }
      }
  
      /**
       * Copies all of the elements from one list into another.  After the
       * operation, the index of each copied element in the destination list
       * will be identical to its index in the source list.  The destination
       * list must be at least as long as the source list.  If it is longer, the
       * remaining elements in the destination list are unaffected. <p>
       *
       * This method runs in linear time.
       *
       * @param  <T> the class of the objects in the lists
       * @param  dest The destination list.
       * @param  src The source list.
       * @throws IndexOutOfBoundsException if the destination list is too small
       *         to contain the entire source List.
       * @throws UnsupportedOperationException if the destination list's
       *         list-iterator does not support the <tt>set</tt> operation.
       */
      public static <T> void copy(List<? super T> dest, List<? extends T> src) {
          int srcSize = src.size();
          if (srcSize > dest.size())
              throw new IndexOutOfBoundsException("Source does not fit in dest");
  
          if (srcSize < COPY_THRESHOLD ||
              (src instanceof RandomAccess && dest instanceof RandomAccess)) {
              for (int i=0; i<srcSize; i++)
                  dest.set(i, src.get(i));
          } else {
              ListIterator<? super T> di=dest.listIterator();
              ListIterator<? extends T> si=src.listIterator();
              for (int i=0; i<srcSize; i++) {
                  di.next();
                  di.set(si.next());
              }
          }
      }
  
      /**
       * Returns the minimum element of the given collection, according to the
       * <i>natural ordering</i> of its elements.  All elements in the
       * collection must implement the <tt>Comparable</tt> interface.
       * Furthermore, all elements in the collection must be <i>mutually
       * comparable</i> (that is, <tt>e1.compareTo(e2)</tt> must not throw a
       * <tt>ClassCastException</tt> for any elements <tt>e1</tt> and
       * <tt>e2</tt> in the collection).<p>
       *
       * This method iterates over the entire collection, hence it requires
       * time proportional to the size of the collection.
       *
       * @param  <T> the class of the objects in the collection
       * @param  coll the collection whose minimum element is to be determined.
       * @return the minimum element of the given collection, according
       *         to the <i>natural ordering</i> of its elements.
       * @throws ClassCastException if the collection contains elements that are
       *         not <i>mutually comparable</i> (for example, strings and
       *         integers).
       * @throws NoSuchElementException if the collection is empty.
       * @see Comparable
       */
      public static <T extends Object & Comparable<? super T>> T min(Collection<? extends T> coll) {
          Iterator<? extends T> i = coll.iterator();
          T candidate = i.next();
  
          while (i.hasNext()) {
              T next = i.next();
              if (next.compareTo(candidate) < 0)
                  candidate = next;
          }
          return candidate;
      }
  
      /**
       * Returns the minimum element of the given collection, according to the
       * order induced by the specified comparator.  All elements in the
       * collection must be <i>mutually comparable</i> by the specified
       * comparator (that is, <tt>comp.compare(e1, e2)</tt> must not throw a
       * <tt>ClassCastException</tt> for any elements <tt>e1</tt> and
       * <tt>e2</tt> in the collection).<p>
       *
       * This method iterates over the entire collection, hence it requires
       * time proportional to the size of the collection.
       *
       * @param  <T> the class of the objects in the collection
       * @param  coll the collection whose minimum element is to be determined.
       * @param  comp the comparator with which to determine the minimum element.
       *         A <tt>null</tt> value indicates that the elements' <i>natural
       *         ordering</i> should be used.
       * @return the minimum element of the given collection, according
       *         to the specified comparator.
       * @throws ClassCastException if the collection contains elements that are
       *         not <i>mutually comparable</i> using the specified comparator.
       * @throws NoSuchElementException if the collection is empty.
       * @see Comparable
       */
      @SuppressWarnings({"unchecked", "rawtypes"})
      public static <T> T min(Collection<? extends T> coll, Comparator<? super T> comp) {
          if (comp==null)
              return (T)min((Collection) coll);
  
          Iterator<? extends T> i = coll.iterator();
          T candidate = i.next();
  
          while (i.hasNext()) {
              T next = i.next();
              if (comp.compare(next, candidate) < 0)
                  candidate = next;
          }
          return candidate;
      }
  
      /**
       * Returns the maximum element of the given collection, according to the
       * <i>natural ordering</i> of its elements.  All elements in the
       * collection must implement the <tt>Comparable</tt> interface.
       * Furthermore, all elements in the collection must be <i>mutually
       * comparable</i> (that is, <tt>e1.compareTo(e2)</tt> must not throw a
       * <tt>ClassCastException</tt> for any elements <tt>e1</tt> and
       * <tt>e2</tt> in the collection).<p>
       *
       * This method iterates over the entire collection, hence it requires
       * time proportional to the size of the collection.
       *
       * @param  <T> the class of the objects in the collection
       * @param  coll the collection whose maximum element is to be determined.
       * @return the maximum element of the given collection, according
       *         to the <i>natural ordering</i> of its elements.
       * @throws ClassCastException if the collection contains elements that are
       *         not <i>mutually comparable</i> (for example, strings and
       *         integers).
       * @throws NoSuchElementException if the collection is empty.
       * @see Comparable
       */
      public static <T extends Object & Comparable<? super T>> T max(Collection<? extends T> coll) {
          Iterator<? extends T> i = coll.iterator();
          T candidate = i.next();
  
          while (i.hasNext()) {
              T next = i.next();
              if (next.compareTo(candidate) > 0)
                  candidate = next;
          }
          return candidate;
      }
  
      /**
       * Returns the maximum element of the given collection, according to the
       * order induced by the specified comparator.  All elements in the
       * collection must be <i>mutually comparable</i> by the specified
       * comparator (that is, <tt>comp.compare(e1, e2)</tt> must not throw a
       * <tt>ClassCastException</tt> for any elements <tt>e1</tt> and
       * <tt>e2</tt> in the collection).<p>
       *
       * This method iterates over the entire collection, hence it requires
       * time proportional to the size of the collection.
       *
       * @param  <T> the class of the objects in the collection
       * @param  coll the collection whose maximum element is to be determined.
       * @param  comp the comparator with which to determine the maximum element.
       *         A <tt>null</tt> value indicates that the elements' <i>natural
       *        ordering</i> should be used.
       * @return the maximum element of the given collection, according
       *         to the specified comparator.
       * @throws ClassCastException if the collection contains elements that are
       *         not <i>mutually comparable</i> using the specified comparator.
       * @throws NoSuchElementException if the collection is empty.
       * @see Comparable
       */
      @SuppressWarnings({"unchecked", "rawtypes"})
      public static <T> T max(Collection<? extends T> coll, Comparator<? super T> comp) {
          if (comp==null)
              return (T)max((Collection) coll);
  
          Iterator<? extends T> i = coll.iterator();
          T candidate = i.next();
  
          while (i.hasNext()) {
              T next = i.next();
              if (comp.compare(next, candidate) > 0)
                  candidate = next;
          }
          return candidate;
      }
  
      /**
       * Rotates the elements in the specified list by the specified distance.
       * After calling this method, the element at index <tt>i</tt> will be
       * the element previously at index <tt>(i - distance)</tt> mod
       * <tt>list.size()</tt>, for all values of <tt>i</tt> between <tt>0</tt>
       * and <tt>list.size()-1</tt>, inclusive.  (This method has no effect on
       * the size of the list.)
       *
       * <p>For example, suppose <tt>list</tt> comprises<tt> [t, a, n, k, s]</tt>.
       * After invoking <tt>Collections.rotate(list, 1)</tt> (or
       * <tt>Collections.rotate(list, -4)</tt>), <tt>list</tt> will comprise
       * <tt>[s, t, a, n, k]</tt>.
       *
       * <p>Note that this method can usefully be applied to sublists to
       * move one or more elements within a list while preserving the
       * order of the remaining elements.  For example, the following idiom
       * moves the element at index <tt>j</tt> forward to position
       * <tt>k</tt> (which must be greater than or equal to <tt>j</tt>):
       * <pre>
       *     Collections.rotate(list.subList(j, k+1), -1);
       * </pre>
       * To make this concrete, suppose <tt>list</tt> comprises
       * <tt>[a, b, c, d, e]</tt>.  To move the element at index <tt>1</tt>
       * (<tt>b</tt>) forward two positions, perform the following invocation:
       * <pre>
       *     Collections.rotate(l.subList(1, 4), -1);
       * </pre>
       * The resulting list is <tt>[a, c, d, b, e]</tt>.
       *
       * <p>To move more than one element forward, increase the absolute value
       * of the rotation distance.  To move elements backward, use a positive
       * shift distance.
       *
       * <p>If the specified list is small or implements the {@link
       * RandomAccess} interface, this implementation exchanges the first
       * element into the location it should go, and then repeatedly exchanges
       * the displaced element into the location it should go until a displaced
       * element is swapped into the first element.  If necessary, the process
       * is repeated on the second and successive elements, until the rotation
       * is complete.  If the specified list is large and doesn't implement the
       * <tt>RandomAccess</tt> interface, this implementation breaks the
       * list into two sublist views around index <tt>-distance mod size</tt>.
       * Then the {@link #reverse(List)} method is invoked on each sublist view,
       * and finally it is invoked on the entire list.  For a more complete
       * description of both algorithms, see Section 2.3 of Jon Bentley's
       * <i>Programming Pearls</i> (Addison-Wesley, 1986).
       *
       * @param list the list to be rotated.
       * @param distance the distance to rotate the list.  There are no
       *        constraints on this value; it may be zero, negative, or
       *        greater than <tt>list.size()</tt>.
       * @throws UnsupportedOperationException if the specified list or
       *         its list-iterator does not support the <tt>set</tt> operation.
       * @since 1.4
       */
      public static void rotate(List<?> list, int distance) {
          if (list instanceof RandomAccess || list.size() < ROTATE_THRESHOLD)
              rotate1(list, distance);
          else
              rotate2(list, distance);
      }
  
      private static <T> void rotate1(List<T> list, int distance) {
          int size = list.size();
          if (size == 0)
              return;
          distance = distance % size;
          if (distance < 0)
              distance += size;
          if (distance == 0)
              return;
  
          for (int cycleStart = 0, nMoved = 0; nMoved != size; cycleStart++) {
              T displaced = list.get(cycleStart);
              int i = cycleStart;
              do {
                  i += distance;
                  if (i >= size)
                      i -= size;
                  displaced = list.set(i, displaced);
                  nMoved ++;
              } while (i != cycleStart);
          }
      }
  
      private static void rotate2(List<?> list, int distance) {
          int size = list.size();
          if (size == 0)
              return;
          int mid =  -distance % size;
          if (mid < 0)
              mid += size;
          if (mid == 0)
              return;
  
          reverse(list.subList(0, mid));
          reverse(list.subList(mid, size));
          reverse(list);
      }
  
      /**
       * Replaces all occurrences of one specified value in a list with another.
       * More formally, replaces with <tt>newVal</tt> each element <tt>e</tt>
       * in <tt>list</tt> such that
       * <tt>(oldVal==null ? e==null : oldVal.equals(e))</tt>.
       * (This method has no effect on the size of the list.)
       *
       * @param  <T> the class of the objects in the list
       * @param list the list in which replacement is to occur.
       * @param oldVal the old value to be replaced.
       * @param newVal the new value with which <tt>oldVal</tt> is to be
       *        replaced.
       * @return <tt>true</tt> if <tt>list</tt> contained one or more elements
       *         <tt>e</tt> such that
       *         <tt>(oldVal==null ?  e==null : oldVal.equals(e))</tt>.
       * @throws UnsupportedOperationException if the specified list or
       *         its list-iterator does not support the <tt>set</tt> operation.
       * @since  1.4
       */
      public static <T> boolean replaceAll(List<T> list, T oldVal, T newVal) {
          boolean result = false;
          int size = list.size();
          if (size < REPLACEALL_THRESHOLD || list instanceof RandomAccess) {
              if (oldVal==null) {
                  for (int i=0; i<size; i++) {
                      if (list.get(i)==null) {
                          list.set(i, newVal);
                          result = true;
                      }
                  }
              } else {
                  for (int i=0; i<size; i++) {
                      if (oldVal.equals(list.get(i))) {
                          list.set(i, newVal);
                          result = true;
                      }
                  }
              }
          } else {
              ListIterator<T> itr=list.listIterator();
              if (oldVal==null) {
                  for (int i=0; i<size; i++) {
                      if (itr.next()==null) {
                          itr.set(newVal);
                          result = true;
                      }
                  }
              } else {
                  for (int i=0; i<size; i++) {
                      if (oldVal.equals(itr.next())) {
                          itr.set(newVal);
                          result = true;
                      }
                  }
              }
          }
          return result;
      }
  
      /**
       * Returns the starting position of the first occurrence of the specified
       * target list within the specified source list, or -1 if there is no
       * such occurrence.  More formally, returns the lowest index <tt>i</tt>
       * such that {@code source.subList(i, i+target.size()).equals(target)},
       * or -1 if there is no such index.  (Returns -1 if
       * {@code target.size() > source.size()})
       *
       * <p>This implementation uses the "brute force" technique of scanning
       * over the source list, looking for a match with the target at each
       * location in turn.
       *
       * @param source the list in which to search for the first occurrence
       *        of <tt>target</tt>.
       * @param target the list to search for as a subList of <tt>source</tt>.
       * @return the starting position of the first occurrence of the specified
       *         target list within the specified source list, or -1 if there
       *         is no such occurrence.
       * @since  1.4
       */
      public static int indexOfSubList(List<?> source, List<?> target) {
          int sourceSize = source.size();
          int targetSize = target.size();
          int maxCandidate = sourceSize - targetSize;
  
          if (sourceSize < INDEXOFSUBLIST_THRESHOLD ||
              (source instanceof RandomAccess&&target instanceof RandomAccess)) {
          nextCand:
              for (int candidate = 0; candidate <= maxCandidate; candidate++) {
                  for (int i=0, j=candidate; i<targetSize; i++, j++)
                      if (!eq(target.get(i), source.get(j)))
                          continue nextCand;  // Element mismatch, try next cand
                  return candidate;  // All elements of candidate matched target
              }
          } else {  // Iterator version of above algorithm
              ListIterator<?> si = source.listIterator();
          nextCand:
              for (int candidate = 0; candidate <= maxCandidate; candidate++) {
                  ListIterator<?> ti = target.listIterator();
                  for (int i=0; i<targetSize; i++) {
                      if (!eq(ti.next(), si.next())) {
                          // Back up source iterator to next candidate
                          for (int j=0; j<i; j++)
                              si.previous();
                          continue nextCand;
                      }
                  }
                  return candidate;
              }
          }
          return -1;  // No candidate matched the target
      }
  
      /**
       * Returns the starting position of the last occurrence of the specified
       * target list within the specified source list, or -1 if there is no such
       * occurrence.  More formally, returns the highest index <tt>i</tt>
       * such that {@code source.subList(i, i+target.size()).equals(target)},
       * or -1 if there is no such index.  (Returns -1 if
       * {@code target.size() > source.size()})
       *
       * <p>This implementation uses the "brute force" technique of iterating
       * over the source list, looking for a match with the target at each
       * location in turn.
       *
       * @param source the list in which to search for the last occurrence
       *        of <tt>target</tt>.
       * @param target the list to search for as a subList of <tt>source</tt>.
       * @return the starting position of the last occurrence of the specified
       *         target list within the specified source list, or -1 if there
       *         is no such occurrence.
       * @since  1.4
       */
      public static int lastIndexOfSubList(List<?> source, List<?> target) {
          int sourceSize = source.size();
          int targetSize = target.size();
          int maxCandidate = sourceSize - targetSize;
  
          if (sourceSize < INDEXOFSUBLIST_THRESHOLD ||
              source instanceof RandomAccess) {   // Index access version
          nextCand:
              for (int candidate = maxCandidate; candidate >= 0; candidate--) {
                  for (int i=0, j=candidate; i<targetSize; i++, j++)
                      if (!eq(target.get(i), source.get(j)))
                          continue nextCand;  // Element mismatch, try next cand
                  return candidate;  // All elements of candidate matched target
              }
          } else {  // Iterator version of above algorithm
              if (maxCandidate < 0)
                  return -1;
              ListIterator<?> si = source.listIterator(maxCandidate);
          nextCand:
              for (int candidate = maxCandidate; candidate >= 0; candidate--) {
                  ListIterator<?> ti = target.listIterator();
                  for (int i=0; i<targetSize; i++) {
                      if (!eq(ti.next(), si.next())) {
                          if (candidate != 0) {
                              // Back up source iterator to next candidate
                              for (int j=0; j<=i+1; j++)
                                  si.previous();
                          }
                          continue nextCand;
                      }
                  }
                  return candidate;
              }
          }
          return -1;  // No candidate matched the target
      }
  
  
      // Unmodifiable Wrappers
  
      /**
       * Returns an unmodifiable view of the specified collection.  This method
       * allows modules to provide users with "read-only" access to internal
       * collections.  Query operations on the returned collection "read through"
       * to the specified collection, and attempts to modify the returned
       * collection, whether direct or via its iterator, result in an
       * <tt>UnsupportedOperationException</tt>.<p>
       *
       * The returned collection does <i>not</i> pass the hashCode and equals
       * operations through to the backing collection, but relies on
       * <tt>Object</tt>'s <tt>equals</tt> and <tt>hashCode</tt> methods.  This
       * is necessary to preserve the contracts of these operations in the case
       * that the backing collection is a set or a list.<p>
       *
       * The returned collection will be serializable if the specified collection
       * is serializable.
       *
       * @param  <T> the class of the objects in the collection
       * @param  c the collection for which an unmodifiable view is to be
       *         returned.
       * @return an unmodifiable view of the specified collection.
       */
      public static <T> Collection<T> unmodifiableCollection(Collection<? extends T> c) {
          return new UnmodifiableCollection<>(c);
      }
  
      /**
       * @serial include
       */
      static class UnmodifiableCollection<E> implements Collection<E>, Serializable {
          private static final long serialVersionUID = 1820017752578914078L;
  
          final Collection<? extends E> c;
  
          UnmodifiableCollection(Collection<? extends E> c) {
              if (c==null)
                  throw new NullPointerException();
              this.c = c;
          }
  
          public int size()                   {return c.size();}
          public boolean isEmpty()            {return c.isEmpty();}
          public boolean contains(Object o)   {return c.contains(o);}
          public Object[] toArray()           {return c.toArray();}
          public <T> T[] toArray(T[] a)       {return c.toArray(a);}
          public String toString()            {return c.toString();}
  
          public Iterator<E> iterator() {
              return new Iterator<E>() {
                  private final Iterator<? extends E> i = c.iterator();
  
                  public boolean hasNext() {return i.hasNext();}
                  public E next()          {return i.next();}
                  public void remove() {
                      throw new UnsupportedOperationException();
                  }
                  @Override
                  public void forEachRemaining(Consumer<? super E> action) {
                      // Use backing collection version
                      i.forEachRemaining(action);
                  }
              };
          }
  
          public boolean add(E e) {
              throw new UnsupportedOperationException();
          }
          public boolean remove(Object o) {
              throw new UnsupportedOperationException();
          }
  
          public boolean containsAll(Collection<?> coll) {
              return c.containsAll(coll);
          }
          public boolean addAll(Collection<? extends E> coll) {
              throw new UnsupportedOperationException();
          }
          public boolean removeAll(Collection<?> coll) {
              throw new UnsupportedOperationException();
          }
          public boolean retainAll(Collection<?> coll) {
              throw new UnsupportedOperationException();
          }
          public void clear() {
              throw new UnsupportedOperationException();
          }
  
          // Override default methods in Collection
          @Override
          public void forEach(Consumer<? super E> action) {
              c.forEach(action);
          }
          @Override
          public boolean removeIf(Predicate<? super E> filter) {
              throw new UnsupportedOperationException();
          }
          @SuppressWarnings("unchecked")
          @Override
          public Spliterator<E> spliterator() {
              return (Spliterator<E>)c.spliterator();
          }
          @SuppressWarnings("unchecked")
          @Override
          public Stream<E> stream() {
              return (Stream<E>)c.stream();
          }
          @SuppressWarnings("unchecked")
          @Override
          public Stream<E> parallelStream() {
              return (Stream<E>)c.parallelStream();
          }
      }
  
      /**
       * Returns an unmodifiable view of the specified set.  This method allows
       * modules to provide users with "read-only" access to internal sets.
       * Query operations on the returned set "read through" to the specified
       * set, and attempts to modify the returned set, whether direct or via its
       * iterator, result in an <tt>UnsupportedOperationException</tt>.<p>
       *
       * The returned set will be serializable if the specified set
       * is serializable.
       *
       * @param  <T> the class of the objects in the set
       * @param  s the set for which an unmodifiable view is to be returned.
       * @return an unmodifiable view of the specified set.
       */
      public static <T> Set<T> unmodifiableSet(Set<? extends T> s) {
          return new UnmodifiableSet<>(s);
      }
  
      /**
       * @serial include
       */
      static class UnmodifiableSet<E> extends UnmodifiableCollection<E>
                                   implements Set<E>, Serializable {
          private static final long serialVersionUID = -9215047833775013803L;
  
          UnmodifiableSet(Set<? extends E> s)     {super(s);}
          public boolean equals(Object o) {return o == this || c.equals(o);}
          public int hashCode()           {return c.hashCode();}
      }
  
      /**
       * Returns an unmodifiable view of the specified sorted set.  This method
       * allows modules to provide users with "read-only" access to internal
       * sorted sets.  Query operations on the returned sorted set "read
       * through" to the specified sorted set.  Attempts to modify the returned
       * sorted set, whether direct, via its iterator, or via its
       * <tt>subSet</tt>, <tt>headSet</tt>, or <tt>tailSet</tt> views, result in
       * an <tt>UnsupportedOperationException</tt>.<p>
       *
       * The returned sorted set will be serializable if the specified sorted set
       * is serializable.
       *
       * @param  <T> the class of the objects in the set
       * @param s the sorted set for which an unmodifiable view is to be
       *        returned.
       * @return an unmodifiable view of the specified sorted set.
       */
      public static <T> SortedSet<T> unmodifiableSortedSet(SortedSet<T> s) {
          return new UnmodifiableSortedSet<>(s);
      }
  
      /**
       * @serial include
       */
      static class UnmodifiableSortedSet<E>
                               extends UnmodifiableSet<E>
                               implements SortedSet<E>, Serializable {
          private static final long serialVersionUID = -4929149591599911165L;
          private final SortedSet<E> ss;
  
          UnmodifiableSortedSet(SortedSet<E> s) {super(s); ss = s;}
  
          public Comparator<? super E> comparator() {return ss.comparator();}
  
          public SortedSet<E> subSet(E fromElement, E toElement) {
              return new UnmodifiableSortedSet<>(ss.subSet(fromElement,toElement));
          }
          public SortedSet<E> headSet(E toElement) {
              return new UnmodifiableSortedSet<>(ss.headSet(toElement));
          }
          public SortedSet<E> tailSet(E fromElement) {
              return new UnmodifiableSortedSet<>(ss.tailSet(fromElement));
          }
  
          public E first()                   {return ss.first();}
          public E last()                    {return ss.last();}
      }
  
      /**
       * Returns an unmodifiable view of the specified navigable set.  This method
       * allows modules to provide users with "read-only" access to internal
       * navigable sets.  Query operations on the returned navigable set "read
       * through" to the specified navigable set.  Attempts to modify the returned
       * navigable set, whether direct, via its iterator, or via its
       * {@code subSet}, {@code headSet}, or {@code tailSet} views, result in
       * an {@code UnsupportedOperationException}.<p>
       *
       * The returned navigable set will be serializable if the specified
       * navigable set is serializable.
       *
       * @param  <T> the class of the objects in the set
       * @param s the navigable set for which an unmodifiable view is to be
       *        returned
       * @return an unmodifiable view of the specified navigable set
       * @since 1.8
       */
      public static <T> NavigableSet<T> unmodifiableNavigableSet(NavigableSet<T> s) {
          return new UnmodifiableNavigableSet<>(s);
      }
  
      /**
       * Wraps a navigable set and disables all of the mutative operations.
       *
       * @param <E> type of elements
       * @serial include
       */
      static class UnmodifiableNavigableSet<E>
                               extends UnmodifiableSortedSet<E>
                               implements NavigableSet<E>, Serializable {
  
          private static final long serialVersionUID = -6027448201786391929L;
  
          /**
           * A singleton empty unmodifiable navigable set used for
           * {@link #emptyNavigableSet()}.
           *
           * @param <E> type of elements, if there were any, and bounds
           */
          private static class EmptyNavigableSet<E> extends UnmodifiableNavigableSet<E>
              implements Serializable {
              private static final long serialVersionUID = -6291252904449939134L;
  
              public EmptyNavigableSet() {
                  super(new TreeSet<E>());
              }
  
              private Object readResolve()        { return EMPTY_NAVIGABLE_SET; }
          }
  
          @SuppressWarnings("rawtypes")
          private static final NavigableSet<?> EMPTY_NAVIGABLE_SET =
                  new EmptyNavigableSet<>();
  
          /**
           * The instance we are protecting.
           */
          private final NavigableSet<E> ns;
  
          UnmodifiableNavigableSet(NavigableSet<E> s)         {super(s); ns = s;}
  
          public E lower(E e)                             { return ns.lower(e); }
          public E floor(E e)                             { return ns.floor(e); }
          public E ceiling(E e)                         { return ns.ceiling(e); }
          public E higher(E e)                           { return ns.higher(e); }
          public E pollFirst()     { throw new UnsupportedOperationException(); }
          public E pollLast()      { throw new UnsupportedOperationException(); }
          public NavigableSet<E> descendingSet()
                   { return new UnmodifiableNavigableSet<>(ns.descendingSet()); }
          public Iterator<E> descendingIterator()
                                           { return descendingSet().iterator(); }
  
          public NavigableSet<E> subSet(E fromElement, boolean fromInclusive, E toElement, boolean toInclusive) {
              return new UnmodifiableNavigableSet<>(
                  ns.subSet(fromElement, fromInclusive, toElement, toInclusive));
          }
  
          public NavigableSet<E> headSet(E toElement, boolean inclusive) {
              return new UnmodifiableNavigableSet<>(
                  ns.headSet(toElement, inclusive));
          }
  
          public NavigableSet<E> tailSet(E fromElement, boolean inclusive) {
              return new UnmodifiableNavigableSet<>(
                  ns.tailSet(fromElement, inclusive));
          }
      }
  
      /**
       * Returns an unmodifiable view of the specified list.  This method allows
       * modules to provide users with "read-only" access to internal
       * lists.  Query operations on the returned list "read through" to the
       * specified list, and attempts to modify the returned list, whether
       * direct or via its iterator, result in an
       * <tt>UnsupportedOperationException</tt>.<p>
       *
       * The returned list will be serializable if the specified list
       * is serializable. Similarly, the returned list will implement
       * {@link RandomAccess} if the specified list does.
       *
       * @param  <T> the class of the objects in the list
       * @param  list the list for which an unmodifiable view is to be returned.
       * @return an unmodifiable view of the specified list.
       */
      public static <T> List<T> unmodifiableList(List<? extends T> list) {
          return (list instanceof RandomAccess ?
                  new UnmodifiableRandomAccessList<>(list) :
                  new UnmodifiableList<>(list));
      }
  
      /**
       * @serial include
       */
      static class UnmodifiableList<E> extends UnmodifiableCollection<E>
                                    implements List<E> {
          private static final long serialVersionUID = -283967356065247728L;
  
          final List<? extends E> list;
  
          UnmodifiableList(List<? extends E> list) {
              super(list);
              this.list = list;
          }
  
          public boolean equals(Object o) {return o == this || list.equals(o);}
          public int hashCode()           {return list.hashCode();}
  
          public E get(int index) {return list.get(index);}
          public E set(int index, E element) {
              throw new UnsupportedOperationException();
          }
          public void add(int index, E element) {
              throw new UnsupportedOperationException();
          }
          public E remove(int index) {
              throw new UnsupportedOperationException();
          }
          public int indexOf(Object o)            {return list.indexOf(o);}
          public int lastIndexOf(Object o)        {return list.lastIndexOf(o);}
          public boolean addAll(int index, Collection<? extends E> c) {
              throw new UnsupportedOperationException();
          }
  
          @Override
          public void replaceAll(UnaryOperator<E> operator) {
              throw new UnsupportedOperationException();
          }
          @Override
          public void sort(Comparator<? super E> c) {
              throw new UnsupportedOperationException();
          }
  
          public ListIterator<E> listIterator()   {return listIterator(0);}
  
          public ListIterator<E> listIterator(final int index) {
              return new ListIterator<E>() {
                  private final ListIterator<? extends E> i
                      = list.listIterator(index);
  
                  public boolean hasNext()     {return i.hasNext();}
                  public E next()              {return i.next();}
                  public boolean hasPrevious() {return i.hasPrevious();}
                  public E previous()          {return i.previous();}
                  public int nextIndex()       {return i.nextIndex();}
                  public int previousIndex()   {return i.previousIndex();}
  
                  public void remove() {
                      throw new UnsupportedOperationException();
                  }
                  public void set(E e) {
                      throw new UnsupportedOperationException();
                  }
                  public void add(E e) {
                      throw new UnsupportedOperationException();
                  }
  
                  @Override
                  public void forEachRemaining(Consumer<? super E> action) {
                      i.forEachRemaining(action);
                  }
              };
          }
  
          public List<E> subList(int fromIndex, int toIndex) {
              return new UnmodifiableList<>(list.subList(fromIndex, toIndex));
          }
  
          /**
           * UnmodifiableRandomAccessList instances are serialized as
           * UnmodifiableList instances to allow them to be deserialized
           * in pre-1.4 JREs (which do not have UnmodifiableRandomAccessList).
           * This method inverts the transformation.  As a beneficial
           * side-effect, it also grafts the RandomAccess marker onto
           * UnmodifiableList instances that were serialized in pre-1.4 JREs.
           *
           * Note: Unfortunately, UnmodifiableRandomAccessList instances
           * serialized in 1.4.1 and deserialized in 1.4 will become
           * UnmodifiableList instances, as this method was missing in 1.4.
           */
          private Object readResolve() {
              return (list instanceof RandomAccess
                      ? new UnmodifiableRandomAccessList<>(list)
                      : this);
          }
      }
  
      /**
       * @serial include
       */
      static class UnmodifiableRandomAccessList<E> extends UnmodifiableList<E>
                                                implements RandomAccess
      {
          UnmodifiableRandomAccessList(List<? extends E> list) {
              super(list);
          }
  
          public List<E> subList(int fromIndex, int toIndex) {
              return new UnmodifiableRandomAccessList<>(
                  list.subList(fromIndex, toIndex));
          }
  
          private static final long serialVersionUID = -2542308836966382001L;
  
          /**
           * Allows instances to be deserialized in pre-1.4 JREs (which do
           * not have UnmodifiableRandomAccessList).  UnmodifiableList has
           * a readResolve method that inverts this transformation upon
           * deserialization.
           */
          private Object writeReplace() {
              return new UnmodifiableList<>(list);
          }
      }
  
      /**
       * Returns an unmodifiable view of the specified map.  This method
       * allows modules to provide users with "read-only" access to internal
       * maps.  Query operations on the returned map "read through"
       * to the specified map, and attempts to modify the returned
       * map, whether direct or via its collection views, result in an
       * <tt>UnsupportedOperationException</tt>.<p>
       *
       * The returned map will be serializable if the specified map
       * is serializable.
       *
       * @param <K> the class of the map keys
       * @param <V> the class of the map values
       * @param  m the map for which an unmodifiable view is to be returned.
       * @return an unmodifiable view of the specified map.
       */
      public static <K,V> Map<K,V> unmodifiableMap(Map<? extends K, ? extends V> m) {
          return new UnmodifiableMap<>(m);
      }
  
      /**
       * @serial include
       */
      private static class UnmodifiableMap<K,V> implements Map<K,V>, Serializable {
          private static final long serialVersionUID = -1034234728574286014L;
  
          private final Map<? extends K, ? extends V> m;
  
          UnmodifiableMap(Map<? extends K, ? extends V> m) {
              if (m==null)
                  throw new NullPointerException();
              this.m = m;
          }
  
          public int size()                        {return m.size();}
          public boolean isEmpty()                 {return m.isEmpty();}
          public boolean containsKey(Object key)   {return m.containsKey(key);}
          public boolean containsValue(Object val) {return m.containsValue(val);}
          public V get(Object key)                 {return m.get(key);}
  
          public V put(K key, V value) {
              throw new UnsupportedOperationException();
          }
          public V remove(Object key) {
              throw new UnsupportedOperationException();
          }
          public void putAll(Map<? extends K, ? extends V> m) {
              throw new UnsupportedOperationException();
          }
          public void clear() {
              throw new UnsupportedOperationException();
          }
  
          private transient Set<K> keySet;
          private transient Set<Entry<K,V>> entrySet;
          private transient Collection<V> values;
  
          public Set<K> keySet() {
              if (keySet==null)
                  keySet = unmodifiableSet(m.keySet());
              return keySet;
          }
  
          public Set<Entry<K,V>> entrySet() {
              if (entrySet==null)
                  entrySet = new UnmodifiableEntrySet<>(m.entrySet());
              return entrySet;
          }
  
          public Collection<V> values() {
              if (values==null)
                  values = unmodifiableCollection(m.values());
              return values;
          }
  
          public boolean equals(Object o) {return o == this || m.equals(o);}
          public int hashCode()           {return m.hashCode();}
          public String toString()        {return m.toString();}
  
          // Override default methods in Map
          @Override
          @SuppressWarnings("unchecked")
          public V getOrDefault(Object k, V defaultValue) {
              // Safe cast as we don't change the value
              return ((Map<K, V>)m).getOrDefault(k, defaultValue);
          }
  
          @Override
          public void forEach(BiConsumer<? super K, ? super V> action) {
              m.forEach(action);
          }
  
          @Override
          public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
              throw new UnsupportedOperationException();
          }
  
          @Override
          public V putIfAbsent(K key, V value) {
              throw new UnsupportedOperationException();
          }
  
          @Override
          public boolean remove(Object key, Object value) {
              throw new UnsupportedOperationException();
          }
  
          @Override
          public boolean replace(K key, V oldValue, V newValue) {
              throw new UnsupportedOperationException();
          }
  
          @Override
          public V replace(K key, V value) {
              throw new UnsupportedOperationException();
          }
  
          @Override
          public V computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction) {
              throw new UnsupportedOperationException();
          }
  
          @Override
          public V computeIfPresent(K key,
                  BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
              throw new UnsupportedOperationException();
          }
  
          @Override
          public V compute(K key,
                  BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
              throw new UnsupportedOperationException();
          }
  
          @Override
          public V merge(K key, V value,
                  BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
              throw new UnsupportedOperationException();
          }
  
          /**
           * We need this class in addition to UnmodifiableSet as
           * Map.Entries themselves permit modification of the backing Map
           * via their setValue operation.  This class is subtle: there are
           * many possible attacks that must be thwarted.
           *
           * @serial include
           */
          static class UnmodifiableEntrySet<K,V>
              extends UnmodifiableSet<Entry<K,V>> {
              private static final long serialVersionUID = 7854390611657943733L;
  
              @SuppressWarnings({"unchecked", "rawtypes"})
              UnmodifiableEntrySet(Set<? extends Entry<? extends K, ? extends V>> s) {
                  // Need to cast to raw in order to work around a limitation in the type system
                  super((Set)s);
              }
  
              static <K, V> Consumer<Entry<K, V>> entryConsumer(Consumer<? super Entry<K, V>> action) {
                  return e -> action.accept(new UnmodifiableEntry<>(e));
              }
  
              public void forEach(Consumer<? super Entry<K, V>> action) {
                  Objects.requireNonNull(action);
                  c.forEach(entryConsumer(action));
              }
  
              static final class UnmodifiableEntrySetSpliterator<K, V>
                      implements Spliterator<Entry<K,V>> {
                  final Spliterator<Entry<K, V>> s;
  
                  UnmodifiableEntrySetSpliterator(Spliterator<Entry<K, V>> s) {
                      this.s = s;
                  }
  
                  @Override
                  public boolean tryAdvance(Consumer<? super Entry<K, V>> action) {
                      Objects.requireNonNull(action);
                      return s.tryAdvance(entryConsumer(action));
                  }
  
                  @Override
                  public void forEachRemaining(Consumer<? super Entry<K, V>> action) {
                      Objects.requireNonNull(action);
                      s.forEachRemaining(entryConsumer(action));
                  }
  
                  @Override
                  public Spliterator<Entry<K, V>> trySplit() {
                      Spliterator<Entry<K, V>> split = s.trySplit();
                      return split == null
                             ? null
                             : new UnmodifiableEntrySetSpliterator<>(split);
                  }
  
                  @Override
                  public long estimateSize() {
                      return s.estimateSize();
                  }
  
                  @Override
                  public long getExactSizeIfKnown() {
                      return s.getExactSizeIfKnown();
                  }
  
                  @Override
                  public int characteristics() {
                      return s.characteristics();
                  }
  
                  @Override
                  public boolean hasCharacteristics(int characteristics) {
                      return s.hasCharacteristics(characteristics);
                  }
  
                  @Override
                  public Comparator<? super Entry<K, V>> getComparator() {
                      return s.getComparator();
                  }
              }
  
              @SuppressWarnings("unchecked")
              public Spliterator<Entry<K,V>> spliterator() {
                  return new UnmodifiableEntrySetSpliterator<>(
                          (Spliterator<Entry<K, V>>) c.spliterator());
              }
  
              @Override
              public Stream<Entry<K,V>> stream() {
                  return StreamSupport.stream(spliterator(), false);
              }
  
              @Override
              public Stream<Entry<K,V>> parallelStream() {
                  return StreamSupport.stream(spliterator(), true);
              }
  
              public Iterator<Entry<K,V>> iterator() {
                  return new Iterator<Entry<K,V>>() {
                      private final Iterator<? extends Entry<? extends K, ? extends V>> i = c.iterator();
  
                      public boolean hasNext() {
                          return i.hasNext();
                      }
                      public Entry<K,V> next() {
                          return new UnmodifiableEntry<>(i.next());
                      }
                      public void remove() {
                          throw new UnsupportedOperationException();
                      }
                      // Android-note: Oversight of Iterator.forEachRemaining().
                      // This seems pretty inconsistent. Unlike other subclasses,
                      // we aren't delegating to the subclass iterator here.
                      // Seems like an oversight. http://b/110351017
                  };
              }
  
              @SuppressWarnings("unchecked")
              public Object[] toArray() {
                  Object[] a = c.toArray();
                  for (int i=0; i<a.length; i++)
                      a[i] = new UnmodifiableEntry<>((Entry<? extends K, ? extends V>)a[i]);
                  return a;
              }
  
              @SuppressWarnings("unchecked")
              public <T> T[] toArray(T[] a) {
                  // We don't pass a to c.toArray, to avoid window of
                  // vulnerability wherein an unscrupulous multithreaded client
                  // could get his hands on raw (unwrapped) Entries from c.
                  Object[] arr = c.toArray(a.length==0 ? a : Arrays.copyOf(a, 0));
  
                  for (int i=0; i<arr.length; i++)
                      arr[i] = new UnmodifiableEntry<>((Entry<? extends K, ? extends V>)arr[i]);
  
                  if (arr.length > a.length)
                      return (T[])arr;
  
                  System.arraycopy(arr, 0, a, 0, arr.length);
                  if (a.length > arr.length)
                      a[arr.length] = null;
                  return a;
              }
  
              /**
               * This method is overridden to protect the backing set against
               * an object with a nefarious equals function that senses
               * that the equality-candidate is Map.Entry and calls its
               * setValue method.
               */
              public boolean contains(Object o) {
                  if (!(o instanceof Map.Entry))
                      return false;
                  return c.contains(
                      new UnmodifiableEntry<>((Entry<?,?>) o));
              }
  
              /**
               * The next two methods are overridden to protect against
               * an unscrupulous List whose contains(Object o) method senses
               * when o is a Map.Entry, and calls o.setValue.
               */
              public boolean containsAll(Collection<?> coll) {
                  for (Object e : coll) {
                      if (!contains(e)) // Invokes safe contains() above
                          return false;
                  }
                  return true;
              }
              public boolean equals(Object o) {
                  if (o == this)
                      return true;
  
                  if (!(o instanceof Set))
                      return false;
                  Set<?> s = (Set<?>) o;
                  if (s.size() != c.size())
                      return false;
                  return containsAll(s); // Invokes safe containsAll() above
              }
  
              /**
               * This "wrapper class" serves two purposes: it prevents
               * the client from modifying the backing Map, by short-circuiting
               * the setValue method, and it protects the backing Map against
               * an ill-behaved Map.Entry that attempts to modify another
               * Map Entry when asked to perform an equality check.
               */
              private static class UnmodifiableEntry<K,V> implements Entry<K,V> {
                  private Entry<? extends K, ? extends V> e;
  
                  UnmodifiableEntry(Entry<? extends K, ? extends V> e)
                          {this.e = Objects.requireNonNull(e);}
  
                  public K getKey()        {return e.getKey();}
                  public V getValue()      {return e.getValue();}
                  public V setValue(V value) {
                      throw new UnsupportedOperationException();
                  }
                  public int hashCode()    {return e.hashCode();}
                  public boolean equals(Object o) {
                      if (this == o)
                          return true;
                      if (!(o instanceof Map.Entry))
                          return false;
                      Entry<?,?> t = (Entry<?,?>)o;
                      return eq(e.getKey(),   t.getKey()) &&
                             eq(e.getValue(), t.getValue());
                  }
                  public String toString() {return e.toString();}
              }
          }
      }
  
      /**
       * Returns an unmodifiable view of the specified sorted map.  This method
       * allows modules to provide users with "read-only" access to internal
       * sorted maps.  Query operations on the returned sorted map "read through"
       * to the specified sorted map.  Attempts to modify the returned
       * sorted map, whether direct, via its collection views, or via its
       * <tt>subMap</tt>, <tt>headMap</tt>, or <tt>tailMap</tt> views, result in
       * an <tt>UnsupportedOperationException</tt>.<p>
       *
       * The returned sorted map will be serializable if the specified sorted map
       * is serializable.
       *
       * @param <K> the class of the map keys
       * @param <V> the class of the map values
       * @param m the sorted map for which an unmodifiable view is to be
       *        returned.
       * @return an unmodifiable view of the specified sorted map.
       */
      public static <K,V> SortedMap<K,V> unmodifiableSortedMap(SortedMap<K, ? extends V> m) {
          return new UnmodifiableSortedMap<>(m);
      }
  
      /**
       * @serial include
       */
      static class UnmodifiableSortedMap<K,V>
            extends UnmodifiableMap<K,V>
            implements SortedMap<K,V>, Serializable {
          private static final long serialVersionUID = -8806743815996713206L;
  
          private final SortedMap<K, ? extends V> sm;
  
          UnmodifiableSortedMap(SortedMap<K, ? extends V> m) {super(m); sm = m; }
          public Comparator<? super K> comparator()   { return sm.comparator(); }
          public SortedMap<K,V> subMap(K fromKey, K toKey)
               { return new UnmodifiableSortedMap<>(sm.subMap(fromKey, toKey)); }
          public SortedMap<K,V> headMap(K toKey)
                       { return new UnmodifiableSortedMap<>(sm.headMap(toKey)); }
          public SortedMap<K,V> tailMap(K fromKey)
                     { return new UnmodifiableSortedMap<>(sm.tailMap(fromKey)); }
          public K firstKey()                           { return sm.firstKey(); }
          public K lastKey()                             { return sm.lastKey(); }
      }
  
      /**
       * Returns an unmodifiable view of the specified navigable map.  This method
       * allows modules to provide users with "read-only" access to internal
       * navigable maps.  Query operations on the returned navigable map "read
       * through" to the specified navigable map.  Attempts to modify the returned
       * navigable map, whether direct, via its collection views, or via its
       * {@code subMap}, {@code headMap}, or {@code tailMap} views, result in
       * an {@code UnsupportedOperationException}.<p>
       *
       * The returned navigable map will be serializable if the specified
       * navigable map is serializable.
       *
       * @param <K> the class of the map keys
       * @param <V> the class of the map values
       * @param m the navigable map for which an unmodifiable view is to be
       *        returned
       * @return an unmodifiable view of the specified navigable map
       * @since 1.8
       */
      public static <K,V> NavigableMap<K,V> unmodifiableNavigableMap(NavigableMap<K, ? extends V> m) {
          return new UnmodifiableNavigableMap<>(m);
      }
  
      /**
       * @serial include
       */
      static class UnmodifiableNavigableMap<K,V>
            extends UnmodifiableSortedMap<K,V>
            implements NavigableMap<K,V>, Serializable {
          private static final long serialVersionUID = -4858195264774772197L;
  
          /**
           * A class for the {@link EMPTY_NAVIGABLE_MAP} which needs readResolve
           * to preserve singleton property.
           *
           * @param <K> type of keys, if there were any, and of bounds
           * @param <V> type of values, if there were any
           */
          private static class EmptyNavigableMap<K,V> extends UnmodifiableNavigableMap<K,V>
              implements Serializable {
  
              private static final long serialVersionUID = -2239321462712562324L;
  
              EmptyNavigableMap()                       { super(new TreeMap<K,V>()); }
  
              @Override
              public NavigableSet<K> navigableKeySet()
                                                  { return emptyNavigableSet(); }
  
              private Object readResolve()        { return EMPTY_NAVIGABLE_MAP; }
          }
  
          /**
           * Singleton for {@link emptyNavigableMap()} which is also immutable.
           */
          private static final EmptyNavigableMap<?,?> EMPTY_NAVIGABLE_MAP =
              new EmptyNavigableMap<>();
  
          /**
           * The instance we wrap and protect.
           */
          private final NavigableMap<K, ? extends V> nm;
  
          UnmodifiableNavigableMap(NavigableMap<K, ? extends V> m)
                                                              {super(m); nm = m;}
  
          public K lowerKey(K key)                   { return nm.lowerKey(key); }
          public K floorKey(K key)                   { return nm.floorKey(key); }
          public K ceilingKey(K key)               { return nm.ceilingKey(key); }
          public K higherKey(K key)                 { return nm.higherKey(key); }
  
          @SuppressWarnings("unchecked")
          public Entry<K, V> lowerEntry(K key) {
              Entry<K,V> lower = (Entry<K, V>) nm.lowerEntry(key);
              return (null != lower)
                  ? new UnmodifiableEntrySet.UnmodifiableEntry<>(lower)
                  : null;
          }
  
          @SuppressWarnings("unchecked")
          public Entry<K, V> floorEntry(K key) {
              Entry<K,V> floor = (Entry<K, V>) nm.floorEntry(key);
              return (null != floor)
                  ? new UnmodifiableEntrySet.UnmodifiableEntry<>(floor)
                  : null;
          }
  
          @SuppressWarnings("unchecked")
          public Entry<K, V> ceilingEntry(K key) {
              Entry<K,V> ceiling = (Entry<K, V>) nm.ceilingEntry(key);
              return (null != ceiling)
                  ? new UnmodifiableEntrySet.UnmodifiableEntry<>(ceiling)
                  : null;
          }
  
  
          @SuppressWarnings("unchecked")
          public Entry<K, V> higherEntry(K key) {
              Entry<K,V> higher = (Entry<K, V>) nm.higherEntry(key);
              return (null != higher)
                  ? new UnmodifiableEntrySet.UnmodifiableEntry<>(higher)
                  : null;
          }
  
          @SuppressWarnings("unchecked")
          public Entry<K, V> firstEntry() {
              Entry<K,V> first = (Entry<K, V>) nm.firstEntry();
              return (null != first)
                  ? new UnmodifiableEntrySet.UnmodifiableEntry<>(first)
                  : null;
          }
  
          @SuppressWarnings("unchecked")
          public Entry<K, V> lastEntry() {
              Entry<K,V> last = (Entry<K, V>) nm.lastEntry();
              return (null != last)
                  ? new UnmodifiableEntrySet.UnmodifiableEntry<>(last)
                  : null;
          }
  
          public Entry<K, V> pollFirstEntry()
                                   { throw new UnsupportedOperationException(); }
          public Entry<K, V> pollLastEntry()
                                   { throw new UnsupportedOperationException(); }
          public NavigableMap<K, V> descendingMap()
                         { return unmodifiableNavigableMap(nm.descendingMap()); }
          public NavigableSet<K> navigableKeySet()
                       { return unmodifiableNavigableSet(nm.navigableKeySet()); }
          public NavigableSet<K> descendingKeySet()
                      { return unmodifiableNavigableSet(nm.descendingKeySet()); }
  
          public NavigableMap<K, V> subMap(K fromKey, boolean fromInclusive, K toKey, boolean toInclusive) {
              return unmodifiableNavigableMap(
                  nm.subMap(fromKey, fromInclusive, toKey, toInclusive));
          }
  
          public NavigableMap<K, V> headMap(K toKey, boolean inclusive)
               { return unmodifiableNavigableMap(nm.headMap(toKey, inclusive)); }
          public NavigableMap<K, V> tailMap(K fromKey, boolean inclusive)
             { return unmodifiableNavigableMap(nm.tailMap(fromKey, inclusive)); }
      }
  
      // Synch Wrappers
  
      /**
       * Returns a synchronized (thread-safe) collection backed by the specified
       * collection.  In order to guarantee serial access, it is critical that
       * <strong>all</strong> access to the backing collection is accomplished
       * through the returned collection.<p>
       *
       * It is imperative that the user manually synchronize on the returned
       * collection when traversing it via {@link Iterator}, {@link Spliterator}
       * or {@link Stream}:
       * <pre>
       *  Collection c = Collections.synchronizedCollection(myCollection);
       *     ...
       *  synchronized (c) {
       *      Iterator i = c.iterator(); // Must be in the synchronized block
       *      while (i.hasNext())
       *         foo(i.next());
       *  }
       * </pre>
       * Failure to follow this advice may result in non-deterministic behavior.
       *
       * <p>The returned collection does <i>not</i> pass the {@code hashCode}
       * and {@code equals} operations through to the backing collection, but
       * relies on {@code Object}'s equals and hashCode methods.  This is
       * necessary to preserve the contracts of these operations in the case
       * that the backing collection is a set or a list.<p>
       *
       * The returned collection will be serializable if the specified collection
       * is serializable.
       *
       * @param  <T> the class of the objects in the collection
       * @param  c the collection to be "wrapped" in a synchronized collection.
       * @return a synchronized view of the specified collection.
       */
      public static <T> Collection<T> synchronizedCollection(Collection<T> c) {
          return new SynchronizedCollection<>(c);
      }
  
      static <T> Collection<T> synchronizedCollection(Collection<T> c, Object mutex) {
          return new SynchronizedCollection<>(c, mutex);
      }
  
      /**
       * @serial include
       */
      static class SynchronizedCollection<E> implements Collection<E>, Serializable {
          private static final long serialVersionUID = 3053995032091335093L;
  
          final Collection<E> c;  // Backing Collection
          final Object mutex;     // Object on which to synchronize
  
          SynchronizedCollection(Collection<E> c) {
              this.c = Objects.requireNonNull(c);
              mutex = this;
          }
  
          SynchronizedCollection(Collection<E> c, Object mutex) {
              this.c = Objects.requireNonNull(c);
              this.mutex = Objects.requireNonNull(mutex);
          }
  
          public int size() {
              synchronized (mutex) {return c.size();}
          }
          public boolean isEmpty() {
              synchronized (mutex) {return c.isEmpty();}
          }
          public boolean contains(Object o) {
              synchronized (mutex) {return c.contains(o);}
          }
          public Object[] toArray() {
              synchronized (mutex) {return c.toArray();}
          }
          public <T> T[] toArray(T[] a) {
              synchronized (mutex) {return c.toArray(a);}
          }
  
          public Iterator<E> iterator() {
              return c.iterator(); // Must be manually synched by user!
          }
  
          public boolean add(E e) {
              synchronized (mutex) {return c.add(e);}
          }
          public boolean remove(Object o) {
              synchronized (mutex) {return c.remove(o);}
          }
  
          public boolean containsAll(Collection<?> coll) {
              synchronized (mutex) {return c.containsAll(coll);}
          }
          public boolean addAll(Collection<? extends E> coll) {
              synchronized (mutex) {return c.addAll(coll);}
          }
          public boolean removeAll(Collection<?> coll) {
              synchronized (mutex) {return c.removeAll(coll);}
          }
          public boolean retainAll(Collection<?> coll) {
              synchronized (mutex) {return c.retainAll(coll);}
          }
          public void clear() {
              synchronized (mutex) {c.clear();}
          }
          public String toString() {
              synchronized (mutex) {return c.toString();}
          }
          // Override default methods in Collection
          @Override
          public void forEach(Consumer<? super E> consumer) {
              synchronized (mutex) {c.forEach(consumer);}
          }
          @Override
          public boolean removeIf(Predicate<? super E> filter) {
              synchronized (mutex) {return c.removeIf(filter);}
          }
          @Override
          public Spliterator<E> spliterator() {
              return c.spliterator(); // Must be manually synched by user!
          }
          @Override
          public Stream<E> stream() {
              return c.stream(); // Must be manually synched by user!
          }
          @Override
          public Stream<E> parallelStream() {
              return c.parallelStream(); // Must be manually synched by user!
          }
          private void writeObject(ObjectOutputStream s) throws IOException {
              synchronized (mutex) {s.defaultWriteObject();}
          }
      }
  
      /**
       * Returns a synchronized (thread-safe) set backed by the specified
       * set.  In order to guarantee serial access, it is critical that
       * <strong>all</strong> access to the backing set is accomplished
       * through the returned set.<p>
       *
       * It is imperative that the user manually synchronize on the returned
       * set when iterating over it:
       * <pre>
       *  Set s = Collections.synchronizedSet(new HashSet());
       *      ...
       *  synchronized (s) {
       *      Iterator i = s.iterator(); // Must be in the synchronized block
       *      while (i.hasNext())
       *          foo(i.next());
       *  }
       * </pre>
       * Failure to follow this advice may result in non-deterministic behavior.
       *
       * <p>The returned set will be serializable if the specified set is
       * serializable.
       *
       * @param  <T> the class of the objects in the set
       * @param  s the set to be "wrapped" in a synchronized set.
       * @return a synchronized view of the specified set.
       */
      public static <T> Set<T> synchronizedSet(Set<T> s) {
          return new SynchronizedSet<>(s);
      }
  
      static <T> Set<T> synchronizedSet(Set<T> s, Object mutex) {
          return new SynchronizedSet<>(s, mutex);
      }
  
      /**
       * @serial include
       */
      static class SynchronizedSet<E>
            extends SynchronizedCollection<E>
            implements Set<E> {
          private static final long serialVersionUID = 487447009682186044L;
  
          SynchronizedSet(Set<E> s) {
              super(s);
          }
          SynchronizedSet(Set<E> s, Object mutex) {
              super(s, mutex);
          }
  
          public boolean equals(Object o) {
              if (this == o)
                  return true;
              synchronized (mutex) {return c.equals(o);}
          }
          public int hashCode() {
              synchronized (mutex) {return c.hashCode();}
          }
      }
  
      /**
       * Returns a synchronized (thread-safe) sorted set backed by the specified
       * sorted set.  In order to guarantee serial access, it is critical that
       * <strong>all</strong> access to the backing sorted set is accomplished
       * through the returned sorted set (or its views).<p>
       *
       * It is imperative that the user manually synchronize on the returned
       * sorted set when iterating over it or any of its <tt>subSet</tt>,
       * <tt>headSet</tt>, or <tt>tailSet</tt> views.
       * <pre>
       *  SortedSet s = Collections.synchronizedSortedSet(new TreeSet());
       *      ...
       *  synchronized (s) {
       *      Iterator i = s.iterator(); // Must be in the synchronized block
       *      while (i.hasNext())
       *          foo(i.next());
       *  }
       * </pre>
       * or:
       * <pre>
       *  SortedSet s = Collections.synchronizedSortedSet(new TreeSet());
       *  SortedSet s2 = s.headSet(foo);
       *      ...
       *  synchronized (s) {  // Note: s, not s2!!!
       *      Iterator i = s2.iterator(); // Must be in the synchronized block
       *      while (i.hasNext())
       *          foo(i.next());
       *  }
       * </pre>
       * Failure to follow this advice may result in non-deterministic behavior.
       *
       * <p>The returned sorted set will be serializable if the specified
       * sorted set is serializable.
       *
       * @param  <T> the class of the objects in the set
       * @param  s the sorted set to be "wrapped" in a synchronized sorted set.
       * @return a synchronized view of the specified sorted set.
       */
      public static <T> SortedSet<T> synchronizedSortedSet(SortedSet<T> s) {
          return new SynchronizedSortedSet<>(s);
      }
  
      /**
       * @serial include
       */
      static class SynchronizedSortedSet<E>
          extends SynchronizedSet<E>
          implements SortedSet<E>
      {
          private static final long serialVersionUID = 8695801310862127406L;
  
          private final SortedSet<E> ss;
  
          SynchronizedSortedSet(SortedSet<E> s) {
              super(s);
              ss = s;
          }
          SynchronizedSortedSet(SortedSet<E> s, Object mutex) {
              super(s, mutex);
              ss = s;
          }
  
          public Comparator<? super E> comparator() {
              synchronized (mutex) {return ss.comparator();}
          }
  
          public SortedSet<E> subSet(E fromElement, E toElement) {
              synchronized (mutex) {
                  return new SynchronizedSortedSet<>(
                      ss.subSet(fromElement, toElement), mutex);
              }
          }
          public SortedSet<E> headSet(E toElement) {
              synchronized (mutex) {
                  return new SynchronizedSortedSet<>(ss.headSet(toElement), mutex);
              }
          }
          public SortedSet<E> tailSet(E fromElement) {
              synchronized (mutex) {
                 return new SynchronizedSortedSet<>(ss.tailSet(fromElement),mutex);
              }
          }
  
          public E first() {
              synchronized (mutex) {return ss.first();}
          }
          public E last() {
              synchronized (mutex) {return ss.last();}
          }
      }
  
      /**
       * Returns a synchronized (thread-safe) navigable set backed by the
       * specified navigable set.  In order to guarantee serial access, it is
       * critical that <strong>all</strong> access to the backing navigable set is
       * accomplished through the returned navigable set (or its views).<p>
       *
       * It is imperative that the user manually synchronize on the returned
       * navigable set when iterating over it or any of its {@code subSet},
       * {@code headSet}, or {@code tailSet} views.
       * <pre>
       *  NavigableSet s = Collections.synchronizedNavigableSet(new TreeSet());
       *      ...
       *  synchronized (s) {
       *      Iterator i = s.iterator(); // Must be in the synchronized block
       *      while (i.hasNext())
       *          foo(i.next());
       *  }
       * </pre>
       * or:
       * <pre>
       *  NavigableSet s = Collections.synchronizedNavigableSet(new TreeSet());
       *  NavigableSet s2 = s.headSet(foo, true);
       *      ...
       *  synchronized (s) {  // Note: s, not s2!!!
       *      Iterator i = s2.iterator(); // Must be in the synchronized block
       *      while (i.hasNext())
       *          foo(i.next());
       *  }
       * </pre>
       * Failure to follow this advice may result in non-deterministic behavior.
       *
       * <p>The returned navigable set will be serializable if the specified
       * navigable set is serializable.
       *
       * @param  <T> the class of the objects in the set
       * @param  s the navigable set to be "wrapped" in a synchronized navigable
       * set
       * @return a synchronized view of the specified navigable set
       * @since 1.8
       */
      public static <T> NavigableSet<T> synchronizedNavigableSet(NavigableSet<T> s) {
          return new SynchronizedNavigableSet<>(s);
      }
  
      /**
       * @serial include
       */
      static class SynchronizedNavigableSet<E>
          extends SynchronizedSortedSet<E>
          implements NavigableSet<E>
      {
          private static final long serialVersionUID = -5505529816273629798L;
  
          private final NavigableSet<E> ns;
  
          SynchronizedNavigableSet(NavigableSet<E> s) {
              super(s);
              ns = s;
          }
  
          SynchronizedNavigableSet(NavigableSet<E> s, Object mutex) {
              super(s, mutex);
              ns = s;
          }
          public E lower(E e)      { synchronized (mutex) {return ns.lower(e);} }
          public E floor(E e)      { synchronized (mutex) {return ns.floor(e);} }
          public E ceiling(E e)  { synchronized (mutex) {return ns.ceiling(e);} }
          public E higher(E e)    { synchronized (mutex) {return ns.higher(e);} }
          public E pollFirst()  { synchronized (mutex) {return ns.pollFirst();} }
          public E pollLast()    { synchronized (mutex) {return ns.pollLast();} }
  
          public NavigableSet<E> descendingSet() {
              synchronized (mutex) {
                  return new SynchronizedNavigableSet<>(ns.descendingSet(), mutex);
              }
          }
  
          public Iterator<E> descendingIterator()
                   { synchronized (mutex) { return descendingSet().iterator(); } }
  
          public NavigableSet<E> subSet(E fromElement, E toElement) {
              synchronized (mutex) {
                  return new SynchronizedNavigableSet<>(ns.subSet(fromElement, true, toElement, false), mutex);
              }
          }
          public NavigableSet<E> headSet(E toElement) {
              synchronized (mutex) {
                  return new SynchronizedNavigableSet<>(ns.headSet(toElement, false), mutex);
              }
          }
          public NavigableSet<E> tailSet(E fromElement) {
              synchronized (mutex) {
                  return new SynchronizedNavigableSet<>(ns.tailSet(fromElement, true), mutex);
              }
          }
  
          public NavigableSet<E> subSet(E fromElement, boolean fromInclusive, E toElement, boolean toInclusive) {
              synchronized (mutex) {
                  return new SynchronizedNavigableSet<>(ns.subSet(fromElement, fromInclusive, toElement, toInclusive), mutex);
              }
          }
  
          public NavigableSet<E> headSet(E toElement, boolean inclusive) {
              synchronized (mutex) {
                  return new SynchronizedNavigableSet<>(ns.headSet(toElement, inclusive), mutex);
              }
          }
  
          public NavigableSet<E> tailSet(E fromElement, boolean inclusive) {
              synchronized (mutex) {
                  return new SynchronizedNavigableSet<>(ns.tailSet(fromElement, inclusive), mutex);
              }
          }
      }
  
      /**
       * Returns a synchronized (thread-safe) list backed by the specified
       * list.  In order to guarantee serial access, it is critical that
       * <strong>all</strong> access to the backing list is accomplished
       * through the returned list.<p>
       *
       * It is imperative that the user manually synchronize on the returned
       * list when iterating over it:
       * <pre>
       *  List list = Collections.synchronizedList(new ArrayList());
       *      ...
       *  synchronized (list) {
       *      Iterator i = list.iterator(); // Must be in synchronized block
       *      while (i.hasNext())
       *          foo(i.next());
       *  }
       * </pre>
       * Failure to follow this advice may result in non-deterministic behavior.
       *
       * <p>The returned list will be serializable if the specified list is
       * serializable.
       *
       * @param  <T> the class of the objects in the list
       * @param  list the list to be "wrapped" in a synchronized list.
       * @return a synchronized view of the specified list.
       */
      public static <T> List<T> synchronizedList(List<T> list) {
          return (list instanceof RandomAccess ?
                  new SynchronizedRandomAccessList<>(list) :
                  new SynchronizedList<>(list));
      }
  
      static <T> List<T> synchronizedList(List<T> list, Object mutex) {
          return (list instanceof RandomAccess ?
                  new SynchronizedRandomAccessList<>(list, mutex) :
                  new SynchronizedList<>(list, mutex));
      }
  
      /**
       * @serial include
       */
      static class SynchronizedList<E>
          extends SynchronizedCollection<E>
          implements List<E> {
          private static final long serialVersionUID = -7754090372962971524L;
  
          final List<E> list;
  
          SynchronizedList(List<E> list) {
              super(list);
              this.list = list;
          }
          SynchronizedList(List<E> list, Object mutex) {
              super(list, mutex);
              this.list = list;
          }
  
          public boolean equals(Object o) {
              if (this == o)
                  return true;
              synchronized (mutex) {return list.equals(o);}
          }
          public int hashCode() {
              synchronized (mutex) {return list.hashCode();}
          }
  
          public E get(int index) {
              synchronized (mutex) {return list.get(index);}
          }
          public E set(int index, E element) {
              synchronized (mutex) {return list.set(index, element);}
          }
          public void add(int index, E element) {
              synchronized (mutex) {list.add(index, element);}
          }
          public E remove(int index) {
              synchronized (mutex) {return list.remove(index);}
          }
  
          public int indexOf(Object o) {
              synchronized (mutex) {return list.indexOf(o);}
          }
          public int lastIndexOf(Object o) {
              synchronized (mutex) {return list.lastIndexOf(o);}
          }
  
          public boolean addAll(int index, Collection<? extends E> c) {
              synchronized (mutex) {return list.addAll(index, c);}
          }
  
          public ListIterator<E> listIterator() {
              return list.listIterator(); // Must be manually synched by user
          }
  
          public ListIterator<E> listIterator(int index) {
              return list.listIterator(index); // Must be manually synched by user
          }
  
          public List<E> subList(int fromIndex, int toIndex) {
              synchronized (mutex) {
                  return new SynchronizedList<>(list.subList(fromIndex, toIndex),
                                              mutex);
              }
          }
  
          @Override
          public void replaceAll(UnaryOperator<E> operator) {
              synchronized (mutex) {list.replaceAll(operator);}
          }
          @Override
          public void sort(Comparator<? super E> c) {
              synchronized (mutex) {list.sort(c);}
          }
  
          /**
           * SynchronizedRandomAccessList instances are serialized as
           * SynchronizedList instances to allow them to be deserialized
           * in pre-1.4 JREs (which do not have SynchronizedRandomAccessList).
           * This method inverts the transformation.  As a beneficial
           * side-effect, it also grafts the RandomAccess marker onto
           * SynchronizedList instances that were serialized in pre-1.4 JREs.
           *
           * Note: Unfortunately, SynchronizedRandomAccessList instances
           * serialized in 1.4.1 and deserialized in 1.4 will become
           * SynchronizedList instances, as this method was missing in 1.4.
           */
          private Object readResolve() {
              return (list instanceof RandomAccess
                      ? new SynchronizedRandomAccessList<>(list)
                      : this);
          }
      }
  
      /**
       * @serial include
       */
      static class SynchronizedRandomAccessList<E>
          extends SynchronizedList<E>
          implements RandomAccess {
  
          SynchronizedRandomAccessList(List<E> list) {
              super(list);
          }
  
          SynchronizedRandomAccessList(List<E> list, Object mutex) {
              super(list, mutex);
          }
  
          public List<E> subList(int fromIndex, int toIndex) {
              synchronized (mutex) {
                  return new SynchronizedRandomAccessList<>(
                      list.subList(fromIndex, toIndex), mutex);
              }
          }
  
          private static final long serialVersionUID = 1530674583602358482L;
  
          /**
           * Allows instances to be deserialized in pre-1.4 JREs (which do
           * not have SynchronizedRandomAccessList).  SynchronizedList has
           * a readResolve method that inverts this transformation upon
           * deserialization.
           */
          private Object writeReplace() {
              return new SynchronizedList<>(list);
          }
      }
  
      /**
       * Returns a synchronized (thread-safe) map backed by the specified
       * map.  In order to guarantee serial access, it is critical that
       * <strong>all</strong> access to the backing map is accomplished
       * through the returned map.<p>
       *
       * It is imperative that the user manually synchronize on the returned
       * map when iterating over any of its collection views:
       * <pre>
       *  Map m = Collections.synchronizedMap(new HashMap());
       *      ...
       *  Set s = m.keySet();  // Needn't be in synchronized block
       *      ...
       *  synchronized (m) {  // Synchronizing on m, not s!
       *      Iterator i = s.iterator(); // Must be in synchronized block
       *      while (i.hasNext())
       *          foo(i.next());
       *  }
       * </pre>
       * Failure to follow this advice may result in non-deterministic behavior.
       *
       * <p>The returned map will be serializable if the specified map is
       * serializable.
       *
       * @param <K> the class of the map keys
       * @param <V> the class of the map values
       * @param  m the map to be "wrapped" in a synchronized map.
       * @return a synchronized view of the specified map.
       */
      public static <K,V> Map<K,V> synchronizedMap(Map<K,V> m) {
          return new SynchronizedMap<>(m);
      }
  
      /**
       * @serial include
       */
      private static class SynchronizedMap<K,V>
          implements Map<K,V>, Serializable {
          private static final long serialVersionUID = 1978198479659022715L;
  
          private final Map<K,V> m;     // Backing Map
          final Object      mutex;        // Object on which to synchronize
  
          SynchronizedMap(Map<K,V> m) {
              this.m = Objects.requireNonNull(m);
              mutex = this;
          }
  
          SynchronizedMap(Map<K,V> m, Object mutex) {
              this.m = m;
              this.mutex = mutex;
          }
  
          public int size() {
              synchronized (mutex) {return m.size();}
          }
          public boolean isEmpty() {
              synchronized (mutex) {return m.isEmpty();}
          }
          public boolean containsKey(Object key) {
              synchronized (mutex) {return m.containsKey(key);}
          }
          public boolean containsValue(Object value) {
              synchronized (mutex) {return m.containsValue(value);}
          }
          public V get(Object key) {
              synchronized (mutex) {return m.get(key);}
          }
  
          public V put(K key, V value) {
              synchronized (mutex) {return m.put(key, value);}
          }
          public V remove(Object key) {
              synchronized (mutex) {return m.remove(key);}
          }
          public void putAll(Map<? extends K, ? extends V> map) {
              synchronized (mutex) {m.putAll(map);}
          }
          public void clear() {
              synchronized (mutex) {m.clear();}
          }
  
          private transient Set<K> keySet;
          private transient Set<Entry<K,V>> entrySet;
          private transient Collection<V> values;
  
          public Set<K> keySet() {
              synchronized (mutex) {
                  if (keySet==null)
                      keySet = new SynchronizedSet<>(m.keySet(), mutex);
                  return keySet;
              }
          }
  
          public Set<Entry<K,V>> entrySet() {
              synchronized (mutex) {
                  if (entrySet==null)
                      entrySet = new SynchronizedSet<>(m.entrySet(), mutex);
                  return entrySet;
              }
          }
  
          public Collection<V> values() {
              synchronized (mutex) {
                  if (values==null)
                      values = new SynchronizedCollection<>(m.values(), mutex);
                  return values;
              }
          }
  
          public boolean equals(Object o) {
              if (this == o)
                  return true;
              synchronized (mutex) {return m.equals(o);}
          }
          public int hashCode() {
              synchronized (mutex) {return m.hashCode();}
          }
          public String toString() {
              synchronized (mutex) {return m.toString();}
          }
  
          // Override default methods in Map
          @Override
          public V getOrDefault(Object k, V defaultValue) {
              synchronized (mutex) {return m.getOrDefault(k, defaultValue);}
          }
          @Override
          public void forEach(BiConsumer<? super K, ? super V> action) {
              synchronized (mutex) {m.forEach(action);}
          }
          @Override
          public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
              synchronized (mutex) {m.replaceAll(function);}
          }
          @Override
          public V putIfAbsent(K key, V value) {
              synchronized (mutex) {return m.putIfAbsent(key, value);}
          }
          @Override
          public boolean remove(Object key, Object value) {
              synchronized (mutex) {return m.remove(key, value);}
          }
          @Override
          public boolean replace(K key, V oldValue, V newValue) {
              synchronized (mutex) {return m.replace(key, oldValue, newValue);}
          }
          @Override
          public V replace(K key, V value) {
              synchronized (mutex) {return m.replace(key, value);}
          }
          @Override
          public V computeIfAbsent(K key,
                  Function<? super K, ? extends V> mappingFunction) {
              synchronized (mutex) {return m.computeIfAbsent(key, mappingFunction);}
          }
          @Override
          public V computeIfPresent(K key,
                  BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
              synchronized (mutex) {return m.computeIfPresent(key, remappingFunction);}
          }
          @Override
          public V compute(K key,
                  BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
              synchronized (mutex) {return m.compute(key, remappingFunction);}
          }
          @Override
          public V merge(K key, V value,
                  BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
              synchronized (mutex) {return m.merge(key, value, remappingFunction);}
          }
  
          private void writeObject(ObjectOutputStream s) throws IOException {
              synchronized (mutex) {s.defaultWriteObject();}
          }
      }
  
      /**
       * Returns a synchronized (thread-safe) sorted map backed by the specified
       * sorted map.  In order to guarantee serial access, it is critical that
       * <strong>all</strong> access to the backing sorted map is accomplished
       * through the returned sorted map (or its views).<p>
       *
       * It is imperative that the user manually synchronize on the returned
       * sorted map when iterating over any of its collection views, or the
       * collections views of any of its <tt>subMap</tt>, <tt>headMap</tt> or
       * <tt>tailMap</tt> views.
       * <pre>
       *  SortedMap m = Collections.synchronizedSortedMap(new TreeMap());
       *      ...
       *  Set s = m.keySet();  // Needn't be in synchronized block
       *      ...
       *  synchronized (m) {  // Synchronizing on m, not s!
       *      Iterator i = s.iterator(); // Must be in synchronized block
       *      while (i.hasNext())
       *          foo(i.next());
       *  }
       * </pre>
       * or:
       * <pre>
       *  SortedMap m = Collections.synchronizedSortedMap(new TreeMap());
       *  SortedMap m2 = m.subMap(foo, bar);
       *      ...
       *  Set s2 = m2.keySet();  // Needn't be in synchronized block
       *      ...
       *  synchronized (m) {  // Synchronizing on m, not m2 or s2!
       *      Iterator i = s.iterator(); // Must be in synchronized block
       *      while (i.hasNext())
       *          foo(i.next());
       *  }
       * </pre>
       * Failure to follow this advice may result in non-deterministic behavior.
       *
       * <p>The returned sorted map will be serializable if the specified
       * sorted map is serializable.
       *
       * @param <K> the class of the map keys
       * @param <V> the class of the map values
       * @param  m the sorted map to be "wrapped" in a synchronized sorted map.
       * @return a synchronized view of the specified sorted map.
       */
      public static <K,V> SortedMap<K,V> synchronizedSortedMap(SortedMap<K,V> m) {
          return new SynchronizedSortedMap<>(m);
      }
  
      /**
       * @serial include
       */
      static class SynchronizedSortedMap<K,V>
          extends SynchronizedMap<K,V>
          implements SortedMap<K,V>
      {
          private static final long serialVersionUID = -8798146769416483793L;
  
          private final SortedMap<K,V> sm;
  
          SynchronizedSortedMap(SortedMap<K,V> m) {
              super(m);
              sm = m;
          }
          SynchronizedSortedMap(SortedMap<K,V> m, Object mutex) {
              super(m, mutex);
              sm = m;
          }
  
          public Comparator<? super K> comparator() {
              synchronized (mutex) {return sm.comparator();}
          }
  
          public SortedMap<K,V> subMap(K fromKey, K toKey) {
              synchronized (mutex) {
                  return new SynchronizedSortedMap<>(
                      sm.subMap(fromKey, toKey), mutex);
              }
          }
          public SortedMap<K,V> headMap(K toKey) {
              synchronized (mutex) {
                  return new SynchronizedSortedMap<>(sm.headMap(toKey), mutex);
              }
          }
          public SortedMap<K,V> tailMap(K fromKey) {
              synchronized (mutex) {
                 return new SynchronizedSortedMap<>(sm.tailMap(fromKey),mutex);
              }
          }
  
          public K firstKey() {
              synchronized (mutex) {return sm.firstKey();}
          }
          public K lastKey() {
              synchronized (mutex) {return sm.lastKey();}
          }
      }
  
      /**
       * Returns a synchronized (thread-safe) navigable map backed by the
       * specified navigable map.  In order to guarantee serial access, it is
       * critical that <strong>all</strong> access to the backing navigable map is
       * accomplished through the returned navigable map (or its views).<p>
       *
       * It is imperative that the user manually synchronize on the returned
       * navigable map when iterating over any of its collection views, or the
       * collections views of any of its {@code subMap}, {@code headMap} or
       * {@code tailMap} views.
       * <pre>
       *  NavigableMap m = Collections.synchronizedNavigableMap(new TreeMap());
       *      ...
       *  Set s = m.keySet();  // Needn't be in synchronized block
       *      ...
       *  synchronized (m) {  // Synchronizing on m, not s!
       *      Iterator i = s.iterator(); // Must be in synchronized block
       *      while (i.hasNext())
       *          foo(i.next());
       *  }
       * </pre>
       * or:
       * <pre>
       *  NavigableMap m = Collections.synchronizedNavigableMap(new TreeMap());
       *  NavigableMap m2 = m.subMap(foo, true, bar, false);
       *      ...
       *  Set s2 = m2.keySet();  // Needn't be in synchronized block
       *      ...
       *  synchronized (m) {  // Synchronizing on m, not m2 or s2!
       *      Iterator i = s.iterator(); // Must be in synchronized block
       *      while (i.hasNext())
       *          foo(i.next());
       *  }
       * </pre>
       * Failure to follow this advice may result in non-deterministic behavior.
       *
       * <p>The returned navigable map will be serializable if the specified
       * navigable map is serializable.
       *
       * @param <K> the class of the map keys
       * @param <V> the class of the map values
       * @param  m the navigable map to be "wrapped" in a synchronized navigable
       *              map
       * @return a synchronized view of the specified navigable map.
       * @since 1.8
       */
      public static <K,V> NavigableMap<K,V> synchronizedNavigableMap(NavigableMap<K,V> m) {
          return new SynchronizedNavigableMap<>(m);
      }
  
      /**
       * A synchronized NavigableMap.
       *
       * @serial include
       */
      static class SynchronizedNavigableMap<K,V>
          extends SynchronizedSortedMap<K,V>
          implements NavigableMap<K,V>
      {
          private static final long serialVersionUID = 699392247599746807L;
  
          private final NavigableMap<K,V> nm;
  
          SynchronizedNavigableMap(NavigableMap<K,V> m) {
              super(m);
              nm = m;
          }
          SynchronizedNavigableMap(NavigableMap<K,V> m, Object mutex) {
              super(m, mutex);
              nm = m;
          }
  
          public Entry<K, V> lowerEntry(K key)
                          { synchronized (mutex) { return nm.lowerEntry(key); } }
          public K lowerKey(K key)
                            { synchronized (mutex) { return nm.lowerKey(key); } }
          public Entry<K, V> floorEntry(K key)
                          { synchronized (mutex) { return nm.floorEntry(key); } }
          public K floorKey(K key)
                            { synchronized (mutex) { return nm.floorKey(key); } }
          public Entry<K, V> ceilingEntry(K key)
                        { synchronized (mutex) { return nm.ceilingEntry(key); } }
          public K ceilingKey(K key)
                          { synchronized (mutex) { return nm.ceilingKey(key); } }
          public Entry<K, V> higherEntry(K key)
                         { synchronized (mutex) { return nm.higherEntry(key); } }
          public K higherKey(K key)
                           { synchronized (mutex) { return nm.higherKey(key); } }
          public Entry<K, V> firstEntry()
                             { synchronized (mutex) { return nm.firstEntry(); } }
          public Entry<K, V> lastEntry()
                              { synchronized (mutex) { return nm.lastEntry(); } }
          public Entry<K, V> pollFirstEntry()
                         { synchronized (mutex) { return nm.pollFirstEntry(); } }
          public Entry<K, V> pollLastEntry()
                          { synchronized (mutex) { return nm.pollLastEntry(); } }
  
          public NavigableMap<K, V> descendingMap() {
              synchronized (mutex) {
                  return
                      new SynchronizedNavigableMap<>(nm.descendingMap(), mutex);
              }
          }
  
          public NavigableSet<K> keySet() {
              return navigableKeySet();
          }
  
          public NavigableSet<K> navigableKeySet() {
              synchronized (mutex) {
                  return new SynchronizedNavigableSet<>(nm.navigableKeySet(), mutex);
              }
          }
  
          public NavigableSet<K> descendingKeySet() {
              synchronized (mutex) {
                  return new SynchronizedNavigableSet<>(nm.descendingKeySet(), mutex);
              }
          }
  
  
          public SortedMap<K,V> subMap(K fromKey, K toKey) {
              synchronized (mutex) {
                  return new SynchronizedNavigableMap<>(
                      nm.subMap(fromKey, true, toKey, false), mutex);
              }
          }
          public SortedMap<K,V> headMap(K toKey) {
              synchronized (mutex) {
                  return new SynchronizedNavigableMap<>(nm.headMap(toKey, false), mutex);
              }
          }
          public SortedMap<K,V> tailMap(K fromKey) {
              synchronized (mutex) {
          return new SynchronizedNavigableMap<>(nm.tailMap(fromKey, true),mutex);
              }
          }
  
          public NavigableMap<K, V> subMap(K fromKey, boolean fromInclusive, K toKey, boolean toInclusive) {
              synchronized (mutex) {
                  return new SynchronizedNavigableMap<>(
                      nm.subMap(fromKey, fromInclusive, toKey, toInclusive), mutex);
              }
          }
  
          public NavigableMap<K, V> headMap(K toKey, boolean inclusive) {
              synchronized (mutex) {
                  return new SynchronizedNavigableMap<>(
                          nm.headMap(toKey, inclusive), mutex);
              }
          }
  
          public NavigableMap<K, V> tailMap(K fromKey, boolean inclusive) {
              synchronized (mutex) {
                  return new SynchronizedNavigableMap<>(
                      nm.tailMap(fromKey, inclusive), mutex);
              }
          }
      }
  
      // Dynamically typesafe collection wrappers
  
      /**
       * Returns a dynamically typesafe view of the specified collection.
       * Any attempt to insert an element of the wrong type will result in an
       * immediate {@link ClassCastException}.  Assuming a collection
       * contains no incorrectly typed elements prior to the time a
       * dynamically typesafe view is generated, and that all subsequent
       * access to the collection takes place through the view, it is
       * <i>guaranteed</i> that the collection cannot contain an incorrectly
       * typed element.
       *
       * <p>The generics mechanism in the language provides compile-time
       * (static) type checking, but it is possible to defeat this mechanism
       * with unchecked casts.  Usually this is not a problem, as the compiler
       * issues warnings on all such unchecked operations.  There are, however,
       * times when static type checking alone is not sufficient.  For example,
       * suppose a collection is passed to a third-party library and it is
       * imperative that the library code not corrupt the collection by
       * inserting an element of the wrong type.
       *
       * <p>Another use of dynamically typesafe views is debugging.  Suppose a
       * program fails with a {@code ClassCastException}, indicating that an
       * incorrectly typed element was put into a parameterized collection.
       * Unfortunately, the exception can occur at any time after the erroneous
       * element is inserted, so it typically provides little or no information
       * as to the real source of the problem.  If the problem is reproducible,
       * one can quickly determine its source by temporarily modifying the
       * program to wrap the collection with a dynamically typesafe view.
       * For example, this declaration:
       *  <pre> {@code
       *     Collection<String> c = new HashSet<>();
       * }</pre>
       * may be replaced temporarily by this one:
       *  <pre> {@code
       *     Collection<String> c = Collections.checkedCollection(
       *         new HashSet<>(), String.class);
       * }</pre>
       * Running the program again will cause it to fail at the point where
       * an incorrectly typed element is inserted into the collection, clearly
       * identifying the source of the problem.  Once the problem is fixed, the
       * modified declaration may be reverted back to the original.
       *
       * <p>The returned collection does <i>not</i> pass the hashCode and equals
       * operations through to the backing collection, but relies on
       * {@code Object}'s {@code equals} and {@code hashCode} methods.  This
       * is necessary to preserve the contracts of these operations in the case
       * that the backing collection is a set or a list.
       *
       * <p>The returned collection will be serializable if the specified
       * collection is serializable.
       *
       * <p>Since {@code null} is considered to be a value of any reference
       * type, the returned collection permits insertion of null elements
       * whenever the backing collection does.
       *
       * @param <E> the class of the objects in the collection
       * @param c the collection for which a dynamically typesafe view is to be
       *          returned
       * @param type the type of element that {@code c} is permitted to hold
       * @return a dynamically typesafe view of the specified collection
       * @since 1.5
       */
      public static <E> Collection<E> checkedCollection(Collection<E> c,
                                                        Class<E> type) {
          return new CheckedCollection<>(c, type);
      }
  
      @SuppressWarnings("unchecked")
      static <T> T[] zeroLengthArray(Class<T> type) {
          return (T[]) Array.newInstance(type, 0);
      }
  
      /**
       * @serial include
       */
      static class CheckedCollection<E> implements Collection<E>, Serializable {
          private static final long serialVersionUID = 1578914078182001775L;
  
          final Collection<E> c;
          final Class<E> type;
  
          @SuppressWarnings("unchecked")
          E typeCheck(Object o) {
              if (o != null && !type.isInstance(o))
                  throw new ClassCastException(badElementMsg(o));
              return (E) o;
          }
  
          private String badElementMsg(Object o) {
              return "Attempt to insert " + o.getClass() +
                  " element into collection with element type " + type;
          }
  
          CheckedCollection(Collection<E> c, Class<E> type) {
              this.c = Objects.requireNonNull(c, "c");
              this.type = Objects.requireNonNull(type, "type");
          }
  
          public int size()                 { return c.size(); }
          public boolean isEmpty()          { return c.isEmpty(); }
          public boolean contains(Object o) { return c.contains(o); }
          public Object[] toArray()         { return c.toArray(); }
          public <T> T[] toArray(T[] a)     { return c.toArray(a); }
          public String toString()          { return c.toString(); }
          public boolean remove(Object o)   { return c.remove(o); }
          public void clear()               {        c.clear(); }
  
          public boolean containsAll(Collection<?> coll) {
              return c.containsAll(coll);
          }
          public boolean removeAll(Collection<?> coll) {
              return c.removeAll(coll);
          }
          public boolean retainAll(Collection<?> coll) {
              return c.retainAll(coll);
          }
  
          public Iterator<E> iterator() {
              // JDK-6363904 - unwrapped iterator could be typecast to
              // ListIterator with unsafe set()
              final Iterator<E> it = c.iterator();
              return new Iterator<E>() {
                  public boolean hasNext() { return it.hasNext(); }
                  public E next()          { return it.next(); }
                  public void remove()     {        it.remove(); }};
              // Android-note: Oversight of Iterator.forEachRemaining().
              // http://b/110351017
          }
  
          public boolean add(E e)          { return c.add(typeCheck(e)); }
  
          private E[] zeroLengthElementArray; // Lazily initialized
  
          private E[] zeroLengthElementArray() {
              return zeroLengthElementArray != null ? zeroLengthElementArray :
                  (zeroLengthElementArray = zeroLengthArray(type));
          }
  
          @SuppressWarnings("unchecked")
          Collection<E> checkedCopyOf(Collection<? extends E> coll) {
              Object[] a;
              try {
                  E[] z = zeroLengthElementArray();
                  a = coll.toArray(z);
                  // Defend against coll violating the toArray contract
                  if (a.getClass() != z.getClass())
                      a = Arrays.copyOf(a, a.length, z.getClass());
              } catch (ArrayStoreException ignore) {
                  // To get better and consistent diagnostics,
                  // we call typeCheck explicitly on each element.
                  // We call clone() to defend against coll retaining a
                  // reference to the returned array and storing a bad
                  // element into it after it has been type checked.
                  a = coll.toArray().clone();
                  for (Object o : a)
                      typeCheck(o);
              }
              // A slight abuse of the type system, but safe here.
              return (Collection<E>) Arrays.asList(a);
          }
  
          public boolean addAll(Collection<? extends E> coll) {
              // Doing things this way insulates us from concurrent changes
              // in the contents of coll and provides all-or-nothing
              // semantics (which we wouldn't get if we type-checked each
              // element as we added it)
              return c.addAll(checkedCopyOf(coll));
          }
  
          // Override default methods in Collection
          @Override
          public void forEach(Consumer<? super E> action) {c.forEach(action);}
          @Override
          public boolean removeIf(Predicate<? super E> filter) {
              return c.removeIf(filter);
          }
          @Override
          public Spliterator<E> spliterator() {return c.spliterator();}
          @Override
          public Stream<E> stream()           {return c.stream();}
          @Override
          public Stream<E> parallelStream()   {return c.parallelStream();}
      }
  
      /**
       * Returns a dynamically typesafe view of the specified queue.
       * Any attempt to insert an element of the wrong type will result in
       * an immediate {@link ClassCastException}.  Assuming a queue contains
       * no incorrectly typed elements prior to the time a dynamically typesafe
       * view is generated, and that all subsequent access to the queue
       * takes place through the view, it is <i>guaranteed</i> that the
       * queue cannot contain an incorrectly typed element.
       *
       * <p>A discussion of the use of dynamically typesafe views may be
       * found in the documentation for the {@link #checkedCollection
       * checkedCollection} method.
       *
       * <p>The returned queue will be serializable if the specified queue
       * is serializable.
       *
       * <p>Since {@code null} is considered to be a value of any reference
       * type, the returned queue permits insertion of {@code null} elements
       * whenever the backing queue does.
       *
       * @param <E> the class of the objects in the queue
       * @param queue the queue for which a dynamically typesafe view is to be
       *             returned
       * @param type the type of element that {@code queue} is permitted to hold
       * @return a dynamically typesafe view of the specified queue
       * @since 1.8
       */
      public static <E> Queue<E> checkedQueue(Queue<E> queue, Class<E> type) {
          return new CheckedQueue<>(queue, type);
      }
  
      /**
       * @serial include
       */
      static class CheckedQueue<E>
          extends CheckedCollection<E>
          implements Queue<E>, Serializable
      {
          private static final long serialVersionUID = 1433151992604707767L;
          final Queue<E> queue;
  
          CheckedQueue(Queue<E> queue, Class<E> elementType) {
              super(queue, elementType);
              this.queue = queue;
          }
  
          public E element()              {return queue.element();}
          public boolean equals(Object o) {return o == this || c.equals(o);}
          public int hashCode()           {return c.hashCode();}
          public E peek()                 {return queue.peek();}
          public E poll()                 {return queue.poll();}
          public E remove()               {return queue.remove();}
          public boolean offer(E e)       {return queue.offer(typeCheck(e));}
      }
  
      /**
       * Returns a dynamically typesafe view of the specified set.
       * Any attempt to insert an element of the wrong type will result in
       * an immediate {@link ClassCastException}.  Assuming a set contains
       * no incorrectly typed elements prior to the time a dynamically typesafe
       * view is generated, and that all subsequent access to the set
       * takes place through the view, it is <i>guaranteed</i> that the
       * set cannot contain an incorrectly typed element.
       *
       * <p>A discussion of the use of dynamically typesafe views may be
       * found in the documentation for the {@link #checkedCollection
       * checkedCollection} method.
       *
       * <p>The returned set will be serializable if the specified set is
       * serializable.
       *
       * <p>Since {@code null} is considered to be a value of any reference
       * type, the returned set permits insertion of null elements whenever
       * the backing set does.
       *
       * @param <E> the class of the objects in the set
       * @param s the set for which a dynamically typesafe view is to be
       *          returned
       * @param type the type of element that {@code s} is permitted to hold
       * @return a dynamically typesafe view of the specified set
       * @since 1.5
       */
      public static <E> Set<E> checkedSet(Set<E> s, Class<E> type) {
          return new CheckedSet<>(s, type);
      }
  
      /**
       * @serial include
       */
      static class CheckedSet<E> extends CheckedCollection<E>
                                   implements Set<E>, Serializable
      {
          private static final long serialVersionUID = 4694047833775013803L;
  
          CheckedSet(Set<E> s, Class<E> elementType) { super(s, elementType); }
  
          public boolean equals(Object o) { return o == this || c.equals(o); }
          public int hashCode()           { return c.hashCode(); }
      }
  
      /**
       * Returns a dynamically typesafe view of the specified sorted set.
       * Any attempt to insert an element of the wrong type will result in an
       * immediate {@link ClassCastException}.  Assuming a sorted set
       * contains no incorrectly typed elements prior to the time a
       * dynamically typesafe view is generated, and that all subsequent
       * access to the sorted set takes place through the view, it is
       * <i>guaranteed</i> that the sorted set cannot contain an incorrectly
       * typed element.
       *
       * <p>A discussion of the use of dynamically typesafe views may be
       * found in the documentation for the {@link #checkedCollection
       * checkedCollection} method.
       *
       * <p>The returned sorted set will be serializable if the specified sorted
       * set is serializable.
       *
       * <p>Since {@code null} is considered to be a value of any reference
       * type, the returned sorted set permits insertion of null elements
       * whenever the backing sorted set does.
       *
       * @param <E> the class of the objects in the set
       * @param s the sorted set for which a dynamically typesafe view is to be
       *          returned
       * @param type the type of element that {@code s} is permitted to hold
       * @return a dynamically typesafe view of the specified sorted set
       * @since 1.5
       */
      public static <E> SortedSet<E> checkedSortedSet(SortedSet<E> s,
                                                      Class<E> type) {
          return new CheckedSortedSet<>(s, type);
      }
  
      /**
       * @serial include
       */
      static class CheckedSortedSet<E> extends CheckedSet<E>
          implements SortedSet<E>, Serializable
      {
          private static final long serialVersionUID = 1599911165492914959L;
  
          private final SortedSet<E> ss;
  
          CheckedSortedSet(SortedSet<E> s, Class<E> type) {
              super(s, type);
              ss = s;
          }
  
          public Comparator<? super E> comparator() { return ss.comparator(); }
          public E first()                   { return ss.first(); }
          public E last()                    { return ss.last(); }
  
          public SortedSet<E> subSet(E fromElement, E toElement) {
              return checkedSortedSet(ss.subSet(fromElement,toElement), type);
          }
          public SortedSet<E> headSet(E toElement) {
              return checkedSortedSet(ss.headSet(toElement), type);
          }
          public SortedSet<E> tailSet(E fromElement) {
              return checkedSortedSet(ss.tailSet(fromElement), type);
          }
      }
  
  /**
       * Returns a dynamically typesafe view of the specified navigable set.
       * Any attempt to insert an element of the wrong type will result in an
       * immediate {@link ClassCastException}.  Assuming a navigable set
       * contains no incorrectly typed elements prior to the time a
       * dynamically typesafe view is generated, and that all subsequent
       * access to the navigable set takes place through the view, it is
       * <em>guaranteed</em> that the navigable set cannot contain an incorrectly
       * typed element.
       *
       * <p>A discussion of the use of dynamically typesafe views may be
       * found in the documentation for the {@link #checkedCollection
       * checkedCollection} method.
       *
       * <p>The returned navigable set will be serializable if the specified
       * navigable set is serializable.
       *
       * <p>Since {@code null} is considered to be a value of any reference
       * type, the returned navigable set permits insertion of null elements
       * whenever the backing sorted set does.
       *
       * @param <E> the class of the objects in the set
       * @param s the navigable set for which a dynamically typesafe view is to be
       *          returned
       * @param type the type of element that {@code s} is permitted to hold
       * @return a dynamically typesafe view of the specified navigable set
       * @since 1.8
       */
      public static <E> NavigableSet<E> checkedNavigableSet(NavigableSet<E> s,
                                                      Class<E> type) {
          return new CheckedNavigableSet<>(s, type);
      }
  
      /**
       * @serial include
       */
      static class CheckedNavigableSet<E> extends CheckedSortedSet<E>
          implements NavigableSet<E>, Serializable
      {
          private static final long serialVersionUID = -5429120189805438922L;
  
          private final NavigableSet<E> ns;
  
          CheckedNavigableSet(NavigableSet<E> s, Class<E> type) {
              super(s, type);
              ns = s;
          }
  
          public E lower(E e)                             { return ns.lower(e); }
          public E floor(E e)                             { return ns.floor(e); }
          public E ceiling(E e)                         { return ns.ceiling(e); }
          public E higher(E e)                           { return ns.higher(e); }
          public E pollFirst()                         { return ns.pollFirst(); }
          public E pollLast()                            {return ns.pollLast(); }
          public NavigableSet<E> descendingSet()
                        { return checkedNavigableSet(ns.descendingSet(), type); }
          public Iterator<E> descendingIterator()
              {return checkedNavigableSet(ns.descendingSet(), type).iterator(); }
  
          public NavigableSet<E> subSet(E fromElement, E toElement) {
              return checkedNavigableSet(ns.subSet(fromElement, true, toElement, false), type);
          }
          public NavigableSet<E> headSet(E toElement) {
              return checkedNavigableSet(ns.headSet(toElement, false), type);
          }
          public NavigableSet<E> tailSet(E fromElement) {
              return checkedNavigableSet(ns.tailSet(fromElement, true), type);
          }
  
          public NavigableSet<E> subSet(E fromElement, boolean fromInclusive, E toElement, boolean toInclusive) {
              return checkedNavigableSet(ns.subSet(fromElement, fromInclusive, toElement, toInclusive), type);
          }
  
          public NavigableSet<E> headSet(E toElement, boolean inclusive) {
              return checkedNavigableSet(ns.headSet(toElement, inclusive), type);
          }
  
          public NavigableSet<E> tailSet(E fromElement, boolean inclusive) {
              return checkedNavigableSet(ns.tailSet(fromElement, inclusive), type);
          }
      }
  
      /**
       * Returns a dynamically typesafe view of the specified list.
       * Any attempt to insert an element of the wrong type will result in
       * an immediate {@link ClassCastException}.  Assuming a list contains
       * no incorrectly typed elements prior to the time a dynamically typesafe
       * view is generated, and that all subsequent access to the list
       * takes place through the view, it is <i>guaranteed</i> that the
       * list cannot contain an incorrectly typed element.
       *
       * <p>A discussion of the use of dynamically typesafe views may be
       * found in the documentation for the {@link #checkedCollection
       * checkedCollection} method.
       *
       * <p>The returned list will be serializable if the specified list
       * is serializable.
       *
       * <p>Since {@code null} is considered to be a value of any reference
       * type, the returned list permits insertion of null elements whenever
       * the backing list does.
       *
       * @param <E> the class of the objects in the list
       * @param list the list for which a dynamically typesafe view is to be
       *             returned
       * @param type the type of element that {@code list} is permitted to hold
       * @return a dynamically typesafe view of the specified list
       * @since 1.5
       */
      public static <E> List<E> checkedList(List<E> list, Class<E> type) {
          return (list instanceof RandomAccess ?
                  new CheckedRandomAccessList<>(list, type) :
                  new CheckedList<>(list, type));
      }
  
      /**
       * @serial include
       */
      static class CheckedList<E>
          extends CheckedCollection<E>
          implements List<E>
      {
          private static final long serialVersionUID = 65247728283967356L;
          final List<E> list;
  
          CheckedList(List<E> list, Class<E> type) {
              super(list, type);
              this.list = list;
          }
  
          public boolean equals(Object o)  { return o == this || list.equals(o); }
          public int hashCode()            { return list.hashCode(); }
          public E get(int index)          { return list.get(index); }
          public E remove(int index)       { return list.remove(index); }
          public int indexOf(Object o)     { return list.indexOf(o); }
          public int lastIndexOf(Object o) { return list.lastIndexOf(o); }
  
          public E set(int index, E element) {
              return list.set(index, typeCheck(element));
          }
  
          public void add(int index, E element) {
              list.add(index, typeCheck(element));
          }
  
          public boolean addAll(int index, Collection<? extends E> c) {
              return list.addAll(index, checkedCopyOf(c));
          }
          public ListIterator<E> listIterator()   { return listIterator(0); }
  
          public ListIterator<E> listIterator(final int index) {
              final ListIterator<E> i = list.listIterator(index);
  
              return new ListIterator<E>() {
                  public boolean hasNext()     { return i.hasNext(); }
                  public E next()              { return i.next(); }
                  public boolean hasPrevious() { return i.hasPrevious(); }
                  public E previous()          { return i.previous(); }
                  public int nextIndex()       { return i.nextIndex(); }
                  public int previousIndex()   { return i.previousIndex(); }
                  public void remove()         {        i.remove(); }
  
                  public void set(E e) {
                      i.set(typeCheck(e));
                  }
  
                  public void add(E e) {
                      i.add(typeCheck(e));
                  }
  
                  @Override
                  public void forEachRemaining(Consumer<? super E> action) {
                      i.forEachRemaining(action);
                  }
              };
          }
  
          public List<E> subList(int fromIndex, int toIndex) {
              return new CheckedList<>(list.subList(fromIndex, toIndex), type);
          }
  
          /**
           * {@inheritDoc}
           *
           * @throws ClassCastException if the class of an element returned by the
           *         operator prevents it from being added to this collection. The
           *         exception may be thrown after some elements of the list have
           *         already been replaced.
           */
          @Override
          public void replaceAll(UnaryOperator<E> operator) {
              Objects.requireNonNull(operator);
              list.replaceAll(e -> typeCheck(operator.apply(e)));
          }
  
          @Override
          public void sort(Comparator<? super E> c) {
              list.sort(c);
          }
      }
  
      /**
       * @serial include
       */
      static class CheckedRandomAccessList<E> extends CheckedList<E>
                                              implements RandomAccess
      {
          private static final long serialVersionUID = 1638200125423088369L;
  
          CheckedRandomAccessList(List<E> list, Class<E> type) {
              super(list, type);
          }
  
          public List<E> subList(int fromIndex, int toIndex) {
              return new CheckedRandomAccessList<>(
                      list.subList(fromIndex, toIndex), type);
          }
      }
  
      /**
       * Returns a dynamically typesafe view of the specified map.
       * Any attempt to insert a mapping whose key or value have the wrong
       * type will result in an immediate {@link ClassCastException}.
       * Similarly, any attempt to modify the value currently associated with
       * a key will result in an immediate {@link ClassCastException},
       * whether the modification is attempted directly through the map
       * itself, or through a {@link Map.Entry} instance obtained from the
       * map's {@link Map#entrySet() entry set} view.
       *
       * <p>Assuming a map contains no incorrectly typed keys or values
       * prior to the time a dynamically typesafe view is generated, and
       * that all subsequent access to the map takes place through the view
       * (or one of its collection views), it is <i>guaranteed</i> that the
       * map cannot contain an incorrectly typed key or value.
       *
       * <p>A discussion of the use of dynamically typesafe views may be
       * found in the documentation for the {@link #checkedCollection
       * checkedCollection} method.
       *
       * <p>The returned map will be serializable if the specified map is
       * serializable.
       *
       * <p>Since {@code null} is considered to be a value of any reference
       * type, the returned map permits insertion of null keys or values
       * whenever the backing map does.
       *
       * @param <K> the class of the map keys
       * @param <V> the class of the map values
       * @param m the map for which a dynamically typesafe view is to be
       *          returned
       * @param keyType the type of key that {@code m} is permitted to hold
       * @param valueType the type of value that {@code m} is permitted to hold
       * @return a dynamically typesafe view of the specified map
       * @since 1.5
       */
      public static <K, V> Map<K, V> checkedMap(Map<K, V> m,
                                                Class<K> keyType,
                                                Class<V> valueType) {
          return new CheckedMap<>(m, keyType, valueType);
      }
  
  
      /**
       * @serial include
       */
      private static class CheckedMap<K,V>
          implements Map<K,V>, Serializable
      {
          private static final long serialVersionUID = 5742860141034234728L;
  
          private final Map<K, V> m;
          final Class<K> keyType;
          final Class<V> valueType;
  
          private void typeCheck(Object key, Object value) {
              if (key != null && !keyType.isInstance(key))
                  throw new ClassCastException(badKeyMsg(key));
  
              if (value != null && !valueType.isInstance(value))
                  throw new ClassCastException(badValueMsg(value));
          }
  
          private BiFunction<? super K, ? super V, ? extends V> typeCheck(
                  BiFunction<? super K, ? super V, ? extends V> func) {
              Objects.requireNonNull(func);
              return (k, v) -> {
                  V newValue = func.apply(k, v);
                  typeCheck(k, newValue);
                  return newValue;
              };
          }
  
          private String badKeyMsg(Object key) {
              return "Attempt to insert " + key.getClass() +
                      " key into map with key type " + keyType;
          }
  
          private String badValueMsg(Object value) {
              return "Attempt to insert " + value.getClass() +
                      " value into map with value type " + valueType;
          }
  
          CheckedMap(Map<K, V> m, Class<K> keyType, Class<V> valueType) {
              this.m = Objects.requireNonNull(m);
              this.keyType = Objects.requireNonNull(keyType);
              this.valueType = Objects.requireNonNull(valueType);
          }
  
          public int size()                      { return m.size(); }
          public boolean isEmpty()               { return m.isEmpty(); }
          public boolean containsKey(Object key) { return m.containsKey(key); }
          public boolean containsValue(Object v) { return m.containsValue(v); }
          public V get(Object key)               { return m.get(key); }
          public V remove(Object key)            { return m.remove(key); }
          public void clear()                    { m.clear(); }
          public Set<K> keySet()                 { return m.keySet(); }
          public Collection<V> values()          { return m.values(); }
          public boolean equals(Object o)        { return o == this || m.equals(o); }
          public int hashCode()                  { return m.hashCode(); }
          public String toString()               { return m.toString(); }
  
          public V put(K key, V value) {
              typeCheck(key, value);
              return m.put(key, value);
          }
  
          @SuppressWarnings("unchecked")
          public void putAll(Map<? extends K, ? extends V> t) {
              // Satisfy the following goals:
              // - good diagnostics in case of type mismatch
              // - all-or-nothing semantics
              // - protection from malicious t
              // - correct behavior if t is a concurrent map
              Object[] entries = t.entrySet().toArray();
              List<Entry<K,V>> checked = new ArrayList<>(entries.length);
              for (Object o : entries) {
                  Entry<?,?> e = (Entry<?,?>) o;
                  Object k = e.getKey();
                  Object v = e.getValue();
                  typeCheck(k, v);
                  checked.add(
                          new AbstractMap.SimpleImmutableEntry<>((K)k, (V)v));
              }
              for (Entry<K,V> e : checked)
                  m.put(e.getKey(), e.getValue());
          }
  
          private transient Set<Entry<K,V>> entrySet;
  
          public Set<Entry<K,V>> entrySet() {
              if (entrySet==null)
                  entrySet = new CheckedEntrySet<>(m.entrySet(), valueType);
              return entrySet;
          }
  
          // Override default methods in Map
          @Override
          public void forEach(BiConsumer<? super K, ? super V> action) {
              m.forEach(action);
          }
  
          @Override
          public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
              m.replaceAll(typeCheck(function));
          }
  
          @Override
          public V putIfAbsent(K key, V value) {
              typeCheck(key, value);
              return m.putIfAbsent(key, value);
          }
  
          @Override
          public boolean remove(Object key, Object value) {
              return m.remove(key, value);
          }
  
          @Override
          public boolean replace(K key, V oldValue, V newValue) {
              typeCheck(key, newValue);
              return m.replace(key, oldValue, newValue);
          }
  
          @Override
          public V replace(K key, V value) {
              typeCheck(key, value);
              return m.replace(key, value);
          }
  
          @Override
          public V computeIfAbsent(K key,
                  Function<? super K, ? extends V> mappingFunction) {
              Objects.requireNonNull(mappingFunction);
              return m.computeIfAbsent(key, k -> {
                  V value = mappingFunction.apply(k);
                  typeCheck(k, value);
                  return value;
              });
          }
  
          @Override
          public V computeIfPresent(K key,
                  BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
              return m.computeIfPresent(key, typeCheck(remappingFunction));
          }
  
          @Override
          public V compute(K key,
                  BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
              return m.compute(key, typeCheck(remappingFunction));
          }
  
          @Override
          public V merge(K key, V value,
                  BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
              Objects.requireNonNull(remappingFunction);
              return m.merge(key, value, (v1, v2) -> {
                  V newValue = remappingFunction.apply(v1, v2);
                  typeCheck(null, newValue);
                  return newValue;
              });
          }
  
          /**
           * We need this class in addition to CheckedSet as Map.Entry permits
           * modification of the backing Map via the setValue operation.  This
           * class is subtle: there are many possible attacks that must be
           * thwarted.
           *
           * @serial exclude
           */
          static class CheckedEntrySet<K,V> implements Set<Entry<K,V>> {
              private final Set<Entry<K,V>> s;
              private final Class<V> valueType;
  
              CheckedEntrySet(Set<Entry<K, V>> s, Class<V> valueType) {
                  this.s = s;
                  this.valueType = valueType;
              }
  
              public int size()        { return s.size(); }
              public boolean isEmpty() { return s.isEmpty(); }
              public String toString() { return s.toString(); }
              public int hashCode()    { return s.hashCode(); }
              public void clear()      {        s.clear(); }
  
              public boolean add(Entry<K, V> e) {
                  throw new UnsupportedOperationException();
              }
              public boolean addAll(Collection<? extends Entry<K, V>> coll) {
                  throw new UnsupportedOperationException();
              }
  
              public Iterator<Entry<K,V>> iterator() {
                  final Iterator<Entry<K, V>> i = s.iterator();
                  final Class<V> valueType = this.valueType;
  
                  return new Iterator<Entry<K,V>>() {
                      public boolean hasNext() { return i.hasNext(); }
                      public void remove()     { i.remove(); }
  
                      public Entry<K,V> next() {
                          return checkedEntry(i.next(), valueType);
                      }
                      // Android-note: Oversight of Iterator.forEachRemaining().
                      // http://b/110351017
                  };
              }
  
              // Android-changed: Ignore IsInstanceOfClass warning. b/73288967, b/73344263.
              // @SuppressWarnings("unchecked")
              @SuppressWarnings({ "unchecked", "IsInstanceOfClass" })
              public Object[] toArray() {
                  Object[] source = s.toArray();
  
                  /*
                   * Ensure that we don't get an ArrayStoreException even if
                   * s.toArray returns an array of something other than Object
                   */
                  Object[] dest = (CheckedEntry.class.isInstance(
                      source.getClass().getComponentType()) ? source :
                                   new Object[source.length]);
  
                  for (int i = 0; i < source.length; i++)
                      dest[i] = checkedEntry((Entry<K,V>)source[i],
                                             valueType);
                  return dest;
              }
  
              @SuppressWarnings("unchecked")
              public <T> T[] toArray(T[] a) {
                  // We don't pass a to s.toArray, to avoid window of
                  // vulnerability wherein an unscrupulous multithreaded client
                  // could get his hands on raw (unwrapped) Entries from s.
                  T[] arr = s.toArray(a.length==0 ? a : Arrays.copyOf(a, 0));
  
                  for (int i=0; i<arr.length; i++)
                      arr[i] = (T) checkedEntry((Entry<K,V>)arr[i],
                                                valueType);
                  if (arr.length > a.length)
                      return arr;
  
                  System.arraycopy(arr, 0, a, 0, arr.length);
                  if (a.length > arr.length)
                      a[arr.length] = null;
                  return a;
              }
  
              /**
               * This method is overridden to protect the backing set against
               * an object with a nefarious equals function that senses
               * that the equality-candidate is Map.Entry and calls its
               * setValue method.
               */
              public boolean contains(Object o) {
                  if (!(o instanceof Map.Entry))
                      return false;
                  Entry<?,?> e = (Entry<?,?>) o;
                  return s.contains(
                      (e instanceof CheckedEntry) ? e : checkedEntry(e, valueType));
              }
  
              /**
               * The bulk collection methods are overridden to protect
               * against an unscrupulous collection whose contains(Object o)
               * method senses when o is a Map.Entry, and calls o.setValue.
               */
              public boolean containsAll(Collection<?> c) {
                  for (Object o : c)
                      if (!contains(o)) // Invokes safe contains() above
                          return false;
                  return true;
              }
  
              public boolean remove(Object o) {
                  if (!(o instanceof Map.Entry))
                      return false;
                  return s.remove(new AbstractMap.SimpleImmutableEntry
                                  <>((Entry<?,?>)o));
              }
  
              public boolean removeAll(Collection<?> c) {
                  return batchRemove(c, false);
              }
              public boolean retainAll(Collection<?> c) {
                  return batchRemove(c, true);
              }
              private boolean batchRemove(Collection<?> c, boolean complement) {
                  Objects.requireNonNull(c);
                  boolean modified = false;
                  Iterator<Entry<K,V>> it = iterator();
                  while (it.hasNext()) {
                      if (c.contains(it.next()) != complement) {
                          it.remove();
                          modified = true;
                      }
                  }
                  return modified;
              }
  
              public boolean equals(Object o) {
                  if (o == this)
                      return true;
                  if (!(o instanceof Set))
                      return false;
                  Set<?> that = (Set<?>) o;
                  return that.size() == s.size()
                      && containsAll(that); // Invokes safe containsAll() above
              }
  
              static <K,V,T> CheckedEntry<K,V,T> checkedEntry(Entry<K,V> e,
                                                              Class<T> valueType) {
                  return new CheckedEntry<>(e, valueType);
              }
  
              /**
               * This "wrapper class" serves two purposes: it prevents
               * the client from modifying the backing Map, by short-circuiting
               * the setValue method, and it protects the backing Map against
               * an ill-behaved Map.Entry that attempts to modify another
               * Map.Entry when asked to perform an equality check.
               */
              private static class CheckedEntry<K,V,T> implements Entry<K,V> {
                  private final Entry<K, V> e;
                  private final Class<T> valueType;
  
                  CheckedEntry(Entry<K, V> e, Class<T> valueType) {
                      this.e = Objects.requireNonNull(e);
                      this.valueType = Objects.requireNonNull(valueType);
                  }
  
                  public K getKey()        { return e.getKey(); }
                  public V getValue()      { return e.getValue(); }
                  public int hashCode()    { return e.hashCode(); }
                  public String toString() { return e.toString(); }
  
                  public V setValue(V value) {
                      if (value != null && !valueType.isInstance(value))
                          throw new ClassCastException(badValueMsg(value));
                      return e.setValue(value);
                  }
  
                  private String badValueMsg(Object value) {
                      return "Attempt to insert " + value.getClass() +
                          " value into map with value type " + valueType;
                  }
  
                  public boolean equals(Object o) {
                      if (o == this)
                          return true;
                      if (!(o instanceof Map.Entry))
                          return false;
                      return e.equals(new AbstractMap.SimpleImmutableEntry
                                      <>((Entry<?,?>)o));
                  }
              }
          }
      }
  
      /**
       * Returns a dynamically typesafe view of the specified sorted map.
       * Any attempt to insert a mapping whose key or value have the wrong
       * type will result in an immediate {@link ClassCastException}.
       * Similarly, any attempt to modify the value currently associated with
       * a key will result in an immediate {@link ClassCastException},
       * whether the modification is attempted directly through the map
       * itself, or through a {@link Map.Entry} instance obtained from the
       * map's {@link Map#entrySet() entry set} view.
       *
       * <p>Assuming a map contains no incorrectly typed keys or values
       * prior to the time a dynamically typesafe view is generated, and
       * that all subsequent access to the map takes place through the view
       * (or one of its collection views), it is <i>guaranteed</i> that the
       * map cannot contain an incorrectly typed key or value.
       *
       * <p>A discussion of the use of dynamically typesafe views may be
       * found in the documentation for the {@link #checkedCollection
       * checkedCollection} method.
       *
       * <p>The returned map will be serializable if the specified map is
       * serializable.
       *
       * <p>Since {@code null} is considered to be a value of any reference
       * type, the returned map permits insertion of null keys or values
       * whenever the backing map does.
       *
       * @param <K> the class of the map keys
       * @param <V> the class of the map values
       * @param m the map for which a dynamically typesafe view is to be
       *          returned
       * @param keyType the type of key that {@code m} is permitted to hold
       * @param valueType the type of value that {@code m} is permitted to hold
       * @return a dynamically typesafe view of the specified map
       * @since 1.5
       */
      public static <K,V> SortedMap<K,V> checkedSortedMap(SortedMap<K, V> m,
                                                          Class<K> keyType,
                                                          Class<V> valueType) {
          return new CheckedSortedMap<>(m, keyType, valueType);
      }
  
      /**
       * @serial include
       */
      static class CheckedSortedMap<K,V> extends CheckedMap<K,V>
          implements SortedMap<K,V>, Serializable
      {
          private static final long serialVersionUID = 1599671320688067438L;
  
          private final SortedMap<K, V> sm;
  
          CheckedSortedMap(SortedMap<K, V> m,
                           Class<K> keyType, Class<V> valueType) {
              super(m, keyType, valueType);
              sm = m;
          }
  
          public Comparator<? super K> comparator() { return sm.comparator(); }
          public K firstKey()                       { return sm.firstKey(); }
          public K lastKey()                        { return sm.lastKey(); }
  
          public SortedMap<K,V> subMap(K fromKey, K toKey) {
              return checkedSortedMap(sm.subMap(fromKey, toKey),
                                      keyType, valueType);
          }
          public SortedMap<K,V> headMap(K toKey) {
              return checkedSortedMap(sm.headMap(toKey), keyType, valueType);
          }
          public SortedMap<K,V> tailMap(K fromKey) {
              return checkedSortedMap(sm.tailMap(fromKey), keyType, valueType);
          }
      }
  
      /**
       * Returns a dynamically typesafe view of the specified navigable map.
       * Any attempt to insert a mapping whose key or value have the wrong
       * type will result in an immediate {@link ClassCastException}.
       * Similarly, any attempt to modify the value currently associated with
       * a key will result in an immediate {@link ClassCastException},
       * whether the modification is attempted directly through the map
       * itself, or through a {@link Map.Entry} instance obtained from the
       * map's {@link Map#entrySet() entry set} view.
       *
       * <p>Assuming a map contains no incorrectly typed keys or values
       * prior to the time a dynamically typesafe view is generated, and
       * that all subsequent access to the map takes place through the view
       * (or one of its collection views), it is <em>guaranteed</em> that the
       * map cannot contain an incorrectly typed key or value.
       *
       * <p>A discussion of the use of dynamically typesafe views may be
       * found in the documentation for the {@link #checkedCollection
       * checkedCollection} method.
       *
       * <p>The returned map will be serializable if the specified map is
       * serializable.
       *
       * <p>Since {@code null} is considered to be a value of any reference
       * type, the returned map permits insertion of null keys or values
       * whenever the backing map does.
       *
       * @param <K> type of map keys
       * @param <V> type of map values
       * @param m the map for which a dynamically typesafe view is to be
       *          returned
       * @param keyType the type of key that {@code m} is permitted to hold
       * @param valueType the type of value that {@code m} is permitted to hold
       * @return a dynamically typesafe view of the specified map
       * @since 1.8
       */
      public static <K,V> NavigableMap<K,V> checkedNavigableMap(NavigableMap<K, V> m,
                                                          Class<K> keyType,
                                                          Class<V> valueType) {
          return new CheckedNavigableMap<>(m, keyType, valueType);
      }
  
      /**
       * @serial include
       */
      static class CheckedNavigableMap<K,V> extends CheckedSortedMap<K,V>
          implements NavigableMap<K,V>, Serializable
      {
          private static final long serialVersionUID = -4852462692372534096L;
  
          private final NavigableMap<K, V> nm;
  
          CheckedNavigableMap(NavigableMap<K, V> m,
                           Class<K> keyType, Class<V> valueType) {
              super(m, keyType, valueType);
              nm = m;
          }
  
          public Comparator<? super K> comparator()   { return nm.comparator(); }
          public K firstKey()                           { return nm.firstKey(); }
          public K lastKey()                             { return nm.lastKey(); }
  
          public Entry<K, V> lowerEntry(K key) {
              Entry<K,V> lower = nm.lowerEntry(key);
              return (null != lower)
                  ? new CheckedMap.CheckedEntrySet.CheckedEntry<>(lower, valueType)
                  : null;
          }
  
          public K lowerKey(K key)                   { return nm.lowerKey(key); }
  
          public Entry<K, V> floorEntry(K key) {
              Entry<K,V> floor = nm.floorEntry(key);
              return (null != floor)
                  ? new CheckedMap.CheckedEntrySet.CheckedEntry<>(floor, valueType)
                  : null;
          }
  
          public K floorKey(K key)                   { return nm.floorKey(key); }
  
          public Entry<K, V> ceilingEntry(K key) {
              Entry<K,V> ceiling = nm.ceilingEntry(key);
              return (null != ceiling)
                  ? new CheckedMap.CheckedEntrySet.CheckedEntry<>(ceiling, valueType)
                  : null;
          }
  
          public K ceilingKey(K key)               { return nm.ceilingKey(key); }
  
          public Entry<K, V> higherEntry(K key) {
              Entry<K,V> higher = nm.higherEntry(key);
              return (null != higher)
                  ? new CheckedMap.CheckedEntrySet.CheckedEntry<>(higher, valueType)
                  : null;
          }
  
          public K higherKey(K key)                 { return nm.higherKey(key); }
  
          public Entry<K, V> firstEntry() {
              Entry<K,V> first = nm.firstEntry();
              return (null != first)
                  ? new CheckedMap.CheckedEntrySet.CheckedEntry<>(first, valueType)
                  : null;
          }
  
          public Entry<K, V> lastEntry() {
              Entry<K,V> last = nm.lastEntry();
              return (null != last)
                  ? new CheckedMap.CheckedEntrySet.CheckedEntry<>(last, valueType)
                  : null;
          }
  
          public Entry<K, V> pollFirstEntry() {
              Entry<K,V> entry = nm.pollFirstEntry();
              return (null == entry)
                  ? null
                  : new CheckedMap.CheckedEntrySet.CheckedEntry<>(entry, valueType);
          }
  
          public Entry<K, V> pollLastEntry() {
              Entry<K,V> entry = nm.pollLastEntry();
              return (null == entry)
                  ? null
                  : new CheckedMap.CheckedEntrySet.CheckedEntry<>(entry, valueType);
          }
  
          public NavigableMap<K, V> descendingMap() {
              return checkedNavigableMap(nm.descendingMap(), keyType, valueType);
          }
  
          public NavigableSet<K> keySet() {
              return navigableKeySet();
          }
  
          public NavigableSet<K> navigableKeySet() {
              return checkedNavigableSet(nm.navigableKeySet(), keyType);
          }
  
          public NavigableSet<K> descendingKeySet() {
              return checkedNavigableSet(nm.descendingKeySet(), keyType);
          }
  
          @Override
          public NavigableMap<K,V> subMap(K fromKey, K toKey) {
              return checkedNavigableMap(nm.subMap(fromKey, true, toKey, false),
                                      keyType, valueType);
          }
  
          @Override
          public NavigableMap<K,V> headMap(K toKey) {
              return checkedNavigableMap(nm.headMap(toKey, false), keyType, valueType);
          }
  
          @Override
          public NavigableMap<K,V> tailMap(K fromKey) {
              return checkedNavigableMap(nm.tailMap(fromKey, true), keyType, valueType);
          }
  
          public NavigableMap<K, V> subMap(K fromKey, boolean fromInclusive, K toKey, boolean toInclusive) {
              return checkedNavigableMap(nm.subMap(fromKey, fromInclusive, toKey, toInclusive), keyType, valueType);
          }
  
          public NavigableMap<K, V> headMap(K toKey, boolean inclusive) {
              return checkedNavigableMap(nm.headMap(toKey, inclusive), keyType, valueType);
          }
  
          public NavigableMap<K, V> tailMap(K fromKey, boolean inclusive) {
              return checkedNavigableMap(nm.tailMap(fromKey, inclusive), keyType, valueType);
          }
      }
  
      // Empty collections
  
      /**
       * Returns an iterator that has no elements.  More precisely,
       *
       * <ul>
       * <li>{@link Iterator#hasNext hasNext} always returns {@code
       * false}.</li>
       * <li>{@link Iterator#next next} always throws {@link
       * NoSuchElementException}.</li>
       * <li>{@link Iterator#remove remove} always throws {@link
       * IllegalStateException}.</li>
       * </ul>
       *
       * <p>Implementations of this method are permitted, but not
       * required, to return the same object from multiple invocations.
       *
       * @param <T> type of elements, if there were any, in the iterator
       * @return an empty iterator
       * @since 1.7
       */
      @SuppressWarnings("unchecked")
      public static <T> Iterator<T> emptyIterator() {
          return (Iterator<T>) EmptyIterator.EMPTY_ITERATOR;
      }
  
      private static class EmptyIterator<E> implements Iterator<E> {
          static final EmptyIterator<Object> EMPTY_ITERATOR
              = new EmptyIterator<>();
  
          public boolean hasNext() { return false; }
          public E next() { throw new NoSuchElementException(); }
          public void remove() { throw new IllegalStateException(); }
          @Override
          public void forEachRemaining(Consumer<? super E> action) {
              Objects.requireNonNull(action);
          }
      }
  
      /**
       * Returns a list iterator that has no elements.  More precisely,
       *
       * <ul>
       * <li>{@link Iterator#hasNext hasNext} and {@link
       * ListIterator#hasPrevious hasPrevious} always return {@code
       * false}.</li>
       * <li>{@link Iterator#next next} and {@link ListIterator#previous
       * previous} always throw {@link NoSuchElementException}.</li>
       * <li>{@link Iterator#remove remove} and {@link ListIterator#set
       * set} always throw {@link IllegalStateException}.</li>
       * <li>{@link ListIterator#add add} always throws {@link
       * UnsupportedOperationException}.</li>
       * <li>{@link ListIterator#nextIndex nextIndex} always returns
       * {@code 0}.</li>
       * <li>{@link ListIterator#previousIndex previousIndex} always
       * returns {@code -1}.</li>
       * </ul>
       *
       * <p>Implementations of this method are permitted, but not
       * required, to return the same object from multiple invocations.
       *
       * @param <T> type of elements, if there were any, in the iterator
       * @return an empty list iterator
       * @since 1.7
       */
      @SuppressWarnings("unchecked")
      public static <T> ListIterator<T> emptyListIterator() {
          return (ListIterator<T>) EmptyListIterator.EMPTY_ITERATOR;
      }
  
      private static class EmptyListIterator<E>
          extends EmptyIterator<E>
          implements ListIterator<E>
      {
          static final EmptyListIterator<Object> EMPTY_ITERATOR
              = new EmptyListIterator<>();
  
          public boolean hasPrevious() { return false; }
          public E previous() { throw new NoSuchElementException(); }
          public int nextIndex()     { return 0; }
          public int previousIndex() { return -1; }
          public void set(E e) { throw new IllegalStateException(); }
          public void add(E e) { throw new UnsupportedOperationException(); }
      }
  
      /**
       * Returns an enumeration that has no elements.  More precisely,
       *
       * <ul>
       * <li>{@link Enumeration#hasMoreElements hasMoreElements} always
       * returns {@code false}.</li>
       * <li> {@link Enumeration#nextElement nextElement} always throws
       * {@link NoSuchElementException}.</li>
       * </ul>
       *
       * <p>Implementations of this method are permitted, but not
       * required, to return the same object from multiple invocations.
       *
       * @param  <T> the class of the objects in the enumeration
       * @return an empty enumeration
       * @since 1.7
       */
      @SuppressWarnings("unchecked")
      public static <T> Enumeration<T> emptyEnumeration() {
          return (Enumeration<T>) EmptyEnumeration.EMPTY_ENUMERATION;
      }
  
      private static class EmptyEnumeration<E> implements Enumeration<E> {
          static final EmptyEnumeration<Object> EMPTY_ENUMERATION
              = new EmptyEnumeration<>();
  
          public boolean hasMoreElements() { return false; }
          public E nextElement() { throw new NoSuchElementException(); }
      }
  
      /**
       * The empty set (immutable).  This set is serializable.
       *
       * @see #emptySet()
       */
      @SuppressWarnings("rawtypes")
      public static final Set EMPTY_SET = new EmptySet<>();
  
      /**
       * Returns an empty set (immutable).  This set is serializable.
       * Unlike the like-named field, this method is parameterized.
       *
       * <p>This example illustrates the type-safe way to obtain an empty set:
       * <pre>
       *     Set&lt;String&gt; s = Collections.emptySet();
       * </pre>
       * @implNote Implementations of this method need not create a separate
       * {@code Set} object for each call.  Using this method is likely to have
       * comparable cost to using the like-named field.  (Unlike this method, the
       * field does not provide type safety.)
       *
       * @param  <T> the class of the objects in the set
       * @return the empty set
       *
       * @see #EMPTY_SET
       * @since 1.5
       */
      @SuppressWarnings("unchecked")
      public static final <T> Set<T> emptySet() {
          return (Set<T>) EMPTY_SET;
      }
  
      /**
       * @serial include
       */
      private static class EmptySet<E>
          extends AbstractSet<E>
          implements Serializable
      {
          private static final long serialVersionUID = 1582296315990362920L;
  
          public Iterator<E> iterator() { return emptyIterator(); }
  
          public int size() {return 0;}
          public boolean isEmpty() {return true;}
  
          public boolean contains(Object obj) {return false;}
          public boolean containsAll(Collection<?> c) { return c.isEmpty(); }
  
          public Object[] toArray() { return new Object[0]; }
  
          public <T> T[] toArray(T[] a) {
              if (a.length > 0)
                  a[0] = null;
              return a;
          }
  
          // Override default methods in Collection
          @Override
          public void forEach(Consumer<? super E> action) {
              Objects.requireNonNull(action);
          }
          @Override
          public boolean removeIf(Predicate<? super E> filter) {
              Objects.requireNonNull(filter);
              return false;
          }
          @Override
          public Spliterator<E> spliterator() { return Spliterators.emptySpliterator(); }
  
          // Preserves singleton property
          private Object readResolve() {
              return EMPTY_SET;
          }
      }
  
      /**
       * Returns an empty sorted set (immutable).  This set is serializable.
       *
       * <p>This example illustrates the type-safe way to obtain an empty
       * sorted set:
       * <pre> {@code
       *     SortedSet<String> s = Collections.emptySortedSet();
       * }</pre>
       *
       * @implNote Implementations of this method need not create a separate
       * {@code SortedSet} object for each call.
       *
       * @param <E> type of elements, if there were any, in the set
       * @return the empty sorted set
       * @since 1.8
       */
      @SuppressWarnings("unchecked")
      public static <E> SortedSet<E> emptySortedSet() {
          return (SortedSet<E>) UnmodifiableNavigableSet.EMPTY_NAVIGABLE_SET;
      }
  
      /**
       * Returns an empty navigable set (immutable).  This set is serializable.
       *
       * <p>This example illustrates the type-safe way to obtain an empty
       * navigable set:
       * <pre> {@code
       *     NavigableSet<String> s = Collections.emptyNavigableSet();
       * }</pre>
       *
       * @implNote Implementations of this method need not
       * create a separate {@code NavigableSet} object for each call.
       *
       * @param <E> type of elements, if there were any, in the set
       * @return the empty navigable set
       * @since 1.8
       */
      @SuppressWarnings("unchecked")
      public static <E> NavigableSet<E> emptyNavigableSet() {
          return (NavigableSet<E>) UnmodifiableNavigableSet.EMPTY_NAVIGABLE_SET;
      }
  
      /**
       * The empty list (immutable).  This list is serializable.
       *
       * @see #emptyList()
       */
      @SuppressWarnings("rawtypes")
      public static final List EMPTY_LIST = new EmptyList<>();
  
      /**
       * Returns an empty list (immutable).  This list is serializable.
       *
       * <p>This example illustrates the type-safe way to obtain an empty list:
       * <pre>
       *     List&lt;String&gt; s = Collections.emptyList();
       * </pre>
       *
       * @implNote
       * Implementations of this method need not create a separate <tt>List</tt>
       * object for each call.   Using this method is likely to have comparable
       * cost to using the like-named field.  (Unlike this method, the field does
       * not provide type safety.)
       *
       * @param <T> type of elements, if there were any, in the list
       * @return an empty immutable list
       *
       * @see #EMPTY_LIST
       * @since 1.5
       */
      @SuppressWarnings("unchecked")
      public static final <T> List<T> emptyList() {
          return (List<T>) EMPTY_LIST;
      }
  
      /**
       * @serial include
       */
      private static class EmptyList<E>
          extends AbstractList<E>
          implements RandomAccess, Serializable {
          private static final long serialVersionUID = 8842843931221139166L;
  
          public Iterator<E> iterator() {
              return emptyIterator();
          }
          public ListIterator<E> listIterator() {
              return emptyListIterator();
          }
  
          public int size() {return 0;}
          public boolean isEmpty() {return true;}
  
          public boolean contains(Object obj) {return false;}
          public boolean containsAll(Collection<?> c) { return c.isEmpty(); }
  
          public Object[] toArray() { return new Object[0]; }
  
          public <T> T[] toArray(T[] a) {
              if (a.length > 0)
                  a[0] = null;
              return a;
          }
  
          public E get(int index) {
              throw new IndexOutOfBoundsException("Index: "+index);
          }
  
          public boolean equals(Object o) {
              return (o instanceof List) && ((List<?>)o).isEmpty();
          }
  
          public int hashCode() { return 1; }
  
          @Override
          public boolean removeIf(Predicate<? super E> filter) {
              Objects.requireNonNull(filter);
              return false;
          }
          @Override
          public void replaceAll(UnaryOperator<E> operator) {
              Objects.requireNonNull(operator);
          }
          @Override
          public void sort(Comparator<? super E> c) {
          }
  
          // Override default methods in Collection
          @Override
          public void forEach(Consumer<? super E> action) {
              Objects.requireNonNull(action);
          }
  
          @Override
          public Spliterator<E> spliterator() { return Spliterators.emptySpliterator(); }
  
          // Preserves singleton property
          private Object readResolve() {
              return EMPTY_LIST;
          }
      }
  
      /**
       * The empty map (immutable).  This map is serializable.
       *
       * @see #emptyMap()
       * @since 1.3
       */
      @SuppressWarnings("rawtypes")
      public static final Map EMPTY_MAP = new EmptyMap<>();
  
      /**
       * Returns an empty map (immutable).  This map is serializable.
       *
       * <p>This example illustrates the type-safe way to obtain an empty map:
       * <pre>
       *     Map&lt;String, Date&gt; s = Collections.emptyMap();
       * </pre>
       * @implNote Implementations of this method need not create a separate
       * {@code Map} object for each call.  Using this method is likely to have
       * comparable cost to using the like-named field.  (Unlike this method, the
       * field does not provide type safety.)
       *
       * @param <K> the class of the map keys
       * @param <V> the class of the map values
       * @return an empty map
       * @see #EMPTY_MAP
       * @since 1.5
       */
      @SuppressWarnings("unchecked")
      public static final <K,V> Map<K,V> emptyMap() {
          return (Map<K,V>) EMPTY_MAP;
      }
  
      /**
       * Returns an empty sorted map (immutable).  This map is serializable.
       *
       * <p>This example illustrates the type-safe way to obtain an empty map:
       * <pre> {@code
       *     SortedMap<String, Date> s = Collections.emptySortedMap();
       * }</pre>
       *
       * @implNote Implementations of this method need not create a separate
       * {@code SortedMap} object for each call.
       *
       * @param <K> the class of the map keys
       * @param <V> the class of the map values
       * @return an empty sorted map
       * @since 1.8
       */
      @SuppressWarnings("unchecked")
      public static final <K,V> SortedMap<K,V> emptySortedMap() {
          return (SortedMap<K,V>) UnmodifiableNavigableMap.EMPTY_NAVIGABLE_MAP;
      }
  
      /**
       * Returns an empty navigable map (immutable).  This map is serializable.
       *
       * <p>This example illustrates the type-safe way to obtain an empty map:
       * <pre> {@code
       *     NavigableMap<String, Date> s = Collections.emptyNavigableMap();
       * }</pre>
       *
       * @implNote Implementations of this method need not create a separate
       * {@code NavigableMap} object for each call.
       *
       * @param <K> the class of the map keys
       * @param <V> the class of the map values
       * @return an empty navigable map
       * @since 1.8
       */
      @SuppressWarnings("unchecked")
      public static final <K,V> NavigableMap<K,V> emptyNavigableMap() {
          return (NavigableMap<K,V>) UnmodifiableNavigableMap.EMPTY_NAVIGABLE_MAP;
      }
  
      /**
       * @serial include
       */
      private static class EmptyMap<K,V>
          extends AbstractMap<K,V>
          implements Serializable
      {
          private static final long serialVersionUID = 6428348081105594320L;
  
          public int size()                          {return 0;}
          public boolean isEmpty()                   {return true;}
          public boolean containsKey(Object key)     {return false;}
          public boolean containsValue(Object value) {return false;}
          public V get(Object key)                   {return null;}
          public Set<K> keySet()                     {return emptySet();}
          public Collection<V> values()              {return emptySet();}
          public Set<Entry<K,V>> entrySet()      {return emptySet();}
  
          public boolean equals(Object o) {
              return (o instanceof Map) && ((Map<?,?>)o).isEmpty();
          }
  
          public int hashCode()                      {return 0;}
  
          // Override default methods in Map
          @Override
          @SuppressWarnings("unchecked")
          public V getOrDefault(Object k, V defaultValue) {
              return defaultValue;
          }
  
          @Override
          public void forEach(BiConsumer<? super K, ? super V> action) {
              Objects.requireNonNull(action);
          }
  
          @Override
          public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
              Objects.requireNonNull(function);
          }
  
          @Override
          public V putIfAbsent(K key, V value) {
              throw new UnsupportedOperationException();
          }
  
          @Override
          public boolean remove(Object key, Object value) {
              throw new UnsupportedOperationException();
          }
  
          @Override
          public boolean replace(K key, V oldValue, V newValue) {
              throw new UnsupportedOperationException();
          }
  
          @Override
          public V replace(K key, V value) {
              throw new UnsupportedOperationException();
          }
  
          @Override
          public V computeIfAbsent(K key,
                  Function<? super K, ? extends V> mappingFunction) {
              throw new UnsupportedOperationException();
          }
  
          @Override
          public V computeIfPresent(K key,
                  BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
              throw new UnsupportedOperationException();
          }
  
          @Override
          public V compute(K key,
                  BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
              throw new UnsupportedOperationException();
          }
  
          @Override
          public V merge(K key, V value,
                  BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
              throw new UnsupportedOperationException();
          }
  
          // Preserves singleton property
          private Object readResolve() {
              return EMPTY_MAP;
          }
      }
  
      // Singleton collections
  
      /**
       * Returns an immutable set containing only the specified object.
       * The returned set is serializable.
       *
       * @param  <T> the class of the objects in the set
       * @param o the sole object to be stored in the returned set.
       * @return an immutable set containing only the specified object.
       */
      public static <T> Set<T> singleton(T o) {
          return new SingletonSet<>(o);
      }
  
      static <E> Iterator<E> singletonIterator(final E e) {
          return new Iterator<E>() {
              private boolean hasNext = true;
              public boolean hasNext() {
                  return hasNext;
              }
              public E next() {
                  if (hasNext) {
                      hasNext = false;
                      return e;
                  }
                  throw new NoSuchElementException();
              }
              public void remove() {
                  throw new UnsupportedOperationException();
              }
              @Override
              public void forEachRemaining(Consumer<? super E> action) {
                  Objects.requireNonNull(action);
                  if (hasNext) {
                      action.accept(e);
                      hasNext = false;
                  }
              }
          };
      }
  
      /**
       * Creates a {@code Spliterator} with only the specified element
       *
       * @param <T> Type of elements
       * @return A singleton {@code Spliterator}
       */
      static <T> Spliterator<T> singletonSpliterator(final T element) {
          return new Spliterator<T>() {
              long est = 1;
  
              @Override
              public Spliterator<T> trySplit() {
                  return null;
              }
  
              @Override
              public boolean tryAdvance(Consumer<? super T> consumer) {
                  Objects.requireNonNull(consumer);
                  if (est > 0) {
                      est--;
                      consumer.accept(element);
                      return true;
                  }
                  return false;
              }
  
              @Override
              public void forEachRemaining(Consumer<? super T> consumer) {
                  tryAdvance(consumer);
              }
  
              @Override
              public long estimateSize() {
                  return est;
              }
  
              @Override
              public int characteristics() {
                  int value = (element != null) ? Spliterator.NONNULL : 0;
  
                  return value | Spliterator.SIZED | Spliterator.SUBSIZED | Spliterator.IMMUTABLE |
                         Spliterator.DISTINCT | Spliterator.ORDERED;
              }
          };
      }
  
      /**
       * @serial include
       */
      private static class SingletonSet<E>
          extends AbstractSet<E>
          implements Serializable
      {
          private static final long serialVersionUID = 3193687207550431679L;
  
          private final E element;
  
          SingletonSet(E e) {element = e;}
  
          public Iterator<E> iterator() {
              return singletonIterator(element);
          }
  
          public int size() {return 1;}
  
          public boolean contains(Object o) {return eq(o, element);}
  
          // Override default methods for Collection
          @Override
          public void forEach(Consumer<? super E> action) {
              action.accept(element);
          }
          @Override
          public Spliterator<E> spliterator() {
              return singletonSpliterator(element);
          }
          @Override
          public boolean removeIf(Predicate<? super E> filter) {
              throw new UnsupportedOperationException();
          }
      }
  
      /**
       * Returns an immutable list containing only the specified object.
       * The returned list is serializable.
       *
       * @param  <T> the class of the objects in the list
       * @param o the sole object to be stored in the returned list.
       * @return an immutable list containing only the specified object.
       * @since 1.3
       */
      public static <T> List<T> singletonList(T o) {
          return new SingletonList<>(o);
      }
  
      /**
       * @serial include
       */
      private static class SingletonList<E>
          extends AbstractList<E>
          implements RandomAccess, Serializable {
  
          private static final long serialVersionUID = 3093736618740652951L;
  
          private final E element;
  
          SingletonList(E obj)                {element = obj;}
  
          public Iterator<E> iterator() {
              return singletonIterator(element);
          }
  
          public int size()                   {return 1;}
  
          public boolean contains(Object obj) {return eq(obj, element);}
  
          public E get(int index) {
              if (index != 0)
                throw new IndexOutOfBoundsException("Index: "+index+", Size: 1");
              return element;
          }
  
          // Override default methods for Collection
          @Override
          public void forEach(Consumer<? super E> action) {
              action.accept(element);
          }
          @Override
          public boolean removeIf(Predicate<? super E> filter) {
              throw new UnsupportedOperationException();
          }
          @Override
          public void replaceAll(UnaryOperator<E> operator) {
              throw new UnsupportedOperationException();
          }
          @Override
          public void sort(Comparator<? super E> c) {
          }
          @Override
          public Spliterator<E> spliterator() {
              return singletonSpliterator(element);
          }
      }
  
      /**
       * Returns an immutable map, mapping only the specified key to the
       * specified value.  The returned map is serializable.
       *
       * @param <K> the class of the map keys
       * @param <V> the class of the map values
       * @param key the sole key to be stored in the returned map.
       * @param value the value to which the returned map maps <tt>key</tt>.
       * @return an immutable map containing only the specified key-value
       *         mapping.
       * @since 1.3
       */
      public static <K,V> Map<K,V> singletonMap(K key, V value) {
          return new SingletonMap<>(key, value);
      }
  
      /**
       * @serial include
       */
      private static class SingletonMap<K,V>
            extends AbstractMap<K,V>
            implements Serializable {
          private static final long serialVersionUID = -6979724477215052911L;
  
          private final K k;
          private final V v;
  
          SingletonMap(K key, V value) {
              k = key;
              v = value;
          }
  
          public int size()                                           {return 1;}
          public boolean isEmpty()                                {return false;}
          public boolean containsKey(Object key)             {return eq(key, k);}
          public boolean containsValue(Object value)       {return eq(value, v);}
          public V get(Object key)              {return (eq(key, k) ? v : null);}
  
          private transient Set<K> keySet;
          private transient Set<Entry<K,V>> entrySet;
          private transient Collection<V> values;
  
          public Set<K> keySet() {
              if (keySet==null)
                  keySet = singleton(k);
              return keySet;
          }
  
          public Set<Entry<K,V>> entrySet() {
              if (entrySet==null)
                  entrySet = Collections.<Entry<K,V>>singleton(
                      new SimpleImmutableEntry<>(k, v));
              return entrySet;
          }
  
          public Collection<V> values() {
              if (values==null)
                  values = singleton(v);
              return values;
          }
  
          // Override default methods in Map
          @Override
          public V getOrDefault(Object key, V defaultValue) {
              return eq(key, k) ? v : defaultValue;
          }
  
          @Override
          public void forEach(BiConsumer<? super K, ? super V> action) {
              action.accept(k, v);
          }
  
          @Override
          public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
              throw new UnsupportedOperationException();
          }
  
          @Override
          public V putIfAbsent(K key, V value) {
              throw new UnsupportedOperationException();
          }
  
          @Override
          public boolean remove(Object key, Object value) {
              throw new UnsupportedOperationException();
          }
  
          @Override
          public boolean replace(K key, V oldValue, V newValue) {
              throw new UnsupportedOperationException();
          }
  
          @Override
          public V replace(K key, V value) {
              throw new UnsupportedOperationException();
          }
  
          @Override
          public V computeIfAbsent(K key,
                  Function<? super K, ? extends V> mappingFunction) {
              throw new UnsupportedOperationException();
          }
  
          @Override
          public V computeIfPresent(K key,
                  BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
              throw new UnsupportedOperationException();
          }
  
          @Override
          public V compute(K key,
                  BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
              throw new UnsupportedOperationException();
          }
  
          @Override
          public V merge(K key, V value,
                  BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
              throw new UnsupportedOperationException();
          }
      }
  
      // Miscellaneous
  
      /**
       * Returns an immutable list consisting of <tt>n</tt> copies of the
       * specified object.  The newly allocated data object is tiny (it contains
       * a single reference to the data object).  This method is useful in
       * combination with the <tt>List.addAll</tt> method to grow lists.
       * The returned list is serializable.
       *
       * @param  <T> the class of the object to copy and of the objects
       *         in the returned list.
       * @param  n the number of elements in the returned list.
       * @param  o the element to appear repeatedly in the returned list.
       * @return an immutable list consisting of <tt>n</tt> copies of the
       *         specified object.
       * @throws IllegalArgumentException if {@code n < 0}
       * @see    List#addAll(Collection)
       * @see    List#addAll(int, Collection)
       */
      public static <T> List<T> nCopies(int n, T o) {
          if (n < 0)
              throw new IllegalArgumentException("List length = " + n);
          return new CopiesList<>(n, o);
      }
  
      /**
       * @serial include
       */
      private static class CopiesList<E>
          extends AbstractList<E>
          implements RandomAccess, Serializable
      {
          private static final long serialVersionUID = 2739099268398711800L;
  
          final int n;
          final E element;
  
          CopiesList(int n, E e) {
              assert n >= 0;
              this.n = n;
              element = e;
          }
  
          public int size() {
              return n;
          }
  
          public boolean contains(Object obj) {
              return n != 0 && eq(obj, element);
          }
  
          public int indexOf(Object o) {
              return contains(o) ? 0 : -1;
          }
  
          public int lastIndexOf(Object o) {
              return contains(o) ? n - 1 : -1;
          }
  
          public E get(int index) {
              if (index < 0 || index >= n)
                  throw new IndexOutOfBoundsException("Index: "+index+
                                                      ", Size: "+n);
              return element;
          }
  
          public Object[] toArray() {
              final Object[] a = new Object[n];
              if (element != null)
                  Arrays.fill(a, 0, n, element);
              return a;
          }
  
          @SuppressWarnings("unchecked")
          public <T> T[] toArray(T[] a) {
              final int n = this.n;
              if (a.length < n) {
                  a = (T[]) Array
                      .newInstance(a.getClass().getComponentType(), n);
                  if (element != null)
                      Arrays.fill(a, 0, n, element);
              } else {
                  Arrays.fill(a, 0, n, element);
                  if (a.length > n)
                      a[n] = null;
              }
              return a;
          }
  
          public List<E> subList(int fromIndex, int toIndex) {
              if (fromIndex < 0)
                  throw new IndexOutOfBoundsException("fromIndex = " + fromIndex);
              if (toIndex > n)
                  throw new IndexOutOfBoundsException("toIndex = " + toIndex);
              if (fromIndex > toIndex)
                  throw new IllegalArgumentException("fromIndex(" + fromIndex +
                                                     ") > toIndex(" + toIndex + ")");
              return new CopiesList<>(toIndex - fromIndex, element);
          }
  
          // Override default methods in Collection
          @Override
          public Stream<E> stream() {
              return IntStream.range(0, n).mapToObj(i -> element);
          }
  
          @Override
          public Stream<E> parallelStream() {
              return IntStream.range(0, n).parallel().mapToObj(i -> element);
          }
  
          @Override
          public Spliterator<E> spliterator() {
              return stream().spliterator();
          }
      }
  
      /**
       * Returns a comparator that imposes the reverse of the <em>natural
       * ordering</em> on a collection of objects that implement the
       * {@code Comparable} interface.  (The natural ordering is the ordering
       * imposed by the objects' own {@code compareTo} method.)  This enables a
       * simple idiom for sorting (or maintaining) collections (or arrays) of
       * objects that implement the {@code Comparable} interface in
       * reverse-natural-order.  For example, suppose {@code a} is an array of
       * strings. Then: <pre>
       *          Arrays.sort(a, Collections.reverseOrder());
       * </pre> sorts the array in reverse-lexicographic (alphabetical) order.<p>
       *
       * The returned comparator is serializable.
       *
       * @param  <T> the class of the objects compared by the comparator
       * @return A comparator that imposes the reverse of the <i>natural
       *         ordering</i> on a collection of objects that implement
       *         the <tt>Comparable</tt> interface.
       * @see Comparable
       */
      @SuppressWarnings("unchecked")
      public static <T> Comparator<T> reverseOrder() {
          return (Comparator<T>) ReverseComparator.REVERSE_ORDER;
      }
  
      /**
       * @serial include
       */
      private static class ReverseComparator
          implements Comparator<Comparable<Object>>, Serializable {
  
          private static final long serialVersionUID = 7207038068494060240L;
  
          static final ReverseComparator REVERSE_ORDER
              = new ReverseComparator();
  
          public int compare(Comparable<Object> c1, Comparable<Object> c2) {
              return c2.compareTo(c1);
          }
  
          private Object readResolve() { return Collections.reverseOrder(); }
  
          @Override
          public Comparator<Comparable<Object>> reversed() {
              return Comparator.naturalOrder();
          }
      }
  
      /**
       * Returns a comparator that imposes the reverse ordering of the specified
       * comparator.  If the specified comparator is {@code null}, this method is
       * equivalent to {@link #reverseOrder()} (in other words, it returns a
       * comparator that imposes the reverse of the <em>natural ordering</em> on
       * a collection of objects that implement the Comparable interface).
       *
       * <p>The returned comparator is serializable (assuming the specified
       * comparator is also serializable or {@code null}).
       *
       * @param <T> the class of the objects compared by the comparator
       * @param cmp a comparator who's ordering is to be reversed by the returned
       * comparator or {@code null}
       * @return A comparator that imposes the reverse ordering of the
       *         specified comparator.
       * @since 1.5
       */
      public static <T> Comparator<T> reverseOrder(Comparator<T> cmp) {
          if (cmp == null)
              return reverseOrder();
  
          if (cmp instanceof ReverseComparator2)
              return ((ReverseComparator2<T>)cmp).cmp;
  
          return new ReverseComparator2<>(cmp);
      }
  
      /**
       * @serial include
       */
      private static class ReverseComparator2<T> implements Comparator<T>,
          Serializable
      {
          private static final long serialVersionUID = 4374092139857L;
  
          /**
           * The comparator specified in the static factory.  This will never
           * be null, as the static factory returns a ReverseComparator
           * instance if its argument is null.
           *
           * @serial
           */
          final Comparator<T> cmp;
  
          ReverseComparator2(Comparator<T> cmp) {
              assert cmp != null;
              this.cmp = cmp;
          }
  
          public int compare(T t1, T t2) {
              return cmp.compare(t2, t1);
          }
  
          public boolean equals(Object o) {
              return (o == this) ||
                  (o instanceof ReverseComparator2 &&
                   cmp.equals(((ReverseComparator2)o).cmp));
          }
  
          public int hashCode() {
              return cmp.hashCode() ^ Integer.MIN_VALUE;
          }
  
          @Override
          public Comparator<T> reversed() {
              return cmp;
          }
      }
  
      /**
       * Returns an enumeration over the specified collection.  This provides
       * interoperability with legacy APIs that require an enumeration
       * as input.
       *
       * @param  <T> the class of the objects in the collection
       * @param c the collection for which an enumeration is to be returned.
       * @return an enumeration over the specified collection.
       * @see Enumeration
       */
      public static <T> Enumeration<T> enumeration(final Collection<T> c) {
          return new Enumeration<T>() {
              private final Iterator<T> i = c.iterator();
  
              public boolean hasMoreElements() {
                  return i.hasNext();
              }
  
              public T nextElement() {
                  return i.next();
              }
          };
      }
  
      /**
       * Returns an array list containing the elements returned by the
       * specified enumeration in the order they are returned by the
       * enumeration.  This method provides interoperability between
       * legacy APIs that return enumerations and new APIs that require
       * collections.
       *
       * @param <T> the class of the objects returned by the enumeration
       * @param e enumeration providing elements for the returned
       *          array list
       * @return an array list containing the elements returned
       *         by the specified enumeration.
       * @since 1.4
       * @see Enumeration
       * @see ArrayList
       */
      public static <T> ArrayList<T> list(Enumeration<T> e) {
          ArrayList<T> l = new ArrayList<>();
          while (e.hasMoreElements())
              l.add(e.nextElement());
          return l;
      }
  
      /**
       * Returns true if the specified arguments are equal, or both null.
       *
       * NB: Do not replace with Object.equals until JDK-8015417 is resolved.
       */
      static boolean eq(Object o1, Object o2) {
          return o1==null ? o2==null : o1.equals(o2);
      }
  
      /**
       * Returns the number of elements in the specified collection equal to the
       * specified object.  More formally, returns the number of elements
       * <tt>e</tt> in the collection such that
       * <tt>(o == null ? e == null : o.equals(e))</tt>.
       *
       * @param c the collection in which to determine the frequency
       *     of <tt>o</tt>
       * @param o the object whose frequency is to be determined
       * @return the number of elements in {@code c} equal to {@code o}
       * @throws NullPointerException if <tt>c</tt> is null
       * @since 1.5
       */
      public static int frequency(Collection<?> c, Object o) {
          int result = 0;
          if (o == null) {
              for (Object e : c)
                  if (e == null)
                      result++;
          } else {
              for (Object e : c)
                  if (o.equals(e))
                      result++;
          }
          return result;
      }
  
      /**
       * Returns {@code true} if the two specified collections have no
       * elements in common.
       *
       * <p>Care must be exercised if this method is used on collections that
       * do not comply with the general contract for {@code Collection}.
       * Implementations may elect to iterate over either collection and test
       * for containment in the other collection (or to perform any equivalent
       * computation).  If either collection uses a nonstandard equality test
       * (as does a {@link SortedSet} whose ordering is not <em>compatible with
       * equals</em>, or the key set of an {@link IdentityHashMap}), both
       * collections must use the same nonstandard equality test, or the
       * result of this method is undefined.
       *
       * <p>Care must also be exercised when using collections that have
       * restrictions on the elements that they may contain. Collection
       * implementations are allowed to throw exceptions for any operation
       * involving elements they deem ineligible. For absolute safety the
       * specified collections should contain only elements which are
       * eligible elements for both collections.
       *
       * <p>Note that it is permissible to pass the same collection in both
       * parameters, in which case the method will return {@code true} if and
       * only if the collection is empty.
       *
       * @param c1 a collection
       * @param c2 a collection
       * @return {@code true} if the two specified collections have no
       * elements in common.
       * @throws NullPointerException if either collection is {@code null}.
       * @throws NullPointerException if one collection contains a {@code null}
       * element and {@code null} is not an eligible element for the other collection.
       * (<a href="Collection.html#optional-restrictions">optional</a>)
       * @throws ClassCastException if one collection contains an element that is
       * of a type which is ineligible for the other collection.
       * (<a href="Collection.html#optional-restrictions">optional</a>)
       * @since 1.5
       */
      public static boolean disjoint(Collection<?> c1, Collection<?> c2) {
          // The collection to be used for contains(). Preference is given to
          // the collection who's contains() has lower O() complexity.
          Collection<?> contains = c2;
          // The collection to be iterated. If the collections' contains() impl
          // are of different O() complexity, the collection with slower
          // contains() will be used for iteration. For collections who's
          // contains() are of the same complexity then best performance is
          // achieved by iterating the smaller collection.
          Collection<?> iterate = c1;
  
          // Performance optimization cases. The heuristics:
          //   1. Generally iterate over c1.
          //   2. If c1 is a Set then iterate over c2.
          //   3. If either collection is empty then result is always true.
          //   4. Iterate over the smaller Collection.
          if (c1 instanceof Set) {
              // Use c1 for contains as a Set's contains() is expected to perform
              // better than O(N/2)
              iterate = c2;
              contains = c1;
          } else if (!(c2 instanceof Set)) {
              // Both are mere Collections. Iterate over smaller collection.
              // Example: If c1 contains 3 elements and c2 contains 50 elements and
              // assuming contains() requires ceiling(N/2) comparisons then
              // checking for all c1 elements in c2 would require 75 comparisons
              // (3 * ceiling(50/2)) vs. checking all c2 elements in c1 requiring
              // 100 comparisons (50 * ceiling(3/2)).
              int c1size = c1.size();
              int c2size = c2.size();
              if (c1size == 0 || c2size == 0) {
                  // At least one collection is empty. Nothing will match.
                  return true;
              }
  
              if (c1size > c2size) {
                  iterate = c2;
                  contains = c1;
              }
          }
  
          for (Object e : iterate) {
              if (contains.contains(e)) {
                 // Found a common element. Collections are not disjoint.
                  return false;
              }
          }
  
          // No common elements were found.
          return true;
      }
  
      /**
       * Adds all of the specified elements to the specified collection.
       * Elements to be added may be specified individually or as an array.
       * The behavior of this convenience method is identical to that of
       * <tt>c.addAll(Arrays.asList(elements))</tt>, but this method is likely
       * to run significantly faster under most implementations.
       *
       * <p>When elements are specified individually, this method provides a
       * convenient way to add a few elements to an existing collection:
       * <pre>
       *     Collections.addAll(flavors, "Peaches 'n Plutonium", "Rocky Racoon");
       * </pre>
       *
       * @param  <T> the class of the elements to add and of the collection
       * @param c the collection into which <tt>elements</tt> are to be inserted
       * @param elements the elements to insert into <tt>c</tt>
       * @return <tt>true</tt> if the collection changed as a result of the call
       * @throws UnsupportedOperationException if <tt>c</tt> does not support
       *         the <tt>add</tt> operation
       * @throws NullPointerException if <tt>elements</tt> contains one or more
       *         null values and <tt>c</tt> does not permit null elements, or
       *         if <tt>c</tt> or <tt>elements</tt> are <tt>null</tt>
       * @throws IllegalArgumentException if some property of a value in
       *         <tt>elements</tt> prevents it from being added to <tt>c</tt>
       * @see Collection#addAll(Collection)
       * @since 1.5
       */
      @SafeVarargs
      public static <T> boolean addAll(Collection<? super T> c, T... elements) {
          boolean result = false;
          for (T element : elements)
              result |= c.add(element);
          return result;
      }
  
      /**
       * Returns a set backed by the specified map.  The resulting set displays
       * the same ordering, concurrency, and performance characteristics as the
       * backing map.  In essence, this factory method provides a {@link Set}
       * implementation corresponding to any {@link Map} implementation.  There
       * is no need to use this method on a {@link Map} implementation that
       * already has a corresponding {@link Set} implementation (such as {@link
       * HashMap} or {@link TreeMap}).
       *
       * <p>Each method invocation on the set returned by this method results in
       * exactly one method invocation on the backing map or its <tt>keySet</tt>
       * view, with one exception.  The <tt>addAll</tt> method is implemented
       * as a sequence of <tt>put</tt> invocations on the backing map.
       *
       * <p>The specified map must be empty at the time this method is invoked,
       * and should not be accessed directly after this method returns.  These
       * conditions are ensured if the map is created empty, passed directly
       * to this method, and no reference to the map is retained, as illustrated
       * in the following code fragment:
       * <pre>
       *    Set&lt;Object&gt; weakHashSet = Collections.newSetFromMap(
       *        new WeakHashMap&lt;Object, Boolean&gt;());
       * </pre>
       *
       * @param <E> the class of the map keys and of the objects in the
       *        returned set
       * @param map the backing map
       * @return the set backed by the map
       * @throws IllegalArgumentException if <tt>map</tt> is not empty
       * @since 1.6
       */
      public static <E> Set<E> newSetFromMap(Map<E, Boolean> map) {
          return new SetFromMap<>(map);
      }
  
      /**
       * @serial include
       */
      private static class SetFromMap<E> extends AbstractSet<E>
          implements Set<E>, Serializable
      {
          private final Map<E, Boolean> m;  // The backing map
          private transient Set<E> s;       // Its keySet
  
          SetFromMap(Map<E, Boolean> map) {
              if (!map.isEmpty())
                  throw new IllegalArgumentException("Map is non-empty");
              m = map;
              s = map.keySet();
          }
  
          public void clear()               {        m.clear(); }
          public int size()                 { return m.size(); }
          public boolean isEmpty()          { return m.isEmpty(); }
          public boolean contains(Object o) { return m.containsKey(o); }
          public boolean remove(Object o)   { return m.remove(o) != null; }
          public boolean add(E e) { return m.put(e, Boolean.TRUE) == null; }
          public Iterator<E> iterator()     { return s.iterator(); }
          public Object[] toArray()         { return s.toArray(); }
          public <T> T[] toArray(T[] a)     { return s.toArray(a); }
          public String toString()          { return s.toString(); }
          public int hashCode()             { return s.hashCode(); }
          public boolean equals(Object o)   { return o == this || s.equals(o); }
          public boolean containsAll(Collection<?> c) {return s.containsAll(c);}
          public boolean removeAll(Collection<?> c)   {return s.removeAll(c);}
          public boolean retainAll(Collection<?> c)   {return s.retainAll(c);}
          // addAll is the only inherited implementation
  
          // Override default methods in Collection
          @Override
          public void forEach(Consumer<? super E> action) {
              s.forEach(action);
          }
          @Override
          public boolean removeIf(Predicate<? super E> filter) {
              return s.removeIf(filter);
          }
  
          @Override
          public Spliterator<E> spliterator() {return s.spliterator();}
          @Override
          public Stream<E> stream()           {return s.stream();}
          @Override
          public Stream<E> parallelStream()   {return s.parallelStream();}
  
          private static final long serialVersionUID = 2454657854757543876L;
  
          private void readObject(java.io.ObjectInputStream stream)
              throws IOException, ClassNotFoundException
          {
              stream.defaultReadObject();
              s = m.keySet();
          }
      }
  
      /**
       * Returns a view of a {@link Deque} as a Last-in-first-out (Lifo)
       * {@link Queue}. Method <tt>add</tt> is mapped to <tt>push</tt>,
       * <tt>remove</tt> is mapped to <tt>pop</tt> and so on. This
       * view can be useful when you would like to use a method
       * requiring a <tt>Queue</tt> but you need Lifo ordering.
       *
       * <p>Each method invocation on the queue returned by this method
       * results in exactly one method invocation on the backing deque, with
       * one exception.  The {@link Queue#addAll addAll} method is
       * implemented as a sequence of {@link Deque#addFirst addFirst}
       * invocations on the backing deque.
       *
       * @param  <T> the class of the objects in the deque
       * @param deque the deque
       * @return the queue
       * @since  1.6
       */
      public static <T> Queue<T> asLifoQueue(Deque<T> deque) {
          return new AsLIFOQueue<>(deque);
      }
  
      /**
       * @serial include
       */
      static class AsLIFOQueue<E> extends AbstractQueue<E>
          implements Queue<E>, Serializable {
          private static final long serialVersionUID = 1802017725587941708L;
          private final Deque<E> q;
          AsLIFOQueue(Deque<E> q)           { this.q = q; }
          public boolean add(E e)           { q.addFirst(e); return true; }
          public boolean offer(E e)         { return q.offerFirst(e); }
          public E poll()                   { return q.pollFirst(); }
          public E remove()                 { return q.removeFirst(); }
          public E peek()                   { return q.peekFirst(); }
          public E element()                { return q.getFirst(); }
          public void clear()               {        q.clear(); }
          public int size()                 { return q.size(); }
          public boolean isEmpty()          { return q.isEmpty(); }
          public boolean contains(Object o) { return q.contains(o); }
          public boolean remove(Object o)   { return q.remove(o); }
          public Iterator<E> iterator()     { return q.iterator(); }
          public Object[] toArray()         { return q.toArray(); }
          public <T> T[] toArray(T[] a)     { return q.toArray(a); }
          public String toString()          { return q.toString(); }
          public boolean containsAll(Collection<?> c) {return q.containsAll(c);}
          public boolean removeAll(Collection<?> c)   {return q.removeAll(c);}
          public boolean retainAll(Collection<?> c)   {return q.retainAll(c);}
          // We use inherited addAll; forwarding addAll would be wrong
  
          // Override default methods in Collection
          @Override
          public void forEach(Consumer<? super E> action) {q.forEach(action);}
          @Override
          public boolean removeIf(Predicate<? super E> filter) {
              return q.removeIf(filter);
          }
          @Override
          public Spliterator<E> spliterator() {return q.spliterator();}
          @Override
          public Stream<E> stream()           {return q.stream();}
          @Override
          public Stream<E> parallelStream()   {return q.parallelStream();}
      }
  }
  