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   * Copyright (c) 1997, 2017, 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.util.function.BiConsumer;
  import java.util.function.BiFunction;
  import java.util.function.Function;
  import java.io.Serializable;
  
  // Android-changed: removed link to collections framework docs
  
  /**
   * An object that maps keys to values.  A map cannot contain duplicate keys;
   * each key can map to at most one value.
   *
   * <p>This interface takes the place of the {@code Dictionary} class, which
   * was a totally abstract class rather than an interface.
   *
   * <p>The {@code Map} interface provides three <i>collection views</i>, which
   * allow a map's contents to be viewed as a set of keys, collection of values,
   * or set of key-value mappings.  The <i>order</i> of a map is defined as
   * the order in which the iterators on the map's collection views return their
   * elements.  Some map implementations, like the {@code TreeMap} class, make
   * specific guarantees as to their order; others, like the {@code HashMap}
   * class, do not.
   *
   * <p>Note: great care must be exercised if mutable objects are used as map
   * keys.  The behavior of a map is not specified if the value of an object is
   * changed in a manner that affects {@code equals} comparisons while the
   * object is a key in the map.  A special case of this prohibition is that it
   * is not permissible for a map to contain itself as a key.  While it is
   * permissible for a map to contain itself as a value, extreme caution is
   * advised: the {@code equals} and {@code hashCode} methods are no longer
   * well defined on such a map.
   *
   * <p>All general-purpose map implementation classes should provide two
   * "standard" constructors: a void (no arguments) constructor which creates an
   * empty map, and a constructor with a single argument of type {@code Map},
   * which creates a new map with the same key-value mappings as its argument.
   * In effect, the latter constructor allows the user to copy any map,
   * producing an equivalent map of the desired class.  There is no way to
   * enforce this recommendation (as interfaces cannot contain constructors) but
   * all of the general-purpose map implementations in the JDK comply.
   *
   * <p>The "destructive" methods contained in this interface, that is, the
   * methods that modify the map on which they operate, are specified to throw
   * {@code UnsupportedOperationException} if this map does not support the
   * operation.  If this is the case, these methods may, but are not required
   * to, throw an {@code UnsupportedOperationException} if the invocation would
   * have no effect on the map.  For example, invoking the {@link #putAll(Map)}
   * method on an unmodifiable map may, but is not required to, throw the
   * exception if the map whose mappings are to be "superimposed" is empty.
   *
   * <p>Some map implementations have restrictions on the keys and values they
   * may contain.  For example, some implementations prohibit null keys and
   * values, and some have restrictions on the types of their keys.  Attempting
   * to insert an ineligible key or value throws an unchecked exception,
   * typically {@code NullPointerException} or {@code ClassCastException}.
   * Attempting to query the presence of an ineligible key or value may throw an
   * exception, or it may simply return false; some implementations will exhibit
   * the former behavior and some will exhibit the latter.  More generally,
   * attempting an operation on an ineligible key or value whose completion
   * would not result in the insertion of an ineligible element into the map may
   * throw an exception or it may succeed, at the option of the implementation.
   * Such exceptions are marked as "optional" in the specification for this
   * interface.
   *
   * <p>Many methods in Collections Framework interfaces are defined
   * in terms of the {@link Object#equals(Object) equals} method.  For
   * example, the specification for the {@link #containsKey(Object)
   * containsKey(Object key)} method says: "returns {@code true} if and
   * only if this map contains a mapping for a key {@code k} such that
   * {@code (key==null ? k==null : key.equals(k))}." This specification should
   * <i>not</i> be construed to imply that invoking {@code Map.containsKey}
   * with a non-null argument {@code key} will cause {@code key.equals(k)} to
   * be invoked for any key {@code k}.  Implementations are free to
   * implement optimizations whereby the {@code equals} invocation is avoided,
   * for example, by first comparing the hash codes of the two keys.  (The
   * {@link Object#hashCode()} specification guarantees that two objects with
   * unequal hash codes cannot be equal.)  More generally, implementations of
   * the various Collections Framework interfaces are free to take advantage of
   * the specified behavior of underlying {@link Object} methods wherever the
   * implementor deems it appropriate.
   *
   * <p>Some map operations which perform recursive traversal of the map may fail
   * with an exception for self-referential instances where the map directly or
   * indirectly contains itself. This includes the {@code clone()},
   * {@code equals()}, {@code hashCode()} and {@code toString()} methods.
   * Implementations may optionally handle the self-referential scenario, however
   * most current implementations do not do so.
   *
   * <h2><a id="immutable">Immutable Map Static Factory Methods</a></h2>
   * <p>The {@link Map#of() Map.of()} and
   * {@link Map#ofEntries(Entry...) Map.ofEntries()}
   * static factory methods provide a convenient way to create immutable maps.
   * The {@code Map}
   * instances created by these methods have the following characteristics:
   *
   * <ul>
   * <li>They are <em>structurally immutable</em>. Keys and values cannot be added,
   * removed, or updated. Calling any mutator method will always cause
   * {@code UnsupportedOperationException} to be thrown.
   * However, if the contained keys or values are themselves mutable, this may cause the
   * Map to behave inconsistently or its contents to appear to change.
   * <li>They disallow {@code null} keys and values. Attempts to create them with
   * {@code null} keys or values result in {@code NullPointerException}.
   * <li>They are serializable if all keys and values are serializable.
   * <li>They reject duplicate keys at creation time. Duplicate keys
   * passed to a static factory method result in {@code IllegalArgumentException}.
   * <li>The iteration order of mappings is unspecified and is subject to change.
   * <li>They are <a href="../lang/doc-files/ValueBased.html">value-based</a>.
   * Callers should make no assumptions about the identity of the returned instances.
   * Factories are free to create new instances or reuse existing ones. Therefore,
   * identity-sensitive operations on these instances (reference equality ({@code ==}),
   * identity hash code, and synchronization) are unreliable and should be avoided.
   * <li>They are serialized as specified on the
   * <a href="{@docRoot}/serialized-form.html#java.util.CollSer">Serialized Form</a>
   * page.
   * </ul>
   *
   * @param <K> the type of keys maintained by this map
   * @param <V> the type of mapped values
   *
   * @author  Josh Bloch
   * @see HashMap
   * @see TreeMap
   * @see Hashtable
   * @see SortedMap
   * @see Collection
   * @see Set
   * @since 1.2
   */
  // Android-changed: fix doc links to Collection#optional-restrictions
  public interface Map<K, V> {
      // Query Operations
  
      /**
       * Returns the number of key-value mappings in this map.  If the
       * map contains more than {@code Integer.MAX_VALUE} elements, returns
       * {@code Integer.MAX_VALUE}.
       *
       * @return the number of key-value mappings in this map
       */
      int size();
  
      /**
       * Returns {@code true} if this map contains no key-value mappings.
       *
       * @return {@code true} if this map contains no key-value mappings
       */
      boolean isEmpty();
  
      /**
       * Returns {@code true} if this map contains a mapping for the specified
       * key.  More formally, returns {@code true} if and only if
       * this map contains a mapping for a key {@code k} such that
       * {@code Objects.equals(key, k)}.  (There can be
       * at most one such mapping.)
       *
       * @param key key whose presence in this map is to be tested
       * @return {@code true} if this map contains a mapping for the specified
       *         key
       * @throws ClassCastException if the key is of an inappropriate type for
       *         this map
       * (<a href="Collection.html#optional-restrictions">optional</a>)
       * @throws NullPointerException if the specified key is null and this map
       *         does not permit null keys
       * (<a href="Collection.html#optional-restrictions">optional</a>)
       */
      boolean containsKey(Object key);
  
      /**
       * Returns {@code true} if this map maps one or more keys to the
       * specified value.  More formally, returns {@code true} if and only if
       * this map contains at least one mapping to a value {@code v} such that
       * {@code Objects.equals(value, v)}.  This operation
       * will probably require time linear in the map size for most
       * implementations of the {@code Map} interface.
       *
       * @param value value whose presence in this map is to be tested
       * @return {@code true} if this map maps one or more keys to the
       *         specified value
       * @throws ClassCastException if the value is of an inappropriate type for
       *         this map
       * (<a href="Collection.html#optional-restrictions">optional</a>)
       * @throws NullPointerException if the specified value is null and this
       *         map does not permit null values
       * (<a href="Collection.html#optional-restrictions">optional</a>)
       */
      boolean containsValue(Object value);
  
      /**
       * Returns the value to which the specified key is mapped,
       * or {@code null} if this map contains no mapping for the key.
       *
       * <p>More formally, if this map contains a mapping from a key
       * {@code k} to a value {@code v} such that
       * {@code Objects.equals(key, k)},
       * then this method returns {@code v}; otherwise
       * it returns {@code null}.  (There can be at most one such mapping.)
       *
       * <p>If this map permits null values, then a return value of
       * {@code null} does not <i>necessarily</i> indicate that the map
       * contains no mapping for the key; it's also possible that the map
       * explicitly maps the key to {@code null}.  The {@link #containsKey
       * containsKey} operation may be used to distinguish these two cases.
       *
       * @param key the key whose associated value is to be returned
       * @return the value to which the specified key is mapped, or
       *         {@code null} if this map contains no mapping for the key
       * @throws ClassCastException if the key is of an inappropriate type for
       *         this map
       * (<a href="Collection.html#optional-restrictions">optional</a>)
       * @throws NullPointerException if the specified key is null and this map
       *         does not permit null keys
       * (<a href="Collection.html#optional-restrictions">optional</a>)
       */
      V get(Object key);
  
      // Modification Operations
  
      /**
       * Associates the specified value with the specified key in this map
       * (optional operation).  If the map previously contained a mapping for
       * the key, the old value is replaced by the specified value.  (A map
       * {@code m} is said to contain a mapping for a key {@code k} if and only
       * if {@link #containsKey(Object) m.containsKey(k)} would return
       * {@code true}.)
       *
       * @param key key with which the specified value is to be associated
       * @param value value to be associated with the specified key
       * @return the previous value associated with {@code key}, or
       *         {@code null} if there was no mapping for {@code key}.
       *         (A {@code null} return can also indicate that the map
       *         previously associated {@code null} with {@code key},
       *         if the implementation supports {@code null} values.)
       * @throws UnsupportedOperationException if the {@code put} operation
       *         is not supported by this map
       * @throws ClassCastException if the class of the specified key or value
       *         prevents it from being stored in this map
       * @throws NullPointerException if the specified key or value is null
       *         and this map does not permit null keys or values
       * @throws IllegalArgumentException if some property of the specified key
       *         or value prevents it from being stored in this map
       */
      V put(K key, V value);
  
      /**
       * Removes the mapping for a key from this map if it is present
       * (optional operation).   More formally, if this map contains a mapping
       * from key {@code k} to value {@code v} such that
       * {@code Objects.equals(key, k)}, that mapping
       * is removed.  (The map can contain at most one such mapping.)
       *
       * <p>Returns the value to which this map previously associated the key,
       * or {@code null} if the map contained no mapping for the key.
       *
       * <p>If this map permits null values, then a return value of
       * {@code null} does not <i>necessarily</i> indicate that the map
       * contained no mapping for the key; it's also possible that the map
       * explicitly mapped the key to {@code null}.
       *
       * <p>The map will not contain a mapping for the specified key once the
       * call returns.
       *
       * @param key key whose mapping is to be removed from the map
       * @return the previous value associated with {@code key}, or
       *         {@code null} if there was no mapping for {@code key}.
       * @throws UnsupportedOperationException if the {@code remove} operation
       *         is not supported by this map
       * @throws ClassCastException if the key is of an inappropriate type for
       *         this map
       * (<a href="Collection.html#optional-restrictions">optional</a>)
       * @throws NullPointerException if the specified key is null and this
       *         map does not permit null keys
       * (<a href="Collection.html#optional-restrictions">optional</a>)
       */
      V remove(Object key);
  
  
      // Bulk Operations
  
      /**
       * Copies all of the mappings from the specified map to this map
       * (optional operation).  The effect of this call is equivalent to that
       * of calling {@link #put(Object,Object) put(k, v)} on this map once
       * for each mapping from key {@code k} to value {@code v} in the
       * specified map.  The behavior of this operation is undefined if the
       * specified map is modified while the operation is in progress.
       *
       * @param m mappings to be stored in this map
       * @throws UnsupportedOperationException if the {@code putAll} operation
       *         is not supported by this map
       * @throws ClassCastException if the class of a key or value in the
       *         specified map prevents it from being stored in this map
       * @throws NullPointerException if the specified map is null, or if
       *         this map does not permit null keys or values, and the
       *         specified map contains null keys or values
       * @throws IllegalArgumentException if some property of a key or value in
       *         the specified map prevents it from being stored in this map
       */
      void putAll(Map<? extends K, ? extends V> m);
  
      /**
       * Removes all of the mappings from this map (optional operation).
       * The map will be empty after this call returns.
       *
       * @throws UnsupportedOperationException if the {@code clear} operation
       *         is not supported by this map
       */
      void clear();
  
  
      // Views
  
      /**
       * Returns a {@link Set} view of the keys contained in this map.
       * The set is backed by the map, so changes to the map are
       * reflected in the set, and vice-versa.  If the map is modified
       * while an iteration over the set is in progress (except through
       * the iterator's own {@code remove} operation), the results of
       * the iteration are undefined.  The set supports element removal,
       * which removes the corresponding mapping from the map, via the
       * {@code Iterator.remove}, {@code Set.remove},
       * {@code removeAll}, {@code retainAll}, and {@code clear}
       * operations.  It does not support the {@code add} or {@code addAll}
       * operations.
       *
       * @return a set view of the keys contained in this map
       */
      Set<K> keySet();
  
      /**
       * Returns a {@link Collection} view of the values contained in this map.
       * The collection is backed by the map, so changes to the map are
       * reflected in the collection, and vice-versa.  If the map is
       * modified while an iteration over the collection is in progress
       * (except through the iterator's own {@code remove} operation),
       * the results of the iteration are undefined.  The collection
       * supports element removal, which removes the corresponding
       * mapping from the map, via the {@code Iterator.remove},
       * {@code Collection.remove}, {@code removeAll},
       * {@code retainAll} and {@code clear} operations.  It does not
       * support the {@code add} or {@code addAll} operations.
       *
       * @return a collection view of the values contained in this map
       */
      Collection<V> values();
  
      /**
       * Returns a {@link Set} view of the mappings contained in this map.
       * The set is backed by the map, so changes to the map are
       * reflected in the set, and vice-versa.  If the map is modified
       * while an iteration over the set is in progress (except through
       * the iterator's own {@code remove} operation, or through the
       * {@code setValue} operation on a map entry returned by the
       * iterator) the results of the iteration are undefined.  The set
       * supports element removal, which removes the corresponding
       * mapping from the map, via the {@code Iterator.remove},
       * {@code Set.remove}, {@code removeAll}, {@code retainAll} and
       * {@code clear} operations.  It does not support the
       * {@code add} or {@code addAll} operations.
       *
       * @return a set view of the mappings contained in this map
       */
      Set<Entry<K, V>> entrySet();
  
      /**
       * A map entry (key-value pair).  The {@code Map.entrySet} method returns
       * a collection-view of the map, whose elements are of this class.  The
       * <i>only</i> way to obtain a reference to a map entry is from the
       * iterator of this collection-view.  These {@code Map.Entry} objects are
       * valid <i>only</i> for the duration of the iteration; more formally,
       * the behavior of a map entry is undefined if the backing map has been
       * modified after the entry was returned by the iterator, except through
       * the {@code setValue} operation on the map entry.
       *
       * @see Map#entrySet()
       * @since 1.2
       */
      interface Entry<K, V> {
          /**
           * Returns the key corresponding to this entry.
           *
           * @return the key corresponding to this entry
           * @throws IllegalStateException implementations may, but are not
           *         required to, throw this exception if the entry has been
           *         removed from the backing map.
           */
          K getKey();
  
          /**
           * Returns the value corresponding to this entry.  If the mapping
           * has been removed from the backing map (by the iterator's
           * {@code remove} operation), the results of this call are undefined.
           *
           * @return the value corresponding to this entry
           * @throws IllegalStateException implementations may, but are not
           *         required to, throw this exception if the entry has been
           *         removed from the backing map.
           */
          V getValue();
  
          /**
           * Replaces the value corresponding to this entry with the specified
           * value (optional operation).  (Writes through to the map.)  The
           * behavior of this call is undefined if the mapping has already been
           * removed from the map (by the iterator's {@code remove} operation).
           *
           * @param value new value to be stored in this entry
           * @return old value corresponding to the entry
           * @throws UnsupportedOperationException if the {@code put} operation
           *         is not supported by the backing map
           * @throws ClassCastException if the class of the specified value
           *         prevents it from being stored in the backing map
           * @throws NullPointerException if the backing map does not permit
           *         null values, and the specified value is null
           * @throws IllegalArgumentException if some property of this value
           *         prevents it from being stored in the backing map
           * @throws IllegalStateException implementations may, but are not
           *         required to, throw this exception if the entry has been
           *         removed from the backing map.
           */
          V setValue(V value);
  
          /**
           * Compares the specified object with this entry for equality.
           * Returns {@code true} if the given object is also a map entry and
           * the two entries represent the same mapping.  More formally, two
           * entries {@code e1} and {@code e2} represent the same mapping
           * if<pre>
           *     (e1.getKey()==null ?
           *      e2.getKey()==null : e1.getKey().equals(e2.getKey()))  &amp;&amp;
           *     (e1.getValue()==null ?
           *      e2.getValue()==null : e1.getValue().equals(e2.getValue()))
           * </pre>
           * This ensures that the {@code equals} method works properly across
           * different implementations of the {@code Map.Entry} interface.
           *
           * @param o object to be compared for equality with this map entry
           * @return {@code true} if the specified object is equal to this map
           *         entry
           */
          boolean equals(Object o);
  
          /**
           * Returns the hash code value for this map entry.  The hash code
           * of a map entry {@code e} is defined to be: <pre>
           *     (e.getKey()==null   ? 0 : e.getKey().hashCode()) ^
           *     (e.getValue()==null ? 0 : e.getValue().hashCode())
           * </pre>
           * This ensures that {@code e1.equals(e2)} implies that
           * {@code e1.hashCode()==e2.hashCode()} for any two Entries
           * {@code e1} and {@code e2}, as required by the general
           * contract of {@code Object.hashCode}.
           *
           * @return the hash code value for this map entry
           * @see Object#hashCode()
           * @see Object#equals(Object)
           * @see #equals(Object)
           */
          int hashCode();
  
          /**
           * Returns a comparator that compares {@link Entry} in natural order on key.
           *
           * <p>The returned comparator is serializable and throws {@link
           * NullPointerException} when comparing an entry with a null key.
           *
           * @param  <K> the {@link Comparable} type of then map keys
           * @param  <V> the type of the map values
           * @return a comparator that compares {@link Entry} in natural order on key.
           * @see Comparable
           * @since 1.8
           */
          public static <K extends Comparable<? super K>, V> Comparator<Entry<K, V>> comparingByKey() {
              return (Comparator<Entry<K, V>> & Serializable)
                  (c1, c2) -> c1.getKey().compareTo(c2.getKey());
          }
  
          /**
           * Returns a comparator that compares {@link Entry} in natural order on value.
           *
           * <p>The returned comparator is serializable and throws {@link
           * NullPointerException} when comparing an entry with null values.
           *
           * @param <K> the type of the map keys
           * @param <V> the {@link Comparable} type of the map values
           * @return a comparator that compares {@link Entry} in natural order on value.
           * @see Comparable
           * @since 1.8
           */
          public static <K, V extends Comparable<? super V>> Comparator<Entry<K, V>> comparingByValue() {
              return (Comparator<Entry<K, V>> & Serializable)
                  (c1, c2) -> c1.getValue().compareTo(c2.getValue());
          }
  
          /**
           * Returns a comparator that compares {@link Entry} by key using the given
           * {@link Comparator}.
           *
           * <p>The returned comparator is serializable if the specified comparator
           * is also serializable.
           *
           * @param  <K> the type of the map keys
           * @param  <V> the type of the map values
           * @param  cmp the key {@link Comparator}
           * @return a comparator that compares {@link Entry} by the key.
           * @since 1.8
           */
          public static <K, V> Comparator<Entry<K, V>> comparingByKey(Comparator<? super K> cmp) {
              Objects.requireNonNull(cmp);
              return (Comparator<Entry<K, V>> & Serializable)
                  (c1, c2) -> cmp.compare(c1.getKey(), c2.getKey());
          }
  
          /**
           * Returns a comparator that compares {@link Entry} by value using the given
           * {@link Comparator}.
           *
           * <p>The returned comparator is serializable if the specified comparator
           * is also serializable.
           *
           * @param  <K> the type of the map keys
           * @param  <V> the type of the map values
           * @param  cmp the value {@link Comparator}
           * @return a comparator that compares {@link Entry} by the value.
           * @since 1.8
           */
          public static <K, V> Comparator<Entry<K, V>> comparingByValue(Comparator<? super V> cmp) {
              Objects.requireNonNull(cmp);
              return (Comparator<Entry<K, V>> & Serializable)
                  (c1, c2) -> cmp.compare(c1.getValue(), c2.getValue());
          }
      }
  
      // Comparison and hashing
  
      /**
       * Compares the specified object with this map for equality.  Returns
       * {@code true} if the given object is also a map and the two maps
       * represent the same mappings.  More formally, two maps {@code m1} and
       * {@code m2} represent the same mappings if
       * {@code m1.entrySet().equals(m2.entrySet())}.  This ensures that the
       * {@code equals} method works properly across different implementations
       * of the {@code Map} interface.
       *
       * @param o object to be compared for equality with this map
       * @return {@code true} if the specified object is equal to this map
       */
      boolean equals(Object o);
  
      /**
       * Returns the hash code value for this map.  The hash code of a map is
       * defined to be the sum of the hash codes of each entry in the map's
       * {@code entrySet()} view.  This ensures that {@code m1.equals(m2)}
       * implies that {@code m1.hashCode()==m2.hashCode()} for any two maps
       * {@code m1} and {@code m2}, as required by the general contract of
       * {@link Object#hashCode}.
       *
       * @return the hash code value for this map
       * @see Entry#hashCode()
       * @see Object#equals(Object)
       * @see #equals(Object)
       */
      int hashCode();
  
      // Defaultable methods
  
      /**
       * Returns the value to which the specified key is mapped, or
       * {@code defaultValue} if this map contains no mapping for the key.
       *
       * @implSpec
       * The default implementation makes no guarantees about synchronization
       * or atomicity properties of this method. Any implementation providing
       * atomicity guarantees must override this method and document its
       * concurrency properties.
       *
       * @param key the key whose associated value is to be returned
       * @param defaultValue the default mapping of the key
       * @return the value to which the specified key is mapped, or
       * {@code defaultValue} if this map contains no mapping for the key
       * @throws ClassCastException if the key is of an inappropriate type for
       * this map
       * (<a href="Collection.html#optional-restrictions">optional</a>)
       * @throws NullPointerException if the specified key is null and this map
       * does not permit null keys
       * (<a href="Collection.html#optional-restrictions">optional</a>)
       * @since 1.8
       */
      default V getOrDefault(Object key, V defaultValue) {
          V v;
          return (((v = get(key)) != null) || containsKey(key))
              ? v
              : defaultValue;
      }
  
      /**
       * Performs the given action for each entry in this map until all entries
       * have been processed or the action throws an exception.   Unless
       * otherwise specified by the implementing class, actions are performed in
       * the order of entry set iteration (if an iteration order is specified.)
       * Exceptions thrown by the action are relayed to the caller.
       *
       * @implSpec
       * The default implementation is equivalent to, for this {@code map}:
       * <pre> {@code
       * for (Map.Entry<K, V> entry : map.entrySet())
       *     action.accept(entry.getKey(), entry.getValue());
       * }</pre>
       *
       * The default implementation makes no guarantees about synchronization
       * or atomicity properties of this method. Any implementation providing
       * atomicity guarantees must override this method and document its
       * concurrency properties.
       *
       * @param action The action to be performed for each entry
       * @throws NullPointerException if the specified action is null
       * @throws ConcurrentModificationException if an entry is found to be
       * removed during iteration
       * @since 1.8
       */
      default void forEach(BiConsumer<? super K, ? super V> action) {
          Objects.requireNonNull(action);
          for (Entry<K, V> entry : entrySet()) {
              K k;
              V v;
              try {
                  k = entry.getKey();
                  v = entry.getValue();
              } catch (IllegalStateException ise) {
                  // this usually means the entry is no longer in the map.
                  throw new ConcurrentModificationException(ise);
              }
              action.accept(k, v);
          }
      }
  
      /**
       * Replaces each entry's value with the result of invoking the given
       * function on that entry until all entries have been processed or the
       * function throws an exception.  Exceptions thrown by the function are
       * relayed to the caller.
       *
       * @implSpec
       * <p>The default implementation is equivalent to, for this {@code map}:
       * <pre> {@code
       * for (Map.Entry<K, V> entry : map.entrySet())
       *     entry.setValue(function.apply(entry.getKey(), entry.getValue()));
       * }</pre>
       *
       * <p>The default implementation makes no guarantees about synchronization
       * or atomicity properties of this method. Any implementation providing
       * atomicity guarantees must override this method and document its
       * concurrency properties.
       *
       * @param function the function to apply to each entry
       * @throws UnsupportedOperationException if the {@code set} operation
       * is not supported by this map's entry set iterator.
       * @throws ClassCastException if the class of a replacement value
       * prevents it from being stored in this map
       * @throws NullPointerException if the specified function is null, or the
       * specified replacement value is null, and this map does not permit null
       * values
       * @throws ClassCastException if a replacement value is of an inappropriate
       *         type for this map
       *         (<a href="Collection.html#optional-restrictions">optional</a>)
       * @throws NullPointerException if function or a replacement value is null,
       *         and this map does not permit null keys or values
       *         (<a href="Collection.html#optional-restrictions">optional</a>)
       * @throws IllegalArgumentException if some property of a replacement value
       *         prevents it from being stored in this map
       *         (<a href="Collection.html#optional-restrictions">optional</a>)
       * @throws ConcurrentModificationException if an entry is found to be
       * removed during iteration
       * @since 1.8
       */
      default void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
          Objects.requireNonNull(function);
          for (Entry<K, V> entry : entrySet()) {
              K k;
              V v;
              try {
                  k = entry.getKey();
                  v = entry.getValue();
              } catch (IllegalStateException ise) {
                  // this usually means the entry is no longer in the map.
                  throw new ConcurrentModificationException(ise);
              }
  
              // ise thrown from function is not a cme.
              v = function.apply(k, v);
  
              try {
                  entry.setValue(v);
              } catch (IllegalStateException ise) {
                  // this usually means the entry is no longer in the map.
                  throw new ConcurrentModificationException(ise);
              }
          }
      }
  
      /**
       * If the specified key is not already associated with a value (or is mapped
       * to {@code null}) associates it with the given value and returns
       * {@code null}, else returns the current value.
       *
       * @implSpec
       * The default implementation is equivalent to, for this {@code
       * map}:
       *
       * <pre> {@code
       * V v = map.get(key);
       * if (v == null)
       *     v = map.put(key, value);
       *
       * return v;
       * }</pre>
       *
       * <p>The default implementation makes no guarantees about synchronization
       * or atomicity properties of this method. Any implementation providing
       * atomicity guarantees must override this method and document its
       * concurrency properties.
       *
       * @param key key with which the specified value is to be associated
       * @param value value to be associated with the specified key
       * @return the previous value associated with the specified key, or
       *         {@code null} if there was no mapping for the key.
       *         (A {@code null} return can also indicate that the map
       *         previously associated {@code null} with the key,
       *         if the implementation supports null values.)
       * @throws UnsupportedOperationException if the {@code put} operation
       *         is not supported by this map
       *         (<a href="Collection.html#optional-restrictions">optional</a>)
       * @throws ClassCastException if the key or value is of an inappropriate
       *         type for this map
       *         (<a href="Collection.html#optional-restrictions">optional</a>)
       * @throws NullPointerException if the specified key or value is null,
       *         and this map does not permit null keys or values
       *         (<a href="Collection.html#optional-restrictions">optional</a>)
       * @throws IllegalArgumentException if some property of the specified key
       *         or value prevents it from being stored in this map
       *         (<a href="Collection.html#optional-restrictions">optional</a>)
       * @since 1.8
       */
      default V putIfAbsent(K key, V value) {
          V v = get(key);
          if (v == null) {
              v = put(key, value);
          }
  
          return v;
      }
  
      /**
       * Removes the entry for the specified key only if it is currently
       * mapped to the specified value.
       *
       * @implSpec
       * The default implementation is equivalent to, for this {@code map}:
       *
       * <pre> {@code
       * if (map.containsKey(key) && Objects.equals(map.get(key), value)) {
       *     map.remove(key);
       *     return true;
       * } else
       *     return false;
       * }</pre>
       *
       * <p>The default implementation makes no guarantees about synchronization
       * or atomicity properties of this method. Any implementation providing
       * atomicity guarantees must override this method and document its
       * concurrency properties.
       *
       * @param key key with which the specified value is associated
       * @param value value expected to be associated with the specified key
       * @return {@code true} if the value was removed
       * @throws UnsupportedOperationException if the {@code remove} operation
       *         is not supported by this map
       *         (<a href="Collection.html#optional-restrictions">optional</a>)
       * @throws ClassCastException if the key or value is of an inappropriate
       *         type for this map
       *         (<a href="Collection.html#optional-restrictions">optional</a>)
       * @throws NullPointerException if the specified key or value is null,
       *         and this map does not permit null keys or values
       *         (<a href="Collection.html#optional-restrictions">optional</a>)
       * @since 1.8
       */
      default boolean remove(Object key, Object value) {
          Object curValue = get(key);
          if (!Objects.equals(curValue, value) ||
              (curValue == null && !containsKey(key))) {
              return false;
          }
          remove(key);
          return true;
      }
  
      /**
       * Replaces the entry for the specified key only if currently
       * mapped to the specified value.
       *
       * @implSpec
       * The default implementation is equivalent to, for this {@code map}:
       *
       * <pre> {@code
       * if (map.containsKey(key) && Objects.equals(map.get(key), value)) {
       *     map.put(key, newValue);
       *     return true;
       * } else
       *     return false;
       * }</pre>
       *
       * The default implementation does not throw NullPointerException
       * for maps that do not support null values if oldValue is null unless
       * newValue is also null.
       *
       * <p>The default implementation makes no guarantees about synchronization
       * or atomicity properties of this method. Any implementation providing
       * atomicity guarantees must override this method and document its
       * concurrency properties.
       *
       * @param key key with which the specified value is associated
       * @param oldValue value expected to be associated with the specified key
       * @param newValue value to be associated with the specified key
       * @return {@code true} if the value was replaced
       * @throws UnsupportedOperationException if the {@code put} operation
       *         is not supported by this map
       *         (<a href="Collection.html#optional-restrictions">optional</a>)
       * @throws ClassCastException if the class of a specified key or value
       *         prevents it from being stored in this map
       * @throws NullPointerException if a specified key or newValue is null,
       *         and this map does not permit null keys or values
       * @throws NullPointerException if oldValue is null and this map does not
       *         permit null values
       *         (<a href="Collection.html#optional-restrictions">optional</a>)
       * @throws IllegalArgumentException if some property of a specified key
       *         or value prevents it from being stored in this map
       * @since 1.8
       */
      default boolean replace(K key, V oldValue, V newValue) {
          Object curValue = get(key);
          if (!Objects.equals(curValue, oldValue) ||
              (curValue == null && !containsKey(key))) {
              return false;
          }
          put(key, newValue);
          return true;
      }
  
      /**
       * Replaces the entry for the specified key only if it is
       * currently mapped to some value.
       *
       * @implSpec
       * The default implementation is equivalent to, for this {@code map}:
       *
       * <pre> {@code
       * if (map.containsKey(key)) {
       *     return map.put(key, value);
       * } else
       *     return null;
       * }</pre>
       *
       * <p>The default implementation makes no guarantees about synchronization
       * or atomicity properties of this method. Any implementation providing
       * atomicity guarantees must override this method and document its
       * concurrency properties.
       *
       * @param key key with which the specified value is associated
       * @param value value to be associated with the specified key
       * @return the previous value associated with the specified key, or
       *         {@code null} if there was no mapping for the key.
       *         (A {@code null} return can also indicate that the map
       *         previously associated {@code null} with the key,
       *         if the implementation supports null values.)
       * @throws UnsupportedOperationException if the {@code put} operation
       *         is not supported by this map
       *         (<a href="Collection.html#optional-restrictions">optional</a>)
       * @throws ClassCastException if the class of the specified key or value
       *         prevents it from being stored in this map
       *         (<a href="Collection.html#optional-restrictions">optional</a>)
       * @throws NullPointerException if the specified key or value is null,
       *         and this map does not permit null keys or values
       * @throws IllegalArgumentException if some property of the specified key
       *         or value prevents it from being stored in this map
       * @since 1.8
       */
      default V replace(K key, V value) {
          V curValue;
          if (((curValue = get(key)) != null) || containsKey(key)) {
              curValue = put(key, value);
          }
          return curValue;
      }
  
      /**
       * If the specified key is not already associated with a value (or is mapped
       * to {@code null}), attempts to compute its value using the given mapping
       * function and enters it into this map unless {@code null}.
       *
       * <p>If the mapping function returns {@code null}, no mapping is recorded.
       * If the mapping function itself throws an (unchecked) exception, the
       * exception is rethrown, and no mapping is recorded.  The most
       * common usage is to construct a new object serving as an initial
       * mapped value or memoized result, as in:
       *
       * <pre> {@code
       * map.computeIfAbsent(key, k -> new Value(f(k)));
       * }</pre>
       *
       * <p>Or to implement a multi-value map, {@code Map<K,Collection<V>>},
       * supporting multiple values per key:
       *
       * <pre> {@code
       * map.computeIfAbsent(key, k -> new HashSet<V>()).add(v);
       * }</pre>
       *
       * <p>The mapping function should not modify this map during computation.
       *
       * @implSpec
       * The default implementation is equivalent to the following steps for this
       * {@code map}, then returning the current value or {@code null} if now
       * absent:
       *
       * <pre> {@code
       * if (map.get(key) == null) {
       *     V newValue = mappingFunction.apply(key);
       *     if (newValue != null)
       *         map.put(key, newValue);
       * }
       * }</pre>
       *
       * <p>The default implementation makes no guarantees about detecting if the
       * mapping function modifies this map during computation and, if
       * appropriate, reporting an error. Non-concurrent implementations should
       * override this method and, on a best-effort basis, throw a
       * {@code ConcurrentModificationException} if it is detected that the
       * mapping function modifies this map during computation. Concurrent
       * implementations should override this method and, on a best-effort basis,
       * throw an {@code IllegalStateException} if it is detected that the
       * mapping function modifies this map during computation and as a result
       * computation would never complete.
       *
       * <p>The default implementation makes no guarantees about synchronization
       * or atomicity properties of this method. Any implementation providing
       * atomicity guarantees must override this method and document its
       * concurrency properties. In particular, all implementations of
       * subinterface {@link java.util.concurrent.ConcurrentMap} must document
       * whether the mapping function is applied once atomically only if the value
       * is not present.
       *
       * @param key key with which the specified value is to be associated
       * @param mappingFunction the mapping function to compute a value
       * @return the current (existing or computed) value associated with
       *         the specified key, or null if the computed value is null
       * @throws NullPointerException if the specified key is null and
       *         this map does not support null keys, or the mappingFunction
       *         is null
       * @throws UnsupportedOperationException if the {@code put} operation
       *         is not supported by this map
       *         (<a href="Collection.html#optional-restrictions">optional</a>)
       * @throws ClassCastException if the class of the specified key or value
       *         prevents it from being stored in this map
       *         (<a href="Collection.html#optional-restrictions">optional</a>)
       * @throws IllegalArgumentException if some property of the specified key
       *         or value prevents it from being stored in this map
       *         (<a href="Collection.html#optional-restrictions">optional</a>)
       * @since 1.8
       */
      default V computeIfAbsent(K key,
              Function<? super K, ? extends V> mappingFunction) {
          Objects.requireNonNull(mappingFunction);
          V v;
          if ((v = get(key)) == null) {
              V newValue;
              if ((newValue = mappingFunction.apply(key)) != null) {
                  put(key, newValue);
                  return newValue;
              }
          }
  
          return v;
      }
  
      /**
       * If the value for the specified key is present and non-null, attempts to
       * compute a new mapping given the key and its current mapped value.
       *
       * <p>If the remapping function returns {@code null}, the mapping is removed.
       * If the remapping function itself throws an (unchecked) exception, the
       * exception is rethrown, and the current mapping is left unchanged.
       *
       * <p>The remapping function should not modify this map during computation.
       *
       * @implSpec
       * The default implementation is equivalent to performing the following
       * steps for this {@code map}, then returning the current value or
       * {@code null} if now absent:
       *
       * <pre> {@code
       * if (map.get(key) != null) {
       *     V oldValue = map.get(key);
       *     V newValue = remappingFunction.apply(key, oldValue);
       *     if (newValue != null)
       *         map.put(key, newValue);
       *     else
       *         map.remove(key);
       * }
       * }</pre>
       *
       * <p>The default implementation makes no guarantees about detecting if the
       * remapping function modifies this map during computation and, if
       * appropriate, reporting an error. Non-concurrent implementations should
       * override this method and, on a best-effort basis, throw a
       * {@code ConcurrentModificationException} if it is detected that the
       * remapping function modifies this map during computation. Concurrent
       * implementations should override this method and, on a best-effort basis,
       * throw an {@code IllegalStateException} if it is detected that the
       * remapping function modifies this map during computation and as a result
       * computation would never complete.
       *
       * <p>The default implementation makes no guarantees about synchronization
       * or atomicity properties of this method. Any implementation providing
       * atomicity guarantees must override this method and document its
       * concurrency properties. In particular, all implementations of
       * subinterface {@link java.util.concurrent.ConcurrentMap} must document
       * whether the remapping function is applied once atomically only if the
       * value is not present.
       *
       * @param key key with which the specified value is to be associated
       * @param remappingFunction the remapping function to compute a value
       * @return the new value associated with the specified key, or null if none
       * @throws NullPointerException if the specified key is null and
       *         this map does not support null keys, or the
       *         remappingFunction is null
       * @throws UnsupportedOperationException if the {@code put} operation
       *         is not supported by this map
       *         (<a href="Collection.html#optional-restrictions">optional</a>)
       * @throws ClassCastException if the class of the specified key or value
       *         prevents it from being stored in this map
       *         (<a href="Collection.html#optional-restrictions">optional</a>)
       * @throws IllegalArgumentException if some property of the specified key
       *         or value prevents it from being stored in this map
       *         (<a href="Collection.html#optional-restrictions">optional</a>)
       * @since 1.8
       */
      default V computeIfPresent(K key,
              BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
          Objects.requireNonNull(remappingFunction);
          V oldValue;
          if ((oldValue = get(key)) != null) {
              V newValue = remappingFunction.apply(key, oldValue);
              if (newValue != null) {
                  put(key, newValue);
                  return newValue;
              } else {
                  remove(key);
                  return null;
              }
          } else {
              return null;
          }
      }
  
      /**
       * Attempts to compute a mapping for the specified key and its current
       * mapped value (or {@code null} if there is no current mapping). For
       * example, to either create or append a {@code String} msg to a value
       * mapping:
       *
       * <pre> {@code
       * map.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg))}</pre>
       * (Method {@link #merge merge()} is often simpler to use for such purposes.)
       *
       * <p>If the remapping function returns {@code null}, the mapping is removed
       * (or remains absent if initially absent).  If the remapping function
       * itself throws an (unchecked) exception, the exception is rethrown, and
       * the current mapping is left unchanged.
       *
       * <p>The remapping function should not modify this map during computation.
       *
       * @implSpec
       * The default implementation is equivalent to performing the following
       * steps for this {@code map}, then returning the current value or
       * {@code null} if absent:
       *
       * <pre> {@code
       * V oldValue = map.get(key);
       * V newValue = remappingFunction.apply(key, oldValue);
       * if (oldValue != null) {
       *    if (newValue != null)
       *       map.put(key, newValue);
       *    else
       *       map.remove(key);
       * } else {
       *    if (newValue != null)
       *       map.put(key, newValue);
       *    else
       *       return null;
       * }
       * }</pre>
       *
       * <p>The default implementation makes no guarantees about detecting if the
       * remapping function modifies this map during computation and, if
       * appropriate, reporting an error. Non-concurrent implementations should
       * override this method and, on a best-effort basis, throw a
       * {@code ConcurrentModificationException} if it is detected that the
       * remapping function modifies this map during computation. Concurrent
       * implementations should override this method and, on a best-effort basis,
       * throw an {@code IllegalStateException} if it is detected that the
       * remapping function modifies this map during computation and as a result
       * computation would never complete.
       *
       * <p>The default implementation makes no guarantees about synchronization
       * or atomicity properties of this method. Any implementation providing
       * atomicity guarantees must override this method and document its
       * concurrency properties. In particular, all implementations of
       * subinterface {@link java.util.concurrent.ConcurrentMap} must document
       * whether the remapping function is applied once atomically only if the
       * value is not present.
       *
       * @param key key with which the specified value is to be associated
       * @param remappingFunction the remapping function to compute a value
       * @return the new value associated with the specified key, or null if none
       * @throws NullPointerException if the specified key is null and
       *         this map does not support null keys, or the
       *         remappingFunction is null
       * @throws UnsupportedOperationException if the {@code put} operation
       *         is not supported by this map
       *         (<a href="Collection.html#optional-restrictions">optional</a>)
       * @throws ClassCastException if the class of the specified key or value
       *         prevents it from being stored in this map
       *         (<a href="Collection.html#optional-restrictions">optional</a>)
       * @throws IllegalArgumentException if some property of the specified key
       *         or value prevents it from being stored in this map
       *         (<a href="Collection.html#optional-restrictions">optional</a>)
       * @since 1.8
       */
      default V compute(K key,
              BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
          Objects.requireNonNull(remappingFunction);
          V oldValue = get(key);
  
          V newValue = remappingFunction.apply(key, oldValue);
          if (newValue == null) {
              // delete mapping
              if (oldValue != null || containsKey(key)) {
                  // something to remove
                  remove(key);
                  return null;
              } else {
                  // nothing to do. Leave things as they were.
                  return null;
              }
          } else {
              // add or replace old mapping
              put(key, newValue);
              return newValue;
          }
      }
  
      /**
       * If the specified key is not already associated with a value or is
       * associated with null, associates it with the given non-null value.
       * Otherwise, replaces the associated value with the results of the given
       * remapping function, or removes if the result is {@code null}. This
       * method may be of use when combining multiple mapped values for a key.
       * For example, to either create or append a {@code String msg} to a
       * value mapping:
       *
       * <pre> {@code
       * map.merge(key, msg, String::concat)
       * }</pre>
       *
       * <p>If the remapping function returns {@code null}, the mapping is removed.
       * If the remapping function itself throws an (unchecked) exception, the
       * exception is rethrown, and the current mapping is left unchanged.
       *
       * <p>The remapping function should not modify this map during computation.
       *
       * @implSpec
       * The default implementation is equivalent to performing the following
       * steps for this {@code map}, then returning the current value or
       * {@code null} if absent:
       *
       * <pre> {@code
       * V oldValue = map.get(key);
       * V newValue = (oldValue == null) ? value :
       *              remappingFunction.apply(oldValue, value);
       * if (newValue == null)
       *     map.remove(key);
       * else
       *     map.put(key, newValue);
       * }</pre>
       *
       * <p>The default implementation makes no guarantees about detecting if the
       * remapping function modifies this map during computation and, if
       * appropriate, reporting an error. Non-concurrent implementations should
       * override this method and, on a best-effort basis, throw a
       * {@code ConcurrentModificationException} if it is detected that the
       * remapping function modifies this map during computation. Concurrent
       * implementations should override this method and, on a best-effort basis,
       * throw an {@code IllegalStateException} if it is detected that the
       * remapping function modifies this map during computation and as a result
       * computation would never complete.
       *
       * <p>The default implementation makes no guarantees about synchronization
       * or atomicity properties of this method. Any implementation providing
       * atomicity guarantees must override this method and document its
       * concurrency properties. In particular, all implementations of
       * subinterface {@link java.util.concurrent.ConcurrentMap} must document
       * whether the remapping function is applied once atomically only if the
       * value is not present.
       *
       * @param key key with which the resulting value is to be associated
       * @param value the non-null value to be merged with the existing value
       *        associated with the key or, if no existing value or a null value
       *        is associated with the key, to be associated with the key
       * @param remappingFunction the remapping function to recompute a value if
       *        present
       * @return the new value associated with the specified key, or null if no
       *         value is associated with the key
       * @throws UnsupportedOperationException if the {@code put} operation
       *         is not supported by this map
       *         (<a href="Collection.html#optional-restrictions">optional</a>)
       * @throws ClassCastException if the class of the specified key or value
       *         prevents it from being stored in this map
       *         (<a href="Collection.html#optional-restrictions">optional</a>)
       * @throws IllegalArgumentException if some property of the specified key
       *         or value prevents it from being stored in this map
       *         (<a href="Collection.html#optional-restrictions">optional</a>)
       * @throws NullPointerException if the specified key is null and this map
       *         does not support null keys or the value or remappingFunction is
       *         null
       * @since 1.8
       */
      default V merge(K key, V value,
              BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
          Objects.requireNonNull(remappingFunction);
          Objects.requireNonNull(value);
          V oldValue = get(key);
          V newValue = (oldValue == null) ? value :
                     remappingFunction.apply(oldValue, value);
          if (newValue == null) {
              remove(key);
          } else {
              put(key, newValue);
          }
          return newValue;
      }
  
      /**
       * Returns an immutable map containing zero mappings.
       * See <a href="#immutable">Immutable Map Static Factory Methods</a> for details.
       *
       * @param <K> the {@code Map}'s key type
       * @param <V> the {@code Map}'s value type
       * @return an empty {@code Map}
       *
       * @since 9
       */
      static <K, V> Map<K, V> of() {
          return ImmutableCollections.Map0.instance();
      }
  
      /**
       * Returns an immutable map containing a single mapping.
       * See <a href="#immutable">Immutable Map Static Factory Methods</a> for details.
       *
       * @param <K> the {@code Map}'s key type
       * @param <V> the {@code Map}'s value type
       * @param k1 the mapping's key
       * @param v1 the mapping's value
       * @return a {@code Map} containing the specified mapping
       * @throws NullPointerException if the key or the value is {@code null}
       *
       * @since 9
       */
      static <K, V> Map<K, V> of(K k1, V v1) {
          return new ImmutableCollections.Map1<>(k1, v1);
      }
  
      /**
       * Returns an immutable map containing two mappings.
       * See <a href="#immutable">Immutable Map Static Factory Methods</a> for details.
       *
       * @param <K> the {@code Map}'s key type
       * @param <V> the {@code Map}'s value type
       * @param k1 the first mapping's key
       * @param v1 the first mapping's value
       * @param k2 the second mapping's key
       * @param v2 the second mapping's value
       * @return a {@code Map} containing the specified mappings
       * @throws IllegalArgumentException if the keys are duplicates
       * @throws NullPointerException if any key or value is {@code null}
       *
       * @since 9
       */
      static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2) {
          return new ImmutableCollections.MapN<>(k1, v1, k2, v2);
      }
  
      /**
       * Returns an immutable map containing three mappings.
       * See <a href="#immutable">Immutable Map Static Factory Methods</a> for details.
       *
       * @param <K> the {@code Map}'s key type
       * @param <V> the {@code Map}'s value type
       * @param k1 the first mapping's key
       * @param v1 the first mapping's value
       * @param k2 the second mapping's key
       * @param v2 the second mapping's value
       * @param k3 the third mapping's key
       * @param v3 the third mapping's value
       * @return a {@code Map} containing the specified mappings
       * @throws IllegalArgumentException if there are any duplicate keys
       * @throws NullPointerException if any key or value is {@code null}
       *
       * @since 9
       */
      static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3) {
          return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3);
      }
  
      /**
       * Returns an immutable map containing four mappings.
       * See <a href="#immutable">Immutable Map Static Factory Methods</a> for details.
       *
       * @param <K> the {@code Map}'s key type
       * @param <V> the {@code Map}'s value type
       * @param k1 the first mapping's key
       * @param v1 the first mapping's value
       * @param k2 the second mapping's key
       * @param v2 the second mapping's value
       * @param k3 the third mapping's key
       * @param v3 the third mapping's value
       * @param k4 the fourth mapping's key
       * @param v4 the fourth mapping's value
       * @return a {@code Map} containing the specified mappings
       * @throws IllegalArgumentException if there are any duplicate keys
       * @throws NullPointerException if any key or value is {@code null}
       *
       * @since 9
       */
      static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4) {
          return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3, k4, v4);
      }
  
      /**
       * Returns an immutable map containing five mappings.
       * See <a href="#immutable">Immutable Map Static Factory Methods</a> for details.
       *
       * @param <K> the {@code Map}'s key type
       * @param <V> the {@code Map}'s value type
       * @param k1 the first mapping's key
       * @param v1 the first mapping's value
       * @param k2 the second mapping's key
       * @param v2 the second mapping's value
       * @param k3 the third mapping's key
       * @param v3 the third mapping's value
       * @param k4 the fourth mapping's key
       * @param v4 the fourth mapping's value
       * @param k5 the fifth mapping's key
       * @param v5 the fifth mapping's value
       * @return a {@code Map} containing the specified mappings
       * @throws IllegalArgumentException if there are any duplicate keys
       * @throws NullPointerException if any key or value is {@code null}
       *
       * @since 9
       */
      static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5) {
          return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3, k4, v4, k5, v5);
      }
  
      /**
       * Returns an immutable map containing six mappings.
       * See <a href="#immutable">Immutable Map Static Factory Methods</a> for details.
       *
       * @param <K> the {@code Map}'s key type
       * @param <V> the {@code Map}'s value type
       * @param k1 the first mapping's key
       * @param v1 the first mapping's value
       * @param k2 the second mapping's key
       * @param v2 the second mapping's value
       * @param k3 the third mapping's key
       * @param v3 the third mapping's value
       * @param k4 the fourth mapping's key
       * @param v4 the fourth mapping's value
       * @param k5 the fifth mapping's key
       * @param v5 the fifth mapping's value
       * @param k6 the sixth mapping's key
       * @param v6 the sixth mapping's value
       * @return a {@code Map} containing the specified mappings
       * @throws IllegalArgumentException if there are any duplicate keys
       * @throws NullPointerException if any key or value is {@code null}
       *
       * @since 9
       */
      static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5,
                                 K k6, V v6) {
          return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3, k4, v4, k5, v5,
                                                 k6, v6);
      }
  
      /**
       * Returns an immutable map containing seven mappings.
       * See <a href="#immutable">Immutable Map Static Factory Methods</a> for details.
       *
       * @param <K> the {@code Map}'s key type
       * @param <V> the {@code Map}'s value type
       * @param k1 the first mapping's key
       * @param v1 the first mapping's value
       * @param k2 the second mapping's key
       * @param v2 the second mapping's value
       * @param k3 the third mapping's key
       * @param v3 the third mapping's value
       * @param k4 the fourth mapping's key
       * @param v4 the fourth mapping's value
       * @param k5 the fifth mapping's key
       * @param v5 the fifth mapping's value
       * @param k6 the sixth mapping's key
       * @param v6 the sixth mapping's value
       * @param k7 the seventh mapping's key
       * @param v7 the seventh mapping's value
       * @return a {@code Map} containing the specified mappings
       * @throws IllegalArgumentException if there are any duplicate keys
       * @throws NullPointerException if any key or value is {@code null}
       *
       * @since 9
       */
      static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5,
                                 K k6, V v6, K k7, V v7) {
          return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3, k4, v4, k5, v5,
                                                 k6, v6, k7, v7);
      }
  
      /**
       * Returns an immutable map containing eight mappings.
       * See <a href="#immutable">Immutable Map Static Factory Methods</a> for details.
       *
       * @param <K> the {@code Map}'s key type
       * @param <V> the {@code Map}'s value type
       * @param k1 the first mapping's key
       * @param v1 the first mapping's value
       * @param k2 the second mapping's key
       * @param v2 the second mapping's value
       * @param k3 the third mapping's key
       * @param v3 the third mapping's value
       * @param k4 the fourth mapping's key
       * @param v4 the fourth mapping's value
       * @param k5 the fifth mapping's key
       * @param v5 the fifth mapping's value
       * @param k6 the sixth mapping's key
       * @param v6 the sixth mapping's value
       * @param k7 the seventh mapping's key
       * @param v7 the seventh mapping's value
       * @param k8 the eighth mapping's key
       * @param v8 the eighth mapping's value
       * @return a {@code Map} containing the specified mappings
       * @throws IllegalArgumentException if there are any duplicate keys
       * @throws NullPointerException if any key or value is {@code null}
       *
       * @since 9
       */
      static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5,
                                 K k6, V v6, K k7, V v7, K k8, V v8) {
          return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3, k4, v4, k5, v5,
                                                 k6, v6, k7, v7, k8, v8);
      }
  
      /**
       * Returns an immutable map containing nine mappings.
       * See <a href="#immutable">Immutable Map Static Factory Methods</a> for details.
       *
       * @param <K> the {@code Map}'s key type
       * @param <V> the {@code Map}'s value type
       * @param k1 the first mapping's key
       * @param v1 the first mapping's value
       * @param k2 the second mapping's key
       * @param v2 the second mapping's value
       * @param k3 the third mapping's key
       * @param v3 the third mapping's value
       * @param k4 the fourth mapping's key
       * @param v4 the fourth mapping's value
       * @param k5 the fifth mapping's key
       * @param v5 the fifth mapping's value
       * @param k6 the sixth mapping's key
       * @param v6 the sixth mapping's value
       * @param k7 the seventh mapping's key
       * @param v7 the seventh mapping's value
       * @param k8 the eighth mapping's key
       * @param v8 the eighth mapping's value
       * @param k9 the ninth mapping's key
       * @param v9 the ninth mapping's value
       * @return a {@code Map} containing the specified mappings
       * @throws IllegalArgumentException if there are any duplicate keys
       * @throws NullPointerException if any key or value is {@code null}
       *
       * @since 9
       */
      static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5,
                                 K k6, V v6, K k7, V v7, K k8, V v8, K k9, V v9) {
          return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3, k4, v4, k5, v5,
                                                 k6, v6, k7, v7, k8, v8, k9, v9);
      }
  
      /**
       * Returns an immutable map containing ten mappings.
       * See <a href="#immutable">Immutable Map Static Factory Methods</a> for details.
       *
       * @param <K> the {@code Map}'s key type
       * @param <V> the {@code Map}'s value type
       * @param k1 the first mapping's key
       * @param v1 the first mapping's value
       * @param k2 the second mapping's key
       * @param v2 the second mapping's value
       * @param k3 the third mapping's key
       * @param v3 the third mapping's value
       * @param k4 the fourth mapping's key
       * @param v4 the fourth mapping's value
       * @param k5 the fifth mapping's key
       * @param v5 the fifth mapping's value
       * @param k6 the sixth mapping's key
       * @param v6 the sixth mapping's value
       * @param k7 the seventh mapping's key
       * @param v7 the seventh mapping's value
       * @param k8 the eighth mapping's key
       * @param v8 the eighth mapping's value
       * @param k9 the ninth mapping's key
       * @param v9 the ninth mapping's value
       * @param k10 the tenth mapping's key
       * @param v10 the tenth mapping's value
       * @return a {@code Map} containing the specified mappings
       * @throws IllegalArgumentException if there are any duplicate keys
       * @throws NullPointerException if any key or value is {@code null}
       *
       * @since 9
       */
      static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5,
                                 K k6, V v6, K k7, V v7, K k8, V v8, K k9, V v9, K k10, V v10) {
          return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3, k4, v4, k5, v5,
                                                 k6, v6, k7, v7, k8, v8, k9, v9, k10, v10);
      }
  
      /**
       * Returns an immutable map containing keys and values extracted from the given entries.
       * The entries themselves are not stored in the map.
       * See <a href="#immutable">Immutable Map Static Factory Methods</a> for details.
       *
       * @apiNote
       * It is convenient to create the map entries using the {@link Map#entry Map.entry()} method.
       * For example,
       *
       * <pre>{@code
       *     import static java.util.Map.entry;
       *
       *     Map<Integer,String> map = Map.ofEntries(
       *         entry(1, "a"),
       *         entry(2, "b"),
       *         entry(3, "c"),
       *         ...
       *         entry(26, "z"));
       * }</pre>
       *
       * @param <K> the {@code Map}'s key type
       * @param <V> the {@code Map}'s value type
       * @param entries {@code Map.Entry}s containing the keys and values from which the map is populated
       * @return a {@code Map} containing the specified mappings
       * @throws IllegalArgumentException if there are any duplicate keys
       * @throws NullPointerException if any entry, key, or value is {@code null}, or if
       *         the {@code entries} array is {@code null}
       *
       * @see Map#entry Map.entry()
       * @since 9
       */
      @SafeVarargs
      @SuppressWarnings("varargs")
      static <K, V> Map<K, V> ofEntries(Entry<? extends K, ? extends V>... entries) {
          if (entries.length == 0) { // implicit null check of entries
              return ImmutableCollections.Map0.instance();
          } else if (entries.length == 1) {
              return new ImmutableCollections.Map1<>(entries[0].getKey(),
                                                     entries[0].getValue());
          } else {
              Object[] kva = new Object[entries.length << 1];
              int a = 0;
              for (Entry<? extends K, ? extends V> entry : entries) {
                  kva[a++] = entry.getKey();
                  kva[a++] = entry.getValue();
              }
              return new ImmutableCollections.MapN<>(kva);
          }
      }
  
      /**
       * Returns an immutable {@link Entry} containing the given key and value.
       * These entries are suitable for populating {@code Map} instances using the
       * {@link Map#ofEntries Map.ofEntries()} method.
       * The {@code Entry} instances created by this method have the following characteristics:
       *
       * <ul>
       * <li>They disallow {@code null} keys and values. Attempts to create them using a {@code null}
       * key or value result in {@code NullPointerException}.
       * <li>They are immutable. Calls to {@link Entry#setValue Entry.setValue()}
       * on a returned {@code Entry} result in {@code UnsupportedOperationException}.
       * <li>They are not serializable.
       * <li>They are <a href="../lang/doc-files/ValueBased.html">value-based</a>.
       * Callers should make no assumptions about the identity of the returned instances.
       * This method is free to create new instances or reuse existing ones. Therefore,
       * identity-sensitive operations on these instances (reference equality ({@code ==}),
       * identity hash code, and synchronization) are unreliable and should be avoided.
       * </ul>
       *
       * @apiNote
       * For a serializable {@code Entry}, see {@link AbstractMap.SimpleEntry} or
       * {@link AbstractMap.SimpleImmutableEntry}.
       *
       * @param <K> the key's type
       * @param <V> the value's type
       * @param k the key
       * @param v the value
       * @return an {@code Entry} containing the specified key and value
       * @throws NullPointerException if the key or value is {@code null}
       *
       * @see Map#ofEntries Map.ofEntries()
       * @since 9
       */
      static <K, V> Entry<K, V> entry(K k, V v) {
          // KeyValueHolder checks for nulls
          return new KeyValueHolder<>(k, v);
      }
  }
  