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   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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   * published by the Free Software Foundation.  Oracle designates this
   * particular file as subject to the "Classpath" exception as provided
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   *
   * 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
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   * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
   *
   * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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  package java.util;
  
  import java.util.function.Predicate;
  import java.util.stream.Stream;
  import java.util.stream.StreamSupport;
  
  /**
   * The root interface in the <i>collection hierarchy</i>.  A collection
   * represents a group of objects, known as its <i>elements</i>.  Some
   * collections allow duplicate elements and others do not.  Some are ordered
   * and others unordered.  The JDK does not provide any <i>direct</i>
   * implementations of this interface: it provides implementations of more
   * specific subinterfaces like <tt>Set</tt> and <tt>List</tt>.  This interface
   * is typically used to pass collections around and manipulate them where
   * maximum generality is desired.
   *
   * <p><i>Bags</i> or <i>multisets</i> (unordered collections that may contain
   * duplicate elements) should implement this interface directly.
   *
   * <p>All general-purpose <tt>Collection</tt> implementation classes (which
   * typically implement <tt>Collection</tt> indirectly through one of its
   * subinterfaces) should provide two "standard" constructors: a void (no
   * arguments) constructor, which creates an empty collection, and a
   * constructor with a single argument of type <tt>Collection</tt>, which
   * creates a new collection with the same elements as its argument.  In
   * effect, the latter constructor allows the user to copy any collection,
   * producing an equivalent collection of the desired implementation type.
   * There is no way to enforce this convention (as interfaces cannot contain
   * constructors) but all of the general-purpose <tt>Collection</tt>
   * implementations in the Java platform libraries comply.
   *
   * <p>The "destructive" methods contained in this interface, that is, the
   * methods that modify the collection on which they operate, are specified to
   * throw <tt>UnsupportedOperationException</tt> if this collection does not
   * support the operation.  If this is the case, these methods may, but are not
   * required to, throw an <tt>UnsupportedOperationException</tt> if the
   * invocation would have no effect on the collection.  For example, invoking
   * the {@link #addAll(Collection)} method on an unmodifiable collection may,
   * but is not required to, throw the exception if the collection to be added
   * is empty.
   *
   * <p><a name="optional-restrictions">
   * Some collection implementations have restrictions on the elements that
   * they may contain.</a>  For example, some implementations prohibit null elements,
   * and some have restrictions on the types of their elements.  Attempting to
   * add an ineligible element throws an unchecked exception, typically
   * <tt>NullPointerException</tt> or <tt>ClassCastException</tt>.  Attempting
   * to query the presence of an ineligible element 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 element whose completion would not result in
   * the insertion of an ineligible element into the collection 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>It is up to each collection to determine its own synchronization
   * policy.  In the absence of a stronger guarantee by the
   * implementation, undefined behavior may result from the invocation
   * of any method on a collection that is being mutated by another
   * thread; this includes direct invocations, passing the collection to
   * a method that might perform invocations, and using an existing
   * iterator to examine the collection.
   *
   * <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 #contains(Object) contains(Object o)}
   * method says: "returns <tt>true</tt> if and only if this collection
   * contains at least one element <tt>e</tt> such that
   * <tt>(o==null ? e==null : o.equals(e))</tt>."  This specification should
   * <i>not</i> be construed to imply that invoking <tt>Collection.contains</tt>
   * with a non-null argument <tt>o</tt> will cause <tt>o.equals(e)</tt> to be
   * invoked for any element <tt>e</tt>.  Implementations are free to implement
   * optimizations whereby the <tt>equals</tt> invocation is avoided, for
   * example, by first comparing the hash codes of the two elements.  (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 collection operations which perform recursive traversal of the
   * collection may fail with an exception for self-referential instances where
   * the collection 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.
   *
   * <p>This interface is a member of the
   * <a href="{@docRoot}/../technotes/guides/collections/index.html">
   * Java Collections Framework</a>.
   *
   * @implSpec
   * The default method implementations (inherited or otherwise) do not apply any
   * synchronization protocol.  If a {@code Collection} implementation has a
   * specific synchronization protocol, then it must override default
   * implementations to apply that protocol.
   *
   * @param <E> the type of elements in this collection
   *
   * @author  Josh Bloch
   * @author  Neal Gafter
   * @see     Set
   * @see     List
   * @see     Map
   * @see     SortedSet
   * @see     SortedMap
   * @see     HashSet
   * @see     TreeSet
   * @see     ArrayList
   * @see     LinkedList
   * @see     Vector
   * @see     Collections
   * @see     Arrays
   * @see     AbstractCollection
   * @since 1.2
   */
  
  public interface Collection<E> extends Iterable<E> {
      // Query Operations
  
      /**
       * Returns the number of elements in this collection.  If this collection
       * contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
       * <tt>Integer.MAX_VALUE</tt>.
       *
       * @return the number of elements in this collection
       */
      int size();
  
      /**
       * Returns <tt>true</tt> if this collection contains no elements.
       *
       * @return <tt>true</tt> if this collection contains no elements
       */
      boolean isEmpty();
  
      /**
       * Returns <tt>true</tt> if this collection contains the specified element.
       * More formally, returns <tt>true</tt> if and only if this collection
       * contains at least one element <tt>e</tt> such that
       * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
       *
       * @param o element whose presence in this collection is to be tested
       * @return <tt>true</tt> if this collection contains the specified
       *         element
       * @throws ClassCastException if the type of the specified element
       *         is incompatible with this collection
       *         (<a href="#optional-restrictions">optional</a>)
       * @throws NullPointerException if the specified element is null and this
       *         collection does not permit null elements
       *         (<a href="#optional-restrictions">optional</a>)
       */
      boolean contains(Object o);
  
      /**
       * Returns an iterator over the elements in this collection.  There are no
       * guarantees concerning the order in which the elements are returned
       * (unless this collection is an instance of some class that provides a
       * guarantee).
       *
       * @return an <tt>Iterator</tt> over the elements in this collection
       */
      Iterator<E> iterator();
  
      /**
       * Returns an array containing all of the elements in this collection.
       * If this collection makes any guarantees as to what order its elements
       * are returned by its iterator, this method must return the elements in
       * the same order.
       *
       * <p>The returned array will be "safe" in that no references to it are
       * maintained by this collection.  (In other words, this method must
       * allocate a new array even if this collection is backed by an array).
       * The caller is thus free to modify the returned array.
       *
       * <p>This method acts as bridge between array-based and collection-based
       * APIs.
       *
       * @return an array containing all of the elements in this collection
       */
      Object[] toArray();
  
      /**
       * Returns an array containing all of the elements in this collection;
       * the runtime type of the returned array is that of the specified array.
       * If the collection fits in the specified array, it is returned therein.
       * Otherwise, a new array is allocated with the runtime type of the
       * specified array and the size of this collection.
       *
       * <p>If this collection fits in the specified array with room to spare
       * (i.e., the array has more elements than this collection), the element
       * in the array immediately following the end of the collection is set to
       * <tt>null</tt>.  (This is useful in determining the length of this
       * collection <i>only</i> if the caller knows that this collection does
       * not contain any <tt>null</tt> elements.)
       *
       * <p>If this collection makes any guarantees as to what order its elements
       * are returned by its iterator, this method must return the elements in
       * the same order.
       *
       * <p>Like the {@link #toArray()} method, this method acts as bridge between
       * array-based and collection-based APIs.  Further, this method allows
       * precise control over the runtime type of the output array, and may,
       * under certain circumstances, be used to save allocation costs.
       *
       * <p>Suppose <tt>x</tt> is a collection known to contain only strings.
       * The following code can be used to dump the collection into a newly
       * allocated array of <tt>String</tt>:
       *
       * <pre>
       *     String[] y = x.toArray(new String[0]);</pre>
       *
       * Note that <tt>toArray(new Object[0])</tt> is identical in function to
       * <tt>toArray()</tt>.
       *
       * @param <T> the runtime type of the array to contain the collection
       * @param a the array into which the elements of this collection are to be
       *        stored, if it is big enough; otherwise, a new array of the same
       *        runtime type is allocated for this purpose.
       * @return an array containing all of the elements in this collection
       * @throws ArrayStoreException if the runtime type of the specified array
       *         is not a supertype of the runtime type of every element in
       *         this collection
       * @throws NullPointerException if the specified array is null
       */
      <T> T[] toArray(T[] a);
  
      // Modification Operations
  
      /**
       * Ensures that this collection contains the specified element (optional
       * operation).  Returns <tt>true</tt> if this collection changed as a
       * result of the call.  (Returns <tt>false</tt> if this collection does
       * not permit duplicates and already contains the specified element.)<p>
       *
       * Collections that support this operation may place limitations on what
       * elements may be added to this collection.  In particular, some
       * collections will refuse to add <tt>null</tt> elements, and others will
       * impose restrictions on the type of elements that may be added.
       * Collection classes should clearly specify in their documentation any
       * restrictions on what elements may be added.<p>
       *
       * If a collection refuses to add a particular element for any reason
       * other than that it already contains the element, it <i>must</i> throw
       * an exception (rather than returning <tt>false</tt>).  This preserves
       * the invariant that a collection always contains the specified element
       * after this call returns.
       *
       * @param e element whose presence in this collection is to be ensured
       * @return <tt>true</tt> if this collection changed as a result of the
       *         call
       * @throws UnsupportedOperationException if the <tt>add</tt> operation
       *         is not supported by this collection
       * @throws ClassCastException if the class of the specified element
       *         prevents it from being added to this collection
       * @throws NullPointerException if the specified element is null and this
       *         collection does not permit null elements
       * @throws IllegalArgumentException if some property of the element
       *         prevents it from being added to this collection
       * @throws IllegalStateException if the element cannot be added at this
       *         time due to insertion restrictions
       */
      boolean add(E e);
  
      /**
       * Removes a single instance of the specified element from this
       * collection, if it is present (optional operation).  More formally,
       * removes an element <tt>e</tt> such that
       * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>, if
       * this collection contains one or more such elements.  Returns
       * <tt>true</tt> if this collection contained the specified element (or
       * equivalently, if this collection changed as a result of the call).
       *
       * @param o element to be removed from this collection, if present
       * @return <tt>true</tt> if an element was removed as a result of this call
       * @throws ClassCastException if the type of the specified element
       *         is incompatible with this collection
       *         (<a href="#optional-restrictions">optional</a>)
       * @throws NullPointerException if the specified element is null and this
       *         collection does not permit null elements
       *         (<a href="#optional-restrictions">optional</a>)
       * @throws UnsupportedOperationException if the <tt>remove</tt> operation
       *         is not supported by this collection
       */
      boolean remove(Object o);
  
  
      // Bulk Operations
  
      /**
       * Returns <tt>true</tt> if this collection contains all of the elements
       * in the specified collection.
       *
       * @param  c collection to be checked for containment in this collection
       * @return <tt>true</tt> if this collection contains all of the elements
       *         in the specified collection
       * @throws ClassCastException if the types of one or more elements
       *         in the specified collection are incompatible with this
       *         collection
       *         (<a href="#optional-restrictions">optional</a>)
       * @throws NullPointerException if the specified collection contains one
       *         or more null elements and this collection does not permit null
       *         elements
       *         (<a href="#optional-restrictions">optional</a>),
       *         or if the specified collection is null.
       * @see    #contains(Object)
       */
      boolean containsAll(Collection<?> c);
  
      /**
       * Adds all of the elements in the specified collection to this collection
       * (optional operation).  The behavior of this operation is undefined if
       * the specified collection is modified while the operation is in progress.
       * (This implies that the behavior of this call is undefined if the
       * specified collection is this collection, and this collection is
       * nonempty.)
       *
       * @param c collection containing elements to be added to this collection
       * @return <tt>true</tt> if this collection changed as a result of the call
       * @throws UnsupportedOperationException if the <tt>addAll</tt> operation
       *         is not supported by this collection
       * @throws ClassCastException if the class of an element of the specified
       *         collection prevents it from being added to this collection
       * @throws NullPointerException if the specified collection contains a
       *         null element and this collection does not permit null elements,
       *         or if the specified collection is null
       * @throws IllegalArgumentException if some property of an element of the
       *         specified collection prevents it from being added to this
       *         collection
       * @throws IllegalStateException if not all the elements can be added at
       *         this time due to insertion restrictions
       * @see #add(Object)
       */
      boolean addAll(Collection<? extends E> c);
  
      /**
       * Removes all of this collection's elements that are also contained in the
       * specified collection (optional operation).  After this call returns,
       * this collection will contain no elements in common with the specified
       * collection.
       *
       * @param c collection containing elements to be removed from this collection
       * @return <tt>true</tt> if this collection changed as a result of the
       *         call
       * @throws UnsupportedOperationException if the <tt>removeAll</tt> method
       *         is not supported by this collection
       * @throws ClassCastException if the types of one or more elements
       *         in this collection are incompatible with the specified
       *         collection
       *         (<a href="#optional-restrictions">optional</a>)
       * @throws NullPointerException if this collection contains one or more
       *         null elements and the specified collection does not support
       *         null elements
       *         (<a href="#optional-restrictions">optional</a>),
       *         or if the specified collection is null
       * @see #remove(Object)
       * @see #contains(Object)
       */
      boolean removeAll(Collection<?> c);
  
      /**
       * Removes all of the elements of this collection that satisfy the given
       * predicate.  Errors or runtime exceptions thrown during iteration or by
       * the predicate are relayed to the caller.
       *
       * @implSpec
       * The default implementation traverses all elements of the collection using
       * its {@link #iterator}.  Each matching element is removed using
       * {@link Iterator#remove()}.  If the collection's iterator does not
       * support removal then an {@code UnsupportedOperationException} will be
       * thrown on the first matching element.
       *
       * @param filter a predicate which returns {@code true} for elements to be
       *        removed
       * @return {@code true} if any elements were removed
       * @throws NullPointerException if the specified filter is null
       * @throws UnsupportedOperationException if elements cannot be removed
       *         from this collection.  Implementations may throw this exception if a
       *         matching element cannot be removed or if, in general, removal is not
       *         supported.
       * @since 1.8
       */
      default boolean removeIf(Predicate<? super E> filter) {
          Objects.requireNonNull(filter);
          boolean removed = false;
          final Iterator<E> each = iterator();
          while (each.hasNext()) {
              if (filter.test(each.next())) {
                  each.remove();
                  removed = true;
              }
          }
          return removed;
      }
  
      /**
       * Retains only the elements in this collection that are contained in the
       * specified collection (optional operation).  In other words, removes from
       * this collection all of its elements that are not contained in the
       * specified collection.
       *
       * @param c collection containing elements to be retained in this collection
       * @return <tt>true</tt> if this collection changed as a result of the call
       * @throws UnsupportedOperationException if the <tt>retainAll</tt> operation
       *         is not supported by this collection
       * @throws ClassCastException if the types of one or more elements
       *         in this collection are incompatible with the specified
       *         collection
       *         (<a href="#optional-restrictions">optional</a>)
       * @throws NullPointerException if this collection contains one or more
       *         null elements and the specified collection does not permit null
       *         elements
       *         (<a href="#optional-restrictions">optional</a>),
       *         or if the specified collection is null
       * @see #remove(Object)
       * @see #contains(Object)
       */
      boolean retainAll(Collection<?> c);
  
      /**
       * Removes all of the elements from this collection (optional operation).
       * The collection will be empty after this method returns.
       *
       * @throws UnsupportedOperationException if the <tt>clear</tt> operation
       *         is not supported by this collection
       */
      void clear();
  
  
      // Comparison and hashing
  
      /**
       * Compares the specified object with this collection for equality. <p>
       *
       * While the <tt>Collection</tt> interface adds no stipulations to the
       * general contract for the <tt>Object.equals</tt>, programmers who
       * implement the <tt>Collection</tt> interface "directly" (in other words,
       * create a class that is a <tt>Collection</tt> but is not a <tt>Set</tt>
       * or a <tt>List</tt>) must exercise care if they choose to override the
       * <tt>Object.equals</tt>.  It is not necessary to do so, and the simplest
       * course of action is to rely on <tt>Object</tt>'s implementation, but
       * the implementor may wish to implement a "value comparison" in place of
       * the default "reference comparison."  (The <tt>List</tt> and
       * <tt>Set</tt> interfaces mandate such value comparisons.)<p>
       *
       * The general contract for the <tt>Object.equals</tt> method states that
       * equals must be symmetric (in other words, <tt>a.equals(b)</tt> if and
       * only if <tt>b.equals(a)</tt>).  The contracts for <tt>List.equals</tt>
       * and <tt>Set.equals</tt> state that lists are only equal to other lists,
       * and sets to other sets.  Thus, a custom <tt>equals</tt> method for a
       * collection class that implements neither the <tt>List</tt> nor
       * <tt>Set</tt> interface must return <tt>false</tt> when this collection
       * is compared to any list or set.  (By the same logic, it is not possible
       * to write a class that correctly implements both the <tt>Set</tt> and
       * <tt>List</tt> interfaces.)
       *
       * @param o object to be compared for equality with this collection
       * @return <tt>true</tt> if the specified object is equal to this
       * collection
       *
       * @see Object#equals(Object)
       * @see Set#equals(Object)
       * @see List#equals(Object)
       */
      boolean equals(Object o);
  
      /**
       * Returns the hash code value for this collection.  While the
       * <tt>Collection</tt> interface adds no stipulations to the general
       * contract for the <tt>Object.hashCode</tt> method, programmers should
       * take note that any class that overrides the <tt>Object.equals</tt>
       * method must also override the <tt>Object.hashCode</tt> method in order
       * to satisfy the general contract for the <tt>Object.hashCode</tt> method.
       * In particular, <tt>c1.equals(c2)</tt> implies that
       * <tt>c1.hashCode()==c2.hashCode()</tt>.
       *
       * @return the hash code value for this collection
       *
       * @see Object#hashCode()
       * @see Object#equals(Object)
       */
      int hashCode();
  
      /**
       * Creates a {@link Spliterator} over the elements in this collection.
       *
       * Implementations should document characteristic values reported by the
       * spliterator.  Such characteristic values are not required to be reported
       * if the spliterator reports {@link Spliterator#SIZED} and this collection
       * contains no elements.
       *
       * <p>The default implementation should be overridden by subclasses that
       * can return a more efficient spliterator.  In order to
       * preserve expected laziness behavior for the {@link #stream()} and
       * {@link #parallelStream()}} methods, spliterators should either have the
       * characteristic of {@code IMMUTABLE} or {@code CONCURRENT}, or be
       * <em><a href="Spliterator.html#binding">late-binding</a></em>.
       * If none of these is practical, the overriding class should describe the
       * spliterator's documented policy of binding and structural interference,
       * and should override the {@link #stream()} and {@link #parallelStream()}
       * methods to create streams using a {@code Supplier} of the spliterator,
       * as in:
       * <pre>{@code
       *     Stream<E> s = StreamSupport.stream(() -> spliterator(), spliteratorCharacteristics)
       * }</pre>
       * <p>These requirements ensure that streams produced by the
       * {@link #stream()} and {@link #parallelStream()} methods will reflect the
       * contents of the collection as of initiation of the terminal stream
       * operation.
       *
       * @implSpec
       * The default implementation creates a
       * <em><a href="Spliterator.html#binding">late-binding</a></em> spliterator
       * from the collections's {@code Iterator}.  The spliterator inherits the
       * <em>fail-fast</em> properties of the collection's iterator.
       * <p>
       * The created {@code Spliterator} reports {@link Spliterator#SIZED}.
       *
       * @implNote
       * The created {@code Spliterator} additionally reports
       * {@link Spliterator#SUBSIZED}.
       *
       * <p>If a spliterator covers no elements then the reporting of additional
       * characteristic values, beyond that of {@code SIZED} and {@code SUBSIZED},
       * does not aid clients to control, specialize or simplify computation.
       * However, this does enable shared use of an immutable and empty
       * spliterator instance (see {@link Spliterators#emptySpliterator()}) for
       * empty collections, and enables clients to determine if such a spliterator
       * covers no elements.
       *
       * @return a {@code Spliterator} over the elements in this collection
       * @since 1.8
       */
      @Override
      default Spliterator<E> spliterator() {
          return Spliterators.spliterator(this, 0);
      }
  
      /**
       * Returns a sequential {@code Stream} with this collection as its source.
       *
       * <p>This method should be overridden when the {@link #spliterator()}
       * method cannot return a spliterator that is {@code IMMUTABLE},
       * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()}
       * for details.)
       *
       * @implSpec
       * The default implementation creates a sequential {@code Stream} from the
       * collection's {@code Spliterator}.
       *
       * @return a sequential {@code Stream} over the elements in this collection
       * @since 1.8
       */
      default Stream<E> stream() {
          return StreamSupport.stream(spliterator(), false);
      }
  
      /**
       * Returns a possibly parallel {@code Stream} with this collection as its
       * source.  It is allowable for this method to return a sequential stream.
       *
       * <p>This method should be overridden when the {@link #spliterator()}
       * method cannot return a spliterator that is {@code IMMUTABLE},
       * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()}
       * for details.)
       *
       * @implSpec
       * The default implementation creates a parallel {@code Stream} from the
       * collection's {@code Spliterator}.
       *
       * @return a possibly parallel {@code Stream} over the elements in this
       * collection
       * @since 1.8
       */
      default Stream<E> parallelStream() {
          return StreamSupport.stream(spliterator(), true);
      }
  }
  