    rev 48045 : 8177290: add copy factory methods for unmodifiable List, Set, Map
    8184690: add Collectors for collecting into unmodifiable List, Set, and Map
    Reviewed-by: alanb, briangoetz, dholmes, jrose, rriggs, scolebourne

       1 /*
       2  * Copyright (c) 1997, 2017, Oracle and/or its affiliates. All rights reserved.
       3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
       4  *
       5  * This code is free software; you can redistribute it and/or modify it
       6  * under the terms of the GNU General Public License version 2 only, as
       7  * published by the Free Software Foundation.  Oracle designates this
       8  * particular file as subject to the "Classpath" exception as provided
       9  * by Oracle in the LICENSE file that accompanied this code.
      10  *
      11  * This code is distributed in the hope that it will be useful, but WITHOUT
      12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
      13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
      14  * version 2 for more details (a copy is included in the LICENSE file that
      15  * accompanied this code).
      16  *
      17  * You should have received a copy of the GNU General Public License version
      18  * 2 along with this work; if not, write to the Free Software Foundation,
      19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
      20  *
      21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
      22  * or visit www.oracle.com if you need additional information or have any
      23  * questions.
      24  */
      25
      26 package java.util;
      27
      28 import java.util.function.Predicate;
      29 import java.util.stream.Stream;
      30 import java.util.stream.StreamSupport;
      31
      32 /**
      33  * The root interface in the <i>collection hierarchy</i>.  A collection
      34  * represents a group of objects, known as its <i>elements</i>.  Some
      35  * collections allow duplicate elements and others do not.  Some are ordered
      36  * and others unordered.  The JDK does not provide any <i>direct</i>
      37  * implementations of this interface: it provides implementations of more
      38  * specific subinterfaces like {@code Set} and {@code List}.  This interface
      39  * is typically used to pass collections around and manipulate them where
      40  * maximum generality is desired.
      41  *
      42  * <p><i>Bags</i> or <i>multisets</i> (unordered collections that may contain
      43  * duplicate elements) should implement this interface directly.
      44  *
      45  * <p>All general-purpose {@code Collection} implementation classes (which
      46  * typically implement {@code Collection} indirectly through one of its
      47  * subinterfaces) should provide two "standard" constructors: a void (no
      48  * arguments) constructor, which creates an empty collection, and a
      49  * constructor with a single argument of type {@code Collection}, which
      50  * creates a new collection with the same elements as its argument.  In
      51  * effect, the latter constructor allows the user to copy any collection,
      52  * producing an equivalent collection of the desired implementation type.
      53  * There is no way to enforce this convention (as interfaces cannot contain
      54  * constructors) but all of the general-purpose {@code Collection}
      55  * implementations in the Java platform libraries comply.
      56  *
      57  * <p>Certain methods are specified to be
      58  * <i>optional</i>. If a collection implementation doesn't implement a
      59  * particular operation, it should define the corresponding method to throw
      60  * {@code UnsupportedOperationException}. Such methods are marked "optional
      61  * operation" in method specifications of the collections interfaces.
      62  *
      63  * <p><a id="optional-restrictions"></a>Some collection implementations
      64  * have restrictions on the elements that they may contain.
      65  * For example, some implementations prohibit null elements,

      66  * and some have restrictions on the types of their elements.  Attempting to
      67  * add an ineligible element throws an unchecked exception, typically
      68  * {@code NullPointerException} or {@code ClassCastException}.  Attempting
      69  * to query the presence of an ineligible element may throw an exception,
      70  * or it may simply return false; some implementations will exhibit the former
      71  * behavior and some will exhibit the latter.  More generally, attempting an
      72  * operation on an ineligible element whose completion would not result in
      73  * the insertion of an ineligible element into the collection may throw an
      74  * exception or it may succeed, at the option of the implementation.
      75  * Such exceptions are marked as "optional" in the specification for this
      76  * interface.
      77  *
      78  * <p>It is up to each collection to determine its own synchronization
      79  * policy.  In the absence of a stronger guarantee by the
      80  * implementation, undefined behavior may result from the invocation
      81  * of any method on a collection that is being mutated by another
      82  * thread; this includes direct invocations, passing the collection to
      83  * a method that might perform invocations, and using an existing
      84  * iterator to examine the collection.
      85  *
      86  * <p>Many methods in Collections Framework interfaces are defined in
      87  * terms of the {@link Object#equals(Object) equals} method.  For example,
      88  * the specification for the {@link #contains(Object) contains(Object o)}
      89  * method says: "returns {@code true} if and only if this collection
      90  * contains at least one element {@code e} such that
      91  * {@code (o==null ? e==null : o.equals(e))}."  This specification should
      92  * <i>not</i> be construed to imply that invoking {@code Collection.contains}
      93  * with a non-null argument {@code o} will cause {@code o.equals(e)} to be
      94  * invoked for any element {@code e}.  Implementations are free to implement
      95  * optimizations whereby the {@code equals} invocation is avoided, for
      96  * example, by first comparing the hash codes of the two elements.  (The
      97  * {@link Object#hashCode()} specification guarantees that two objects with
      98  * unequal hash codes cannot be equal.)  More generally, implementations of
      99  * the various Collections Framework interfaces are free to take advantage of
     100  * the specified behavior of underlying {@link Object} methods wherever the
     101  * implementor deems it appropriate.
     102  *
     103  * <p>Some collection operations which perform recursive traversal of the
     104  * collection may fail with an exception for self-referential instances where
     105  * the collection directly or indirectly contains itself. This includes the
     106  * {@code clone()}, {@code equals()}, {@code hashCode()} and {@code toString()}
     107  * methods. Implementations may optionally handle the self-referential scenario,
     108  * however most current implementations do not do so.
     109  *
     110  * <h2><a id="view">View Collections</a></h2>
     111  *
     112  * <p>Most collections manage storage for elements they contain. By contrast, <i>view
     113  * collections</i> themselves do not store elements, but instead they rely on a
     114  * backing collection to store the actual elements. Operations that are not handled
     115  * by the view collection itself are delegated to the backing collection. Examples of
     116  * view collections include the wrapper collections returned by methods such as
     117  * {@link Collections#checkedCollection Collections.checkedCollection},
     118  * {@link Collections#synchronizedCollection Collections.synchronizedCollection}, and
     119  * {@link Collections#unmodifiableCollection Collections.unmodifiableCollection}.
     120  * Other examples of view collections include collections that provide a
     121  * different representation of the same elements, for example, as
     122  * provided by {@link List#subList List.subList},
     123  * {@link NavigableSet#subSet NavigableSet.subSet}, or
     124  * {@link Map#entrySet Map.entrySet}.
     125  * Any changes made to the backing collection are visible in the view collection.
     126  * Correspondingly, any changes made to the view collection &mdash; if changes
     127  * are permitted &mdash; are written through to the backing collection.
     128  * Although they technically aren't collections, instances of
     129  * {@link Iterator} and {@link ListIterator} can also allow modifications
     130  * to be written through to the backing collection, and in some cases,
     131  * modifications to the backing collection will be visible to the Iterator
     132  * during iteration.
     133  *
     134  * <h2><a id="unmodifiable">Unmodifiable Collections</a></h2>
     135  *
     136  * <p>Certain methods of this interface are considered "destructive" and are called
     137  * "mutator" methods in that they modify the group of objects contained within
     138  * the collection on which they operate. They can be specified to throw
     139  * {@code UnsupportedOperationException} if this collection implementation
     140  * does not support the operation. Such methods should (but are not required
     141  * to) throw an {@code UnsupportedOperationException} if the invocation would
     142  * have no effect on the collection. For example, consider a collection that
     143  * does not support the {@link #add add} operation. What will happen if the
     144  * {@link #addAll addAll} method is invoked on this collection, with an empty
     145  * collection as the argument? The addition of zero elements has no effect,
     146  * so it is permissible for this collection simply to do nothing and not to throw
     147  * an exception. However, it is recommended that such cases throw an exception
     148  * unconditionally, as throwing only in certain cases can lead to
     149  * programming errors.
     150  *
     151  * <p>An <i>unmodifiable collection</i> is a collection, all of whose
     152  * mutator methods (as defined above) are specified to throw
     153  * {@code UnsupportedOperationException}. Such a collection thus cannot be
     154  * modified by calling any methods on it. For a collection to be properly
     155  * unmodifiable, any view collections derived from it must also be unmodifiable.
     156  * For example, if a List is unmodifiable, the List returned by
     157  * {@link List#subList List.subList} is also unmodifiable.
     158  *
     159  * <p>An unmodifiable collection is not necessarily immutable. If the
     160  * contained elements are mutable, the entire collection is clearly
     161  * mutable, even though it might be unmodifiable. For example, consider
     162  * two unmodifiable lists containing mutable elements. The result of calling
     163  * {@code list1.equals(list2)} might differ from one call to the next if
     164  * the elements had been mutated, even though both lists are unmodifiable.
     165  * However, if an unmodifiable collection contains all immutable elements,
     166  * it can be considered effectively immutable.
     167  *
     168  * <h2><a id="unmodview">Unmodifiable View Collections</a></h2>
     169  *
     170  * <p>An <i>unmodifiable view collection</i> is a collection that is unmodifiable
     171  * and that is also a view onto a backing collection. Its mutator methods throw
     172  * {@code UnsupportedOperationException}, as described above, while
     173  * reading and querying methods are delegated to the backing collection.
     174  * The effect is to provide read-only access to the backing collection.
     175  * This is useful for a component to provide users with read access to
     176  * an internal collection, while preventing them from modifying such
     177  * collections unexpectedly. Examples of unmodifiable view collections
     178  * are those returned by the
     179  * {@link Collections#unmodifiableCollection Collections.unmodifiableCollection},
     180  * {@link Collections#unmodifiableList Collections.unmodifiableList}, and
     181  * related methods.
     182  *
     183  * <p>Note that changes to the backing collection might still be possible,
     184  * and if they occur, they are visible through the unmodifiable view. Thus,
     185  * an unmodifiable view collection is not necessarily immutable. However,
     186  * if the backing collection of an unmodifiable view is effectively immutable,
     187  * or if the only reference to the backing collection is through an
     188  * unmodifiable view, the view can be considered effectively immutable.
     189  *
     190  * <p>This interface is a member of the
     191  * <a href="{@docRoot}/java/util/package-summary.html#CollectionsFramework">
     192  * Java Collections Framework</a>.
     193  *
     194  * @implSpec
     195  * The default method implementations (inherited or otherwise) do not apply any
     196  * synchronization protocol.  If a {@code Collection} implementation has a
     197  * specific synchronization protocol, then it must override default
     198  * implementations to apply that protocol.
     199  *
     200  * @param <E> the type of elements in this collection
     201  *
     202  * @author  Josh Bloch
     203  * @author  Neal Gafter
     204  * @see     Set
     205  * @see     List
     206  * @see     Map
     207  * @see     SortedSet
     208  * @see     SortedMap
     209  * @see     HashSet
     210  * @see     TreeSet
     211  * @see     ArrayList
     212  * @see     LinkedList
     213  * @see     Vector
     214  * @see     Collections
     215  * @see     Arrays
     216  * @see     AbstractCollection
     217  * @since 1.2
     218  */
     219
     220 public interface Collection<E> extends Iterable<E> {
     221     // Query Operations
     222
     223     /**
     224      * Returns the number of elements in this collection.  If this collection
     225      * contains more than {@code Integer.MAX_VALUE} elements, returns
     226      * {@code Integer.MAX_VALUE}.
     227      *
     228      * @return the number of elements in this collection
     229      */
     230     int size();
     231
     232     /**
     233      * Returns {@code true} if this collection contains no elements.
     234      *
     235      * @return {@code true} if this collection contains no elements
     236      */
     237     boolean isEmpty();
     238
     239     /**
     240      * Returns {@code true} if this collection contains the specified element.
     241      * More formally, returns {@code true} if and only if this collection
     242      * contains at least one element {@code e} such that
     243      * {@code Objects.equals(o, e)}.
     244      *
     245      * @param o element whose presence in this collection is to be tested
     246      * @return {@code true} if this collection contains the specified
     247      *         element
     248      * @throws ClassCastException if the type of the specified element
     249      *         is incompatible with this collection
     250      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     251      * @throws NullPointerException if the specified element is null and this
     252      *         collection does not permit null elements
     253      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     254      */
     255     boolean contains(Object o);
     256
     257     /**
     258      * Returns an iterator over the elements in this collection.  There are no
     259      * guarantees concerning the order in which the elements are returned
     260      * (unless this collection is an instance of some class that provides a
     261      * guarantee).
     262      *
     263      * @return an {@code Iterator} over the elements in this collection
     264      */
     265     Iterator<E> iterator();
     266
     267     /**
     268      * Returns an array containing all of the elements in this collection.
     269      * If this collection makes any guarantees as to what order its elements
     270      * are returned by its iterator, this method must return the elements in
     271      * the same order. The returned array's {@linkplain Class#getComponentType
     272      * runtime component type} is {@code Object}.
     273      *
     274      * <p>The returned array will be "safe" in that no references to it are
     275      * maintained by this collection.  (In other words, this method must
     276      * allocate a new array even if this collection is backed by an array).
     277      * The caller is thus free to modify the returned array.
     278      *
     279      * <p>This method acts as bridge between array-based and collection-based
     280      * APIs.
     281      *
     282      * @return an array, whose {@linkplain Class#getComponentType runtime component
     283      * type} is {@code Object}, containing all of the elements in this collection
     284      */
     285     Object[] toArray();
     286
     287     /**
     288      * Returns an array containing all of the elements in this collection;
     289      * the runtime type of the returned array is that of the specified array.
     290      * If the collection fits in the specified array, it is returned therein.
     291      * Otherwise, a new array is allocated with the runtime type of the
     292      * specified array and the size of this collection.
     293      *
     294      * <p>If this collection fits in the specified array with room to spare
     295      * (i.e., the array has more elements than this collection), the element
     296      * in the array immediately following the end of the collection is set to
     297      * {@code null}.  (This is useful in determining the length of this
     298      * collection <i>only</i> if the caller knows that this collection does
     299      * not contain any {@code null} elements.)
     300      *
     301      * <p>If this collection makes any guarantees as to what order its elements
     302      * are returned by its iterator, this method must return the elements in
     303      * the same order.
     304      *
     305      * <p>Like the {@link #toArray()} method, this method acts as bridge between
     306      * array-based and collection-based APIs.  Further, this method allows
     307      * precise control over the runtime type of the output array, and may,
     308      * under certain circumstances, be used to save allocation costs.
     309      *
     310      * <p>Suppose {@code x} is a collection known to contain only strings.
     311      * The following code can be used to dump the collection into a newly
     312      * allocated array of {@code String}:
     313      *
     314      * <pre>
     315      *     String[] y = x.toArray(new String[0]);</pre>
     316      *
     317      * Note that {@code toArray(new Object[0])} is identical in function to
     318      * {@code toArray()}.
     319      *
     320      * @param <T> the component type of the array to contain the collection
     321      * @param a the array into which the elements of this collection are to be
     322      *        stored, if it is big enough; otherwise, a new array of the same
     323      *        runtime type is allocated for this purpose.
     324      * @return an array containing all of the elements in this collection
     325      * @throws ArrayStoreException if the runtime type of any element in this
     326      *         collection is not assignable to the {@linkplain Class#getComponentType
     327      *         runtime component type} of the specified array
     328      * @throws NullPointerException if the specified array is null
     329      */
     330     <T> T[] toArray(T[] a);
     331
     332     // Modification Operations
     333
     334     /**
     335      * Ensures that this collection contains the specified element (optional
     336      * operation).  Returns {@code true} if this collection changed as a
     337      * result of the call.  (Returns {@code false} if this collection does
     338      * not permit duplicates and already contains the specified element.)<p>
     339      *
     340      * Collections that support this operation may place limitations on what
     341      * elements may be added to this collection.  In particular, some
     342      * collections will refuse to add {@code null} elements, and others will
     343      * impose restrictions on the type of elements that may be added.
     344      * Collection classes should clearly specify in their documentation any
     345      * restrictions on what elements may be added.<p>
     346      *
     347      * If a collection refuses to add a particular element for any reason
     348      * other than that it already contains the element, it <i>must</i> throw
     349      * an exception (rather than returning {@code false}).  This preserves
     350      * the invariant that a collection always contains the specified element
     351      * after this call returns.
     352      *
     353      * @param e element whose presence in this collection is to be ensured
     354      * @return {@code true} if this collection changed as a result of the
     355      *         call
     356      * @throws UnsupportedOperationException if the {@code add} operation
     357      *         is not supported by this collection
     358      * @throws ClassCastException if the class of the specified element
     359      *         prevents it from being added to this collection
     360      * @throws NullPointerException if the specified element is null and this
     361      *         collection does not permit null elements
     362      * @throws IllegalArgumentException if some property of the element
     363      *         prevents it from being added to this collection
     364      * @throws IllegalStateException if the element cannot be added at this
     365      *         time due to insertion restrictions
     366      */
     367     boolean add(E e);
     368
     369     /**
     370      * Removes a single instance of the specified element from this
     371      * collection, if it is present (optional operation).  More formally,
     372      * removes an element {@code e} such that
     373      * {@code Objects.equals(o, e)}, if
     374      * this collection contains one or more such elements.  Returns
     375      * {@code true} if this collection contained the specified element (or
     376      * equivalently, if this collection changed as a result of the call).
     377      *
     378      * @param o element to be removed from this collection, if present
     379      * @return {@code true} if an element was removed as a result of this call
     380      * @throws ClassCastException if the type of the specified element
     381      *         is incompatible with this collection
     382      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     383      * @throws NullPointerException if the specified element is null and this
     384      *         collection does not permit null elements
     385      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     386      * @throws UnsupportedOperationException if the {@code remove} operation
     387      *         is not supported by this collection
     388      */
     389     boolean remove(Object o);
     390
     391
     392     // Bulk Operations
     393
     394     /**
     395      * Returns {@code true} if this collection contains all of the elements
     396      * in the specified collection.
     397      *
     398      * @param  c collection to be checked for containment in this collection
     399      * @return {@code true} if this collection contains all of the elements
     400      *         in the specified collection
     401      * @throws ClassCastException if the types of one or more elements
     402      *         in the specified collection are incompatible with this
     403      *         collection
     404      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     405      * @throws NullPointerException if the specified collection contains one
     406      *         or more null elements and this collection does not permit null
     407      *         elements
     408      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>),
     409      *         or if the specified collection is null.
     410      * @see    #contains(Object)
     411      */
     412     boolean containsAll(Collection<?> c);
     413
     414     /**
     415      * Adds all of the elements in the specified collection to this collection
     416      * (optional operation).  The behavior of this operation is undefined if
     417      * the specified collection is modified while the operation is in progress.
     418      * (This implies that the behavior of this call is undefined if the
     419      * specified collection is this collection, and this collection is
     420      * nonempty.)
     421      *
     422      * @param c collection containing elements to be added to this collection
     423      * @return {@code true} if this collection changed as a result of the call
     424      * @throws UnsupportedOperationException if the {@code addAll} operation
     425      *         is not supported by this collection
     426      * @throws ClassCastException if the class of an element of the specified
     427      *         collection prevents it from being added to this collection
     428      * @throws NullPointerException if the specified collection contains a
     429      *         null element and this collection does not permit null elements,
     430      *         or if the specified collection is null
     431      * @throws IllegalArgumentException if some property of an element of the
     432      *         specified collection prevents it from being added to this
     433      *         collection
     434      * @throws IllegalStateException if not all the elements can be added at
     435      *         this time due to insertion restrictions
     436      * @see #add(Object)
     437      */
     438     boolean addAll(Collection<? extends E> c);
     439
     440     /**
     441      * Removes all of this collection's elements that are also contained in the
     442      * specified collection (optional operation).  After this call returns,
     443      * this collection will contain no elements in common with the specified
     444      * collection.
     445      *
     446      * @param c collection containing elements to be removed from this collection
     447      * @return {@code true} if this collection changed as a result of the
     448      *         call
     449      * @throws UnsupportedOperationException if the {@code removeAll} method
     450      *         is not supported by this collection
     451      * @throws ClassCastException if the types of one or more elements
     452      *         in this collection are incompatible with the specified
     453      *         collection
     454      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     455      * @throws NullPointerException if this collection contains one or more
     456      *         null elements and the specified collection does not support
     457      *         null elements
     458      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>),
     459      *         or if the specified collection is null
     460      * @see #remove(Object)
     461      * @see #contains(Object)
     462      */
     463     boolean removeAll(Collection<?> c);
     464
     465     /**
     466      * Removes all of the elements of this collection that satisfy the given
     467      * predicate.  Errors or runtime exceptions thrown during iteration or by
     468      * the predicate are relayed to the caller.
     469      *
     470      * @implSpec
     471      * The default implementation traverses all elements of the collection using
     472      * its {@link #iterator}.  Each matching element is removed using
     473      * {@link Iterator#remove()}.  If the collection's iterator does not
     474      * support removal then an {@code UnsupportedOperationException} will be
     475      * thrown on the first matching element.
     476      *
     477      * @param filter a predicate which returns {@code true} for elements to be
     478      *        removed
     479      * @return {@code true} if any elements were removed
     480      * @throws NullPointerException if the specified filter is null
     481      * @throws UnsupportedOperationException if elements cannot be removed
     482      *         from this collection.  Implementations may throw this exception if a
     483      *         matching element cannot be removed or if, in general, removal is not
     484      *         supported.
     485      * @since 1.8
     486      */
     487     default boolean removeIf(Predicate<? super E> filter) {
     488         Objects.requireNonNull(filter);
     489         boolean removed = false;
     490         final Iterator<E> each = iterator();
     491         while (each.hasNext()) {
     492             if (filter.test(each.next())) {
     493                 each.remove();
     494                 removed = true;
     495             }
     496         }
     497         return removed;
     498     }
     499
     500     /**
     501      * Retains only the elements in this collection that are contained in the
     502      * specified collection (optional operation).  In other words, removes from
     503      * this collection all of its elements that are not contained in the
     504      * specified collection.
     505      *
     506      * @param c collection containing elements to be retained in this collection
     507      * @return {@code true} if this collection changed as a result of the call
     508      * @throws UnsupportedOperationException if the {@code retainAll} operation
     509      *         is not supported by this collection
     510      * @throws ClassCastException if the types of one or more elements
     511      *         in this collection are incompatible with the specified
     512      *         collection
     513      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     514      * @throws NullPointerException if this collection contains one or more
     515      *         null elements and the specified collection does not permit null
     516      *         elements
     517      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>),
     518      *         or if the specified collection is null
     519      * @see #remove(Object)
     520      * @see #contains(Object)
     521      */
     522     boolean retainAll(Collection<?> c);
     523
     524     /**
     525      * Removes all of the elements from this collection (optional operation).
     526      * The collection will be empty after this method returns.
     527      *
     528      * @throws UnsupportedOperationException if the {@code clear} operation
     529      *         is not supported by this collection
     530      */
     531     void clear();
     532
     533
     534     // Comparison and hashing
     535
     536     /**
     537      * Compares the specified object with this collection for equality. <p>
     538      *
     539      * While the {@code Collection} interface adds no stipulations to the
     540      * general contract for the {@code Object.equals}, programmers who
     541      * implement the {@code Collection} interface "directly" (in other words,
     542      * create a class that is a {@code Collection} but is not a {@code Set}
     543      * or a {@code List}) must exercise care if they choose to override the
     544      * {@code Object.equals}.  It is not necessary to do so, and the simplest
     545      * course of action is to rely on {@code Object}'s implementation, but
     546      * the implementor may wish to implement a "value comparison" in place of
     547      * the default "reference comparison."  (The {@code List} and
     548      * {@code Set} interfaces mandate such value comparisons.)<p>
     549      *
     550      * The general contract for the {@code Object.equals} method states that
     551      * equals must be symmetric (in other words, {@code a.equals(b)} if and
     552      * only if {@code b.equals(a)}).  The contracts for {@code List.equals}
     553      * and {@code Set.equals} state that lists are only equal to other lists,
     554      * and sets to other sets.  Thus, a custom {@code equals} method for a
     555      * collection class that implements neither the {@code List} nor
     556      * {@code Set} interface must return {@code false} when this collection
     557      * is compared to any list or set.  (By the same logic, it is not possible
     558      * to write a class that correctly implements both the {@code Set} and
     559      * {@code List} interfaces.)
     560      *
     561      * @param o object to be compared for equality with this collection
     562      * @return {@code true} if the specified object is equal to this
     563      * collection
     564      *
     565      * @see Object#equals(Object)
     566      * @see Set#equals(Object)
     567      * @see List#equals(Object)
     568      */
     569     boolean equals(Object o);
     570
     571     /**
     572      * Returns the hash code value for this collection.  While the
     573      * {@code Collection} interface adds no stipulations to the general
     574      * contract for the {@code Object.hashCode} method, programmers should
     575      * take note that any class that overrides the {@code Object.equals}
     576      * method must also override the {@code Object.hashCode} method in order
     577      * to satisfy the general contract for the {@code Object.hashCode} method.
     578      * In particular, {@code c1.equals(c2)} implies that
     579      * {@code c1.hashCode()==c2.hashCode()}.
     580      *
     581      * @return the hash code value for this collection
     582      *
     583      * @see Object#hashCode()
     584      * @see Object#equals(Object)
     585      */
     586     int hashCode();
     587
     588     /**
     589      * Creates a {@link Spliterator} over the elements in this collection.
     590      *
     591      * Implementations should document characteristic values reported by the
     592      * spliterator.  Such characteristic values are not required to be reported
     593      * if the spliterator reports {@link Spliterator#SIZED} and this collection
     594      * contains no elements.
     595      *
     596      * <p>The default implementation should be overridden by subclasses that
     597      * can return a more efficient spliterator.  In order to
     598      * preserve expected laziness behavior for the {@link #stream()} and
     599      * {@link #parallelStream()} methods, spliterators should either have the
     600      * characteristic of {@code IMMUTABLE} or {@code CONCURRENT}, or be
     601      * <em><a href="Spliterator.html#binding">late-binding</a></em>.
     602      * If none of these is practical, the overriding class should describe the
     603      * spliterator's documented policy of binding and structural interference,
     604      * and should override the {@link #stream()} and {@link #parallelStream()}
     605      * methods to create streams using a {@code Supplier} of the spliterator,
     606      * as in:
     607      * <pre>{@code
     608      *     Stream<E> s = StreamSupport.stream(() -> spliterator(), spliteratorCharacteristics)
     609      * }</pre>
     610      * <p>These requirements ensure that streams produced by the
     611      * {@link #stream()} and {@link #parallelStream()} methods will reflect the
     612      * contents of the collection as of initiation of the terminal stream
     613      * operation.
     614      *
     615      * @implSpec
     616      * The default implementation creates a
     617      * <em><a href="Spliterator.html#binding">late-binding</a></em> spliterator
     618      * from the collection's {@code Iterator}.  The spliterator inherits the
     619      * <em>fail-fast</em> properties of the collection's iterator.
     620      * <p>
     621      * The created {@code Spliterator} reports {@link Spliterator#SIZED}.
     622      *
     623      * @implNote
     624      * The created {@code Spliterator} additionally reports
     625      * {@link Spliterator#SUBSIZED}.
     626      *
     627      * <p>If a spliterator covers no elements then the reporting of additional
     628      * characteristic values, beyond that of {@code SIZED} and {@code SUBSIZED},
     629      * does not aid clients to control, specialize or simplify computation.
     630      * However, this does enable shared use of an immutable and empty
     631      * spliterator instance (see {@link Spliterators#emptySpliterator()}) for
     632      * empty collections, and enables clients to determine if such a spliterator
     633      * covers no elements.
     634      *
     635      * @return a {@code Spliterator} over the elements in this collection
     636      * @since 1.8
     637      */
     638     @Override
     639     default Spliterator<E> spliterator() {
     640         return Spliterators.spliterator(this, 0);
     641     }
     642
     643     /**
     644      * Returns a sequential {@code Stream} with this collection as its source.
     645      *
     646      * <p>This method should be overridden when the {@link #spliterator()}
     647      * method cannot return a spliterator that is {@code IMMUTABLE},
     648      * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()}
     649      * for details.)
     650      *
     651      * @implSpec
     652      * The default implementation creates a sequential {@code Stream} from the
     653      * collection's {@code Spliterator}.
     654      *
     655      * @return a sequential {@code Stream} over the elements in this collection
     656      * @since 1.8
     657      */
     658     default Stream<E> stream() {
     659         return StreamSupport.stream(spliterator(), false);
     660     }
     661
     662     /**
     663      * Returns a possibly parallel {@code Stream} with this collection as its
     664      * source.  It is allowable for this method to return a sequential stream.
     665      *
     666      * <p>This method should be overridden when the {@link #spliterator()}
     667      * method cannot return a spliterator that is {@code IMMUTABLE},
     668      * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()}
     669      * for details.)
     670      *
     671      * @implSpec
     672      * The default implementation creates a parallel {@code Stream} from the
     673      * collection's {@code Spliterator}.
     674      *
     675      * @return a possibly parallel {@code Stream} over the elements in this
     676      * collection
     677      * @since 1.8
     678      */
     679     default Stream<E> parallelStream() {
     680         return StreamSupport.stream(spliterator(), true);
     681     }
     682 }
    --- EOF ---