       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.
     272      *
     273      * <p>The returned array will be "safe" in that no references to it are
     274      * maintained by this collection.  (In other words, this method must
     275      * allocate a new array even if this collection is backed by an array).
     276      * The caller is thus free to modify the returned array.
     277      *
     278      * <p>This method acts as bridge between array-based and collection-based
     279      * APIs.
     280      *
     281      * @return an array containing all of the elements in this collection
     282      */
     283     Object[] toArray();
     284
     285     /**
     286      * Returns an array containing all of the elements in this collection;
     287      * the runtime type of the returned array is that of the specified array.
     288      * If the collection fits in the specified array, it is returned therein.
     289      * Otherwise, a new array is allocated with the runtime type of the
     290      * specified array and the size of this collection.
     291      *
     292      * <p>If this collection fits in the specified array with room to spare
     293      * (i.e., the array has more elements than this collection), the element
     294      * in the array immediately following the end of the collection is set to
     295      * {@code null}.  (This is useful in determining the length of this
     296      * collection <i>only</i> if the caller knows that this collection does
     297      * not contain any {@code null} elements.)
     298      *
     299      * <p>If this collection makes any guarantees as to what order its elements
     300      * are returned by its iterator, this method must return the elements in
     301      * the same order.
     302      *
     303      * <p>Like the {@link #toArray()} method, this method acts as bridge between
     304      * array-based and collection-based APIs.  Further, this method allows
     305      * precise control over the runtime type of the output array, and may,
     306      * under certain circumstances, be used to save allocation costs.
     307      *
     308      * <p>Suppose {@code x} is a collection known to contain only strings.
     309      * The following code can be used to dump the collection into a newly
     310      * allocated array of {@code String}:
     311      *
     312      * <pre>
     313      *     String[] y = x.toArray(new String[0]);</pre>
     314      *
     315      * Note that {@code toArray(new Object[0])} is identical in function to
     316      * {@code toArray()}.
     317      *
     318      * @param <T> the runtime type of the array to contain the collection
     319      * @param a the array into which the elements of this collection are to be
     320      *        stored, if it is big enough; otherwise, a new array of the same
     321      *        runtime type is allocated for this purpose.
     322      * @return an array containing all of the elements in this collection
     323      * @throws ArrayStoreException if the runtime type of the specified array
     324      *         is not a supertype of the runtime type of every element in
     325      *         this collection
     326      * @throws NullPointerException if the specified array is null
     327      */
     328     <T> T[] toArray(T[] a);
     329
     330     // Modification Operations
     331
     332     /**
     333      * Ensures that this collection contains the specified element (optional
     334      * operation).  Returns {@code true} if this collection changed as a
     335      * result of the call.  (Returns {@code false} if this collection does
     336      * not permit duplicates and already contains the specified element.)<p>
     337      *
     338      * Collections that support this operation may place limitations on what
     339      * elements may be added to this collection.  In particular, some
     340      * collections will refuse to add {@code null} elements, and others will
     341      * impose restrictions on the type of elements that may be added.
     342      * Collection classes should clearly specify in their documentation any
     343      * restrictions on what elements may be added.<p>
     344      *
     345      * If a collection refuses to add a particular element for any reason
     346      * other than that it already contains the element, it <i>must</i> throw
     347      * an exception (rather than returning {@code false}).  This preserves
     348      * the invariant that a collection always contains the specified element
     349      * after this call returns.
     350      *
     351      * @param e element whose presence in this collection is to be ensured
     352      * @return {@code true} if this collection changed as a result of the
     353      *         call
     354      * @throws UnsupportedOperationException if the {@code add} operation
     355      *         is not supported by this collection
     356      * @throws ClassCastException if the class of the specified element
     357      *         prevents it from being added to this collection
     358      * @throws NullPointerException if the specified element is null and this
     359      *         collection does not permit null elements
     360      * @throws IllegalArgumentException if some property of the element
     361      *         prevents it from being added to this collection
     362      * @throws IllegalStateException if the element cannot be added at this
     363      *         time due to insertion restrictions
     364      */
     365     boolean add(E e);
     366
     367     /**
     368      * Removes a single instance of the specified element from this
     369      * collection, if it is present (optional operation).  More formally,
     370      * removes an element {@code e} such that
     371      * {@code Objects.equals(o, e)}, if
     372      * this collection contains one or more such elements.  Returns
     373      * {@code true} if this collection contained the specified element (or
     374      * equivalently, if this collection changed as a result of the call).
     375      *
     376      * @param o element to be removed from this collection, if present
     377      * @return {@code true} if an element was removed as a result of this call
     378      * @throws ClassCastException if the type of the specified element
     379      *         is incompatible with this collection
     380      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     381      * @throws NullPointerException if the specified element is null and this
     382      *         collection does not permit null elements
     383      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     384      * @throws UnsupportedOperationException if the {@code remove} operation
     385      *         is not supported by this collection
     386      */
     387     boolean remove(Object o);
     388
     389
     390     // Bulk Operations
     391
     392     /**
     393      * Returns {@code true} if this collection contains all of the elements
     394      * in the specified collection.
     395      *
     396      * @param  c collection to be checked for containment in this collection
     397      * @return {@code true} if this collection contains all of the elements
     398      *         in the specified collection
     399      * @throws ClassCastException if the types of one or more elements
     400      *         in the specified collection are incompatible with this
     401      *         collection
     402      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     403      * @throws NullPointerException if the specified collection contains one
     404      *         or more null elements and this collection does not permit null
     405      *         elements
     406      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>),
     407      *         or if the specified collection is null.
     408      * @see    #contains(Object)
     409      */
     410     boolean containsAll(Collection<?> c);
     411
     412     /**
     413      * Adds all of the elements in the specified collection to this collection
     414      * (optional operation).  The behavior of this operation is undefined if
     415      * the specified collection is modified while the operation is in progress.
     416      * (This implies that the behavior of this call is undefined if the
     417      * specified collection is this collection, and this collection is
     418      * nonempty.)
     419      *
     420      * @param c collection containing elements to be added to this collection
     421      * @return {@code true} if this collection changed as a result of the call
     422      * @throws UnsupportedOperationException if the {@code addAll} operation
     423      *         is not supported by this collection
     424      * @throws ClassCastException if the class of an element of the specified
     425      *         collection prevents it from being added to this collection
     426      * @throws NullPointerException if the specified collection contains a
     427      *         null element and this collection does not permit null elements,
     428      *         or if the specified collection is null
     429      * @throws IllegalArgumentException if some property of an element of the
     430      *         specified collection prevents it from being added to this
     431      *         collection
     432      * @throws IllegalStateException if not all the elements can be added at
     433      *         this time due to insertion restrictions
     434      * @see #add(Object)
     435      */
     436     boolean addAll(Collection<? extends E> c);
     437
     438     /**
     439      * Removes all of this collection's elements that are also contained in the
     440      * specified collection (optional operation).  After this call returns,
     441      * this collection will contain no elements in common with the specified
     442      * collection.
     443      *
     444      * @param c collection containing elements to be removed from this collection
     445      * @return {@code true} if this collection changed as a result of the
     446      *         call
     447      * @throws UnsupportedOperationException if the {@code removeAll} method
     448      *         is not supported by this collection
     449      * @throws ClassCastException if the types of one or more elements
     450      *         in this collection are incompatible with the specified
     451      *         collection
     452      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     453      * @throws NullPointerException if this collection contains one or more
     454      *         null elements and the specified collection does not support
     455      *         null elements
     456      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>),
     457      *         or if the specified collection is null
     458      * @see #remove(Object)
     459      * @see #contains(Object)
     460      */
     461     boolean removeAll(Collection<?> c);
     462
     463     /**
     464      * Removes all of the elements of this collection that satisfy the given
     465      * predicate.  Errors or runtime exceptions thrown during iteration or by
     466      * the predicate are relayed to the caller.
     467      *
     468      * @implSpec
     469      * The default implementation traverses all elements of the collection using
     470      * its {@link #iterator}.  Each matching element is removed using
     471      * {@link Iterator#remove()}.  If the collection's iterator does not
     472      * support removal then an {@code UnsupportedOperationException} will be
     473      * thrown on the first matching element.
     474      *
     475      * @param filter a predicate which returns {@code true} for elements to be
     476      *        removed
     477      * @return {@code true} if any elements were removed
     478      * @throws NullPointerException if the specified filter is null
     479      * @throws UnsupportedOperationException if elements cannot be removed
     480      *         from this collection.  Implementations may throw this exception if a
     481      *         matching element cannot be removed or if, in general, removal is not
     482      *         supported.
     483      * @since 1.8
     484      */
     485     default boolean removeIf(Predicate<? super E> filter) {
     486         Objects.requireNonNull(filter);
     487         boolean removed = false;
     488         final Iterator<E> each = iterator();
     489         while (each.hasNext()) {
     490             if (filter.test(each.next())) {
     491                 each.remove();
     492                 removed = true;
     493             }
     494         }
     495         return removed;
     496     }
     497
     498     /**
     499      * Retains only the elements in this collection that are contained in the
     500      * specified collection (optional operation).  In other words, removes from
     501      * this collection all of its elements that are not contained in the
     502      * specified collection.
     503      *
     504      * @param c collection containing elements to be retained in this collection
     505      * @return {@code true} if this collection changed as a result of the call
     506      * @throws UnsupportedOperationException if the {@code retainAll} operation
     507      *         is not supported by this collection
     508      * @throws ClassCastException if the types of one or more elements
     509      *         in this collection are incompatible with the specified
     510      *         collection
     511      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     512      * @throws NullPointerException if this collection contains one or more
     513      *         null elements and the specified collection does not permit null
     514      *         elements
     515      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>),
     516      *         or if the specified collection is null
     517      * @see #remove(Object)
     518      * @see #contains(Object)
     519      */
     520     boolean retainAll(Collection<?> c);
     521
     522     /**
     523      * Removes all of the elements from this collection (optional operation).
     524      * The collection will be empty after this method returns.
     525      *
     526      * @throws UnsupportedOperationException if the {@code clear} operation
     527      *         is not supported by this collection
     528      */
     529     void clear();
     530
     531
     532     // Comparison and hashing
     533
     534     /**
     535      * Compares the specified object with this collection for equality. <p>
     536      *
     537      * While the {@code Collection} interface adds no stipulations to the
     538      * general contract for the {@code Object.equals}, programmers who
     539      * implement the {@code Collection} interface "directly" (in other words,
     540      * create a class that is a {@code Collection} but is not a {@code Set}
     541      * or a {@code List}) must exercise care if they choose to override the
     542      * {@code Object.equals}.  It is not necessary to do so, and the simplest
     543      * course of action is to rely on {@code Object}'s implementation, but
     544      * the implementor may wish to implement a "value comparison" in place of
     545      * the default "reference comparison."  (The {@code List} and
     546      * {@code Set} interfaces mandate such value comparisons.)<p>
     547      *
     548      * The general contract for the {@code Object.equals} method states that
     549      * equals must be symmetric (in other words, {@code a.equals(b)} if and
     550      * only if {@code b.equals(a)}).  The contracts for {@code List.equals}
     551      * and {@code Set.equals} state that lists are only equal to other lists,
     552      * and sets to other sets.  Thus, a custom {@code equals} method for a
     553      * collection class that implements neither the {@code List} nor
     554      * {@code Set} interface must return {@code false} when this collection
     555      * is compared to any list or set.  (By the same logic, it is not possible
     556      * to write a class that correctly implements both the {@code Set} and
     557      * {@code List} interfaces.)
     558      *
     559      * @param o object to be compared for equality with this collection
     560      * @return {@code true} if the specified object is equal to this
     561      * collection
     562      *
     563      * @see Object#equals(Object)
     564      * @see Set#equals(Object)
     565      * @see List#equals(Object)
     566      */
     567     boolean equals(Object o);
     568
     569     /**
     570      * Returns the hash code value for this collection.  While the
     571      * {@code Collection} interface adds no stipulations to the general
     572      * contract for the {@code Object.hashCode} method, programmers should
     573      * take note that any class that overrides the {@code Object.equals}
     574      * method must also override the {@code Object.hashCode} method in order
     575      * to satisfy the general contract for the {@code Object.hashCode} method.
     576      * In particular, {@code c1.equals(c2)} implies that
     577      * {@code c1.hashCode()==c2.hashCode()}.
     578      *
     579      * @return the hash code value for this collection
     580      *
     581      * @see Object#hashCode()
     582      * @see Object#equals(Object)
     583      */
     584     int hashCode();
     585
     586     /**
     587      * Creates a {@link Spliterator} over the elements in this collection.
     588      *
     589      * Implementations should document characteristic values reported by the
     590      * spliterator.  Such characteristic values are not required to be reported
     591      * if the spliterator reports {@link Spliterator#SIZED} and this collection
     592      * contains no elements.
     593      *
     594      * <p>The default implementation should be overridden by subclasses that
     595      * can return a more efficient spliterator.  In order to
     596      * preserve expected laziness behavior for the {@link #stream()} and
     597      * {@link #parallelStream()} methods, spliterators should either have the
     598      * characteristic of {@code IMMUTABLE} or {@code CONCURRENT}, or be
     599      * <em><a href="Spliterator.html#binding">late-binding</a></em>.
     600      * If none of these is practical, the overriding class should describe the
     601      * spliterator's documented policy of binding and structural interference,
     602      * and should override the {@link #stream()} and {@link #parallelStream()}
     603      * methods to create streams using a {@code Supplier} of the spliterator,
     604      * as in:
     605      * <pre>{@code
     606      *     Stream<E> s = StreamSupport.stream(() -> spliterator(), spliteratorCharacteristics)
     607      * }</pre>
     608      * <p>These requirements ensure that streams produced by the
     609      * {@link #stream()} and {@link #parallelStream()} methods will reflect the
     610      * contents of the collection as of initiation of the terminal stream
     611      * operation.
     612      *
     613      * @implSpec
     614      * The default implementation creates a
     615      * <em><a href="Spliterator.html#binding">late-binding</a></em> spliterator
     616      * from the collection's {@code Iterator}.  The spliterator inherits the
     617      * <em>fail-fast</em> properties of the collection's iterator.
     618      * <p>
     619      * The created {@code Spliterator} reports {@link Spliterator#SIZED}.
     620      *
     621      * @implNote
     622      * The created {@code Spliterator} additionally reports
     623      * {@link Spliterator#SUBSIZED}.
     624      *
     625      * <p>If a spliterator covers no elements then the reporting of additional
     626      * characteristic values, beyond that of {@code SIZED} and {@code SUBSIZED},
     627      * does not aid clients to control, specialize or simplify computation.
     628      * However, this does enable shared use of an immutable and empty
     629      * spliterator instance (see {@link Spliterators#emptySpliterator()}) for
     630      * empty collections, and enables clients to determine if such a spliterator
     631      * covers no elements.
     632      *
     633      * @return a {@code Spliterator} over the elements in this collection
     634      * @since 1.8
     635      */
     636     @Override
     637     default Spliterator<E> spliterator() {
     638         return Spliterators.spliterator(this, 0);
     639     }
     640
     641     /**
     642      * Returns a sequential {@code Stream} with this collection as its source.
     643      *
     644      * <p>This method should be overridden when the {@link #spliterator()}
     645      * method cannot return a spliterator that is {@code IMMUTABLE},
     646      * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()}
     647      * for details.)
     648      *
     649      * @implSpec
     650      * The default implementation creates a sequential {@code Stream} from the
     651      * collection's {@code Spliterator}.
     652      *
     653      * @return a sequential {@code Stream} over the elements in this collection
     654      * @since 1.8
     655      */
     656     default Stream<E> stream() {
     657         return StreamSupport.stream(spliterator(), false);
     658     }
     659
     660     /**
     661      * Returns a possibly parallel {@code Stream} with this collection as its
     662      * source.  It is allowable for this method to return a sequential stream.
     663      *
     664      * <p>This method should be overridden when the {@link #spliterator()}
     665      * method cannot return a spliterator that is {@code IMMUTABLE},
     666      * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()}
     667      * for details.)
     668      *
     669      * @implSpec
     670      * The default implementation creates a parallel {@code Stream} from the
     671      * collection's {@code Spliterator}.
     672      *
     673      * @return a possibly parallel {@code Stream} over the elements in this
     674      * collection
     675      * @since 1.8
     676      */
     677     default Stream<E> parallelStream() {
     678         return StreamSupport.stream(spliterator(), true);
     679     }
     680 }