    rev 47953 : 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>The "destructive" methods contained in this interface, that is, the
      58  * methods that modify the collection on which they operate, are specified to
      59  * throw {@code UnsupportedOperationException} if this collection does not
      60  * support the operation.  If this is the case, these methods may, but are not
      61  * required to, throw an {@code UnsupportedOperationException} if the
      62  * invocation would have no effect on the collection.  For example, invoking
      63  * the {@link #addAll(Collection)} method on an unmodifiable collection may,
      64  * but is not required to, throw the exception if the collection to be added
      65  * is empty.
      66  *
      67  * <p><a id="optional-restrictions">
      68  * Some collection implementations have restrictions on the elements that
      69  * they may contain.</a>  For example, some implementations prohibit null elements,
      70  * and some have restrictions on the types of their elements.  Attempting to
      71  * add an ineligible element throws an unchecked exception, typically
      72  * {@code NullPointerException} or {@code ClassCastException}.  Attempting
      73  * to query the presence of an ineligible element may throw an exception,
      74  * or it may simply return false; some implementations will exhibit the former
      75  * behavior and some will exhibit the latter.  More generally, attempting an
      76  * operation on an ineligible element whose completion would not result in
      77  * the insertion of an ineligible element into the collection may throw an
      78  * exception or it may succeed, at the option of the implementation.
      79  * Such exceptions are marked as "optional" in the specification for this
      80  * interface.
      81  *
      82  * <p>It is up to each collection to determine its own synchronization
      83  * policy.  In the absence of a stronger guarantee by the
      84  * implementation, undefined behavior may result from the invocation
      85  * of any method on a collection that is being mutated by another
      86  * thread; this includes direct invocations, passing the collection to
      87  * a method that might perform invocations, and using an existing
      88  * iterator to examine the collection.
      89  *
      90  * <p>Many methods in Collections Framework interfaces are defined in
      91  * terms of the {@link Object#equals(Object) equals} method.  For example,
      92  * the specification for the {@link #contains(Object) contains(Object o)}
      93  * method says: "returns {@code true} if and only if this collection
      94  * contains at least one element {@code e} such that
      95  * {@code (o==null ? e==null : o.equals(e))}."  This specification should
      96  * <i>not</i> be construed to imply that invoking {@code Collection.contains}
      97  * with a non-null argument {@code o} will cause {@code o.equals(e)} to be
      98  * invoked for any element {@code e}.  Implementations are free to implement
      99  * optimizations whereby the {@code equals} invocation is avoided, for
     100  * example, by first comparing the hash codes of the two elements.  (The
     101  * {@link Object#hashCode()} specification guarantees that two objects with
     102  * unequal hash codes cannot be equal.)  More generally, implementations of
     103  * the various Collections Framework interfaces are free to take advantage of
     104  * the specified behavior of underlying {@link Object} methods wherever the
     105  * implementor deems it appropriate.
     106  *
     107  * <p>Some collection operations which perform recursive traversal of the
     108  * collection may fail with an exception for self-referential instances where
     109  * the collection directly or indirectly contains itself. This includes the
     110  * {@code clone()}, {@code equals()}, {@code hashCode()} and {@code toString()}
     111  * methods. Implementations may optionally handle the self-referential scenario,
     112  * however most current implementations do not do so.
     113  *

     114  * <p>This interface is a member of the
     115  * <a href="{@docRoot}/java/util/package-summary.html#CollectionsFramework">
     116  * Java Collections Framework</a>.
     117  *
     118  * @implSpec
     119  * The default method implementations (inherited or otherwise) do not apply any
     120  * synchronization protocol.  If a {@code Collection} implementation has a
     121  * specific synchronization protocol, then it must override default
     122  * implementations to apply that protocol.
     123  *
     124  * @param <E> the type of elements in this collection
     125  *
     126  * @author  Josh Bloch
     127  * @author  Neal Gafter
     128  * @see     Set
     129  * @see     List
     130  * @see     Map
     131  * @see     SortedSet
     132  * @see     SortedMap
     133  * @see     HashSet
     134  * @see     TreeSet
     135  * @see     ArrayList
     136  * @see     LinkedList
     137  * @see     Vector
     138  * @see     Collections
     139  * @see     Arrays
     140  * @see     AbstractCollection
     141  * @since 1.2
     142  */
     143
     144 public interface Collection<E> extends Iterable<E> {
     145     // Query Operations
     146
     147     /**
     148      * Returns the number of elements in this collection.  If this collection
     149      * contains more than {@code Integer.MAX_VALUE} elements, returns
     150      * {@code Integer.MAX_VALUE}.
     151      *
     152      * @return the number of elements in this collection
     153      */
     154     int size();
     155
     156     /**
     157      * Returns {@code true} if this collection contains no elements.
     158      *
     159      * @return {@code true} if this collection contains no elements
     160      */
     161     boolean isEmpty();
     162
     163     /**
     164      * Returns {@code true} if this collection contains the specified element.
     165      * More formally, returns {@code true} if and only if this collection
     166      * contains at least one element {@code e} such that
     167      * {@code Objects.equals(o, e)}.
     168      *
     169      * @param o element whose presence in this collection is to be tested
     170      * @return {@code true} if this collection contains the specified
     171      *         element
     172      * @throws ClassCastException if the type of the specified element
     173      *         is incompatible with this collection
     174      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     175      * @throws NullPointerException if the specified element is null and this
     176      *         collection does not permit null elements
     177      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     178      */
     179     boolean contains(Object o);
     180
     181     /**
     182      * Returns an iterator over the elements in this collection.  There are no
     183      * guarantees concerning the order in which the elements are returned
     184      * (unless this collection is an instance of some class that provides a
     185      * guarantee).
     186      *
     187      * @return an {@code Iterator} over the elements in this collection
     188      */
     189     Iterator<E> iterator();
     190
     191     /**
     192      * Returns an array containing all of the elements in this collection.
     193      * If this collection makes any guarantees as to what order its elements
     194      * are returned by its iterator, this method must return the elements in
     195      * the same order.
     196      *
     197      * <p>The returned array will be "safe" in that no references to it are
     198      * maintained by this collection.  (In other words, this method must
     199      * allocate a new array even if this collection is backed by an array).
     200      * The caller is thus free to modify the returned array.
     201      *
     202      * <p>This method acts as bridge between array-based and collection-based
     203      * APIs.
     204      *
     205      * @return an array containing all of the elements in this collection
     206      */
     207     Object[] toArray();
     208
     209     /**
     210      * Returns an array containing all of the elements in this collection;
     211      * the runtime type of the returned array is that of the specified array.
     212      * If the collection fits in the specified array, it is returned therein.
     213      * Otherwise, a new array is allocated with the runtime type of the
     214      * specified array and the size of this collection.
     215      *
     216      * <p>If this collection fits in the specified array with room to spare
     217      * (i.e., the array has more elements than this collection), the element
     218      * in the array immediately following the end of the collection is set to
     219      * {@code null}.  (This is useful in determining the length of this
     220      * collection <i>only</i> if the caller knows that this collection does
     221      * not contain any {@code null} elements.)
     222      *
     223      * <p>If this collection makes any guarantees as to what order its elements
     224      * are returned by its iterator, this method must return the elements in
     225      * the same order.
     226      *
     227      * <p>Like the {@link #toArray()} method, this method acts as bridge between
     228      * array-based and collection-based APIs.  Further, this method allows
     229      * precise control over the runtime type of the output array, and may,
     230      * under certain circumstances, be used to save allocation costs.
     231      *
     232      * <p>Suppose {@code x} is a collection known to contain only strings.
     233      * The following code can be used to dump the collection into a newly
     234      * allocated array of {@code String}:
     235      *
     236      * <pre>
     237      *     String[] y = x.toArray(new String[0]);</pre>
     238      *
     239      * Note that {@code toArray(new Object[0])} is identical in function to
     240      * {@code toArray()}.
     241      *
     242      * @param <T> the runtime type of the array to contain the collection
     243      * @param a the array into which the elements of this collection are to be
     244      *        stored, if it is big enough; otherwise, a new array of the same
     245      *        runtime type is allocated for this purpose.
     246      * @return an array containing all of the elements in this collection
     247      * @throws ArrayStoreException if the runtime type of the specified array
     248      *         is not a supertype of the runtime type of every element in
     249      *         this collection
     250      * @throws NullPointerException if the specified array is null
     251      */
     252     <T> T[] toArray(T[] a);
     253
     254     // Modification Operations
     255
     256     /**
     257      * Ensures that this collection contains the specified element (optional
     258      * operation).  Returns {@code true} if this collection changed as a
     259      * result of the call.  (Returns {@code false} if this collection does
     260      * not permit duplicates and already contains the specified element.)<p>
     261      *
     262      * Collections that support this operation may place limitations on what
     263      * elements may be added to this collection.  In particular, some
     264      * collections will refuse to add {@code null} elements, and others will
     265      * impose restrictions on the type of elements that may be added.
     266      * Collection classes should clearly specify in their documentation any
     267      * restrictions on what elements may be added.<p>
     268      *
     269      * If a collection refuses to add a particular element for any reason
     270      * other than that it already contains the element, it <i>must</i> throw
     271      * an exception (rather than returning {@code false}).  This preserves
     272      * the invariant that a collection always contains the specified element
     273      * after this call returns.
     274      *
     275      * @param e element whose presence in this collection is to be ensured
     276      * @return {@code true} if this collection changed as a result of the
     277      *         call
     278      * @throws UnsupportedOperationException if the {@code add} operation
     279      *         is not supported by this collection
     280      * @throws ClassCastException if the class of the specified element
     281      *         prevents it from being added to this collection
     282      * @throws NullPointerException if the specified element is null and this
     283      *         collection does not permit null elements
     284      * @throws IllegalArgumentException if some property of the element
     285      *         prevents it from being added to this collection
     286      * @throws IllegalStateException if the element cannot be added at this
     287      *         time due to insertion restrictions
     288      */
     289     boolean add(E e);
     290
     291     /**
     292      * Removes a single instance of the specified element from this
     293      * collection, if it is present (optional operation).  More formally,
     294      * removes an element {@code e} such that
     295      * {@code Objects.equals(o, e)}, if
     296      * this collection contains one or more such elements.  Returns
     297      * {@code true} if this collection contained the specified element (or
     298      * equivalently, if this collection changed as a result of the call).
     299      *
     300      * @param o element to be removed from this collection, if present
     301      * @return {@code true} if an element was removed as a result of this call
     302      * @throws ClassCastException if the type of the specified element
     303      *         is incompatible with this collection
     304      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     305      * @throws NullPointerException if the specified element is null and this
     306      *         collection does not permit null elements
     307      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     308      * @throws UnsupportedOperationException if the {@code remove} operation
     309      *         is not supported by this collection
     310      */
     311     boolean remove(Object o);
     312
     313
     314     // Bulk Operations
     315
     316     /**
     317      * Returns {@code true} if this collection contains all of the elements
     318      * in the specified collection.
     319      *
     320      * @param  c collection to be checked for containment in this collection
     321      * @return {@code true} if this collection contains all of the elements
     322      *         in the specified collection
     323      * @throws ClassCastException if the types of one or more elements
     324      *         in the specified collection are incompatible with this
     325      *         collection
     326      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     327      * @throws NullPointerException if the specified collection contains one
     328      *         or more null elements and this collection does not permit null
     329      *         elements
     330      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>),
     331      *         or if the specified collection is null.
     332      * @see    #contains(Object)
     333      */
     334     boolean containsAll(Collection<?> c);
     335
     336     /**
     337      * Adds all of the elements in the specified collection to this collection
     338      * (optional operation).  The behavior of this operation is undefined if
     339      * the specified collection is modified while the operation is in progress.
     340      * (This implies that the behavior of this call is undefined if the
     341      * specified collection is this collection, and this collection is
     342      * nonempty.)
     343      *
     344      * @param c collection containing elements to be added to this collection
     345      * @return {@code true} if this collection changed as a result of the call
     346      * @throws UnsupportedOperationException if the {@code addAll} operation
     347      *         is not supported by this collection
     348      * @throws ClassCastException if the class of an element of the specified
     349      *         collection prevents it from being added to this collection
     350      * @throws NullPointerException if the specified collection contains a
     351      *         null element and this collection does not permit null elements,
     352      *         or if the specified collection is null
     353      * @throws IllegalArgumentException if some property of an element of the
     354      *         specified collection prevents it from being added to this
     355      *         collection
     356      * @throws IllegalStateException if not all the elements can be added at
     357      *         this time due to insertion restrictions
     358      * @see #add(Object)
     359      */
     360     boolean addAll(Collection<? extends E> c);
     361
     362     /**
     363      * Removes all of this collection's elements that are also contained in the
     364      * specified collection (optional operation).  After this call returns,
     365      * this collection will contain no elements in common with the specified
     366      * collection.
     367      *
     368      * @param c collection containing elements to be removed from this collection
     369      * @return {@code true} if this collection changed as a result of the
     370      *         call
     371      * @throws UnsupportedOperationException if the {@code removeAll} method
     372      *         is not supported by this collection
     373      * @throws ClassCastException if the types of one or more elements
     374      *         in this collection are incompatible with the specified
     375      *         collection
     376      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     377      * @throws NullPointerException if this collection contains one or more
     378      *         null elements and the specified collection does not support
     379      *         null elements
     380      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>),
     381      *         or if the specified collection is null
     382      * @see #remove(Object)
     383      * @see #contains(Object)
     384      */
     385     boolean removeAll(Collection<?> c);
     386
     387     /**
     388      * Removes all of the elements of this collection that satisfy the given
     389      * predicate.  Errors or runtime exceptions thrown during iteration or by
     390      * the predicate are relayed to the caller.
     391      *
     392      * @implSpec
     393      * The default implementation traverses all elements of the collection using
     394      * its {@link #iterator}.  Each matching element is removed using
     395      * {@link Iterator#remove()}.  If the collection's iterator does not
     396      * support removal then an {@code UnsupportedOperationException} will be
     397      * thrown on the first matching element.
     398      *
     399      * @param filter a predicate which returns {@code true} for elements to be
     400      *        removed
     401      * @return {@code true} if any elements were removed
     402      * @throws NullPointerException if the specified filter is null
     403      * @throws UnsupportedOperationException if elements cannot be removed
     404      *         from this collection.  Implementations may throw this exception if a
     405      *         matching element cannot be removed or if, in general, removal is not
     406      *         supported.
     407      * @since 1.8
     408      */
     409     default boolean removeIf(Predicate<? super E> filter) {
     410         Objects.requireNonNull(filter);
     411         boolean removed = false;
     412         final Iterator<E> each = iterator();
     413         while (each.hasNext()) {
     414             if (filter.test(each.next())) {
     415                 each.remove();
     416                 removed = true;
     417             }
     418         }
     419         return removed;
     420     }
     421
     422     /**
     423      * Retains only the elements in this collection that are contained in the
     424      * specified collection (optional operation).  In other words, removes from
     425      * this collection all of its elements that are not contained in the
     426      * specified collection.
     427      *
     428      * @param c collection containing elements to be retained in this collection
     429      * @return {@code true} if this collection changed as a result of the call
     430      * @throws UnsupportedOperationException if the {@code retainAll} operation
     431      *         is not supported by this collection
     432      * @throws ClassCastException if the types of one or more elements
     433      *         in this collection are incompatible with the specified
     434      *         collection
     435      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     436      * @throws NullPointerException if this collection contains one or more
     437      *         null elements and the specified collection does not permit null
     438      *         elements
     439      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>),
     440      *         or if the specified collection is null
     441      * @see #remove(Object)
     442      * @see #contains(Object)
     443      */
     444     boolean retainAll(Collection<?> c);
     445
     446     /**
     447      * Removes all of the elements from this collection (optional operation).
     448      * The collection will be empty after this method returns.
     449      *
     450      * @throws UnsupportedOperationException if the {@code clear} operation
     451      *         is not supported by this collection
     452      */
     453     void clear();
     454
     455
     456     // Comparison and hashing
     457
     458     /**
     459      * Compares the specified object with this collection for equality. <p>
     460      *
     461      * While the {@code Collection} interface adds no stipulations to the
     462      * general contract for the {@code Object.equals}, programmers who
     463      * implement the {@code Collection} interface "directly" (in other words,
     464      * create a class that is a {@code Collection} but is not a {@code Set}
     465      * or a {@code List}) must exercise care if they choose to override the
     466      * {@code Object.equals}.  It is not necessary to do so, and the simplest
     467      * course of action is to rely on {@code Object}'s implementation, but
     468      * the implementor may wish to implement a "value comparison" in place of
     469      * the default "reference comparison."  (The {@code List} and
     470      * {@code Set} interfaces mandate such value comparisons.)<p>
     471      *
     472      * The general contract for the {@code Object.equals} method states that
     473      * equals must be symmetric (in other words, {@code a.equals(b)} if and
     474      * only if {@code b.equals(a)}).  The contracts for {@code List.equals}
     475      * and {@code Set.equals} state that lists are only equal to other lists,
     476      * and sets to other sets.  Thus, a custom {@code equals} method for a
     477      * collection class that implements neither the {@code List} nor
     478      * {@code Set} interface must return {@code false} when this collection
     479      * is compared to any list or set.  (By the same logic, it is not possible
     480      * to write a class that correctly implements both the {@code Set} and
     481      * {@code List} interfaces.)
     482      *
     483      * @param o object to be compared for equality with this collection
     484      * @return {@code true} if the specified object is equal to this
     485      * collection
     486      *
     487      * @see Object#equals(Object)
     488      * @see Set#equals(Object)
     489      * @see List#equals(Object)
     490      */
     491     boolean equals(Object o);
     492
     493     /**
     494      * Returns the hash code value for this collection.  While the
     495      * {@code Collection} interface adds no stipulations to the general
     496      * contract for the {@code Object.hashCode} method, programmers should
     497      * take note that any class that overrides the {@code Object.equals}
     498      * method must also override the {@code Object.hashCode} method in order
     499      * to satisfy the general contract for the {@code Object.hashCode} method.
     500      * In particular, {@code c1.equals(c2)} implies that
     501      * {@code c1.hashCode()==c2.hashCode()}.
     502      *
     503      * @return the hash code value for this collection
     504      *
     505      * @see Object#hashCode()
     506      * @see Object#equals(Object)
     507      */
     508     int hashCode();
     509
     510     /**
     511      * Creates a {@link Spliterator} over the elements in this collection.
     512      *
     513      * Implementations should document characteristic values reported by the
     514      * spliterator.  Such characteristic values are not required to be reported
     515      * if the spliterator reports {@link Spliterator#SIZED} and this collection
     516      * contains no elements.
     517      *
     518      * <p>The default implementation should be overridden by subclasses that
     519      * can return a more efficient spliterator.  In order to
     520      * preserve expected laziness behavior for the {@link #stream()} and
     521      * {@link #parallelStream()} methods, spliterators should either have the
     522      * characteristic of {@code IMMUTABLE} or {@code CONCURRENT}, or be
     523      * <em><a href="Spliterator.html#binding">late-binding</a></em>.
     524      * If none of these is practical, the overriding class should describe the
     525      * spliterator's documented policy of binding and structural interference,
     526      * and should override the {@link #stream()} and {@link #parallelStream()}
     527      * methods to create streams using a {@code Supplier} of the spliterator,
     528      * as in:
     529      * <pre>{@code
     530      *     Stream<E> s = StreamSupport.stream(() -> spliterator(), spliteratorCharacteristics)
     531      * }</pre>
     532      * <p>These requirements ensure that streams produced by the
     533      * {@link #stream()} and {@link #parallelStream()} methods will reflect the
     534      * contents of the collection as of initiation of the terminal stream
     535      * operation.
     536      *
     537      * @implSpec
     538      * The default implementation creates a
     539      * <em><a href="Spliterator.html#binding">late-binding</a></em> spliterator
     540      * from the collection's {@code Iterator}.  The spliterator inherits the
     541      * <em>fail-fast</em> properties of the collection's iterator.
     542      * <p>
     543      * The created {@code Spliterator} reports {@link Spliterator#SIZED}.
     544      *
     545      * @implNote
     546      * The created {@code Spliterator} additionally reports
     547      * {@link Spliterator#SUBSIZED}.
     548      *
     549      * <p>If a spliterator covers no elements then the reporting of additional
     550      * characteristic values, beyond that of {@code SIZED} and {@code SUBSIZED},
     551      * does not aid clients to control, specialize or simplify computation.
     552      * However, this does enable shared use of an immutable and empty
     553      * spliterator instance (see {@link Spliterators#emptySpliterator()}) for
     554      * empty collections, and enables clients to determine if such a spliterator
     555      * covers no elements.
     556      *
     557      * @return a {@code Spliterator} over the elements in this collection
     558      * @since 1.8
     559      */
     560     @Override
     561     default Spliterator<E> spliterator() {
     562         return Spliterators.spliterator(this, 0);
     563     }
     564
     565     /**
     566      * Returns a sequential {@code Stream} with this collection as its source.
     567      *
     568      * <p>This method should be overridden when the {@link #spliterator()}
     569      * method cannot return a spliterator that is {@code IMMUTABLE},
     570      * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()}
     571      * for details.)
     572      *
     573      * @implSpec
     574      * The default implementation creates a sequential {@code Stream} from the
     575      * collection's {@code Spliterator}.
     576      *
     577      * @return a sequential {@code Stream} over the elements in this collection
     578      * @since 1.8
     579      */
     580     default Stream<E> stream() {
     581         return StreamSupport.stream(spliterator(), false);
     582     }
     583
     584     /**
     585      * Returns a possibly parallel {@code Stream} with this collection as its
     586      * source.  It is allowable for this method to return a sequential stream.
     587      *
     588      * <p>This method should be overridden when the {@link #spliterator()}
     589      * method cannot return a spliterator that is {@code IMMUTABLE},
     590      * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()}
     591      * for details.)
     592      *
     593      * @implSpec
     594      * The default implementation creates a parallel {@code Stream} from the
     595      * collection's {@code Spliterator}.
     596      *
     597      * @return a possibly parallel {@code Stream} over the elements in this
     598      * collection
     599      * @since 1.8
     600      */
     601     default Stream<E> parallelStream() {
     602         return StreamSupport.stream(spliterator(), true);
     603     }
     604 }
    --- EOF ---