1 /*
   2  * Copyright (c) 1997, 2013, 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 <tt>Set</tt> and <tt>List</tt>.  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 <tt>Collection</tt> implementation classes (which
  46  * typically implement <tt>Collection</tt> 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 <tt>Collection</tt>, 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 <tt>Collection</tt>
  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 <tt>UnsupportedOperationException</tt> 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 <tt>UnsupportedOperationException</tt> 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 name="optional-restrictions"/>
  68  * Some collection implementations have restrictions on the elements that
  69  * they may contain.  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  * <tt>NullPointerException</tt> or <tt>ClassCastException</tt>.  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 <tt>true</tt> if and only if this collection
  94  * contains at least one element <tt>e</tt> such that
  95  * <tt>(o==null ? e==null : o.equals(e))</tt>."  This specification should
  96  * <i>not</i> be construed to imply that invoking <tt>Collection.contains</tt>
  97  * with a non-null argument <tt>o</tt> will cause <tt>o.equals(e)</tt> to be
  98  * invoked for any element <tt>e</tt>.  Implementations are free to implement
  99  * optimizations whereby the <tt>equals</tt> 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>This interface is a member of the
 108  * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 109  * Java Collections Framework</a>.
 110  *
 111  * @implSpec
 112  * The default method implementations (inherited or otherwise) do not apply any
 113  * synchronization protocol.  If a {@code Collection} implementation has a
 114  * specific synchronization protocol, then it must override default
 115  * implementations to apply that protocol.
 116  *
 117  * @param <E> the type of elements in this collection
 118  *
 119  * @author  Josh Bloch
 120  * @author  Neal Gafter
 121  * @see     Set
 122  * @see     List
 123  * @see     Map
 124  * @see     SortedSet
 125  * @see     SortedMap
 126  * @see     HashSet
 127  * @see     TreeSet
 128  * @see     ArrayList
 129  * @see     LinkedList
 130  * @see     Vector
 131  * @see     Collections
 132  * @see     Arrays
 133  * @see     AbstractCollection
 134  * @since 1.2
 135  */
 136 
 137 public interface Collection<E> extends Iterable<E> {
 138     // Query Operations
 139 
 140     /**
 141      * Returns the number of elements in this collection.  If this collection
 142      * contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
 143      * <tt>Integer.MAX_VALUE</tt>.
 144      *
 145      * @return the number of elements in this collection
 146      */
 147     int size();
 148 
 149     /**
 150      * Returns <tt>true</tt> if this collection contains no elements.
 151      *
 152      * @return <tt>true</tt> if this collection contains no elements
 153      */
 154     boolean isEmpty();
 155 
 156     /**
 157      * Returns <tt>true</tt> if this collection contains the specified element.
 158      * More formally, returns <tt>true</tt> if and only if this collection
 159      * contains at least one element <tt>e</tt> such that
 160      * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
 161      *
 162      * @param o element whose presence in this collection is to be tested
 163      * @return <tt>true</tt> if this collection contains the specified
 164      *         element
 165      * @throws ClassCastException if the type of the specified element
 166      *         is incompatible with this collection
 167      *         (<a href="#optional-restrictions">optional</a>)
 168      * @throws NullPointerException if the specified element is null and this
 169      *         collection does not permit null elements
 170      *         (<a href="#optional-restrictions">optional</a>)
 171      */
 172     boolean contains(Object o);
 173 
 174     /**
 175      * Returns an iterator over the elements in this collection.  There are no
 176      * guarantees concerning the order in which the elements are returned
 177      * (unless this collection is an instance of some class that provides a
 178      * guarantee).
 179      *
 180      * @return an <tt>Iterator</tt> over the elements in this collection
 181      */
 182     Iterator<E> iterator();
 183 
 184     /**
 185      * Returns an array containing all of the elements in this collection.
 186      * If this collection makes any guarantees as to what order its elements
 187      * are returned by its iterator, this method must return the elements in
 188      * the same order.
 189      *
 190      * <p>The returned array will be "safe" in that no references to it are
 191      * maintained by this collection.  (In other words, this method must
 192      * allocate a new array even if this collection is backed by an array).
 193      * The caller is thus free to modify the returned array.
 194      *
 195      * <p>This method acts as bridge between array-based and collection-based
 196      * APIs.
 197      *
 198      * @return an array containing all of the elements in this collection
 199      */
 200     Object[] toArray();
 201 
 202     /**
 203      * Returns an array containing all of the elements in this collection;
 204      * the runtime type of the returned array is that of the specified array.
 205      * If the collection fits in the specified array, it is returned therein.
 206      * Otherwise, a new array is allocated with the runtime type of the
 207      * specified array and the size of this collection.
 208      *
 209      * <p>If this collection fits in the specified array with room to spare
 210      * (i.e., the array has more elements than this collection), the element
 211      * in the array immediately following the end of the collection is set to
 212      * <tt>null</tt>.  (This is useful in determining the length of this
 213      * collection <i>only</i> if the caller knows that this collection does
 214      * not contain any <tt>null</tt> elements.)
 215      *
 216      * <p>If this collection makes any guarantees as to what order its elements
 217      * are returned by its iterator, this method must return the elements in
 218      * the same order.
 219      *
 220      * <p>Like the {@link #toArray()} method, this method acts as bridge between
 221      * array-based and collection-based APIs.  Further, this method allows
 222      * precise control over the runtime type of the output array, and may,
 223      * under certain circumstances, be used to save allocation costs.
 224      *
 225      * <p>Suppose <tt>x</tt> is a collection known to contain only strings.
 226      * The following code can be used to dump the collection into a newly
 227      * allocated array of <tt>String</tt>:
 228      *
 229      * <pre>
 230      *     String[] y = x.toArray(new String[0]);</pre>
 231      *
 232      * Note that <tt>toArray(new Object[0])</tt> is identical in function to
 233      * <tt>toArray()</tt>.
 234      *
 235      * @param a the array into which the elements of this collection are to be
 236      *        stored, if it is big enough; otherwise, a new array of the same
 237      *        runtime type is allocated for this purpose.
 238      * @return an array containing all of the elements in this collection
 239      * @throws ArrayStoreException if the runtime type of the specified array
 240      *         is not a supertype of the runtime type of every element in
 241      *         this collection
 242      * @throws NullPointerException if the specified array is null
 243      */
 244     <T> T[] toArray(T[] a);
 245 
 246     // Modification Operations
 247 
 248     /**
 249      * Ensures that this collection contains the specified element (optional
 250      * operation).  Returns <tt>true</tt> if this collection changed as a
 251      * result of the call.  (Returns <tt>false</tt> if this collection does
 252      * not permit duplicates and already contains the specified element.)<p>
 253      *
 254      * Collections that support this operation may place limitations on what
 255      * elements may be added to this collection.  In particular, some
 256      * collections will refuse to add <tt>null</tt> elements, and others will
 257      * impose restrictions on the type of elements that may be added.
 258      * Collection classes should clearly specify in their documentation any
 259      * restrictions on what elements may be added.<p>
 260      *
 261      * If a collection refuses to add a particular element for any reason
 262      * other than that it already contains the element, it <i>must</i> throw
 263      * an exception (rather than returning <tt>false</tt>).  This preserves
 264      * the invariant that a collection always contains the specified element
 265      * after this call returns.
 266      *
 267      * @param e element whose presence in this collection is to be ensured
 268      * @return <tt>true</tt> if this collection changed as a result of the
 269      *         call
 270      * @throws UnsupportedOperationException if the <tt>add</tt> operation
 271      *         is not supported by this collection
 272      * @throws ClassCastException if the class of the specified element
 273      *         prevents it from being added to this collection
 274      * @throws NullPointerException if the specified element is null and this
 275      *         collection does not permit null elements
 276      * @throws IllegalArgumentException if some property of the element
 277      *         prevents it from being added to this collection
 278      * @throws IllegalStateException if the element cannot be added at this
 279      *         time due to insertion restrictions
 280      */
 281     boolean add(E e);
 282 
 283     /**
 284      * Removes a single instance of the specified element from this
 285      * collection, if it is present (optional operation).  More formally,
 286      * removes an element <tt>e</tt> such that
 287      * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>, if
 288      * this collection contains one or more such elements.  Returns
 289      * <tt>true</tt> if this collection contained the specified element (or
 290      * equivalently, if this collection changed as a result of the call).
 291      *
 292      * @param o element to be removed from this collection, if present
 293      * @return <tt>true</tt> if an element was removed as a result of this call
 294      * @throws ClassCastException if the type of the specified element
 295      *         is incompatible with this collection
 296      *         (<a href="#optional-restrictions">optional</a>)
 297      * @throws NullPointerException if the specified element is null and this
 298      *         collection does not permit null elements
 299      *         (<a href="#optional-restrictions">optional</a>)
 300      * @throws UnsupportedOperationException if the <tt>remove</tt> operation
 301      *         is not supported by this collection
 302      */
 303     boolean remove(Object o);
 304 
 305 
 306     // Bulk Operations
 307 
 308     /**
 309      * Returns <tt>true</tt> if this collection contains all of the elements
 310      * in the specified collection.
 311      *
 312      * @param  c collection to be checked for containment in this collection
 313      * @return <tt>true</tt> if this collection contains all of the elements
 314      *         in the specified collection
 315      * @throws ClassCastException if the types of one or more elements
 316      *         in the specified collection are incompatible with this
 317      *         collection
 318      *         (<a href="#optional-restrictions">optional</a>)
 319      * @throws NullPointerException if the specified collection contains one
 320      *         or more null elements and this collection does not permit null
 321      *         elements
 322      *         (<a href="#optional-restrictions">optional</a>),
 323      *         or if the specified collection is null.
 324      * @see    #contains(Object)
 325      */
 326     boolean containsAll(Collection<?> c);
 327 
 328     /**
 329      * Adds all of the elements in the specified collection to this collection
 330      * (optional operation).  The behavior of this operation is undefined if
 331      * the specified collection is modified while the operation is in progress.
 332      * (This implies that the behavior of this call is undefined if the
 333      * specified collection is this collection, and this collection is
 334      * nonempty.)
 335      *
 336      * @param c collection containing elements to be added to this collection
 337      * @return <tt>true</tt> if this collection changed as a result of the call
 338      * @throws UnsupportedOperationException if the <tt>addAll</tt> operation
 339      *         is not supported by this collection
 340      * @throws ClassCastException if the class of an element of the specified
 341      *         collection prevents it from being added to this collection
 342      * @throws NullPointerException if the specified collection contains a
 343      *         null element and this collection does not permit null elements,
 344      *         or if the specified collection is null
 345      * @throws IllegalArgumentException if some property of an element of the
 346      *         specified collection prevents it from being added to this
 347      *         collection
 348      * @throws IllegalStateException if not all the elements can be added at
 349      *         this time due to insertion restrictions
 350      * @see #add(Object)
 351      */
 352     boolean addAll(Collection<? extends E> c);
 353 
 354     /**
 355      * Removes all of this collection's elements that are also contained in the
 356      * specified collection (optional operation).  After this call returns,
 357      * this collection will contain no elements in common with the specified
 358      * collection.
 359      *
 360      * @param c collection containing elements to be removed from this collection
 361      * @return <tt>true</tt> if this collection changed as a result of the
 362      *         call
 363      * @throws UnsupportedOperationException if the <tt>removeAll</tt> method
 364      *         is not supported by this collection
 365      * @throws ClassCastException if the types of one or more elements
 366      *         in this collection are incompatible with the specified
 367      *         collection
 368      *         (<a href="#optional-restrictions">optional</a>)
 369      * @throws NullPointerException if this collection contains one or more
 370      *         null elements and the specified collection does not support
 371      *         null elements
 372      *         (<a href="#optional-restrictions">optional</a>),
 373      *         or if the specified collection is null
 374      * @see #remove(Object)
 375      * @see #contains(Object)
 376      */
 377     boolean removeAll(Collection<?> c);
 378 
 379     /**
 380      * Removes all of the elements of this collection that satisfy the given
 381      * predicate.  Errors or runtime exceptions thrown by the predicate are
 382      * relayed to the caller.
 383      *
 384      * @implSpec
 385      * The default implementation traverses all elements of the collection using
 386      * its {@link #iterator}.  Each matching element is removed using
 387      * {@link Iterator#remove()}.  If the collection's iterator does not
 388      * support removal then an {@code UnsupportedOperationException} will be
 389      * thrown on the first matching element.
 390      *
 391      * @param filter a predicate which returns {@code true} for elements to be
 392      *        removed
 393      * @return {@code true} if any elements were removed
 394      * @throws NullPointerException if the specified filter is null
 395      * @throws UnsupportedOperationException if the {@code remove}
 396      *         method is not supported by this collection's
 397      *         {@link #iterator}
 398      * @since 1.8
 399      */
 400     default boolean removeIf(Predicate<? super E> filter) {
 401         Objects.requireNonNull(filter);
 402         boolean removed = false;
 403         final Iterator<E> each = iterator();
 404         while (each.hasNext()) {
 405             if (filter.test(each.next())) {
 406                 each.remove();
 407                 removed = true;
 408             }
 409         }
 410         return removed;
 411     }
 412 
 413     /**
 414      * Retains only the elements in this collection that are contained in the
 415      * specified collection (optional operation).  In other words, removes from
 416      * this collection all of its elements that are not contained in the
 417      * specified collection.
 418      *
 419      * @param c collection containing elements to be retained in this collection
 420      * @return <tt>true</tt> if this collection changed as a result of the call
 421      * @throws UnsupportedOperationException if the <tt>retainAll</tt> operation
 422      *         is not supported by this collection
 423      * @throws ClassCastException if the types of one or more elements
 424      *         in this collection are incompatible with the specified
 425      *         collection
 426      *         (<a href="#optional-restrictions">optional</a>)
 427      * @throws NullPointerException if this collection contains one or more
 428      *         null elements and the specified collection does not permit null
 429      *         elements
 430      *         (<a href="#optional-restrictions">optional</a>),
 431      *         or if the specified collection is null
 432      * @see #remove(Object)
 433      * @see #contains(Object)
 434      */
 435     boolean retainAll(Collection<?> c);
 436 
 437     /**
 438      * Removes all of the elements from this collection (optional operation).
 439      * The collection will be empty after this method returns.
 440      *
 441      * @throws UnsupportedOperationException if the <tt>clear</tt> operation
 442      *         is not supported by this collection
 443      */
 444     void clear();
 445 
 446 
 447     // Comparison and hashing
 448 
 449     /**
 450      * Compares the specified object with this collection for equality. <p>
 451      *
 452      * While the <tt>Collection</tt> interface adds no stipulations to the
 453      * general contract for the <tt>Object.equals</tt>, programmers who
 454      * implement the <tt>Collection</tt> interface "directly" (in other words,
 455      * create a class that is a <tt>Collection</tt> but is not a <tt>Set</tt>
 456      * or a <tt>List</tt>) must exercise care if they choose to override the
 457      * <tt>Object.equals</tt>.  It is not necessary to do so, and the simplest
 458      * course of action is to rely on <tt>Object</tt>'s implementation, but
 459      * the implementor may wish to implement a "value comparison" in place of
 460      * the default "reference comparison."  (The <tt>List</tt> and
 461      * <tt>Set</tt> interfaces mandate such value comparisons.)<p>
 462      *
 463      * The general contract for the <tt>Object.equals</tt> method states that
 464      * equals must be symmetric (in other words, <tt>a.equals(b)</tt> if and
 465      * only if <tt>b.equals(a)</tt>).  The contracts for <tt>List.equals</tt>
 466      * and <tt>Set.equals</tt> state that lists are only equal to other lists,
 467      * and sets to other sets.  Thus, a custom <tt>equals</tt> method for a
 468      * collection class that implements neither the <tt>List</tt> nor
 469      * <tt>Set</tt> interface must return <tt>false</tt> when this collection
 470      * is compared to any list or set.  (By the same logic, it is not possible
 471      * to write a class that correctly implements both the <tt>Set</tt> and
 472      * <tt>List</tt> interfaces.)
 473      *
 474      * @param o object to be compared for equality with this collection
 475      * @return <tt>true</tt> if the specified object is equal to this
 476      * collection
 477      *
 478      * @see Object#equals(Object)
 479      * @see Set#equals(Object)
 480      * @see List#equals(Object)
 481      */
 482     boolean equals(Object o);
 483 
 484     /**
 485      * Returns the hash code value for this collection.  While the
 486      * <tt>Collection</tt> interface adds no stipulations to the general
 487      * contract for the <tt>Object.hashCode</tt> method, programmers should
 488      * take note that any class that overrides the <tt>Object.equals</tt>
 489      * method must also override the <tt>Object.hashCode</tt> method in order
 490      * to satisfy the general contract for the <tt>Object.hashCode</tt> method.
 491      * In particular, <tt>c1.equals(c2)</tt> implies that
 492      * <tt>c1.hashCode()==c2.hashCode()</tt>.
 493      *
 494      * @return the hash code value for this collection
 495      *
 496      * @see Object#hashCode()
 497      * @see Object#equals(Object)
 498      */
 499     int hashCode();
 500 
 501     /**
 502      * Creates a {@link Spliterator} over the elements in this collection.
 503      *
 504      * <p>The returned {@code Spliterator} must report the characteristic
 505      * {@link Spliterator#SIZED}; implementations should document any additional
 506      * characteristic values reported by the returned Spliterator.
 507      *
 508      * <p>The default implementation should be overridden by subclasses that
 509      * can return a more efficient spliterator.  In order to
 510      * preserve expected laziness behavior for the {@link #stream()} and
 511      * {@link #parallelStream()}} methods, spliterators should either have the
 512      * characteristic of {@code IMMUTABLE} or {@code CONCURRENT}, or be
 513      * <em><a href="Spliterator.html#binding">late-binding</a></em>.
 514      * If none of these is practical, the overriding class should describe the
 515      * spliterator's documented policy of binding and structural interference,
 516      * and should override the {@link #stream()} and {@link #parallelStream()}
 517      * methods to create streams using a {@code Supplier} of the spliterator,
 518      * as in:
 519      * <pre>{@code
 520      *     Stream<E> s = StreamSupport.stream(() -> spliterator(), spliteratorCharacteristics)
 521      * }</pre>
 522      * <p>These requirements ensure that streams produced by the
 523      * {@link #stream()} and {@link #parallelStream()} methods will reflect the
 524      * contents of the collection as of initiation of the terminal stream
 525      * operation.
 526      *
 527      * @implSpec
 528      * The default implementation creates a
 529      * <em><a href="Spliterator.html#binding">late-binding</a></em> spliterator
 530      * from the collections's {@code Iterator}.  The spliterator inherits the
 531      * <em>fail-fast</em> properties of the collection's iterator.
 532      *
 533      * @implNote
 534      * The returned {@code Spliterator} additionally reports
 535      * {@link Spliterator#SUBSIZED}.
 536      *
 537      * @return a {@code Spliterator} over the elements in this collection
 538      * @since 1.8
 539      */
 540     default Spliterator<E> spliterator() {
 541         return Spliterators.spliterator(this, 0);
 542     }
 543 
 544     /**
 545      * Returns a sequential {@code Stream} with this collection as its source.
 546      *
 547      * <p>This method should be overridden when the {@link #spliterator()}
 548      * method cannot return a spliterator that is {@code IMMUTABLE},
 549      * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()}
 550      * for details.)
 551      *
 552      * @implSpec
 553      * The default implementation creates a sequential {@code Stream} from the
 554      * collection's {@code Spliterator}.
 555      *
 556      * @return a sequential {@code Stream} over the elements in this collection
 557      * @since 1.8
 558      */
 559     default Stream<E> stream() {
 560         return StreamSupport.stream(spliterator());
 561     }
 562 
 563     /**
 564      * Returns a possibly parallel {@code Stream} with this collection as its
 565      * source.  It is allowable for this method to return a sequential stream.
 566      *
 567      * <p>This method should be overridden when the {@link #spliterator()}
 568      * method cannot return a spliterator that is {@code IMMUTABLE},
 569      * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()}
 570      * for details.)
 571      *
 572      * @implSpec
 573      * The default implementation creates a parallel {@code Stream} from the
 574      * collection's {@code Spliterator}.
 575      *
 576      * @return a possibly parallel {@code Stream} over the elements in this
 577      * collection
 578      * @since 1.8
 579      */
 580     default Stream<E> parallelStream() {
 581         return StreamSupport.parallelStream(spliterator());
 582     }
 583 }