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