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