1 /*
2 * Copyright (c) 1997, 2017, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
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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 manage storage for elements they contain. By contrast, <i>view
112 * collections</i> themselves do not store elements, but instead they rely on a
113 * backing collection to store the actual elements. Operations that are not handled
114 * by the view 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 visible in the view collection.
125 * Correspondingly, any changes made to the view collection — if changes
126 * are permitted — 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, consider a collection that
141 * does not support the {@link #add add} operation. What will happen if the
142 * {@link #addAll addAll} method is invoked on this collection, with an empty
143 * collection as the argument? The addition of zero elements has no effect,
144 * so it is permissible for this collection simply to do nothing and not to throw
145 * an exception. However, it is recommended that such cases throw an exception
146 * unconditionally, as throwing only in certain cases can lead to
147 * programming errors.
148 *
149 * <p>An <i>unmodifiable collection</i> is a collection, all of whose
150 * mutator methods (as defined above) are specified to throw
151 * {@code UnsupportedOperationException}. Such a collection thus cannot be
152 * modified by calling any methods on it. For a collection to be properly
153 * unmodifiable, any view collections derived from it must also be unmodifiable.
154 * For example, if a List is unmodifiable, the List returned by
155 * {@link List#subList List.subList} is also unmodifiable.
156 *
157 * <p>An unmodifiable collection is not necessarily immutable. If the
158 * contained elements are mutable, the entire collection is clearly
159 * mutable, even though it might be unmodifiable. For example, consider
160 * two unmodifiable lists containing mutable elements. The result of calling
161 * {@code list1.equals(list2)} might differ from one call to the next if
162 * the elements had been mutated, even though both lists are unmodifiable.
163 * However, if an unmodifiable collection contains all immutable elements,
164 * it can be considered effectively immutable.
165 *
166 * <p><a id="unmodview"><b>Unmodifiable view collections.</b></a>
167 * An unmodifiable view is a collection that is unmodifiable and that is
168 * a view onto a backing collection. Mutator methods throw
169 * {@code UnsupportedOperationException}, as described above, while
170 * reading and querying methods are delegated to the backing collection.
171 * The effect is to provide read-only access to the backing collection.
172 * This is useful for a component to provide users with read access to
173 * an internal collection, while preventing them from modifying such
174 * collections unexpectedly. Examples of unmodifiable view collections
175 * are those returned by the
176 * {@link Collections#unmodifiableCollection Collections.unmodifiableCollection},
177 * {@link Collections#unmodifiableList Collections.unmodifiableList}, and
178 * related methods.
179 *
180 * <p>Note that changes to the backing collection might still be possible,
181 * and if they occur, they are visible through the unmodifiable view. Thus,
182 * an unmodifiable view collection is not necessarily immutable. However,
183 * if the backing collection of an unmodifiable view is effectively immutable,
184 * or if the only reference to the backing collection is through an
185 * unmodifiable view, the view can be considered effectively immutable.
186 *
187 * <p>This interface is a member of the
188 * <a href="{@docRoot}/java/util/package-summary.html#CollectionsFramework">
189 * Java Collections Framework</a>.
190 *
191 * @implSpec
192 * The default method implementations (inherited or otherwise) do not apply any
193 * synchronization protocol. If a {@code Collection} implementation has a
194 * specific synchronization protocol, then it must override default
195 * implementations to apply that protocol.
196 *
197 * @param <E> the type of elements in this collection
198 *
199 * @author Josh Bloch
200 * @author Neal Gafter
201 * @see Set
202 * @see List
203 * @see Map
204 * @see SortedSet
205 * @see SortedMap
206 * @see HashSet
207 * @see TreeSet
208 * @see ArrayList
209 * @see LinkedList
210 * @see Vector
211 * @see Collections
212 * @see Arrays
213 * @see AbstractCollection
214 * @since 1.2
215 */
216
217 public interface Collection<E> extends Iterable<E> {
218 // Query Operations
219
220 /**
221 * Returns the number of elements in this collection. If this collection
222 * contains more than {@code Integer.MAX_VALUE} elements, returns
223 * {@code Integer.MAX_VALUE}.
224 *
225 * @return the number of elements in this collection
226 */
227 int size();
228
229 /**
230 * Returns {@code true} if this collection contains no elements.
231 *
232 * @return {@code true} if this collection contains no elements
233 */
234 boolean isEmpty();
235
236 /**
237 * Returns {@code true} if this collection contains the specified element.
238 * More formally, returns {@code true} if and only if this collection
239 * contains at least one element {@code e} such that
240 * {@code Objects.equals(o, e)}.
241 *
242 * @param o element whose presence in this collection is to be tested
243 * @return {@code true} if this collection contains the specified
244 * element
245 * @throws ClassCastException if the type of the specified element
246 * is incompatible with this collection
247 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
248 * @throws NullPointerException if the specified element is null and this
249 * collection does not permit null elements
250 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
251 */
252 boolean contains(Object o);
253
254 /**
255 * Returns an iterator over the elements in this collection. There are no
256 * guarantees concerning the order in which the elements are returned
257 * (unless this collection is an instance of some class that provides a
258 * guarantee).
259 *
260 * @return an {@code Iterator} over the elements in this collection
261 */
262 Iterator<E> iterator();
263
264 /**
265 * Returns an array containing all of the elements in this collection.
266 * If this collection makes any guarantees as to what order its elements
267 * are returned by its iterator, this method must return the elements in
268 * the same order.
269 *
270 * <p>The returned array will be "safe" in that no references to it are
271 * maintained by this collection. (In other words, this method must
272 * allocate a new array even if this collection is backed by an array).
273 * The caller is thus free to modify the returned array.
274 *
275 * <p>This method acts as bridge between array-based and collection-based
276 * APIs.
277 *
278 * @return an array containing all of the elements in this collection
279 */
280 Object[] toArray();
281
282 /**
283 * Returns an array containing all of the elements in this collection;
284 * the runtime type of the returned array is that of the specified array.
285 * If the collection fits in the specified array, it is returned therein.
286 * Otherwise, a new array is allocated with the runtime type of the
287 * specified array and the size of this collection.
288 *
289 * <p>If this collection fits in the specified array with room to spare
290 * (i.e., the array has more elements than this collection), the element
291 * in the array immediately following the end of the collection is set to
292 * {@code null}. (This is useful in determining the length of this
293 * collection <i>only</i> if the caller knows that this collection does
294 * not contain any {@code null} elements.)
295 *
296 * <p>If this collection makes any guarantees as to what order its elements
297 * are returned by its iterator, this method must return the elements in
298 * the same order.
299 *
300 * <p>Like the {@link #toArray()} method, this method acts as bridge between
301 * array-based and collection-based APIs. Further, this method allows
302 * precise control over the runtime type of the output array, and may,
303 * under certain circumstances, be used to save allocation costs.
304 *
305 * <p>Suppose {@code x} is a collection known to contain only strings.
306 * The following code can be used to dump the collection into a newly
307 * allocated array of {@code String}:
308 *
309 * <pre>
310 * String[] y = x.toArray(new String[0]);</pre>
311 *
312 * Note that {@code toArray(new Object[0])} is identical in function to
313 * {@code toArray()}.
314 *
315 * @param <T> the runtime type of the array to contain the collection
316 * @param a the array into which the elements of this collection are to be
317 * stored, if it is big enough; otherwise, a new array of the same
318 * runtime type is allocated for this purpose.
319 * @return an array containing all of the elements in this collection
320 * @throws ArrayStoreException if the runtime type of the specified array
321 * is not a supertype of the runtime type of every element in
322 * this collection
323 * @throws NullPointerException if the specified array is null
324 */
325 <T> T[] toArray(T[] a);
326
327 // Modification Operations
328
329 /**
330 * Ensures that this collection contains the specified element (optional
331 * operation). Returns {@code true} if this collection changed as a
332 * result of the call. (Returns {@code false} if this collection does
333 * not permit duplicates and already contains the specified element.)<p>
334 *
335 * Collections that support this operation may place limitations on what
336 * elements may be added to this collection. In particular, some
337 * collections will refuse to add {@code null} elements, and others will
338 * impose restrictions on the type of elements that may be added.
339 * Collection classes should clearly specify in their documentation any
340 * restrictions on what elements may be added.<p>
341 *
342 * If a collection refuses to add a particular element for any reason
343 * other than that it already contains the element, it <i>must</i> throw
344 * an exception (rather than returning {@code false}). This preserves
345 * the invariant that a collection always contains the specified element
346 * after this call returns.
347 *
348 * @param e element whose presence in this collection is to be ensured
349 * @return {@code true} if this collection changed as a result of the
350 * call
351 * @throws UnsupportedOperationException if the {@code add} operation
352 * is not supported by this collection
353 * @throws ClassCastException if the class of the specified element
354 * prevents it from being added to this collection
355 * @throws NullPointerException if the specified element is null and this
356 * collection does not permit null elements
357 * @throws IllegalArgumentException if some property of the element
358 * prevents it from being added to this collection
359 * @throws IllegalStateException if the element cannot be added at this
360 * time due to insertion restrictions
361 */
362 boolean add(E e);
363
364 /**
365 * Removes a single instance of the specified element from this
366 * collection, if it is present (optional operation). More formally,
367 * removes an element {@code e} such that
368 * {@code Objects.equals(o, e)}, if
369 * this collection contains one or more such elements. Returns
370 * {@code true} if this collection contained the specified element (or
371 * equivalently, if this collection changed as a result of the call).
372 *
373 * @param o element to be removed from this collection, if present
374 * @return {@code true} if an element was removed as a result of this call
375 * @throws ClassCastException if the type of the specified element
376 * is incompatible with this collection
377 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
378 * @throws NullPointerException if the specified element is null and this
379 * collection does not permit null elements
380 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
381 * @throws UnsupportedOperationException if the {@code remove} operation
382 * is not supported by this collection
383 */
384 boolean remove(Object o);
385
386
387 // Bulk Operations
388
389 /**
390 * Returns {@code true} if this collection contains all of the elements
391 * in the specified collection.
392 *
393 * @param c collection to be checked for containment in this collection
394 * @return {@code true} if this collection contains all of the elements
395 * in the specified collection
396 * @throws ClassCastException if the types of one or more elements
397 * in the specified collection are incompatible with this
398 * collection
399 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
400 * @throws NullPointerException if the specified collection contains one
401 * or more null elements and this collection does not permit null
402 * elements
403 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>),
404 * or if the specified collection is null.
405 * @see #contains(Object)
406 */
407 boolean containsAll(Collection<?> c);
408
409 /**
410 * Adds all of the elements in the specified collection to this collection
411 * (optional operation). The behavior of this operation is undefined if
412 * the specified collection is modified while the operation is in progress.
413 * (This implies that the behavior of this call is undefined if the
414 * specified collection is this collection, and this collection is
415 * nonempty.)
416 *
417 * @param c collection containing elements to be added to this collection
418 * @return {@code true} if this collection changed as a result of the call
419 * @throws UnsupportedOperationException if the {@code addAll} operation
420 * is not supported by this collection
421 * @throws ClassCastException if the class of an element of the specified
422 * collection prevents it from being added to this collection
423 * @throws NullPointerException if the specified collection contains a
424 * null element and this collection does not permit null elements,
425 * or if the specified collection is null
426 * @throws IllegalArgumentException if some property of an element of the
427 * specified collection prevents it from being added to this
428 * collection
429 * @throws IllegalStateException if not all the elements can be added at
430 * this time due to insertion restrictions
431 * @see #add(Object)
432 */
433 boolean addAll(Collection<? extends E> c);
434
435 /**
436 * Removes all of this collection's elements that are also contained in the
437 * specified collection (optional operation). After this call returns,
438 * this collection will contain no elements in common with the specified
439 * collection.
440 *
441 * @param c collection containing elements to be removed from this collection
442 * @return {@code true} if this collection changed as a result of the
443 * call
444 * @throws UnsupportedOperationException if the {@code removeAll} method
445 * is not supported by this collection
446 * @throws ClassCastException if the types of one or more elements
447 * in this collection are incompatible with the specified
448 * collection
449 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
450 * @throws NullPointerException if this collection contains one or more
451 * null elements and the specified collection does not support
452 * null elements
453 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>),
454 * or if the specified collection is null
455 * @see #remove(Object)
456 * @see #contains(Object)
457 */
458 boolean removeAll(Collection<?> c);
459
460 /**
461 * Removes all of the elements of this collection that satisfy the given
462 * predicate. Errors or runtime exceptions thrown during iteration or by
463 * the predicate are relayed to the caller.
464 *
465 * @implSpec
466 * The default implementation traverses all elements of the collection using
467 * its {@link #iterator}. Each matching element is removed using
468 * {@link Iterator#remove()}. If the collection's iterator does not
469 * support removal then an {@code UnsupportedOperationException} will be
470 * thrown on the first matching element.
471 *
472 * @param filter a predicate which returns {@code true} for elements to be
473 * removed
474 * @return {@code true} if any elements were removed
475 * @throws NullPointerException if the specified filter is null
476 * @throws UnsupportedOperationException if elements cannot be removed
477 * from this collection. Implementations may throw this exception if a
478 * matching element cannot be removed or if, in general, removal is not
479 * supported.
480 * @since 1.8
481 */
482 default boolean removeIf(Predicate<? super E> filter) {
483 Objects.requireNonNull(filter);
484 boolean removed = false;
485 final Iterator<E> each = iterator();
486 while (each.hasNext()) {
487 if (filter.test(each.next())) {
488 each.remove();
489 removed = true;
490 }
491 }
492 return removed;
493 }
494
495 /**
496 * Retains only the elements in this collection that are contained in the
497 * specified collection (optional operation). In other words, removes from
498 * this collection all of its elements that are not contained in the
499 * specified collection.
500 *
501 * @param c collection containing elements to be retained in this collection
502 * @return {@code true} if this collection changed as a result of the call
503 * @throws UnsupportedOperationException if the {@code retainAll} operation
504 * is not supported by this collection
505 * @throws ClassCastException if the types of one or more elements
506 * in this collection are incompatible with the specified
507 * collection
508 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
509 * @throws NullPointerException if this collection contains one or more
510 * null elements and the specified collection does not permit null
511 * elements
512 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>),
513 * or if the specified collection is null
514 * @see #remove(Object)
515 * @see #contains(Object)
516 */
517 boolean retainAll(Collection<?> c);
518
519 /**
520 * Removes all of the elements from this collection (optional operation).
521 * The collection will be empty after this method returns.
522 *
523 * @throws UnsupportedOperationException if the {@code clear} operation
524 * is not supported by this collection
525 */
526 void clear();
527
528
529 // Comparison and hashing
530
531 /**
532 * Compares the specified object with this collection for equality. <p>
533 *
534 * While the {@code Collection} interface adds no stipulations to the
535 * general contract for the {@code Object.equals}, programmers who
536 * implement the {@code Collection} interface "directly" (in other words,
537 * create a class that is a {@code Collection} but is not a {@code Set}
538 * or a {@code List}) must exercise care if they choose to override the
539 * {@code Object.equals}. It is not necessary to do so, and the simplest
540 * course of action is to rely on {@code Object}'s implementation, but
541 * the implementor may wish to implement a "value comparison" in place of
542 * the default "reference comparison." (The {@code List} and
543 * {@code Set} interfaces mandate such value comparisons.)<p>
544 *
545 * The general contract for the {@code Object.equals} method states that
546 * equals must be symmetric (in other words, {@code a.equals(b)} if and
547 * only if {@code b.equals(a)}). The contracts for {@code List.equals}
548 * and {@code Set.equals} state that lists are only equal to other lists,
549 * and sets to other sets. Thus, a custom {@code equals} method for a
550 * collection class that implements neither the {@code List} nor
551 * {@code Set} interface must return {@code false} when this collection
552 * is compared to any list or set. (By the same logic, it is not possible
553 * to write a class that correctly implements both the {@code Set} and
554 * {@code List} interfaces.)
555 *
556 * @param o object to be compared for equality with this collection
557 * @return {@code true} if the specified object is equal to this
558 * collection
559 *
560 * @see Object#equals(Object)
561 * @see Set#equals(Object)
562 * @see List#equals(Object)
563 */
564 boolean equals(Object o);
565
566 /**
567 * Returns the hash code value for this collection. While the
568 * {@code Collection} interface adds no stipulations to the general
569 * contract for the {@code Object.hashCode} method, programmers should
570 * take note that any class that overrides the {@code Object.equals}
571 * method must also override the {@code Object.hashCode} method in order
572 * to satisfy the general contract for the {@code Object.hashCode} method.
573 * In particular, {@code c1.equals(c2)} implies that
574 * {@code c1.hashCode()==c2.hashCode()}.
575 *
576 * @return the hash code value for this collection
577 *
578 * @see Object#hashCode()
579 * @see Object#equals(Object)
580 */
581 int hashCode();
582
583 /**
584 * Creates a {@link Spliterator} over the elements in this collection.
585 *
586 * Implementations should document characteristic values reported by the
587 * spliterator. Such characteristic values are not required to be reported
588 * if the spliterator reports {@link Spliterator#SIZED} and this collection
589 * contains no elements.
590 *
591 * <p>The default implementation should be overridden by subclasses that
592 * can return a more efficient spliterator. In order to
593 * preserve expected laziness behavior for the {@link #stream()} and
594 * {@link #parallelStream()} methods, spliterators should either have the
595 * characteristic of {@code IMMUTABLE} or {@code CONCURRENT}, or be
596 * <em><a href="Spliterator.html#binding">late-binding</a></em>.
597 * If none of these is practical, the overriding class should describe the
598 * spliterator's documented policy of binding and structural interference,
599 * and should override the {@link #stream()} and {@link #parallelStream()}
600 * methods to create streams using a {@code Supplier} of the spliterator,
601 * as in:
602 * <pre>{@code
603 * Stream<E> s = StreamSupport.stream(() -> spliterator(), spliteratorCharacteristics)
604 * }</pre>
605 * <p>These requirements ensure that streams produced by the
606 * {@link #stream()} and {@link #parallelStream()} methods will reflect the
607 * contents of the collection as of initiation of the terminal stream
608 * operation.
609 *
610 * @implSpec
611 * The default implementation creates a
612 * <em><a href="Spliterator.html#binding">late-binding</a></em> spliterator
613 * from the collection's {@code Iterator}. The spliterator inherits the
614 * <em>fail-fast</em> properties of the collection's iterator.
615 * <p>
616 * The created {@code Spliterator} reports {@link Spliterator#SIZED}.
617 *
618 * @implNote
619 * The created {@code Spliterator} additionally reports
620 * {@link Spliterator#SUBSIZED}.
621 *
622 * <p>If a spliterator covers no elements then the reporting of additional
623 * characteristic values, beyond that of {@code SIZED} and {@code SUBSIZED},
624 * does not aid clients to control, specialize or simplify computation.
625 * However, this does enable shared use of an immutable and empty
626 * spliterator instance (see {@link Spliterators#emptySpliterator()}) for
627 * empty collections, and enables clients to determine if such a spliterator
628 * covers no elements.
629 *
630 * @return a {@code Spliterator} over the elements in this collection
631 * @since 1.8
632 */
633 @Override
634 default Spliterator<E> spliterator() {
635 return Spliterators.spliterator(this, 0);
636 }
637
638 /**
639 * Returns a sequential {@code Stream} with this collection as its source.
640 *
641 * <p>This method should be overridden when the {@link #spliterator()}
642 * method cannot return a spliterator that is {@code IMMUTABLE},
643 * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()}
644 * for details.)
645 *
646 * @implSpec
647 * The default implementation creates a sequential {@code Stream} from the
648 * collection's {@code Spliterator}.
649 *
650 * @return a sequential {@code Stream} over the elements in this collection
651 * @since 1.8
652 */
653 default Stream<E> stream() {
654 return StreamSupport.stream(spliterator(), false);
655 }
656
657 /**
658 * Returns a possibly parallel {@code Stream} with this collection as its
659 * source. It is allowable for this method to return a sequential stream.
660 *
661 * <p>This method should be overridden when the {@link #spliterator()}
662 * method cannot return a spliterator that is {@code IMMUTABLE},
663 * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()}
664 * for details.)
665 *
666 * @implSpec
667 * The default implementation creates a parallel {@code Stream} from the
668 * collection's {@code Spliterator}.
669 *
670 * @return a possibly parallel {@code Stream} over the elements in this
671 * collection
672 * @since 1.8
673 */
674 default Stream<E> parallelStream() {
675 return StreamSupport.stream(spliterator(), true);
676 }
677 }