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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7 * published by the Free Software Foundation. Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
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24 */
25
26 package java.util;
27
28 import java.util.function.IntFunction;
29 import java.util.function.Predicate;
30 import java.util.stream.Stream;
31 import java.util.stream.StreamSupport;
32
33 /**
34 * The root interface in the <i>collection hierarchy</i>. A collection
35 * represents a group of objects, known as its <i>elements</i>. Some
36 * collections allow duplicate elements and others do not. Some are ordered
37 * and others unordered. The JDK does not provide any <i>direct</i>
38 * implementations of this interface: it provides implementations of more
39 * specific subinterfaces like {@code Set} and {@code List}. This interface
40 * is typically used to pass collections around and manipulate them where
41 * maximum generality is desired.
42 *
43 * <p><i>Bags</i> or <i>multisets</i> (unordered collections that may contain
44 * duplicate elements) should implement this interface directly.
45 *
46 * <p>All general-purpose {@code Collection} implementation classes (which
47 * typically implement {@code Collection} indirectly through one of its
48 * subinterfaces) should provide two "standard" constructors: a void (no
49 * arguments) constructor, which creates an empty collection, and a
50 * constructor with a single argument of type {@code Collection}, which
51 * creates a new collection with the same elements as its argument. In
52 * effect, the latter constructor allows the user to copy any collection,
53 * producing an equivalent collection of the desired implementation type.
54 * There is no way to enforce this convention (as interfaces cannot contain
55 * constructors) but all of the general-purpose {@code Collection}
56 * implementations in the Java platform libraries comply.
57 *
58 * <p>Certain methods are specified to be
59 * <i>optional</i>. If a collection implementation doesn't implement a
60 * particular operation, it should define the corresponding method to throw
61 * {@code UnsupportedOperationException}. Such methods are marked "optional
62 * operation" in method specifications of the collections interfaces.
63 *
64 * <p><a id="optional-restrictions"></a>Some collection implementations
65 * have restrictions on the elements that they may contain.
66 * For example, some implementations prohibit null elements,
67 * and some have restrictions on the types of their elements. Attempting to
68 * add an ineligible element throws an unchecked exception, typically
69 * {@code NullPointerException} or {@code ClassCastException}. Attempting
70 * to query the presence of an ineligible element may throw an exception,
71 * or it may simply return false; some implementations will exhibit the former
72 * behavior and some will exhibit the latter. More generally, attempting an
73 * operation on an ineligible element whose completion would not result in
74 * the insertion of an ineligible element into the collection may throw an
75 * exception or it may succeed, at the option of the implementation.
76 * Such exceptions are marked as "optional" in the specification for this
77 * interface.
78 *
79 * <p>It is up to each collection to determine its own synchronization
80 * policy. In the absence of a stronger guarantee by the
81 * implementation, undefined behavior may result from the invocation
82 * of any method on a collection that is being mutated by another
83 * thread; this includes direct invocations, passing the collection to
84 * a method that might perform invocations, and using an existing
85 * iterator to examine the collection.
86 *
87 * <p>Many methods in Collections Framework interfaces are defined in
88 * terms of the {@link Object#equals(Object) equals} method. For example,
89 * the specification for the {@link #contains(Object) contains(Object o)}
90 * method says: "returns {@code true} if and only if this collection
91 * contains at least one element {@code e} such that
92 * {@code (o==null ? e==null : o.equals(e))}." This specification should
93 * <i>not</i> be construed to imply that invoking {@code Collection.contains}
94 * with a non-null argument {@code o} will cause {@code o.equals(e)} to be
95 * invoked for any element {@code e}. Implementations are free to implement
96 * optimizations whereby the {@code equals} invocation is avoided, for
97 * example, by first comparing the hash codes of the two elements. (The
98 * {@link Object#hashCode()} specification guarantees that two objects with
99 * unequal hash codes cannot be equal.) More generally, implementations of
100 * the various Collections Framework interfaces are free to take advantage of
101 * the specified behavior of underlying {@link Object} methods wherever the
102 * implementor deems it appropriate.
103 *
104 * <p>Some collection operations which perform recursive traversal of the
105 * collection may fail with an exception for self-referential instances where
106 * the collection directly or indirectly contains itself. This includes the
107 * {@code clone()}, {@code equals()}, {@code hashCode()} and {@code toString()}
108 * methods. Implementations may optionally handle the self-referential scenario,
109 * however most current implementations do not do so.
110 *
111 * <h2><a id="view">View Collections</a></h2>
112 *
113 * <p>Most collections manage storage for elements they contain. By contrast, <i>view
114 * collections</i> themselves do not store elements, but instead they rely on a
115 * backing collection to store the actual elements. Operations that are not handled
116 * by the view collection itself are delegated to the backing collection. Examples of
117 * view collections include the wrapper collections returned by methods such as
118 * {@link Collections#checkedCollection Collections.checkedCollection},
119 * {@link Collections#synchronizedCollection Collections.synchronizedCollection}, and
120 * {@link Collections#unmodifiableCollection Collections.unmodifiableCollection}.
121 * Other examples of view collections include collections that provide a
122 * different representation of the same elements, for example, as
123 * provided by {@link List#subList List.subList},
124 * {@link NavigableSet#subSet NavigableSet.subSet}, or
125 * {@link Map#entrySet Map.entrySet}.
126 * Any changes made to the backing collection are visible in the view collection.
127 * Correspondingly, any changes made to the view collection — if changes
128 * are permitted — are written through to the backing collection.
129 * Although they technically aren't collections, instances of
130 * {@link Iterator} and {@link ListIterator} can also allow modifications
131 * to be written through to the backing collection, and in some cases,
132 * modifications to the backing collection will be visible to the Iterator
133 * during iteration.
134 *
135 * <h2><a id="unmodifiable">Unmodifiable Collections</a></h2>
136 *
137 * <p>Certain methods of this interface are considered "destructive" and are called
138 * "mutator" methods in that they modify the group of objects contained within
139 * the collection on which they operate. They can be specified to throw
140 * {@code UnsupportedOperationException} if this collection implementation
141 * does not support the operation. Such methods should (but are not required
142 * to) throw an {@code UnsupportedOperationException} if the invocation would
143 * have no effect on the collection. For example, consider a collection that
144 * does not support the {@link #add add} operation. What will happen if the
145 * {@link #addAll addAll} method is invoked on this collection, with an empty
146 * collection as the argument? The addition of zero elements has no effect,
147 * so it is permissible for this collection simply to do nothing and not to throw
148 * an exception. However, it is recommended that such cases throw an exception
149 * unconditionally, as throwing only in certain cases can lead to
150 * programming errors.
151 *
152 * <p>An <i>unmodifiable collection</i> is a collection, all of whose
153 * mutator methods (as defined above) are specified to throw
154 * {@code UnsupportedOperationException}. Such a collection thus cannot be
155 * modified by calling any methods on it. For a collection to be properly
156 * unmodifiable, any view collections derived from it must also be unmodifiable.
157 * For example, if a List is unmodifiable, the List returned by
158 * {@link List#subList List.subList} is also unmodifiable.
159 *
160 * <p>An unmodifiable collection is not necessarily immutable. If the
161 * contained elements are mutable, the entire collection is clearly
162 * mutable, even though it might be unmodifiable. For example, consider
163 * two unmodifiable lists containing mutable elements. The result of calling
164 * {@code list1.equals(list2)} might differ from one call to the next if
165 * the elements had been mutated, even though both lists are unmodifiable.
166 * However, if an unmodifiable collection contains all immutable elements,
167 * it can be considered effectively immutable.
168 *
169 * <h2><a id="unmodview">Unmodifiable View Collections</a></h2>
170 *
171 * <p>An <i>unmodifiable view collection</i> is a collection that is unmodifiable
172 * and that is also a view onto a backing collection. Its mutator methods throw
173 * {@code UnsupportedOperationException}, as described above, while
174 * reading and querying methods are delegated to the backing collection.
175 * The effect is to provide read-only access to the backing collection.
176 * This is useful for a component to provide users with read access to
177 * an internal collection, while preventing them from modifying such
178 * collections unexpectedly. Examples of unmodifiable view collections
179 * are those returned by the
180 * {@link Collections#unmodifiableCollection Collections.unmodifiableCollection},
181 * {@link Collections#unmodifiableList Collections.unmodifiableList}, and
182 * related methods.
183 *
184 * <p>Note that changes to the backing collection might still be possible,
185 * and if they occur, they are visible through the unmodifiable view. Thus,
186 * an unmodifiable view collection is not necessarily immutable. However,
187 * if the backing collection of an unmodifiable view is effectively immutable,
188 * or if the only reference to the backing collection is through an
189 * unmodifiable view, the view can be considered effectively immutable.
190 *
191 * <p>This interface is a member of the
192 * <a href="{@docRoot}/java/util/package-summary.html#CollectionsFramework">
193 * Java Collections Framework</a>.
194 *
195 * @implSpec
196 * The default method implementations (inherited or otherwise) do not apply any
197 * synchronization protocol. If a {@code Collection} implementation has a
198 * specific synchronization protocol, then it must override default
199 * implementations to apply that protocol.
200 *
201 * @param <E> the type of elements in this collection
202 *
203 * @author Josh Bloch
204 * @author Neal Gafter
205 * @see Set
206 * @see List
207 * @see Map
208 * @see SortedSet
209 * @see SortedMap
210 * @see HashSet
211 * @see TreeSet
212 * @see ArrayList
213 * @see LinkedList
214 * @see Vector
215 * @see Collections
216 * @see Arrays
217 * @see AbstractCollection
218 * @since 1.2
219 */
220
221 public interface Collection<E> extends Iterable<E> {
222 // Query Operations
223
224 /**
225 * Returns the number of elements in this collection. If this collection
226 * contains more than {@code Integer.MAX_VALUE} elements, returns
227 * {@code Integer.MAX_VALUE}.
228 *
229 * @return the number of elements in this collection
230 */
231 int size();
232
233 /**
234 * Returns {@code true} if this collection contains no elements.
235 *
236 * @return {@code true} if this collection contains no elements
237 */
238 boolean isEmpty();
239
240 /**
241 * Returns {@code true} if this collection contains the specified element.
242 * More formally, returns {@code true} if and only if this collection
243 * contains at least one element {@code e} such that
244 * {@code Objects.equals(o, e)}.
245 *
246 * @param o element whose presence in this collection is to be tested
247 * @return {@code true} if this collection contains the specified
248 * element
249 * @throws ClassCastException if the type of the specified element
250 * is incompatible with this collection
251 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
252 * @throws NullPointerException if the specified element is null and this
253 * collection does not permit null elements
254 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
255 */
256 boolean contains(Object o);
257
258 /**
259 * Returns an iterator over the elements in this collection. There are no
260 * guarantees concerning the order in which the elements are returned
261 * (unless this collection is an instance of some class that provides a
262 * guarantee).
263 *
264 * @return an {@code Iterator} over the elements in this collection
265 */
266 Iterator<E> iterator();
267
268 /**
269 * Returns an array containing all of the elements in this collection.
270 * If this collection makes any guarantees as to what order its elements
271 * are returned by its iterator, this method must return the elements in
272 * the same order. The returned array's {@linkplain Class#getComponentType
273 * runtime component type} is {@code Object}.
274 *
275 * <p>The returned array will be "safe" in that no references to it are
276 * maintained by this collection. (In other words, this method must
277 * allocate a new array even if this collection is backed by an array).
278 * The caller is thus free to modify the returned array.
279 *
280 * @apiNote
281 * This method acts as a bridge between array-based and collection-based APIs.
282 * It returns an array whose runtime type is {@code Object[]}.
283 * Use {@link #toArray(Object[]) toArray(T[])} to reuse an existing
284 * array, or use {@link #toArray(IntFunction)} to control the runtime type
285 * of the array.
286 *
287 * @return an array, whose {@linkplain Class#getComponentType runtime component
288 * type} is {@code Object}, containing all of the elements in this collection
289 */
290 Object[] toArray();
291
292 /**
293 * Returns an array containing all of the elements in this collection;
294 * the runtime type of the returned array is that of the specified array.
295 * If the collection fits in the specified array, it is returned therein.
296 * Otherwise, a new array is allocated with the runtime type of the
297 * specified array and the size of this collection.
298 *
299 * <p>If this collection fits in the specified array with room to spare
300 * (i.e., the array has more elements than this collection), the element
301 * in the array immediately following the end of the collection is set to
302 * {@code null}. (This is useful in determining the length of this
303 * collection <i>only</i> if the caller knows that this collection does
304 * not contain any {@code null} elements.)
305 *
306 * <p>If this collection makes any guarantees as to what order its elements
307 * are returned by its iterator, this method must return the elements in
308 * the same order.
309 *
310 * @apiNote
311 * This method acts as a bridge between array-based and collection-based APIs.
312 * It allows an existing array to be reused under certain circumstances.
313 * Use {@link #toArray()} to create an array whose runtime type is {@code Object[]},
314 * or use {@link #toArray(IntFunction)} to control the runtime type of
315 * the array.
316 *
317 * <p>Suppose {@code x} is a collection known to contain only strings.
318 * The following code can be used to dump the collection into a previously
319 * allocated {@code String} array:
320 *
321 * <pre>
322 * String[] y = new String[SIZE];
323 * ...
324 * y = x.toArray(y);</pre>
325 *
326 * <p>The return value is reassigned to the variable {@code y}, because a
327 * new array will be allocated and returned if the collection {@code x} has
328 * too many elements to fit into the existing array {@code y}.
329 *
330 * <p>Note that {@code toArray(new Object[0])} is identical in function to
331 * {@code toArray()}.
332 *
333 * @param <T> the component type of the array to contain the collection
334 * @param a the array into which the elements of this collection are to be
335 * stored, if it is big enough; otherwise, a new array of the same
336 * runtime type is allocated for this purpose.
337 * @return an array containing all of the elements in this collection
338 * @throws ArrayStoreException if the runtime type of any element in this
339 * collection is not assignable to the {@linkplain Class#getComponentType
340 * runtime component type} of the specified array
341 * @throws NullPointerException if the specified array is null
342 */
343 <T> T[] toArray(T[] a);
344
345 /**
346 * Returns an array containing all of the elements in this collection,
347 * using the provided {@code generator} function to allocate the returned array.
348 *
349 * <p>If this collection makes any guarantees as to what order its elements
350 * are returned by its iterator, this method must return the elements in
351 * the same order.
352 *
353 * @apiNote
354 * This method acts as a bridge between array-based and collection-based APIs.
355 * It allows creation of an array of a particular runtime type. Use
356 * {@link #toArray()} to create an array whose runtime type is {@code Object[]},
357 * or use {@link #toArray(Object[]) toArray(T[])} to reuse an existing array.
358 *
359 * <p>Suppose {@code x} is a collection known to contain only strings.
360 * The following code can be used to dump the collection into a newly
361 * allocated array of {@code String}:
362 *
363 * <pre>
364 * String[] y = x.toArray(String[]::new);</pre>
365 *
366 * @implSpec
367 * The default implementation calls the generator function with zero
368 * and then passes the resulting array to {@link #toArray(Object[]) toArray(T[])}.
369 *
370 * @param <T> the component type of the array to contain the collection
371 * @param generator a function which produces a new array of the desired
372 * type and the provided length
373 * @return an array containing all of the elements in this collection
374 * @throws ArrayStoreException if the runtime type of any element in this
375 * collection is not assignable to the {@linkplain Class#getComponentType
376 * runtime component type} of the generated array
377 * @throws NullPointerException if the generator function is null
378 * @since 10
379 */
380 default <T> T[] toArray(IntFunction<T[]> generator) {
381 return toArray(generator.apply(0));
382 }
383
384 // Modification Operations
385
386 /**
387 * Ensures that this collection contains the specified element (optional
388 * operation). Returns {@code true} if this collection changed as a
389 * result of the call. (Returns {@code false} if this collection does
390 * not permit duplicates and already contains the specified element.)<p>
391 *
392 * Collections that support this operation may place limitations on what
393 * elements may be added to this collection. In particular, some
394 * collections will refuse to add {@code null} elements, and others will
395 * impose restrictions on the type of elements that may be added.
396 * Collection classes should clearly specify in their documentation any
397 * restrictions on what elements may be added.<p>
398 *
399 * If a collection refuses to add a particular element for any reason
400 * other than that it already contains the element, it <i>must</i> throw
401 * an exception (rather than returning {@code false}). This preserves
402 * the invariant that a collection always contains the specified element
403 * after this call returns.
404 *
405 * @param e element whose presence in this collection is to be ensured
406 * @return {@code true} if this collection changed as a result of the
407 * call
408 * @throws UnsupportedOperationException if the {@code add} operation
409 * is not supported by this collection
410 * @throws ClassCastException if the class of the specified element
411 * prevents it from being added to this collection
412 * @throws NullPointerException if the specified element is null and this
413 * collection does not permit null elements
414 * @throws IllegalArgumentException if some property of the element
415 * prevents it from being added to this collection
416 * @throws IllegalStateException if the element cannot be added at this
417 * time due to insertion restrictions
418 */
419 boolean add(E e);
420
421 /**
422 * Removes a single instance of the specified element from this
423 * collection, if it is present (optional operation). More formally,
424 * removes an element {@code e} such that
425 * {@code Objects.equals(o, e)}, if
426 * this collection contains one or more such elements. Returns
427 * {@code true} if this collection contained the specified element (or
428 * equivalently, if this collection changed as a result of the call).
429 *
430 * @param o element to be removed from this collection, if present
431 * @return {@code true} if an element was removed as a result of this call
432 * @throws ClassCastException if the type of the specified element
433 * is incompatible with this collection
434 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
435 * @throws NullPointerException if the specified element is null and this
436 * collection does not permit null elements
437 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
438 * @throws UnsupportedOperationException if the {@code remove} operation
439 * is not supported by this collection
440 */
441 boolean remove(Object o);
442
443
444 // Bulk Operations
445
446 /**
447 * Returns {@code true} if this collection contains all of the elements
448 * in the specified collection.
449 *
450 * @param c collection to be checked for containment in this collection
451 * @return {@code true} if this collection contains all of the elements
452 * in the specified collection
453 * @throws ClassCastException if the types of one or more elements
454 * in the specified collection are incompatible with this
455 * collection
456 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
457 * @throws NullPointerException if the specified collection contains one
458 * or more null elements and this collection does not permit null
459 * elements
460 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>),
461 * or if the specified collection is null.
462 * @see #contains(Object)
463 */
464 boolean containsAll(Collection<?> c);
465
466 /**
467 * Adds all of the elements in the specified collection to this collection
468 * (optional operation). The behavior of this operation is undefined if
469 * the specified collection is modified while the operation is in progress.
470 * (This implies that the behavior of this call is undefined if the
471 * specified collection is this collection, and this collection is
472 * nonempty.)
473 *
474 * @param c collection containing elements to be added to this collection
475 * @return {@code true} if this collection changed as a result of the call
476 * @throws UnsupportedOperationException if the {@code addAll} operation
477 * is not supported by this collection
478 * @throws ClassCastException if the class of an element of the specified
479 * collection prevents it from being added to this collection
480 * @throws NullPointerException if the specified collection contains a
481 * null element and this collection does not permit null elements,
482 * or if the specified collection is null
483 * @throws IllegalArgumentException if some property of an element of the
484 * specified collection prevents it from being added to this
485 * collection
486 * @throws IllegalStateException if not all the elements can be added at
487 * this time due to insertion restrictions
488 * @see #add(Object)
489 */
490 boolean addAll(Collection<? extends E> c);
491
492 /**
493 * Removes all of this collection's elements that are also contained in the
494 * specified collection (optional operation). After this call returns,
495 * this collection will contain no elements in common with the specified
496 * collection.
497 *
498 * @param c collection containing elements to be removed from this collection
499 * @return {@code true} if this collection changed as a result of the
500 * call
501 * @throws UnsupportedOperationException if the {@code removeAll} method
502 * is not supported by this collection
503 * @throws ClassCastException if the types of one or more elements
504 * in this collection are incompatible with the specified
505 * collection
506 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
507 * @throws NullPointerException if this collection contains one or more
508 * null elements and the specified collection does not support
509 * null elements
510 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>),
511 * or if the specified collection is null
512 * @see #remove(Object)
513 * @see #contains(Object)
514 */
515 boolean removeAll(Collection<?> c);
516
517 /**
518 * Removes all of the elements of this collection that satisfy the given
519 * predicate. Errors or runtime exceptions thrown during iteration or by
520 * the predicate are relayed to the caller.
521 *
522 * @implSpec
523 * The default implementation traverses all elements of the collection using
524 * its {@link #iterator}. Each matching element is removed using
525 * {@link Iterator#remove()}. If the collection's iterator does not
526 * support removal then an {@code UnsupportedOperationException} will be
527 * thrown on the first matching element.
528 *
529 * @param filter a predicate which returns {@code true} for elements to be
530 * removed
531 * @return {@code true} if any elements were removed
532 * @throws NullPointerException if the specified filter is null
533 * @throws UnsupportedOperationException if elements cannot be removed
534 * from this collection. Implementations may throw this exception if a
535 * matching element cannot be removed or if, in general, removal is not
536 * supported.
537 * @since 1.8
538 */
539 default boolean removeIf(Predicate<? super E> filter) {
540 Objects.requireNonNull(filter);
541 boolean removed = false;
542 final Iterator<E> each = iterator();
543 while (each.hasNext()) {
544 if (filter.test(each.next())) {
545 each.remove();
546 removed = true;
547 }
548 }
549 return removed;
550 }
551
552 /**
553 * Retains only the elements in this collection that are contained in the
554 * specified collection (optional operation). In other words, removes from
555 * this collection all of its elements that are not contained in the
556 * specified collection.
557 *
558 * @param c collection containing elements to be retained in this collection
559 * @return {@code true} if this collection changed as a result of the call
560 * @throws UnsupportedOperationException if the {@code retainAll} operation
561 * is not supported by this collection
562 * @throws ClassCastException if the types of one or more elements
563 * in this collection are incompatible with the specified
564 * collection
565 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
566 * @throws NullPointerException if this collection contains one or more
567 * null elements and the specified collection does not permit null
568 * elements
569 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>),
570 * or if the specified collection is null
571 * @see #remove(Object)
572 * @see #contains(Object)
573 */
574 boolean retainAll(Collection<?> c);
575
576 /**
577 * Removes all of the elements from this collection (optional operation).
578 * The collection will be empty after this method returns.
579 *
580 * @throws UnsupportedOperationException if the {@code clear} operation
581 * is not supported by this collection
582 */
583 void clear();
584
585
586 // Comparison and hashing
587
588 /**
589 * Compares the specified object with this collection for equality. <p>
590 *
591 * While the {@code Collection} interface adds no stipulations to the
592 * general contract for the {@code Object.equals}, programmers who
593 * implement the {@code Collection} interface "directly" (in other words,
594 * create a class that is a {@code Collection} but is not a {@code Set}
595 * or a {@code List}) must exercise care if they choose to override the
596 * {@code Object.equals}. It is not necessary to do so, and the simplest
597 * course of action is to rely on {@code Object}'s implementation, but
598 * the implementor may wish to implement a "value comparison" in place of
599 * the default "reference comparison." (The {@code List} and
600 * {@code Set} interfaces mandate such value comparisons.)<p>
601 *
602 * The general contract for the {@code Object.equals} method states that
603 * equals must be symmetric (in other words, {@code a.equals(b)} if and
604 * only if {@code b.equals(a)}). The contracts for {@code List.equals}
605 * and {@code Set.equals} state that lists are only equal to other lists,
606 * and sets to other sets. Thus, a custom {@code equals} method for a
607 * collection class that implements neither the {@code List} nor
608 * {@code Set} interface must return {@code false} when this collection
609 * is compared to any list or set. (By the same logic, it is not possible
610 * to write a class that correctly implements both the {@code Set} and
611 * {@code List} interfaces.)
612 *
613 * @param o object to be compared for equality with this collection
614 * @return {@code true} if the specified object is equal to this
615 * collection
616 *
617 * @see Object#equals(Object)
618 * @see Set#equals(Object)
619 * @see List#equals(Object)
620 */
621 boolean equals(Object o);
622
623 /**
624 * Returns the hash code value for this collection. While the
625 * {@code Collection} interface adds no stipulations to the general
626 * contract for the {@code Object.hashCode} method, programmers should
627 * take note that any class that overrides the {@code Object.equals}
628 * method must also override the {@code Object.hashCode} method in order
629 * to satisfy the general contract for the {@code Object.hashCode} method.
630 * In particular, {@code c1.equals(c2)} implies that
631 * {@code c1.hashCode()==c2.hashCode()}.
632 *
633 * @return the hash code value for this collection
634 *
635 * @see Object#hashCode()
636 * @see Object#equals(Object)
637 */
638 int hashCode();
639
640 /**
641 * Creates a {@link Spliterator} over the elements in this collection.
642 *
643 * Implementations should document characteristic values reported by the
644 * spliterator. Such characteristic values are not required to be reported
645 * if the spliterator reports {@link Spliterator#SIZED} and this collection
646 * contains no elements.
647 *
648 * <p>The default implementation should be overridden by subclasses that
649 * can return a more efficient spliterator. In order to
650 * preserve expected laziness behavior for the {@link #stream()} and
651 * {@link #parallelStream()} methods, spliterators should either have the
652 * characteristic of {@code IMMUTABLE} or {@code CONCURRENT}, or be
653 * <em><a href="Spliterator.html#binding">late-binding</a></em>.
654 * If none of these is practical, the overriding class should describe the
655 * spliterator's documented policy of binding and structural interference,
656 * and should override the {@link #stream()} and {@link #parallelStream()}
657 * methods to create streams using a {@code Supplier} of the spliterator,
658 * as in:
659 * <pre>{@code
660 * Stream<E> s = StreamSupport.stream(() -> spliterator(), spliteratorCharacteristics)
661 * }</pre>
662 * <p>These requirements ensure that streams produced by the
663 * {@link #stream()} and {@link #parallelStream()} methods will reflect the
664 * contents of the collection as of initiation of the terminal stream
665 * operation.
666 *
667 * @implSpec
668 * The default implementation creates a
669 * <em><a href="Spliterator.html#binding">late-binding</a></em> spliterator
670 * from the collection's {@code Iterator}. The spliterator inherits the
671 * <em>fail-fast</em> properties of the collection's iterator.
672 * <p>
673 * The created {@code Spliterator} reports {@link Spliterator#SIZED}.
674 *
675 * @implNote
676 * The created {@code Spliterator} additionally reports
677 * {@link Spliterator#SUBSIZED}.
678 *
679 * <p>If a spliterator covers no elements then the reporting of additional
680 * characteristic values, beyond that of {@code SIZED} and {@code SUBSIZED},
681 * does not aid clients to control, specialize or simplify computation.
682 * However, this does enable shared use of an immutable and empty
683 * spliterator instance (see {@link Spliterators#emptySpliterator()}) for
684 * empty collections, and enables clients to determine if such a spliterator
685 * covers no elements.
686 *
687 * @return a {@code Spliterator} over the elements in this collection
688 * @since 1.8
689 */
690 @Override
691 default Spliterator<E> spliterator() {
692 return Spliterators.spliterator(this, 0);
693 }
694
695 /**
696 * Returns a sequential {@code Stream} with this collection as its source.
697 *
698 * <p>This method should be overridden when the {@link #spliterator()}
699 * method cannot return a spliterator that is {@code IMMUTABLE},
700 * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()}
701 * for details.)
702 *
703 * @implSpec
704 * The default implementation creates a sequential {@code Stream} from the
705 * collection's {@code Spliterator}.
706 *
707 * @return a sequential {@code Stream} over the elements in this collection
708 * @since 1.8
709 */
710 default Stream<E> stream() {
711 return StreamSupport.stream(spliterator(), false);
712 }
713
714 /**
715 * Returns a possibly parallel {@code Stream} with this collection as its
716 * source. It is allowable for this method to return a sequential stream.
717 *
718 * <p>This method should be overridden when the {@link #spliterator()}
719 * method cannot return a spliterator that is {@code IMMUTABLE},
720 * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()}
721 * for details.)
722 *
723 * @implSpec
724 * The default implementation creates a parallel {@code Stream} from the
725 * collection's {@code Spliterator}.
726 *
727 * @return a possibly parallel {@code Stream} over the elements in this
728 * collection
729 * @since 1.8
730 */
731 default Stream<E> parallelStream() {
732 return StreamSupport.stream(spliterator(), true);
733 }
734 }