1 /*
2 * Copyright (c) 1997, 2019, 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
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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.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 interfaces and implementations may specify membership
105 * semantics that are not defined in terms of the {@code equals} method. For
106 * example, the {@link SortedSet#contains(Object) SortedSet.contains} method
107 * is defined in terms of a comparison method provided at construction time
108 * instead of the {@code equals} method. Such collections are well-defined,
109 * although their behavior may be counterintuitive when mixed with collections
110 * that use different membership semantics. For operations that may involve the
111 * membership semantics of more than one collection, it is specified which
112 * collection's membership semantics the operation uses.
113 *
114 * <p>Some collection operations which perform recursive traversal of the
115 * collection may fail with an exception for self-referential instances where
116 * the collection directly or indirectly contains itself. This includes the
117 * {@code clone()}, {@code equals()}, {@code hashCode()} and {@code toString()}
118 * methods. Implementations may optionally handle the self-referential scenario,
119 * however most current implementations do not do so.
120 *
121 * <h2><a id="view">View Collections</a></h2>
122 *
123 * <p>Most collections manage storage for elements they contain. By contrast, <i>view
124 * collections</i> themselves do not store elements, but instead they rely on a
125 * backing collection to store the actual elements. Operations that are not handled
126 * by the view collection itself are delegated to the backing collection. Examples of
127 * view collections include the wrapper collections returned by methods such as
128 * {@link Collections#checkedCollection Collections.checkedCollection},
129 * {@link Collections#synchronizedCollection Collections.synchronizedCollection}, and
130 * {@link Collections#unmodifiableCollection Collections.unmodifiableCollection}.
131 * Other examples of view collections include collections that provide a
132 * different representation of the same elements, for example, as
133 * provided by {@link List#subList List.subList},
134 * {@link NavigableSet#subSet NavigableSet.subSet}, or
135 * {@link Map#entrySet Map.entrySet}.
136 * Any changes made to the backing collection are visible in the view collection.
137 * Correspondingly, any changes made to the view collection — if changes
138 * are permitted — are written through to the backing collection.
139 * Although they technically aren't collections, instances of
140 * {@link Iterator} and {@link ListIterator} can also allow modifications
141 * to be written through to the backing collection, and in some cases,
142 * modifications to the backing collection will be visible to the Iterator
143 * during iteration.
144 *
145 * <h2><a id="unmodifiable">Unmodifiable Collections</a></h2>
146 *
147 * <p>Certain methods of this interface are considered "destructive" and are called
148 * "mutator" methods in that they modify the group of objects contained within
149 * the collection on which they operate. They can be specified to throw
150 * {@code UnsupportedOperationException} if this collection implementation
151 * does not support the operation. Such methods should (but are not required
152 * to) throw an {@code UnsupportedOperationException} if the invocation would
153 * have no effect on the collection. For example, consider a collection that
154 * does not support the {@link #add add} operation. What will happen if the
155 * {@link #addAll addAll} method is invoked on this collection, with an empty
156 * collection as the argument? The addition of zero elements has no effect,
157 * so it is permissible for this collection simply to do nothing and not to throw
158 * an exception. However, it is recommended that such cases throw an exception
159 * unconditionally, as throwing only in certain cases can lead to
160 * programming errors.
161 *
162 * <p>An <i>unmodifiable collection</i> is a collection, all of whose
163 * mutator methods (as defined above) are specified to throw
164 * {@code UnsupportedOperationException}. Such a collection thus cannot be
165 * modified by calling any methods on it. For a collection to be properly
166 * unmodifiable, any view collections derived from it must also be unmodifiable.
167 * For example, if a List is unmodifiable, the List returned by
168 * {@link List#subList List.subList} is also unmodifiable.
169 *
170 * <p>An unmodifiable collection is not necessarily immutable. If the
171 * contained elements are mutable, the entire collection is clearly
172 * mutable, even though it might be unmodifiable. For example, consider
173 * two unmodifiable lists containing mutable elements. The result of calling
174 * {@code list1.equals(list2)} might differ from one call to the next if
175 * the elements had been mutated, even though both lists are unmodifiable.
176 * However, if an unmodifiable collection contains all immutable elements,
177 * it can be considered effectively immutable.
178 *
179 * <h2><a id="unmodview">Unmodifiable View Collections</a></h2>
180 *
181 * <p>An <i>unmodifiable view collection</i> is a collection that is unmodifiable
182 * and that is also a view onto a backing collection. Its mutator methods throw
183 * {@code UnsupportedOperationException}, as described above, while
184 * reading and querying methods are delegated to the backing collection.
185 * The effect is to provide read-only access to the backing collection.
186 * This is useful for a component to provide users with read access to
187 * an internal collection, while preventing them from modifying such
188 * collections unexpectedly. Examples of unmodifiable view collections
189 * are those returned by the
190 * {@link Collections#unmodifiableCollection Collections.unmodifiableCollection},
191 * {@link Collections#unmodifiableList Collections.unmodifiableList}, and
192 * related methods.
193 *
194 * <p>Note that changes to the backing collection might still be possible,
195 * and if they occur, they are visible through the unmodifiable view. Thus,
196 * an unmodifiable view collection is not necessarily immutable. However,
197 * if the backing collection of an unmodifiable view is effectively immutable,
198 * or if the only reference to the backing collection is through an
199 * unmodifiable view, the view can be considered effectively immutable.
200 *
201 * <p>This interface is a member of the
202 * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
203 * Java Collections Framework</a>.
204 *
205 * @implSpec
206 * The default method implementations (inherited or otherwise) do not apply any
207 * synchronization protocol. If a {@code Collection} implementation has a
208 * specific synchronization protocol, then it must override default
209 * implementations to apply that protocol.
210 *
211 * @param <E> the type of elements in this collection
212 *
213 * @author Josh Bloch
214 * @author Neal Gafter
215 * @see Set
216 * @see List
217 * @see Map
218 * @see SortedSet
219 * @see SortedMap
220 * @see HashSet
221 * @see TreeSet
222 * @see ArrayList
223 * @see LinkedList
224 * @see Vector
225 * @see Collections
226 * @see Arrays
227 * @see AbstractCollection
228 * @since 1.2
229 */
230
231 public interface Collection<E> extends Iterable<E> {
232 // Query Operations
233
234 /**
235 * Returns the number of elements in this collection. If this collection
236 * contains more than {@code Integer.MAX_VALUE} elements, returns
237 * {@code Integer.MAX_VALUE}.
238 *
239 * @return the number of elements in this collection
240 */
241 int size();
242
243 /**
244 * Returns {@code true} if this collection contains no elements.
245 *
246 * @return {@code true} if this collection contains no elements
247 */
248 boolean isEmpty();
249
250 /**
251 * Returns {@code true} if this collection contains the specified element.
252 * More formally, returns {@code true} if and only if this collection
253 * contains at least one element {@code e} such that
254 * {@code Objects.equals(o, e)}.
255 *
256 * @param o element whose presence in this collection is to be tested
257 * @return {@code true} if this collection contains the specified
258 * element
259 * @throws ClassCastException if the type of the specified element
260 * is incompatible with this collection
261 * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
262 * @throws NullPointerException if the specified element is null and this
263 * collection does not permit null elements
264 * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
265 */
266 boolean contains(Object o);
267
268 /**
269 * Returns an iterator over the elements in this collection. There are no
270 * guarantees concerning the order in which the elements are returned
271 * (unless this collection is an instance of some class that provides a
272 * guarantee).
273 *
274 * @return an {@code Iterator} over the elements in this collection
275 */
276 Iterator<E> iterator();
277
278 /**
279 * Returns an array containing all of the elements in this collection.
280 * If this collection makes any guarantees as to what order its elements
281 * are returned by its iterator, this method must return the elements in
282 * the same order. The returned array's {@linkplain Class#getComponentType
283 * runtime component type} is {@code Object}.
284 *
285 * <p>The returned array will be "safe" in that no references to it are
286 * maintained by this collection. (In other words, this method must
287 * allocate a new array even if this collection is backed by an array).
288 * The caller is thus free to modify the returned array.
289 *
290 * @apiNote
291 * This method acts as a bridge between array-based and collection-based APIs.
292 * It returns an array whose runtime type is {@code Object[]}.
293 * Use {@link #toArray(Object[]) toArray(T[])} to reuse an existing
294 * array, or use {@link #toArray(IntFunction)} to control the runtime type
295 * of the array.
296 *
297 * @return an array, whose {@linkplain Class#getComponentType runtime component
298 * type} is {@code Object}, containing all of the elements in this collection
299 */
300 Object[] toArray();
301
302 /**
303 * Returns an array containing all of the elements in this collection;
304 * the runtime type of the returned array is that of the specified array.
305 * If the collection fits in the specified array, it is returned therein.
306 * Otherwise, a new array is allocated with the runtime type of the
307 * specified array and the size of this collection.
308 *
309 * <p>If this collection fits in the specified array with room to spare
310 * (i.e., the array has more elements than this collection), the element
311 * in the array immediately following the end of the collection is set to
312 * {@code null}. (This is useful in determining the length of this
313 * collection <i>only</i> if the caller knows that this collection does
314 * not contain any {@code null} elements.)
315 *
316 * <p>If this collection makes any guarantees as to what order its elements
317 * are returned by its iterator, this method must return the elements in
318 * the same order.
319 *
320 * @apiNote
321 * This method acts as a bridge between array-based and collection-based APIs.
322 * It allows an existing array to be reused under certain circumstances.
323 * Use {@link #toArray()} to create an array whose runtime type is {@code Object[]},
324 * or use {@link #toArray(IntFunction)} to control the runtime type of
325 * the array.
326 *
327 * <p>Suppose {@code x} is a collection known to contain only strings.
328 * The following code can be used to dump the collection into a previously
329 * allocated {@code String} array:
330 *
331 * <pre>
332 * String[] y = new String[SIZE];
333 * ...
334 * y = x.toArray(y);</pre>
335 *
336 * <p>The return value is reassigned to the variable {@code y}, because a
337 * new array will be allocated and returned if the collection {@code x} has
338 * too many elements to fit into the existing array {@code y}.
339 *
340 * <p>Note that {@code toArray(new Object[0])} is identical in function to
341 * {@code toArray()}.
342 *
343 * @param <T> the component type of the array to contain the collection
344 * @param a the array into which the elements of this collection are to be
345 * stored, if it is big enough; otherwise, a new array of the same
346 * runtime type is allocated for this purpose.
347 * @return an array containing all of the elements in this collection
348 * @throws ArrayStoreException if the runtime type of any element in this
349 * collection is not assignable to the {@linkplain Class#getComponentType
350 * runtime component type} of the specified array
351 * @throws NullPointerException if the specified array is null
352 */
353 <T> T[] toArray(T[] a);
354
355 /**
356 * Returns an array containing all of the elements in this collection,
357 * using the provided {@code generator} function to allocate the returned array.
358 *
359 * <p>If this collection makes any guarantees as to what order its elements
360 * are returned by its iterator, this method must return the elements in
361 * the same order.
362 *
363 * @apiNote
364 * This method acts as a bridge between array-based and collection-based APIs.
365 * It allows creation of an array of a particular runtime type. Use
366 * {@link #toArray()} to create an array whose runtime type is {@code Object[]},
367 * or use {@link #toArray(Object[]) toArray(T[])} to reuse an existing array.
368 *
369 * <p>Suppose {@code x} is a collection known to contain only strings.
370 * The following code can be used to dump the collection into a newly
371 * allocated array of {@code String}:
372 *
373 * <pre>
374 * String[] y = x.toArray(String[]::new);</pre>
375 *
376 * @implSpec
377 * The default implementation calls the generator function with zero
378 * and then passes the resulting array to {@link #toArray(Object[]) toArray(T[])}.
379 *
380 * @param <T> the component type of the array to contain the collection
381 * @param generator a function which produces a new array of the desired
382 * type and the provided length
383 * @return an array containing all of the elements in this collection
384 * @throws ArrayStoreException if the runtime type of any element in this
385 * collection is not assignable to the {@linkplain Class#getComponentType
386 * runtime component type} of the generated array
387 * @throws NullPointerException if the generator function is null
388 * @since 11
389 */
390 default <T> T[] toArray(IntFunction<T[]> generator) {
391 return toArray(generator.apply(0));
392 }
393
394 // Modification Operations
395
396 /**
397 * Ensures that this collection contains the specified element (optional
398 * operation). Returns {@code true} if this collection changed as a
399 * result of the call. (Returns {@code false} if this collection does
400 * not permit duplicates and already contains the specified element.)<p>
401 *
402 * Collections that support this operation may place limitations on what
403 * elements may be added to this collection. In particular, some
404 * collections will refuse to add {@code null} elements, and others will
405 * impose restrictions on the type of elements that may be added.
406 * Collection classes should clearly specify in their documentation any
407 * restrictions on what elements may be added.<p>
408 *
409 * If a collection refuses to add a particular element for any reason
410 * other than that it already contains the element, it <i>must</i> throw
411 * an exception (rather than returning {@code false}). This preserves
412 * the invariant that a collection always contains the specified element
413 * after this call returns.
414 *
415 * @param e element whose presence in this collection is to be ensured
416 * @return {@code true} if this collection changed as a result of the
417 * call
418 * @throws UnsupportedOperationException if the {@code add} operation
419 * is not supported by this collection
420 * @throws ClassCastException if the class of the specified element
421 * prevents it from being added to this collection
422 * @throws NullPointerException if the specified element is null and this
423 * collection does not permit null elements
424 * @throws IllegalArgumentException if some property of the element
425 * prevents it from being added to this collection
426 * @throws IllegalStateException if the element cannot be added at this
427 * time due to insertion restrictions
428 */
429 boolean add(E e);
430
431 /**
432 * Removes a single instance of the specified element from this
433 * collection, if it is present (optional operation). More formally,
434 * removes an element {@code e} such that
435 * {@code Objects.equals(o, e)}, if
436 * this collection contains one or more such elements. Returns
437 * {@code true} if this collection contained the specified element (or
438 * equivalently, if this collection changed as a result of the call).
439 *
440 * @param o element to be removed from this collection, if present
441 * @return {@code true} if an element was removed as a result of this call
442 * @throws ClassCastException if the type of the specified element
443 * is incompatible with this collection
444 * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
445 * @throws NullPointerException if the specified element is null and this
446 * collection does not permit null elements
447 * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
448 * @throws UnsupportedOperationException if the {@code remove} operation
449 * is not supported by this collection
450 */
451 boolean remove(Object o);
452
453
454 // Bulk Operations
455
456 /**
457 * Returns {@code true} if this collection contains all of the elements
458 * in the specified collection. This operation uses the membership semantics
459 * of this collection.
460 *
461 * @implNote
462 * Most implementations will call this collection's {@code contains}
463 * method repeatedly. This may result in performance problems if the
464 * {@code contains} method has linear or worse performance.
465 *
466 * @param c collection to be checked for containment in this collection
467 * @return {@code true} if this collection contains all of the elements
468 * in the specified collection
469 * @throws ClassCastException if the types of one or more elements
470 * in the specified collection are incompatible with this
471 * collection
472 * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
473 * @throws NullPointerException if the specified collection contains one
474 * or more null elements and this collection does not permit null
475 * elements
476 * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>),
477 * or if the specified collection is null.
478 * @see #contains(Object)
479 */
480 boolean containsAll(Collection<?> c);
481
482 /**
483 * Adds all of the elements in the specified collection to this collection
484 * (optional operation). The behavior of this operation is undefined if
485 * the specified collection is modified while the operation is in progress.
486 * (This implies that the behavior of this call is undefined if the
487 * specified collection is this collection, and this collection is
488 * nonempty.)
489 *
490 * @param c collection containing elements to be added to this collection
491 * @return {@code true} if this collection changed as a result of the call
492 * @throws UnsupportedOperationException if the {@code addAll} operation
493 * is not supported by this collection
494 * @throws ClassCastException if the class of an element of the specified
495 * collection prevents it from being added to this collection
496 * @throws NullPointerException if the specified collection contains a
497 * null element and this collection does not permit null elements,
498 * or if the specified collection is null
499 * @throws IllegalArgumentException if some property of an element of the
500 * specified collection prevents it from being added to this
501 * collection
502 * @throws IllegalStateException if not all the elements can be added at
503 * this time due to insertion restrictions
504 * @see #add(Object)
505 */
506 boolean addAll(Collection<? extends E> c);
507
508 /**
509 * Removes all of this collection's elements that are also contained in the
510 * specified collection (optional operation). After this call returns,
511 * this collection will contain no elements in common with the specified
512 * collection. This operation uses the membership semantics of the specified
513 * collection.
514 *
515 * @implNote
516 * Most implementations will call the specified collection's {@code contains}
517 * method repeatedly. This may result in performance problems if the specified
518 * collection's {@code contains} operation has linear or worse performance.
519 *
520 * @param c collection containing elements to be removed from this collection
521 * @return {@code true} if this collection changed as a result of the
522 * call
523 * @throws UnsupportedOperationException if the {@code removeAll} method
524 * is not supported by this collection
525 * @throws ClassCastException if the types of one or more elements
526 * in this collection are incompatible with the specified
527 * collection
528 * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
529 * @throws NullPointerException if this collection contains one or more
530 * null elements and the specified collection does not support
531 * null elements
532 * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>),
533 * or if the specified collection is null
534 * @see #remove(Object)
535 * @see #contains(Object)
536 */
537 boolean removeAll(Collection<?> c);
538
539 /**
540 * Removes all of the elements of this collection that satisfy the given
541 * predicate. Errors or runtime exceptions thrown during iteration or by
542 * the predicate are relayed to the caller.
543 *
544 * @implSpec
545 * The default implementation traverses all elements of the collection using
546 * its {@link #iterator}. Each matching element is removed using
547 * {@link Iterator#remove()}. If the collection's iterator does not
548 * support removal then an {@code UnsupportedOperationException} will be
549 * thrown on the first matching element.
550 *
551 * @param filter a predicate which returns {@code true} for elements to be
552 * removed
553 * @return {@code true} if any elements were removed
554 * @throws NullPointerException if the specified filter is null
555 * @throws UnsupportedOperationException if elements cannot be removed
556 * from this collection. Implementations may throw this exception if a
557 * matching element cannot be removed or if, in general, removal is not
558 * supported.
559 * @since 1.8
560 */
561 default boolean removeIf(Predicate<? super E> filter) {
562 Objects.requireNonNull(filter);
563 boolean removed = false;
564 final Iterator<E> each = iterator();
565 while (each.hasNext()) {
566 if (filter.test(each.next())) {
567 each.remove();
568 removed = true;
569 }
570 }
571 return removed;
572 }
573
574 /**
575 * Retains only the elements in this collection that are contained in the
576 * specified collection (optional operation). In other words, removes from
577 * this collection all of its elements that are not contained in the
578 * specified collection. This operation uses the membership semantics of the
579 * specified collection.
580 *
581 * @implNote
582 * Most implementations will call the specified collection's {@code contains}
583 * method repeatedly. This may result in performance problems if the specified
584 * collection's {@code contains} operation has linear or worse performance.
585 *
586 * @param c collection containing elements to be retained in this collection
587 * @return {@code true} if this collection changed as a result of the call
588 * @throws UnsupportedOperationException if the {@code retainAll} operation
589 * is not supported by this collection
590 * @throws ClassCastException if the types of one or more elements
591 * in this collection are incompatible with the specified
592 * collection
593 * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
594 * @throws NullPointerException if this collection contains one or more
595 * null elements and the specified collection does not permit null
596 * elements
597 * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>),
598 * or if the specified collection is null
599 * @see #remove(Object)
600 * @see #contains(Object)
601 */
602 boolean retainAll(Collection<?> c);
603
604 /**
605 * Removes all of the elements from this collection (optional operation).
606 * The collection will be empty after this method returns.
607 *
608 * @throws UnsupportedOperationException if the {@code clear} operation
609 * is not supported by this collection
610 */
611 void clear();
612
613
614 // Comparison and hashing
615
616 /**
617 * Compares the specified object with this collection for equality. <p>
618 *
619 * While the {@code Collection} interface adds no stipulations to the
620 * general contract for the {@code Object.equals}, programmers who
621 * implement the {@code Collection} interface "directly" (in other words,
622 * create a class that is a {@code Collection} but is not a {@code Set}
623 * or a {@code List}) must exercise care if they choose to override the
624 * {@code Object.equals}. It is not necessary to do so, and the simplest
625 * course of action is to rely on {@code Object}'s implementation, but
626 * the implementor may wish to implement a "value comparison" in place of
627 * the default "reference comparison." (The {@code List} and
628 * {@code Set} interfaces mandate such value comparisons.)<p>
629 *
630 * The general contract for the {@code Object.equals} method states that
631 * equals must be symmetric (in other words, {@code a.equals(b)} if and
632 * only if {@code b.equals(a)}). The contracts for {@code List.equals}
633 * and {@code Set.equals} state that lists are only equal to other lists,
634 * and sets to other sets. Thus, a custom {@code equals} method for a
635 * collection class that implements neither the {@code List} nor
636 * {@code Set} interface must return {@code false} when this collection
637 * is compared to any list or set. (By the same logic, it is not possible
638 * to write a class that correctly implements both the {@code Set} and
639 * {@code List} interfaces.)
640 *
641 * @param o object to be compared for equality with this collection
642 * @return {@code true} if the specified object is equal to this
643 * collection
644 *
645 * @see Object#equals(Object)
646 * @see Set#equals(Object)
647 * @see List#equals(Object)
648 */
649 boolean equals(Object o);
650
651 /**
652 * Returns the hash code value for this collection. While the
653 * {@code Collection} interface adds no stipulations to the general
654 * contract for the {@code Object.hashCode} method, programmers should
655 * take note that any class that overrides the {@code Object.equals}
656 * method must also override the {@code Object.hashCode} method in order
657 * to satisfy the general contract for the {@code Object.hashCode} method.
658 * In particular, {@code c1.equals(c2)} implies that
659 * {@code c1.hashCode()==c2.hashCode()}.
660 *
661 * @return the hash code value for this collection
662 *
663 * @see Object#hashCode()
664 * @see Object#equals(Object)
665 */
666 int hashCode();
667
668 /**
669 * Creates a {@link Spliterator} over the elements in this collection.
670 *
671 * Implementations should document characteristic values reported by the
672 * spliterator. Such characteristic values are not required to be reported
673 * if the spliterator reports {@link Spliterator#SIZED} and this collection
674 * contains no elements.
675 *
676 * <p>The default implementation should be overridden by subclasses that
677 * can return a more efficient spliterator. In order to
678 * preserve expected laziness behavior for the {@link #stream()} and
679 * {@link #parallelStream()} methods, spliterators should either have the
680 * characteristic of {@code IMMUTABLE} or {@code CONCURRENT}, or be
681 * <em><a href="Spliterator.html#binding">late-binding</a></em>.
682 * If none of these is practical, the overriding class should describe the
683 * spliterator's documented policy of binding and structural interference,
684 * and should override the {@link #stream()} and {@link #parallelStream()}
685 * methods to create streams using a {@code Supplier} of the spliterator,
686 * as in:
687 * <pre>{@code
688 * Stream<E> s = StreamSupport.stream(() -> spliterator(), spliteratorCharacteristics)
689 * }</pre>
690 * <p>These requirements ensure that streams produced by the
691 * {@link #stream()} and {@link #parallelStream()} methods will reflect the
692 * contents of the collection as of initiation of the terminal stream
693 * operation.
694 *
695 * @implSpec
696 * The default implementation creates a
697 * <em><a href="Spliterator.html#binding">late-binding</a></em> spliterator
698 * from the collection's {@code Iterator}. The spliterator inherits the
699 * <em>fail-fast</em> properties of the collection's iterator.
700 * <p>
701 * The created {@code Spliterator} reports {@link Spliterator#SIZED}.
702 *
703 * @implNote
704 * The created {@code Spliterator} additionally reports
705 * {@link Spliterator#SUBSIZED}.
706 *
707 * <p>If a spliterator covers no elements then the reporting of additional
708 * characteristic values, beyond that of {@code SIZED} and {@code SUBSIZED},
709 * does not aid clients to control, specialize or simplify computation.
710 * However, this does enable shared use of an immutable and empty
711 * spliterator instance (see {@link Spliterators#emptySpliterator()}) for
712 * empty collections, and enables clients to determine if such a spliterator
713 * covers no elements.
714 *
715 * @return a {@code Spliterator} over the elements in this collection
716 * @since 1.8
717 */
718 @Override
719 default Spliterator<E> spliterator() {
720 return Spliterators.spliterator(this, 0);
721 }
722
723 /**
724 * Returns a sequential {@code Stream} with this collection as its source.
725 *
726 * <p>This method should be overridden when the {@link #spliterator()}
727 * method cannot return a spliterator that is {@code IMMUTABLE},
728 * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()}
729 * for details.)
730 *
731 * @implSpec
732 * The default implementation creates a sequential {@code Stream} from the
733 * collection's {@code Spliterator}.
734 *
735 * @return a sequential {@code Stream} over the elements in this collection
736 * @since 1.8
737 */
738 default Stream<E> stream() {
739 return StreamSupport.stream(spliterator(), false);
740 }
741
742 /**
743 * Returns a possibly parallel {@code Stream} with this collection as its
744 * source. It is allowable for this method to return a sequential stream.
745 *
746 * <p>This method should be overridden when the {@link #spliterator()}
747 * method cannot return a spliterator that is {@code IMMUTABLE},
748 * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()}
749 * for details.)
750 *
751 * @implSpec
752 * The default implementation creates a parallel {@code Stream} from the
753 * collection's {@code Spliterator}.
754 *
755 * @return a possibly parallel {@code Stream} over the elements in this
756 * collection
757 * @since 1.8
758 */
759 default Stream<E> parallelStream() {
760 return StreamSupport.stream(spliterator(), true);
761 }
762 }