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
6 * under the terms of the GNU General Public License version 2 only, as
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 *
17 * You should have received a copy of the GNU General Public License version
18 * 2 along with this work; if not, write to the Free Software Foundation,
19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22 * or visit www.oracle.com if you need additional information or have any
23 * questions.
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 * <p>This method acts as bridge between array-based and collection-based
281 * APIs.
282 *
283 * @return an array, whose {@linkplain Class#getComponentType runtime component
284 * type} is {@code Object}, containing all of the elements in this collection
285 */
286 Object[] toArray();
287
288 /**
289 * Returns an array containing all of the elements in this collection;
290 * the runtime type of the returned array is that of the specified array.
291 * If the collection fits in the specified array, it is returned therein.
292 * Otherwise, a new array is allocated with the runtime type of the
293 * specified array and the size of this collection.
294 *
295 * <p>If this collection fits in the specified array with room to spare
296 * (i.e., the array has more elements than this collection), the element
297 * in the array immediately following the end of the collection is set to
298 * {@code null}. (This is useful in determining the length of this
299 * collection <i>only</i> if the caller knows that this collection does
300 * not contain any {@code null} elements.)
301 *
302 * <p>If this collection makes any guarantees as to what order its elements
303 * are returned by its iterator, this method must return the elements in
304 * the same order.
305 *
306 * <p>Like the {@link #toArray()} method, this method acts as bridge between
307 * array-based and collection-based APIs. Further, this method allows
308 * precise control over the runtime type of the output array, and may,
309 * under certain circumstances, be used to save allocation costs.
310 *
311 * <p>Suppose {@code x} is a collection known to contain only strings.
312 * The following code can be used to dump the collection into a newly
313 * allocated array of {@code String}:
314 *
315 * <pre>
316 * String[] y = x.toArray(new String[0]);</pre>
317 *
318 * Note that {@code toArray(new Object[0])} is identical in function to
319 * {@code toArray()}.
320 *
321 * @param <T> the component type of the array to contain the collection
322 * @param a the array into which the elements of this collection are to be
323 * stored, if it is big enough; otherwise, a new array of the same
324 * runtime type is allocated for this purpose.
325 * @return an array containing all of the elements in this collection
326 * @throws ArrayStoreException if the runtime type of any element in this
327 * collection is not assignable to the {@linkplain Class#getComponentType
328 * runtime component type} of the specified array
329 * @throws NullPointerException if the specified array is null
330 */
331 <T> T[] toArray(T[] a);
332
333 /**
334 * Returns an array containing all of the elements in this collection,
335 * using the provided {@code generator} function to allocate the returned array.
336 *
337 * <p>If this collection makes any guarantees as to what order its elements
338 * are returned by its iterator, this method must return the elements in
339 * the same order.
340 *
341 * <p>Like the {@link #toArray()} method, this method acts as bridge between
342 * array-based and collection-based APIs. Further, this method allows
343 * precise control over the runtime type of the output array.
344 *
345 * <p>Suppose {@code x} is a collection known to contain only strings.
346 * The following code can be used to dump the collection into a newly
347 * allocated array of {@code String}:
348 *
349 * <pre>
350 * String[] y = x.toArray(String[]::new);</pre>
351 *
352 * @implSpec
353 * The default implementation calls the generator function with zero
354 * and then passes the resulting array to {@link #toArray(T[])}.
355 *
356 * @param <T> the component type of the array to contain the collection
357 * @param generator a function which produces a new array of the desired
358 * type and the provided length
359 * @return an array containing all of the elements in this collection
360 * @throws ArrayStoreException if the runtime type of any element in this
361 * collection is not assignable to the {@linkplain Class#getComponentType
362 * runtime component type} of array returned by the generator function
363 * @throws NullPointerException if the generator function is null
364 * @since 10
365 */
366 default <T> T[] toArray(IntFunction<T[]> generator) {
367 return toArray(generator.apply(0));
368 }
369
370 // Modification Operations
371
372 /**
373 * Ensures that this collection contains the specified element (optional
374 * operation). Returns {@code true} if this collection changed as a
375 * result of the call. (Returns {@code false} if this collection does
376 * not permit duplicates and already contains the specified element.)<p>
377 *
378 * Collections that support this operation may place limitations on what
379 * elements may be added to this collection. In particular, some
380 * collections will refuse to add {@code null} elements, and others will
381 * impose restrictions on the type of elements that may be added.
382 * Collection classes should clearly specify in their documentation any
383 * restrictions on what elements may be added.<p>
384 *
385 * If a collection refuses to add a particular element for any reason
386 * other than that it already contains the element, it <i>must</i> throw
387 * an exception (rather than returning {@code false}). This preserves
388 * the invariant that a collection always contains the specified element
389 * after this call returns.
390 *
391 * @param e element whose presence in this collection is to be ensured
392 * @return {@code true} if this collection changed as a result of the
393 * call
394 * @throws UnsupportedOperationException if the {@code add} operation
395 * is not supported by this collection
396 * @throws ClassCastException if the class of the specified element
397 * prevents it from being added to this collection
398 * @throws NullPointerException if the specified element is null and this
399 * collection does not permit null elements
400 * @throws IllegalArgumentException if some property of the element
401 * prevents it from being added to this collection
402 * @throws IllegalStateException if the element cannot be added at this
403 * time due to insertion restrictions
404 */
405 boolean add(E e);
406
407 /**
408 * Removes a single instance of the specified element from this
409 * collection, if it is present (optional operation). More formally,
410 * removes an element {@code e} such that
411 * {@code Objects.equals(o, e)}, if
412 * this collection contains one or more such elements. Returns
413 * {@code true} if this collection contained the specified element (or
414 * equivalently, if this collection changed as a result of the call).
415 *
416 * @param o element to be removed from this collection, if present
417 * @return {@code true} if an element was removed as a result of this call
418 * @throws ClassCastException if the type of the specified element
419 * is incompatible with this collection
420 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
421 * @throws NullPointerException if the specified element is null and this
422 * collection does not permit null elements
423 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
424 * @throws UnsupportedOperationException if the {@code remove} operation
425 * is not supported by this collection
426 */
427 boolean remove(Object o);
428
429
430 // Bulk Operations
431
432 /**
433 * Returns {@code true} if this collection contains all of the elements
434 * in the specified collection.
435 *
436 * @param c collection to be checked for containment in this collection
437 * @return {@code true} if this collection contains all of the elements
438 * in the specified collection
439 * @throws ClassCastException if the types of one or more elements
440 * in the specified collection are incompatible with this
441 * collection
442 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
443 * @throws NullPointerException if the specified collection contains one
444 * or more null elements and this collection does not permit null
445 * elements
446 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>),
447 * or if the specified collection is null.
448 * @see #contains(Object)
449 */
450 boolean containsAll(Collection<?> c);
451
452 /**
453 * Adds all of the elements in the specified collection to this collection
454 * (optional operation). The behavior of this operation is undefined if
455 * the specified collection is modified while the operation is in progress.
456 * (This implies that the behavior of this call is undefined if the
457 * specified collection is this collection, and this collection is
458 * nonempty.)
459 *
460 * @param c collection containing elements to be added to this collection
461 * @return {@code true} if this collection changed as a result of the call
462 * @throws UnsupportedOperationException if the {@code addAll} operation
463 * is not supported by this collection
464 * @throws ClassCastException if the class of an element of the specified
465 * collection prevents it from being added to this collection
466 * @throws NullPointerException if the specified collection contains a
467 * null element and this collection does not permit null elements,
468 * or if the specified collection is null
469 * @throws IllegalArgumentException if some property of an element of the
470 * specified collection prevents it from being added to this
471 * collection
472 * @throws IllegalStateException if not all the elements can be added at
473 * this time due to insertion restrictions
474 * @see #add(Object)
475 */
476 boolean addAll(Collection<? extends E> c);
477
478 /**
479 * Removes all of this collection's elements that are also contained in the
480 * specified collection (optional operation). After this call returns,
481 * this collection will contain no elements in common with the specified
482 * collection.
483 *
484 * @param c collection containing elements to be removed from this collection
485 * @return {@code true} if this collection changed as a result of the
486 * call
487 * @throws UnsupportedOperationException if the {@code removeAll} method
488 * is not supported by this collection
489 * @throws ClassCastException if the types of one or more elements
490 * in this collection are incompatible with the specified
491 * collection
492 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
493 * @throws NullPointerException if this collection contains one or more
494 * null elements and the specified collection does not support
495 * null elements
496 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>),
497 * or if the specified collection is null
498 * @see #remove(Object)
499 * @see #contains(Object)
500 */
501 boolean removeAll(Collection<?> c);
502
503 /**
504 * Removes all of the elements of this collection that satisfy the given
505 * predicate. Errors or runtime exceptions thrown during iteration or by
506 * the predicate are relayed to the caller.
507 *
508 * @implSpec
509 * The default implementation traverses all elements of the collection using
510 * its {@link #iterator}. Each matching element is removed using
511 * {@link Iterator#remove()}. If the collection's iterator does not
512 * support removal then an {@code UnsupportedOperationException} will be
513 * thrown on the first matching element.
514 *
515 * @param filter a predicate which returns {@code true} for elements to be
516 * removed
517 * @return {@code true} if any elements were removed
518 * @throws NullPointerException if the specified filter is null
519 * @throws UnsupportedOperationException if elements cannot be removed
520 * from this collection. Implementations may throw this exception if a
521 * matching element cannot be removed or if, in general, removal is not
522 * supported.
523 * @since 1.8
524 */
525 default boolean removeIf(Predicate<? super E> filter) {
526 Objects.requireNonNull(filter);
527 boolean removed = false;
528 final Iterator<E> each = iterator();
529 while (each.hasNext()) {
530 if (filter.test(each.next())) {
531 each.remove();
532 removed = true;
533 }
534 }
535 return removed;
536 }
537
538 /**
539 * Retains only the elements in this collection that are contained in the
540 * specified collection (optional operation). In other words, removes from
541 * this collection all of its elements that are not contained in the
542 * specified collection.
543 *
544 * @param c collection containing elements to be retained in this collection
545 * @return {@code true} if this collection changed as a result of the call
546 * @throws UnsupportedOperationException if the {@code retainAll} operation
547 * is not supported by this collection
548 * @throws ClassCastException if the types of one or more elements
549 * in this collection are incompatible with the specified
550 * collection
551 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
552 * @throws NullPointerException if this collection contains one or more
553 * null elements and the specified collection does not permit null
554 * elements
555 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>),
556 * or if the specified collection is null
557 * @see #remove(Object)
558 * @see #contains(Object)
559 */
560 boolean retainAll(Collection<?> c);
561
562 /**
563 * Removes all of the elements from this collection (optional operation).
564 * The collection will be empty after this method returns.
565 *
566 * @throws UnsupportedOperationException if the {@code clear} operation
567 * is not supported by this collection
568 */
569 void clear();
570
571
572 // Comparison and hashing
573
574 /**
575 * Compares the specified object with this collection for equality. <p>
576 *
577 * While the {@code Collection} interface adds no stipulations to the
578 * general contract for the {@code Object.equals}, programmers who
579 * implement the {@code Collection} interface "directly" (in other words,
580 * create a class that is a {@code Collection} but is not a {@code Set}
581 * or a {@code List}) must exercise care if they choose to override the
582 * {@code Object.equals}. It is not necessary to do so, and the simplest
583 * course of action is to rely on {@code Object}'s implementation, but
584 * the implementor may wish to implement a "value comparison" in place of
585 * the default "reference comparison." (The {@code List} and
586 * {@code Set} interfaces mandate such value comparisons.)<p>
587 *
588 * The general contract for the {@code Object.equals} method states that
589 * equals must be symmetric (in other words, {@code a.equals(b)} if and
590 * only if {@code b.equals(a)}). The contracts for {@code List.equals}
591 * and {@code Set.equals} state that lists are only equal to other lists,
592 * and sets to other sets. Thus, a custom {@code equals} method for a
593 * collection class that implements neither the {@code List} nor
594 * {@code Set} interface must return {@code false} when this collection
595 * is compared to any list or set. (By the same logic, it is not possible
596 * to write a class that correctly implements both the {@code Set} and
597 * {@code List} interfaces.)
598 *
599 * @param o object to be compared for equality with this collection
600 * @return {@code true} if the specified object is equal to this
601 * collection
602 *
603 * @see Object#equals(Object)
604 * @see Set#equals(Object)
605 * @see List#equals(Object)
606 */
607 boolean equals(Object o);
608
609 /**
610 * Returns the hash code value for this collection. While the
611 * {@code Collection} interface adds no stipulations to the general
612 * contract for the {@code Object.hashCode} method, programmers should
613 * take note that any class that overrides the {@code Object.equals}
614 * method must also override the {@code Object.hashCode} method in order
615 * to satisfy the general contract for the {@code Object.hashCode} method.
616 * In particular, {@code c1.equals(c2)} implies that
617 * {@code c1.hashCode()==c2.hashCode()}.
618 *
619 * @return the hash code value for this collection
620 *
621 * @see Object#hashCode()
622 * @see Object#equals(Object)
623 */
624 int hashCode();
625
626 /**
627 * Creates a {@link Spliterator} over the elements in this collection.
628 *
629 * Implementations should document characteristic values reported by the
630 * spliterator. Such characteristic values are not required to be reported
631 * if the spliterator reports {@link Spliterator#SIZED} and this collection
632 * contains no elements.
633 *
634 * <p>The default implementation should be overridden by subclasses that
635 * can return a more efficient spliterator. In order to
636 * preserve expected laziness behavior for the {@link #stream()} and
637 * {@link #parallelStream()} methods, spliterators should either have the
638 * characteristic of {@code IMMUTABLE} or {@code CONCURRENT}, or be
639 * <em><a href="Spliterator.html#binding">late-binding</a></em>.
640 * If none of these is practical, the overriding class should describe the
641 * spliterator's documented policy of binding and structural interference,
642 * and should override the {@link #stream()} and {@link #parallelStream()}
643 * methods to create streams using a {@code Supplier} of the spliterator,
644 * as in:
645 * <pre>{@code
646 * Stream<E> s = StreamSupport.stream(() -> spliterator(), spliteratorCharacteristics)
647 * }</pre>
648 * <p>These requirements ensure that streams produced by the
649 * {@link #stream()} and {@link #parallelStream()} methods will reflect the
650 * contents of the collection as of initiation of the terminal stream
651 * operation.
652 *
653 * @implSpec
654 * The default implementation creates a
655 * <em><a href="Spliterator.html#binding">late-binding</a></em> spliterator
656 * from the collection's {@code Iterator}. The spliterator inherits the
657 * <em>fail-fast</em> properties of the collection's iterator.
658 * <p>
659 * The created {@code Spliterator} reports {@link Spliterator#SIZED}.
660 *
661 * @implNote
662 * The created {@code Spliterator} additionally reports
663 * {@link Spliterator#SUBSIZED}.
664 *
665 * <p>If a spliterator covers no elements then the reporting of additional
666 * characteristic values, beyond that of {@code SIZED} and {@code SUBSIZED},
667 * does not aid clients to control, specialize or simplify computation.
668 * However, this does enable shared use of an immutable and empty
669 * spliterator instance (see {@link Spliterators#emptySpliterator()}) for
670 * empty collections, and enables clients to determine if such a spliterator
671 * covers no elements.
672 *
673 * @return a {@code Spliterator} over the elements in this collection
674 * @since 1.8
675 */
676 @Override
677 default Spliterator<E> spliterator() {
678 return Spliterators.spliterator(this, 0);
679 }
680
681 /**
682 * Returns a sequential {@code Stream} with this collection as its source.
683 *
684 * <p>This method should be overridden when the {@link #spliterator()}
685 * method cannot return a spliterator that is {@code IMMUTABLE},
686 * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()}
687 * for details.)
688 *
689 * @implSpec
690 * The default implementation creates a sequential {@code Stream} from the
691 * collection's {@code Spliterator}.
692 *
693 * @return a sequential {@code Stream} over the elements in this collection
694 * @since 1.8
695 */
696 default Stream<E> stream() {
697 return StreamSupport.stream(spliterator(), false);
698 }
699
700 /**
701 * Returns a possibly parallel {@code Stream} with this collection as its
702 * source. It is allowable for this method to return a sequential stream.
703 *
704 * <p>This method should be overridden when the {@link #spliterator()}
705 * method cannot return a spliterator that is {@code IMMUTABLE},
706 * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()}
707 * for details.)
708 *
709 * @implSpec
710 * The default implementation creates a parallel {@code Stream} from the
711 * collection's {@code Spliterator}.
712 *
713 * @return a possibly parallel {@code Stream} over the elements in this
714 * collection
715 * @since 1.8
716 */
717 default Stream<E> parallelStream() {
718 return StreamSupport.stream(spliterator(), true);
719 }
720 }