1 /*
2 * Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
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24 */
25
26 package java.util;
27
28 import java.util.function.Predicate;
29
30 /**
31 * The root interface in the <i>collection hierarchy</i>. A collection
32 * represents a group of objects, known as its <i>elements</i>. Some
33 * collections allow duplicate elements and others do not. Some are ordered
34 * and others unordered. The JDK does not provide any <i>direct</i>
35 * implementations of this interface: it provides implementations of more
36 * specific subinterfaces like <tt>Set</tt> and <tt>List</tt>. This interface
37 * is typically used to pass collections around and manipulate them where
38 * maximum generality is desired.
39 *
40 * <p><i>Bags</i> or <i>multisets</i> (unordered collections that may contain
41 * duplicate elements) should implement this interface directly.
42 *
43 * <p>All general-purpose <tt>Collection</tt> implementation classes (which
44 * typically implement <tt>Collection</tt> indirectly through one of its
45 * subinterfaces) should provide two "standard" constructors: a void (no
46 * arguments) constructor, which creates an empty collection, and a
47 * constructor with a single argument of type <tt>Collection</tt>, which
48 * creates a new collection with the same elements as its argument. In
49 * effect, the latter constructor allows the user to copy any collection,
50 * producing an equivalent collection of the desired implementation type.
51 * There is no way to enforce this convention (as interfaces cannot contain
52 * constructors) but all of the general-purpose <tt>Collection</tt>
53 * implementations in the Java platform libraries comply.
54 *
55 * <p>The "destructive" methods contained in this interface, that is, the
56 * methods that modify the collection on which they operate, are specified to
57 * throw <tt>UnsupportedOperationException</tt> if this collection does not
58 * support the operation. If this is the case, these methods may, but are not
59 * required to, throw an <tt>UnsupportedOperationException</tt> if the
60 * invocation would have no effect on the collection. For example, invoking
61 * the {@link #addAll(Collection)} method on an unmodifiable collection may,
62 * but is not required to, throw the exception if the collection to be added
63 * is empty.
64 *
65 * <p><a name="optional-restrictions"/>
66 * Some collection implementations have restrictions on the elements that
67 * they may contain. For example, some implementations prohibit null elements,
68 * and some have restrictions on the types of their elements. Attempting to
69 * add an ineligible element throws an unchecked exception, typically
70 * <tt>NullPointerException</tt> or <tt>ClassCastException</tt>. Attempting
71 * to query the presence of an ineligible element may throw an exception,
72 * or it may simply return false; some implementations will exhibit the former
73 * behavior and some will exhibit the latter. More generally, attempting an
74 * operation on an ineligible element whose completion would not result in
75 * the insertion of an ineligible element into the collection may throw an
76 * exception or it may succeed, at the option of the implementation.
77 * Such exceptions are marked as "optional" in the specification for this
78 * interface.
79 *
80 * <p>It is up to each collection to determine its own synchronization
81 * policy. In the absence of a stronger guarantee by the
82 * implementation, undefined behavior may result from the invocation
83 * of any method on a collection that is being mutated by another
84 * thread; this includes direct invocations, passing the collection to
85 * a method that might perform invocations, and using an existing
86 * iterator to examine the collection.
87 *
88 * <p>Many methods in Collections Framework interfaces are defined in
89 * terms of the {@link Object#equals(Object) equals} method. For example,
90 * the specification for the {@link #contains(Object) contains(Object o)}
91 * method says: "returns <tt>true</tt> if and only if this collection
92 * contains at least one element <tt>e</tt> such that
93 * <tt>(o==null ? e==null : o.equals(e))</tt>." This specification should
94 * <i>not</i> be construed to imply that invoking <tt>Collection.contains</tt>
95 * with a non-null argument <tt>o</tt> will cause <tt>o.equals(e)</tt> to be
96 * invoked for any element <tt>e</tt>. Implementations are free to implement
97 * optimizations whereby the <tt>equals</tt> invocation is avoided, for
98 * example, by first comparing the hash codes of the two elements. (The
99 * {@link Object#hashCode()} specification guarantees that two objects with
100 * unequal hash codes cannot be equal.) More generally, implementations of
101 * the various Collections Framework interfaces are free to take advantage of
102 * the specified behavior of underlying {@link Object} methods wherever the
103 * implementor deems it appropriate.
104 *
105 * <p>This interface is a member of the
106 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
107 * Java Collections Framework</a>.
108 *
109 * @implSpec
110 * The default method implementations (inherited or otherwise) do not apply any
111 * synchronization protocol. If a {@code Collection} implementation has a
112 * specific synchronization protocol, then it must override default
113 * implementations to apply that protocol.
114 *
115 * @param <E> the type of elements in this collection
116 *
117 * @author Josh Bloch
118 * @author Neal Gafter
119 * @see Set
120 * @see List
121 * @see Map
122 * @see SortedSet
123 * @see SortedMap
124 * @see HashSet
125 * @see TreeSet
126 * @see ArrayList
127 * @see LinkedList
128 * @see Vector
129 * @see Collections
130 * @see Arrays
131 * @see AbstractCollection
132 * @since 1.2
133 */
134
135 public interface Collection<E> extends Iterable<E> {
136 // Query Operations
137
138 /**
139 * Returns the number of elements in this collection. If this collection
140 * contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
141 * <tt>Integer.MAX_VALUE</tt>.
142 *
143 * @return the number of elements in this collection
144 */
145 int size();
146
147 /**
148 * Returns <tt>true</tt> if this collection contains no elements.
149 *
150 * @return <tt>true</tt> if this collection contains no elements
151 */
152 boolean isEmpty();
153
154 /**
155 * Returns <tt>true</tt> if this collection contains the specified element.
156 * More formally, returns <tt>true</tt> if and only if this collection
157 * contains at least one element <tt>e</tt> such that
158 * <tt>(o==null ? e==null : o.equals(e))</tt>.
159 *
160 * @param o element whose presence in this collection is to be tested
161 * @return <tt>true</tt> if this collection contains the specified
162 * element
163 * @throws ClassCastException if the type of the specified element
164 * is incompatible with this collection
165 * (<a href="#optional-restrictions">optional</a>)
166 * @throws NullPointerException if the specified element is null and this
167 * collection does not permit null elements
168 * (<a href="#optional-restrictions">optional</a>)
169 */
170 boolean contains(Object o);
171
172 /**
173 * Returns an iterator over the elements in this collection. There are no
174 * guarantees concerning the order in which the elements are returned
175 * (unless this collection is an instance of some class that provides a
176 * guarantee).
177 *
178 * @return an <tt>Iterator</tt> over the elements in this collection
179 */
180 Iterator<E> iterator();
181
182 /**
183 * Returns an array containing all of the elements in this collection.
184 * If this collection makes any guarantees as to what order its elements
185 * are returned by its iterator, this method must return the elements in
186 * the same order.
187 *
188 * <p>The returned array will be "safe" in that no references to it are
189 * maintained by this collection. (In other words, this method must
190 * allocate a new array even if this collection is backed by an array).
191 * The caller is thus free to modify the returned array.
192 *
193 * <p>This method acts as bridge between array-based and collection-based
194 * APIs.
195 *
196 * @return an array containing all of the elements in this collection
197 */
198 Object[] toArray();
199
200 /**
201 * Returns an array containing all of the elements in this collection;
202 * the runtime type of the returned array is that of the specified array.
203 * If the collection fits in the specified array, it is returned therein.
204 * Otherwise, a new array is allocated with the runtime type of the
205 * specified array and the size of this collection.
206 *
207 * <p>If this collection fits in the specified array with room to spare
208 * (i.e., the array has more elements than this collection), the element
209 * in the array immediately following the end of the collection is set to
210 * <tt>null</tt>. (This is useful in determining the length of this
211 * collection <i>only</i> if the caller knows that this collection does
212 * not contain any <tt>null</tt> elements.)
213 *
214 * <p>If this collection makes any guarantees as to what order its elements
215 * are returned by its iterator, this method must return the elements in
216 * the same order.
217 *
218 * <p>Like the {@link #toArray()} method, this method acts as bridge between
219 * array-based and collection-based APIs. Further, this method allows
220 * precise control over the runtime type of the output array, and may,
221 * under certain circumstances, be used to save allocation costs.
222 *
223 * <p>Suppose <tt>x</tt> is a collection known to contain only strings.
224 * The following code can be used to dump the collection into a newly
225 * allocated array of <tt>String</tt>:
226 *
227 * <pre>
228 * String[] y = x.toArray(new String[0]);</pre>
229 *
230 * Note that <tt>toArray(new Object[0])</tt> is identical in function to
231 * <tt>toArray()</tt>.
232 *
233 * @param a the array into which the elements of this collection are to be
234 * stored, if it is big enough; otherwise, a new array of the same
235 * runtime type is allocated for this purpose.
236 * @return an array containing all of the elements in this collection
237 * @throws ArrayStoreException if the runtime type of the specified array
238 * is not a supertype of the runtime type of every element in
239 * this collection
240 * @throws NullPointerException if the specified array is null
241 */
242 <T> T[] toArray(T[] a);
243
244 // Modification Operations
245
246 /**
247 * Ensures that this collection contains the specified element (optional
248 * operation). Returns <tt>true</tt> if this collection changed as a
249 * result of the call. (Returns <tt>false</tt> if this collection does
250 * not permit duplicates and already contains the specified element.)<p>
251 *
252 * Collections that support this operation may place limitations on what
253 * elements may be added to this collection. In particular, some
254 * collections will refuse to add <tt>null</tt> elements, and others will
255 * impose restrictions on the type of elements that may be added.
256 * Collection classes should clearly specify in their documentation any
257 * restrictions on what elements may be added.<p>
258 *
259 * If a collection refuses to add a particular element for any reason
260 * other than that it already contains the element, it <i>must</i> throw
261 * an exception (rather than returning <tt>false</tt>). This preserves
262 * the invariant that a collection always contains the specified element
263 * after this call returns.
264 *
265 * @param e element whose presence in this collection is to be ensured
266 * @return <tt>true</tt> if this collection changed as a result of the
267 * call
268 * @throws UnsupportedOperationException if the <tt>add</tt> operation
269 * is not supported by this collection
270 * @throws ClassCastException if the class of the specified element
271 * prevents it from being added to this collection
272 * @throws NullPointerException if the specified element is null and this
273 * collection does not permit null elements
274 * @throws IllegalArgumentException if some property of the element
275 * prevents it from being added to this collection
276 * @throws IllegalStateException if the element cannot be added at this
277 * time due to insertion restrictions
278 */
279 boolean add(E e);
280
281 /**
282 * Removes a single instance of the specified element from this
283 * collection, if it is present (optional operation). More formally,
284 * removes an element <tt>e</tt> such that
285 * <tt>(o==null ? e==null : o.equals(e))</tt>, if
286 * this collection contains one or more such elements. Returns
287 * <tt>true</tt> if this collection contained the specified element (or
288 * equivalently, if this collection changed as a result of the call).
289 *
290 * @param o element to be removed from this collection, if present
291 * @return <tt>true</tt> if an element was removed as a result of this call
292 * @throws ClassCastException if the type of the specified element
293 * is incompatible with this collection
294 * (<a href="#optional-restrictions">optional</a>)
295 * @throws NullPointerException if the specified element is null and this
296 * collection does not permit null elements
297 * (<a href="#optional-restrictions">optional</a>)
298 * @throws UnsupportedOperationException if the <tt>remove</tt> operation
299 * is not supported by this collection
300 */
301 boolean remove(Object o);
302
303
304 // Bulk Operations
305
306 /**
307 * Returns <tt>true</tt> if this collection contains all of the elements
308 * in the specified collection.
309 *
310 * @param c collection to be checked for containment in this collection
311 * @return <tt>true</tt> if this collection contains all of the elements
312 * in the specified collection
313 * @throws ClassCastException if the types of one or more elements
314 * in the specified collection are incompatible with this
315 * collection
316 * (<a href="#optional-restrictions">optional</a>)
317 * @throws NullPointerException if the specified collection contains one
318 * or more null elements and this collection does not permit null
319 * elements
320 * (<a href="#optional-restrictions">optional</a>),
321 * or if the specified collection is null.
322 * @see #contains(Object)
323 */
324 boolean containsAll(Collection<?> c);
325
326 /**
327 * Adds all of the elements in the specified collection to this collection
328 * (optional operation). The behavior of this operation is undefined if
329 * the specified collection is modified while the operation is in progress.
330 * (This implies that the behavior of this call is undefined if the
331 * specified collection is this collection, and this collection is
332 * nonempty.)
333 *
334 * @param c collection containing elements to be added to this collection
335 * @return <tt>true</tt> if this collection changed as a result of the call
336 * @throws UnsupportedOperationException if the <tt>addAll</tt> operation
337 * is not supported by this collection
338 * @throws ClassCastException if the class of an element of the specified
339 * collection prevents it from being added to this collection
340 * @throws NullPointerException if the specified collection contains a
341 * null element and this collection does not permit null elements,
342 * or if the specified collection is null
343 * @throws IllegalArgumentException if some property of an element of the
344 * specified collection prevents it from being added to this
345 * collection
346 * @throws IllegalStateException if not all the elements can be added at
347 * this time due to insertion restrictions
348 * @see #add(Object)
349 */
350 boolean addAll(Collection<? extends E> c);
351
352 /**
353 * Removes all of this collection's elements that are also contained in the
354 * specified collection (optional operation). After this call returns,
355 * this collection will contain no elements in common with the specified
356 * collection.
357 *
358 * @param c collection containing elements to be removed from this collection
359 * @return <tt>true</tt> if this collection changed as a result of the
360 * call
361 * @throws UnsupportedOperationException if the <tt>removeAll</tt> method
362 * is not supported by this collection
363 * @throws ClassCastException if the types of one or more elements
364 * in this collection are incompatible with the specified
365 * collection
366 * (<a href="#optional-restrictions">optional</a>)
367 * @throws NullPointerException if this collection contains one or more
368 * null elements and the specified collection does not support
369 * null elements
370 * (<a href="#optional-restrictions">optional</a>),
371 * or if the specified collection is null
372 * @see #remove(Object)
373 * @see #contains(Object)
374 */
375 boolean removeAll(Collection<?> c);
376
377 /**
378 * Removes all of the elements of this collection that satisfy the given
379 * predicate. RuntimeExceptions and Errors thrown by the predicate are
380 * propagated to the caller.
381 *
382 * @implSpec
383 * The default implementation traverses all elements of the collection using
384 * its {@link #iterator}. Each matching element is removed using
385 * {@link Iterator#remove()}. If the collection's iterator does not
386 * support removal then an {@code UnsupportedOperationException} will be
387 * thrown on the first matching element.
388 *
389 * @param filter a predicate which returns {@code true} for elements to be
390 * removed
391 * @return {@code true} if any elements were removed
392 * @throws NullPointerException if the specified filter is null
393 * @throws UnsupportedOperationException if the {@code remove}
394 * method is not supported by this collection's
395 * {@link #iterator}
396 * @since 1.8
397 */
398 default boolean removeIf(Predicate<? super E> filter) {
399 Objects.requireNonNull(filter);
400 boolean removed = false;
401 final Iterator<E> each = iterator();
402 while (each.hasNext()) {
403 if (filter.test(each.next())) {
404 each.remove();
405 removed = true;
406 }
407 }
408 return removed;
409 }
410
411 /**
412 * Retains only the elements in this collection that are contained in the
413 * specified collection (optional operation). In other words, removes from
414 * this collection all of its elements that are not contained in the
415 * specified collection.
416 *
417 * @param c collection containing elements to be retained in this collection
418 * @return <tt>true</tt> if this collection changed as a result of the call
419 * @throws UnsupportedOperationException if the <tt>retainAll</tt> operation
420 * is not supported by this collection
421 * @throws ClassCastException if the types of one or more elements
422 * in this collection are incompatible with the specified
423 * collection
424 * (<a href="#optional-restrictions">optional</a>)
425 * @throws NullPointerException if this collection contains one or more
426 * null elements and the specified collection does not permit null
427 * elements
428 * (<a href="#optional-restrictions">optional</a>),
429 * or if the specified collection is null
430 * @see #remove(Object)
431 * @see #contains(Object)
432 */
433 boolean retainAll(Collection<?> c);
434
435 /**
436 * Removes all of the elements from this collection (optional operation).
437 * The collection will be empty after this method returns.
438 *
439 * @throws UnsupportedOperationException if the <tt>clear</tt> operation
440 * is not supported by this collection
441 */
442 void clear();
443
444
445 // Comparison and hashing
446
447 /**
448 * Compares the specified object with this collection for equality. <p>
449 *
450 * While the <tt>Collection</tt> interface adds no stipulations to the
451 * general contract for the <tt>Object.equals</tt>, programmers who
452 * implement the <tt>Collection</tt> interface "directly" (in other words,
453 * create a class that is a <tt>Collection</tt> but is not a <tt>Set</tt>
454 * or a <tt>List</tt>) must exercise care if they choose to override the
455 * <tt>Object.equals</tt>. It is not necessary to do so, and the simplest
456 * course of action is to rely on <tt>Object</tt>'s implementation, but
457 * the implementor may wish to implement a "value comparison" in place of
458 * the default "reference comparison." (The <tt>List</tt> and
459 * <tt>Set</tt> interfaces mandate such value comparisons.)<p>
460 *
461 * The general contract for the <tt>Object.equals</tt> method states that
462 * equals must be symmetric (in other words, <tt>a.equals(b)</tt> if and
463 * only if <tt>b.equals(a)</tt>). The contracts for <tt>List.equals</tt>
464 * and <tt>Set.equals</tt> state that lists are only equal to other lists,
465 * and sets to other sets. Thus, a custom <tt>equals</tt> method for a
466 * collection class that implements neither the <tt>List</tt> nor
467 * <tt>Set</tt> interface must return <tt>false</tt> when this collection
468 * is compared to any list or set. (By the same logic, it is not possible
469 * to write a class that correctly implements both the <tt>Set</tt> and
470 * <tt>List</tt> interfaces.)
471 *
472 * @param o object to be compared for equality with this collection
473 * @return <tt>true</tt> if the specified object is equal to this
474 * collection
475 *
476 * @see Object#equals(Object)
477 * @see Set#equals(Object)
478 * @see List#equals(Object)
479 */
480 boolean equals(Object o);
481
482 /**
483 * Returns the hash code value for this collection. While the
484 * <tt>Collection</tt> interface adds no stipulations to the general
485 * contract for the <tt>Object.hashCode</tt> method, programmers should
486 * take note that any class that overrides the <tt>Object.equals</tt>
487 * method must also override the <tt>Object.hashCode</tt> method in order
488 * to satisfy the general contract for the <tt>Object.hashCode</tt> method.
489 * In particular, <tt>c1.equals(c2)</tt> implies that
490 * <tt>c1.hashCode()==c2.hashCode()</tt>.
491 *
492 * @return the hash code value for this collection
493 *
494 * @see Object#hashCode()
495 * @see Object#equals(Object)
496 */
497 int hashCode();
498
499 /**
500 * Creates a {@link Spliterator} over the elements in this collection.
501 *
502 * <p>The {@code Spliterator} reports {@link Spliterator#SIZED}.
503 * Implementations should document the reporting of additional
504 * characteristic values.
505 *
506 * @implSpec
507 * The default implementation creates a
508 * <em><a href="Spliterator.html#binding">late-binding</a></em> spliterator
509 * from the collections's {@code Iterator}. The spliterator inherits the
510 * <em>fail-fast</em> properties of the collection's iterator.
511 *
512 * @implNote
513 * The created {@code Spliterator} additionally reports
514 * {@link Spliterator#SUBSIZED}.
515 *
516 * @return a {@code Spliterator} over the elements in this collection
517 * @since 1.8
518 */
519 default Spliterator<E> spliterator() {
520 return Spliterators.spliterator(this, 0);
521 }
522 }