1 /*
2 * Copyright (c) 1997, 2011, 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 *
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20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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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 * @param <E> the type of elements in this collection
110 *
111 * @author Josh Bloch
112 * @author Neal Gafter
113 * @see Set
114 * @see List
115 * @see Map
116 * @see SortedSet
117 * @see SortedMap
118 * @see HashSet
119 * @see TreeSet
120 * @see ArrayList
121 * @see LinkedList
122 * @see Vector
123 * @see Collections
124 * @see Arrays
125 * @see AbstractCollection
126 * @since 1.2
127 */
128
129 public interface Collection<E> extends Iterable<E> {
130 // Query Operations
131
132 /**
133 * Returns the number of elements in this collection. If this collection
134 * contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
135 * <tt>Integer.MAX_VALUE</tt>.
136 *
137 * @return the number of elements in this collection
138 */
139 int size();
140
141 /**
142 * Returns <tt>true</tt> if this collection contains no elements.
143 *
144 * @return <tt>true</tt> if this collection contains no elements
145 */
146 boolean isEmpty();
147
148 /**
149 * Returns <tt>true</tt> if this collection contains the specified element.
150 * More formally, returns <tt>true</tt> if and only if this collection
151 * contains at least one element <tt>e</tt> such that
152 * <tt>(o==null ? e==null : o.equals(e))</tt>.
153 *
154 * @param o element whose presence in this collection is to be tested
155 * @return <tt>true</tt> if this collection contains the specified
156 * element
157 * @throws ClassCastException if the type of the specified element
158 * is incompatible with this collection
159 * (<a href="#optional-restrictions">optional</a>)
160 * @throws NullPointerException if the specified element is null and this
161 * collection does not permit null elements
162 * (<a href="#optional-restrictions">optional</a>)
163 */
164 boolean contains(Object o);
165
166 /**
167 * Returns an iterator over the elements in this collection. There are no
168 * guarantees concerning the order in which the elements are returned
169 * (unless this collection is an instance of some class that provides a
170 * guarantee).
171 *
172 * @return an <tt>Iterator</tt> over the elements in this collection
173 */
174 Iterator<E> iterator();
175
176 /**
177 * Returns an array containing all of the elements in this collection.
178 * If this collection makes any guarantees as to what order its elements
179 * are returned by its iterator, this method must return the elements in
180 * the same order.
181 *
182 * <p>The returned array will be "safe" in that no references to it are
183 * maintained by this collection. (In other words, this method must
184 * allocate a new array even if this collection is backed by an array).
185 * The caller is thus free to modify the returned array.
186 *
187 * <p>This method acts as bridge between array-based and collection-based
188 * APIs.
189 *
190 * @return an array containing all of the elements in this collection
191 */
192 Object[] toArray();
193
194 /**
195 * Returns an array containing all of the elements in this collection;
196 * the runtime type of the returned array is that of the specified array.
197 * If the collection fits in the specified array, it is returned therein.
198 * Otherwise, a new array is allocated with the runtime type of the
199 * specified array and the size of this collection.
200 *
201 * <p>If this collection fits in the specified array with room to spare
202 * (i.e., the array has more elements than this collection), the element
203 * in the array immediately following the end of the collection is set to
204 * <tt>null</tt>. (This is useful in determining the length of this
205 * collection <i>only</i> if the caller knows that this collection does
206 * not contain any <tt>null</tt> elements.)
207 *
208 * <p>If this collection makes any guarantees as to what order its elements
209 * are returned by its iterator, this method must return the elements in
210 * the same order.
211 *
212 * <p>Like the {@link #toArray()} method, this method acts as bridge between
213 * array-based and collection-based APIs. Further, this method allows
214 * precise control over the runtime type of the output array, and may,
215 * under certain circumstances, be used to save allocation costs.
216 *
217 * <p>Suppose <tt>x</tt> is a collection known to contain only strings.
218 * The following code can be used to dump the collection into a newly
219 * allocated array of <tt>String</tt>:
220 *
221 * <pre>
222 * String[] y = x.toArray(new String[0]);</pre>
223 *
224 * Note that <tt>toArray(new Object[0])</tt> is identical in function to
225 * <tt>toArray()</tt>.
226 *
227 * @param a the array into which the elements of this collection are to be
228 * stored, if it is big enough; otherwise, a new array of the same
229 * runtime type is allocated for this purpose.
230 * @return an array containing all of the elements in this collection
231 * @throws ArrayStoreException if the runtime type of the specified array
232 * is not a supertype of the runtime type of every element in
233 * this collection
234 * @throws NullPointerException if the specified array is null
235 */
236 <T> T[] toArray(T[] a);
237
238 // Modification Operations
239
240 /**
241 * Ensures that this collection contains the specified element (optional
242 * operation). Returns <tt>true</tt> if this collection changed as a
243 * result of the call. (Returns <tt>false</tt> if this collection does
244 * not permit duplicates and already contains the specified element.)<p>
245 *
246 * Collections that support this operation may place limitations on what
247 * elements may be added to this collection. In particular, some
248 * collections will refuse to add <tt>null</tt> elements, and others will
249 * impose restrictions on the type of elements that may be added.
250 * Collection classes should clearly specify in their documentation any
251 * restrictions on what elements may be added.<p>
252 *
253 * If a collection refuses to add a particular element for any reason
254 * other than that it already contains the element, it <i>must</i> throw
255 * an exception (rather than returning <tt>false</tt>). This preserves
256 * the invariant that a collection always contains the specified element
257 * after this call returns.
258 *
259 * @param e element whose presence in this collection is to be ensured
260 * @return <tt>true</tt> if this collection changed as a result of the
261 * call
262 * @throws UnsupportedOperationException if the <tt>add</tt> operation
263 * is not supported by this collection
264 * @throws ClassCastException if the class of the specified element
265 * prevents it from being added to this collection
266 * @throws NullPointerException if the specified element is null and this
267 * collection does not permit null elements
268 * @throws IllegalArgumentException if some property of the element
269 * prevents it from being added to this collection
270 * @throws IllegalStateException if the element cannot be added at this
271 * time due to insertion restrictions
272 */
273 boolean add(E e);
274
275 /**
276 * Removes a single instance of the specified element from this
277 * collection, if it is present (optional operation). More formally,
278 * removes an element <tt>e</tt> such that
279 * <tt>(o==null ? e==null : o.equals(e))</tt>, if
280 * this collection contains one or more such elements. Returns
281 * <tt>true</tt> if this collection contained the specified element (or
282 * equivalently, if this collection changed as a result of the call).
283 *
284 * @param o element to be removed from this collection, if present
285 * @return <tt>true</tt> if an element was removed as a result of this call
286 * @throws ClassCastException if the type of the specified element
287 * is incompatible with this collection
288 * (<a href="#optional-restrictions">optional</a>)
289 * @throws NullPointerException if the specified element is null and this
290 * collection does not permit null elements
291 * (<a href="#optional-restrictions">optional</a>)
292 * @throws UnsupportedOperationException if the <tt>remove</tt> operation
293 * is not supported by this collection
294 */
295 boolean remove(Object o);
296
297
298 // Bulk Operations
299
300 /**
301 * Returns <tt>true</tt> if this collection contains all of the elements
302 * in the specified collection.
303 *
304 * @param c collection to be checked for containment in this collection
305 * @return <tt>true</tt> if this collection contains all of the elements
306 * in the specified collection
307 * @throws ClassCastException if the types of one or more elements
308 * in the specified collection are incompatible with this
309 * collection
310 * (<a href="#optional-restrictions">optional</a>)
311 * @throws NullPointerException if the specified collection contains one
312 * or more null elements and this collection does not permit null
313 * elements
314 * (<a href="#optional-restrictions">optional</a>),
315 * or if the specified collection is null.
316 * @see #contains(Object)
317 */
318 boolean containsAll(Collection<?> c);
319
320 /**
321 * Adds all of the elements in the specified collection to this collection
322 * (optional operation). The behavior of this operation is undefined if
323 * the specified collection is modified while the operation is in progress.
324 * (This implies that the behavior of this call is undefined if the
325 * specified collection is this collection, and this collection is
326 * nonempty.)
327 *
328 * @param c collection containing elements to be added to this collection
329 * @return <tt>true</tt> if this collection changed as a result of the call
330 * @throws UnsupportedOperationException if the <tt>addAll</tt> operation
331 * is not supported by this collection
332 * @throws ClassCastException if the class of an element of the specified
333 * collection prevents it from being added to this collection
334 * @throws NullPointerException if the specified collection contains a
335 * null element and this collection does not permit null elements,
336 * or if the specified collection is null
337 * @throws IllegalArgumentException if some property of an element of the
338 * specified collection prevents it from being added to this
339 * collection
340 * @throws IllegalStateException if not all the elements can be added at
341 * this time due to insertion restrictions
342 * @see #add(Object)
343 */
344 boolean addAll(Collection<? extends E> c);
345
346 /**
347 * Removes all of this collection's elements that are also contained in the
348 * specified collection (optional operation). After this call returns,
349 * this collection will contain no elements in common with the specified
350 * collection.
351 *
352 * @param c collection containing elements to be removed from this collection
353 * @return <tt>true</tt> if this collection changed as a result of the
354 * call
355 * @throws UnsupportedOperationException if the <tt>removeAll</tt> method
356 * is not supported by this collection
357 * @throws ClassCastException if the types of one or more elements
358 * in this collection are incompatible with the specified
359 * collection
360 * (<a href="#optional-restrictions">optional</a>)
361 * @throws NullPointerException if this collection contains one or more
362 * null elements and the specified collection does not support
363 * null elements
364 * (<a href="#optional-restrictions">optional</a>),
365 * or if the specified collection is null
366 * @see #remove(Object)
367 * @see #contains(Object)
368 */
369 boolean removeAll(Collection<?> c);
370
371 /**
372 * Retains only the elements in this collection that are contained in the
373 * specified collection (optional operation). In other words, removes from
374 * this collection all of its elements that are not contained in the
375 * specified collection.
376 *
377 * @param c collection containing elements to be retained in this collection
378 * @return <tt>true</tt> if this collection changed as a result of the call
379 * @throws UnsupportedOperationException if the <tt>retainAll</tt> operation
380 * is not supported by this collection
381 * @throws ClassCastException if the types of one or more elements
382 * in this collection are incompatible with the specified
383 * collection
384 * (<a href="#optional-restrictions">optional</a>)
385 * @throws NullPointerException if this collection contains one or more
386 * null elements and the specified collection does not permit null
387 * elements
388 * (<a href="#optional-restrictions">optional</a>),
389 * or if the specified collection is null
390 * @see #remove(Object)
391 * @see #contains(Object)
392 */
393 boolean retainAll(Collection<?> c);
394
395 /**
396 * Removes all of the elements from this collection (optional operation).
397 * The collection will be empty after this method returns.
398 *
399 * @throws UnsupportedOperationException if the <tt>clear</tt> operation
400 * is not supported by this collection
401 */
402 void clear();
403
404
405 // Comparison and hashing
406
407 /**
408 * Compares the specified object with this collection for equality. <p>
409 *
410 * While the <tt>Collection</tt> interface adds no stipulations to the
411 * general contract for the <tt>Object.equals</tt>, programmers who
412 * implement the <tt>Collection</tt> interface "directly" (in other words,
413 * create a class that is a <tt>Collection</tt> but is not a <tt>Set</tt>
414 * or a <tt>List</tt>) must exercise care if they choose to override the
415 * <tt>Object.equals</tt>. It is not necessary to do so, and the simplest
416 * course of action is to rely on <tt>Object</tt>'s implementation, but
417 * the implementor may wish to implement a "value comparison" in place of
418 * the default "reference comparison." (The <tt>List</tt> and
419 * <tt>Set</tt> interfaces mandate such value comparisons.)<p>
420 *
421 * The general contract for the <tt>Object.equals</tt> method states that
422 * equals must be symmetric (in other words, <tt>a.equals(b)</tt> if and
423 * only if <tt>b.equals(a)</tt>). The contracts for <tt>List.equals</tt>
424 * and <tt>Set.equals</tt> state that lists are only equal to other lists,
425 * and sets to other sets. Thus, a custom <tt>equals</tt> method for a
426 * collection class that implements neither the <tt>List</tt> nor
427 * <tt>Set</tt> interface must return <tt>false</tt> when this collection
428 * is compared to any list or set. (By the same logic, it is not possible
429 * to write a class that correctly implements both the <tt>Set</tt> and
430 * <tt>List</tt> interfaces.)
431 *
432 * @param o object to be compared for equality with this collection
433 * @return <tt>true</tt> if the specified object is equal to this
434 * collection
435 *
436 * @see Object#equals(Object)
437 * @see Set#equals(Object)
438 * @see List#equals(Object)
439 */
440 boolean equals(Object o);
441
442 /**
443 * Returns the hash code value for this collection. While the
444 * <tt>Collection</tt> interface adds no stipulations to the general
445 * contract for the <tt>Object.hashCode</tt> method, programmers should
446 * take note that any class that overrides the <tt>Object.equals</tt>
447 * method must also override the <tt>Object.hashCode</tt> method in order
448 * to satisfy the general contract for the <tt>Object.hashCode</tt> method.
449 * In particular, <tt>c1.equals(c2)</tt> implies that
450 * <tt>c1.hashCode()==c2.hashCode()</tt>.
451 *
452 * @return the hash code value for this collection
453 *
454 * @see Object#hashCode()
455 * @see Object#equals(Object)
456 */
457 int hashCode();
458
459 /**
460 * Removes all of the elements of this collection which match the provided
461 * predicate.
462 *
463 * @param filter a predicate which returns {@code true} for elements to be
464 * removed
465 * @return {@code true} if any elements were removed
466 * @throws NullPointerException if the specified predicate is null
467 * @since 1.8
468 */
469 public default boolean removeAll(Predicate<? super E> filter) {
470 Objects.requireNonNull(filter);
471 boolean removed = false;
472 Iterator<E> each = iterator();
473 while (each.hasNext()) {
474 if (filter.test(each.next())) {
475 each.remove();
476 removed = true;
477 }
478 }
479
480 return removed;
481 }
482 }