1 /*
2 * Copyright (c) 1997, 2010, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
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7 * published by the Free Software Foundation. Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
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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 /**
29 * The root interface in the <i>collection hierarchy</i>. A collection
30 * represents a group of objects, known as its <i>elements</i>. Some
31 * collections allow duplicate elements and others do not. Some are ordered
32 * and others unordered. The JDK does not provide any <i>direct</i>
33 * implementations of this interface: it provides implementations of more
34 * specific subinterfaces like <tt>Set</tt> and <tt>List</tt>. This interface
35 * is typically used to pass collections around and manipulate them where
36 * maximum generality is desired.
37 *
38 * <p><i>Bags</i> or <i>multisets</i> (unordered collections that may contain
39 * duplicate elements) should implement this interface directly.
40 *
41 * <p>All general-purpose <tt>Collection</tt> implementation classes (which
42 * typically implement <tt>Collection</tt> indirectly through one of its
43 * subinterfaces) should provide two "standard" constructors: a void (no
44 * arguments) constructor, which creates an empty collection, and a
45 * constructor with a single argument of type <tt>Collection</tt>, which
46 * creates a new collection with the same elements as its argument. In
47 * effect, the latter constructor allows the user to copy any collection,
48 * producing an equivalent collection of the desired implementation type.
49 * There is no way to enforce this convention (as interfaces cannot contain
50 * constructors) but all of the general-purpose <tt>Collection</tt>
51 * implementations in the Java platform libraries comply.
52 *
53 * <p>The "destructive" methods contained in this interface, that is, the
54 * methods that modify the collection on which they operate, are specified to
55 * throw <tt>UnsupportedOperationException</tt> if this collection does not
56 * support the operation. If this is the case, these methods may, but are not
57 * required to, throw an <tt>UnsupportedOperationException</tt> if the
58 * invocation would have no effect on the collection. For example, invoking
59 * the {@link #addAll(Collection)} method on an unmodifiable collection may,
60 * but is not required to, throw the exception if the collection to be added
61 * is empty.
62 *
63 * <p><a name="optional-restrictions"/>
64 * Some collection implementations have restrictions on the elements that
65 * they may contain. For example, some implementations prohibit null elements,
66 * and some have restrictions on the types of their elements. Attempting to
67 * add an ineligible element throws an unchecked exception, typically
68 * <tt>NullPointerException</tt> or <tt>ClassCastException</tt>. Attempting
69 * to query the presence of an ineligible element may throw an exception,
70 * or it may simply return false; some implementations will exhibit the former
71 * behavior and some will exhibit the latter. More generally, attempting an
72 * operation on an ineligible element whose completion would not result in
73 * the insertion of an ineligible element into the collection may throw an
74 * exception or it may succeed, at the option of the implementation.
75 * Such exceptions are marked as "optional" in the specification for this
76 * interface.
77 *
78 * <p>It is up to each collection to determine its own synchronization
79 * policy. In the absence of a stronger guarantee by the
80 * implementation, undefined behavior may result from the invocation
81 * of any method on a collection that is being mutated by another
82 * thread; this includes direct invocations, passing the collection to
83 * a method that might perform invocations, and using an existing
84 * iterator to examine the collection.
85 *
86 * <p>Many methods in Collections Framework interfaces are defined in
87 * terms of the {@link Object#equals(Object) equals} method. For example,
88 * the specification for the {@link #contains(Object) contains(Object o)}
89 * method says: "returns <tt>true</tt> if and only if this collection
90 * contains at least one element <tt>e</tt> such that
91 * <tt>(o==null ? e==null : o.equals(e))</tt>." This specification should
92 * <i>not</i> be construed to imply that invoking <tt>Collection.contains</tt>
93 * with a non-null argument <tt>o</tt> will cause <tt>o.equals(e)</tt> to be
94 * invoked for any element <tt>e</tt>. Implementations are free to implement
95 * optimizations whereby the <tt>equals</tt> invocation is avoided, for
96 * example, by first comparing the hash codes of the two elements. (The
97 * {@link Object#hashCode()} specification guarantees that two objects with
98 * unequal hash codes cannot be equal.) More generally, implementations of
99 * the various Collections Framework interfaces are free to take advantage of
100 * the specified behavior of underlying {@link Object} methods wherever the
101 * implementor deems it appropriate.
102 *
103 * <p>This interface is a member of the
104 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
105 * Java Collections Framework</a>.
106 *
107 * @param <E> the type of elements in this collection
108 *
109 * @author Josh Bloch
110 * @author Neal Gafter
111 * @see Set
112 * @see List
113 * @see Map
114 * @see SortedSet
115 * @see SortedMap
116 * @see HashSet
117 * @see TreeSet
118 * @see ArrayList
119 * @see LinkedList
120 * @see Vector
121 * @see Collections
122 * @see Arrays
123 * @see AbstractCollection
124 * @since 1.2
125 */
126
127 public interface Collection<E> extends Iterable<E> {
128 // Query Operations
129
130 /**
131 * Returns the number of elements in this collection. If this collection
132 * contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
133 * <tt>Integer.MAX_VALUE</tt>.
134 *
135 * @return the number of elements in this collection
136 */
137 int size();
138
139 /**
140 * Returns <tt>true</tt> if this collection contains no elements.
141 *
142 * @return <tt>true</tt> if this collection contains no elements
143 */
144 boolean isEmpty();
145
146 /**
147 * Returns <tt>true</tt> if this collection contains the specified element.
148 * More formally, returns <tt>true</tt> if and only if this collection
149 * contains at least one element <tt>e</tt> such that
150 * <tt>(o==null ? e==null : o.equals(e))</tt>.
151 *
152 * @param o element whose presence in this collection is to be tested
153 * @return <tt>true</tt> if this collection contains the specified
154 * element
155 * @throws ClassCastException if the type of the specified element
156 * is incompatible with this collection
157 * (<a href="#optional-restrictions">optional</a>)
158 * @throws NullPointerException if the specified element is null and this
159 * collection does not permit null elements
160 * (<a href="#optional-restrictions">optional</a>)
161 */
162 boolean contains(Object o);
163
164 /**
165 * Returns an iterator over the elements in this collection. There are no
166 * guarantees concerning the order in which the elements are returned
167 * (unless this collection is an instance of some class that provides a
168 * guarantee).
169 *
170 * @return an <tt>Iterator</tt> over the elements in this collection
171 */
172 Iterator<E> iterator();
173
174 /**
175 * Returns an array containing all of the elements in this collection.
176 * If this collection makes any guarantees as to what order its elements
177 * are returned by its iterator, this method must return the elements in
178 * the same order.
179 *
180 * <p>The returned array will be "safe" in that no references to it are
181 * maintained by this collection. (In other words, this method must
182 * allocate a new array even if this collection is backed by an array).
183 * The caller is thus free to modify the returned array.
184 *
185 * <p>This method acts as bridge between array-based and collection-based
186 * APIs.
187 *
188 * @return an array containing all of the elements in this collection
189 */
190 Object[] toArray();
191
192 /**
193 * Returns an array containing all of the elements in this collection;
194 * the runtime type of the returned array is that of the specified array.
195 * If the collection fits in the specified array, it is returned therein.
196 * Otherwise, a new array is allocated with the runtime type of the
197 * specified array and the size of this collection.
198 *
199 * <p>If this collection fits in the specified array with room to spare
200 * (i.e., the array has more elements than this collection), the element
201 * in the array immediately following the end of the collection is set to
202 * <tt>null</tt>. (This is useful in determining the length of this
203 * collection <i>only</i> if the caller knows that this collection does
204 * not contain any <tt>null</tt> elements.)
205 *
206 * <p>If this collection makes any guarantees as to what order its elements
207 * are returned by its iterator, this method must return the elements in
208 * the same order.
209 *
210 * <p>Like the {@link #toArray()} method, this method acts as bridge between
211 * array-based and collection-based APIs. Further, this method allows
212 * precise control over the runtime type of the output array, and may,
213 * under certain circumstances, be used to save allocation costs.
214 *
215 * <p>Suppose <tt>x</tt> is a collection known to contain only strings.
216 * The following code can be used to dump the collection into a newly
217 * allocated array of <tt>String</tt>:
218 *
219 * <pre>
220 * String[] y = x.toArray(new String[0]);</pre>
221 *
222 * Note that <tt>toArray(new Object[0])</tt> is identical in function to
223 * <tt>toArray()</tt>.
224 *
225 * @param a the array into which the elements of this collection are to be
226 * stored, if it is big enough; otherwise, a new array of the same
227 * runtime type is allocated for this purpose.
228 * @return an array containing all of the elements in this collection
229 * @throws ArrayStoreException if the runtime type of the specified array
230 * is not a supertype of the runtime type of every element in
231 * this collection
232 * @throws NullPointerException if the specified array is null
233 */
234 <T> T[] toArray(T[] a);
235
236 // Modification Operations
237
238 /**
239 * Ensures that this collection contains the specified element (optional
240 * operation). Returns <tt>true</tt> if this collection changed as a
241 * result of the call. (Returns <tt>false</tt> if this collection does
242 * not permit duplicates and already contains the specified element.)<p>
243 *
244 * Collections that support this operation may place limitations on what
245 * elements may be added to this collection. In particular, some
246 * collections will refuse to add <tt>null</tt> elements, and others will
247 * impose restrictions on the type of elements that may be added.
248 * Collection classes should clearly specify in their documentation any
249 * restrictions on what elements may be added.<p>
250 *
251 * If a collection refuses to add a particular element for any reason
252 * other than that it already contains the element, it <i>must</i> throw
253 * an exception (rather than returning <tt>false</tt>). This preserves
254 * the invariant that a collection always contains the specified element
255 * after this call returns.
256 *
257 * @param e element whose presence in this collection is to be ensured
258 * @return <tt>true</tt> if this collection changed as a result of the
259 * call
260 * @throws UnsupportedOperationException if the <tt>add</tt> operation
261 * is not supported by this collection
262 * @throws ClassCastException if the class of the specified element
263 * prevents it from being added to this collection
264 * @throws NullPointerException if the specified element is null and this
265 * collection does not permit null elements
266 * @throws IllegalArgumentException if some property of the element
267 * prevents it from being added to this collection
268 * @throws IllegalStateException if the element cannot be added at this
269 * time due to insertion restrictions
270 */
271 boolean add(E e);
272
273 /**
274 * Removes a single instance of the specified element from this
275 * collection, if it is present (optional operation). More formally,
276 * removes an element <tt>e</tt> such that
277 * <tt>(o==null ? e==null : o.equals(e))</tt>, if
278 * this collection contains one or more such elements. Returns
279 * <tt>true</tt> if this collection contained the specified element (or
280 * equivalently, if this collection changed as a result of the call).
281 *
282 * @param o element to be removed from this collection, if present
283 * @return <tt>true</tt> if an element was removed as a result of this call
284 * @throws ClassCastException if the type of the specified element
285 * is incompatible with this collection
286 * (<a href="#optional-restrictions">optional</a>)
287 * @throws NullPointerException if the specified element is null and this
288 * collection does not permit null elements
289 * (<a href="#optional-restrictions">optional</a>)
290 * @throws UnsupportedOperationException if the <tt>remove</tt> operation
291 * is not supported by this collection
292 */
293 boolean remove(Object o);
294
295
296 // Bulk Operations
297
298 /**
299 * Returns <tt>true</tt> if this collection contains all of the elements
300 * in the specified collection.
301 *
302 * @param c collection to be checked for containment in this collection
303 * @return <tt>true</tt> if this collection contains all of the elements
304 * in the specified collection
305 * @throws ClassCastException if the types of one or more elements
306 * in the specified collection are incompatible with this
307 * collection
308 * (<a href="#optional-restrictions">optional</a>)
309 * @throws NullPointerException if the specified collection contains one
310 * or more null elements and this collection does not permit null
311 * elements
312 * (<a href="#optional-restrictions">optional</a>),
313 * or if the specified collection is null.
314 * @see #contains(Object)
315 */
316 boolean containsAll(Collection<?> c);
317
318 /**
319 * Adds all of the elements in the specified collection to this collection
320 * (optional operation). The behavior of this operation is undefined if
321 * the specified collection is modified while the operation is in progress.
322 * (This implies that the behavior of this call is undefined if the
323 * specified collection is this collection, and this collection is
324 * nonempty.)
325 *
326 * @param c collection containing elements to be added to this collection
327 * @return <tt>true</tt> if this collection changed as a result of the call
328 * @throws UnsupportedOperationException if the <tt>addAll</tt> operation
329 * is not supported by this collection
330 * @throws ClassCastException if the class of an element of the specified
331 * collection prevents it from being added to this collection
332 * @throws NullPointerException if the specified collection contains a
333 * null element and this collection does not permit null elements,
334 * or if the specified collection is null
335 * @throws IllegalArgumentException if some property of an element of the
336 * specified collection prevents it from being added to this
337 * collection
338 * @throws IllegalStateException if not all the elements can be added at
339 * this time due to insertion restrictions
340 * @see #add(Object)
341 */
342 boolean addAll(Collection<? extends E> c);
343
344 /**
345 * Removes all of this collection's elements that are also contained in the
346 * specified collection (optional operation). After this call returns,
347 * this collection will contain no elements in common with the specified
348 * collection.
349 *
350 * @param c collection containing elements to be removed from this collection
351 * @return <tt>true</tt> if this collection changed as a result of the
352 * call
353 * @throws UnsupportedOperationException if the <tt>removeAll</tt> method
354 * is not supported by this collection
355 * @throws ClassCastException if the types of one or more elements
356 * in this collection are incompatible with the specified
357 * collection
358 * (<a href="#optional-restrictions">optional</a>)
359 * @throws NullPointerException if this collection contains one or more
360 * null elements and the specified collection does not support
361 * null elements
362 * (<a href="#optional-restrictions">optional</a>),
363 * or if the specified collection is null
364 * @see #remove(Object)
365 * @see #contains(Object)
366 */
367 boolean removeAll(Collection<?> c);
368
369 /**
370 * Retains only the elements in this collection that are contained in the
371 * specified collection (optional operation). In other words, removes from
372 * this collection all of its elements that are not contained in the
373 * specified collection.
374 *
375 * @param c collection containing elements to be retained in this collection
376 * @return <tt>true</tt> if this collection changed as a result of the call
377 * @throws UnsupportedOperationException if the <tt>retainAll</tt> operation
378 * is not supported by this collection
379 * @throws ClassCastException if the types of one or more elements
380 * in this collection are incompatible with the specified
381 * collection
382 * (<a href="#optional-restrictions">optional</a>)
383 * @throws NullPointerException if this collection contains one or more
384 * null elements and the specified collection does not permit null
385 * elements
386 * (<a href="#optional-restrictions">optional</a>),
387 * or if the specified collection is null
388 * @see #remove(Object)
389 * @see #contains(Object)
390 */
391 boolean retainAll(Collection<?> c);
392
393 /**
394 * Removes all of the elements from this collection (optional operation).
395 * The collection will be empty after this method returns.
396 *
397 * @throws UnsupportedOperationException if the <tt>clear</tt> operation
398 * is not supported by this collection
399 */
400 void clear();
401
402
403 // Comparison and hashing
404
405 /**
406 * Compares the specified object with this collection for equality. <p>
407 *
408 * While the <tt>Collection</tt> interface adds no stipulations to the
409 * general contract for the <tt>Object.equals</tt>, programmers who
410 * implement the <tt>Collection</tt> interface "directly" (in other words,
411 * create a class that is a <tt>Collection</tt> but is not a <tt>Set</tt>
412 * or a <tt>List</tt>) must exercise care if they choose to override the
413 * <tt>Object.equals</tt>. It is not necessary to do so, and the simplest
414 * course of action is to rely on <tt>Object</tt>'s implementation, but
415 * the implementor may wish to implement a "value comparison" in place of
416 * the default "reference comparison." (The <tt>List</tt> and
417 * <tt>Set</tt> interfaces mandate such value comparisons.)<p>
418 *
419 * The general contract for the <tt>Object.equals</tt> method states that
420 * equals must be symmetric (in other words, <tt>a.equals(b)</tt> if and
421 * only if <tt>b.equals(a)</tt>). The contracts for <tt>List.equals</tt>
422 * and <tt>Set.equals</tt> state that lists are only equal to other lists,
423 * and sets to other sets. Thus, a custom <tt>equals</tt> method for a
424 * collection class that implements neither the <tt>List</tt> nor
425 * <tt>Set</tt> interface must return <tt>false</tt> when this collection
426 * is compared to any list or set. (By the same logic, it is not possible
427 * to write a class that correctly implements both the <tt>Set</tt> and
428 * <tt>List</tt> interfaces.)
429 *
430 * @param o object to be compared for equality with this collection
431 * @return <tt>true</tt> if the specified object is equal to this
432 * collection
433 *
434 * @see Object#equals(Object)
435 * @see Set#equals(Object)
436 * @see List#equals(Object)
437 */
438 boolean equals(Object o);
439
440 /**
441 * Returns the hash code value for this collection. While the
442 * <tt>Collection</tt> interface adds no stipulations to the general
443 * contract for the <tt>Object.hashCode</tt> method, programmers should
444 * take note that any class that overrides the <tt>Object.equals</tt>
445 * method must also override the <tt>Object.hashCode</tt> method in order
446 * to satisfy the general contract for the <tt>Object.hashCode</tt> method.
447 * In particular, <tt>c1.equals(c2)</tt> implies that
448 * <tt>c1.hashCode()==c2.hashCode()</tt>.
449 *
450 * @return the hash code value for this collection
451 *
452 * @see Object#hashCode()
453 * @see Object#equals(Object)
454 */
455 int hashCode();
456 }