1 /*
2 * Copyright 1997-2006 Sun Microsystems, Inc. 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. Sun designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Sun 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
22 * CA 95054 USA or visit www.sun.com if you need additional information or
23 * have any questions.
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>Some collection implementations have restrictions on the elements that
64 * they may contain. For example, some implementations prohibit null elements,
65 * and some have restrictions on the types of their elements. Attempting to
66 * add an ineligible element throws an unchecked exception, typically
67 * <tt>NullPointerException</tt> or <tt>ClassCastException</tt>. Attempting
68 * to query the presence of an ineligible element may throw an exception,
69 * or it may simply return false; some implementations will exhibit the former
70 * behavior and some will exhibit the latter. More generally, attempting an
71 * operation on an ineligible element whose completion would not result in
72 * the insertion of an ineligible element into the collection may throw an
73 * exception or it may succeed, at the option of the implementation.
74 * Such exceptions are marked as "optional" in the specification for this
75 * interface.
76 *
77 * <p>It is up to each collection to determine its own synchronization
78 * policy. In the absence of a stronger guarantee by the
79 * implementation, undefined behavior may result from the invocation
80 * of any method on a collection that is being mutated by another
81 * thread; this includes direct invocations, passing the collection to
82 * a method that might perform invocations, and using an existing
83 * iterator to examine the collection.
84 *
85 * <p>Many methods in Collections Framework interfaces are defined in
86 * terms of the {@link Object#equals(Object) equals} method. For example,
87 * the specification for the {@link #contains(Object) contains(Object o)}
88 * method says: "returns <tt>true</tt> if and only if this collection
89 * contains at least one element <tt>e</tt> such that
90 * <tt>(o==null ? e==null : o.equals(e))</tt>." This specification should
91 * <i>not</i> be construed to imply that invoking <tt>Collection.contains</tt>
92 * with a non-null argument <tt>o</tt> will cause <tt>o.equals(e)</tt> to be
93 * invoked for any element <tt>e</tt>. Implementations are free to implement
94 * optimizations whereby the <tt>equals</tt> invocation is avoided, for
95 * example, by first comparing the hash codes of the two elements. (The
96 * {@link Object#hashCode()} specification guarantees that two objects with
97 * unequal hash codes cannot be equal.) More generally, implementations of
98 * the various Collections Framework interfaces are free to take advantage of
99 * the specified behavior of underlying {@link Object} methods wherever the
100 * implementor deems it appropriate.
101 *
102 * <p>This interface is a member of the
103 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
104 * Java Collections Framework</a>.
105 *
106 * @author Josh Bloch
107 * @author Neal Gafter
108 * @see Set
109 * @see List
110 * @see Map
111 * @see SortedSet
112 * @see SortedMap
113 * @see HashSet
114 * @see TreeSet
115 * @see ArrayList
116 * @see LinkedList
117 * @see Vector
118 * @see Collections
119 * @see Arrays
120 * @see AbstractCollection
121 * @since 1.2
122 */
123
124 public interface Collection<E> extends Iterable<E> {
125 // Query Operations
126
127 /**
128 * Returns the number of elements in this collection. If this collection
129 * contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
130 * <tt>Integer.MAX_VALUE</tt>.
131 *
132 * @return the number of elements in this collection
133 */
134 int size();
135
136 /**
137 * Returns <tt>true</tt> if this collection contains no elements.
138 *
139 * @return <tt>true</tt> if this collection contains no elements
140 */
141 boolean isEmpty();
142
143 /**
144 * Returns <tt>true</tt> if this collection contains the specified element.
145 * More formally, returns <tt>true</tt> if and only if this collection
146 * contains at least one element <tt>e</tt> such that
147 * <tt>(o==null ? e==null : o.equals(e))</tt>.
148 *
149 * @param o element whose presence in this collection is to be tested
150 * @return <tt>true</tt> if this collection contains the specified
151 * element
152 * @throws ClassCastException if the type of the specified element
153 * is incompatible with this collection (optional)
154 * @throws NullPointerException if the specified element is null and this
155 * collection does not permit null elements (optional)
156 */
157 boolean contains(Object o);
158
159 /**
160 * Returns an iterator over the elements in this collection. There are no
161 * guarantees concerning the order in which the elements are returned
162 * (unless this collection is an instance of some class that provides a
163 * guarantee).
164 *
165 * @return an <tt>Iterator</tt> over the elements in this collection
166 */
167 Iterator<E> iterator();
168
169 /**
170 * Returns an array containing all of the elements in this collection.
171 * If this collection makes any guarantees as to what order its elements
172 * are returned by its iterator, this method must return the elements in
173 * the same order.
174 *
175 * <p>The returned array will be "safe" in that no references to it are
176 * maintained by this collection. (In other words, this method must
177 * allocate a new array even if this collection is backed by an array).
178 * The caller is thus free to modify the returned array.
179 *
180 * <p>This method acts as bridge between array-based and collection-based
181 * APIs.
182 *
183 * @return an array containing all of the elements in this collection
184 */
185 Object[] toArray();
186
187 /**
188 * Returns an array containing all of the elements in this collection;
189 * the runtime type of the returned array is that of the specified array.
190 * If the collection fits in the specified array, it is returned therein.
191 * Otherwise, a new array is allocated with the runtime type of the
192 * specified array and the size of this collection.
193 *
194 * <p>If this collection fits in the specified array with room to spare
195 * (i.e., the array has more elements than this collection), the element
196 * in the array immediately following the end of the collection is set to
197 * <tt>null</tt>. (This is useful in determining the length of this
198 * collection <i>only</i> if the caller knows that this collection does
199 * not contain any <tt>null</tt> elements.)
200 *
201 * <p>If this collection makes any guarantees as to what order its elements
202 * are returned by its iterator, this method must return the elements in
203 * the same order.
204 *
205 * <p>Like the {@link #toArray()} method, this method acts as bridge between
206 * array-based and collection-based APIs. Further, this method allows
207 * precise control over the runtime type of the output array, and may,
208 * under certain circumstances, be used to save allocation costs.
209 *
210 * <p>Suppose <tt>x</tt> is a collection known to contain only strings.
211 * The following code can be used to dump the collection into a newly
212 * allocated array of <tt>String</tt>:
213 *
214 * <pre>
215 * String[] y = x.toArray(new String[0]);</pre>
216 *
217 * Note that <tt>toArray(new Object[0])</tt> is identical in function to
218 * <tt>toArray()</tt>.
219 *
220 * @param a the array into which the elements of this collection are to be
221 * stored, if it is big enough; otherwise, a new array of the same
222 * runtime type is allocated for this purpose.
223 * @return an array containing all of the elements in this collection
224 * @throws ArrayStoreException if the runtime type of the specified array
225 * is not a supertype of the runtime type of every element in
226 * this collection
227 * @throws NullPointerException if the specified array is null
228 */
229 <T> T[] toArray(T[] a);
230
231 // Modification Operations
232
233 /**
234 * Ensures that this collection contains the specified element (optional
235 * operation). Returns <tt>true</tt> if this collection changed as a
236 * result of the call. (Returns <tt>false</tt> if this collection does
237 * not permit duplicates and already contains the specified element.)<p>
238 *
239 * Collections that support this operation may place limitations on what
240 * elements may be added to this collection. In particular, some
241 * collections will refuse to add <tt>null</tt> elements, and others will
242 * impose restrictions on the type of elements that may be added.
243 * Collection classes should clearly specify in their documentation any
244 * restrictions on what elements may be added.<p>
245 *
246 * If a collection refuses to add a particular element for any reason
247 * other than that it already contains the element, it <i>must</i> throw
248 * an exception (rather than returning <tt>false</tt>). This preserves
249 * the invariant that a collection always contains the specified element
250 * after this call returns.
251 *
252 * @param e element whose presence in this collection is to be ensured
253 * @return <tt>true</tt> if this collection changed as a result of the
254 * call
255 * @throws UnsupportedOperationException if the <tt>add</tt> operation
256 * is not supported by this collection
257 * @throws ClassCastException if the class of the specified element
258 * prevents it from being added to this collection
259 * @throws NullPointerException if the specified element is null and this
260 * collection does not permit null elements
261 * @throws IllegalArgumentException if some property of the element
262 * prevents it from being added to this collection
263 * @throws IllegalStateException if the element cannot be added at this
264 * time due to insertion restrictions
265 */
266 boolean add(E e);
267
268 /**
269 * Removes a single instance of the specified element from this
270 * collection, if it is present (optional operation). More formally,
271 * removes an element <tt>e</tt> such that
272 * <tt>(o==null ? e==null : o.equals(e))</tt>, if
273 * this collection contains one or more such elements. Returns
274 * <tt>true</tt> if this collection contained the specified element (or
275 * equivalently, if this collection changed as a result of the call).
276 *
277 * @param o element to be removed from this collection, if present
278 * @return <tt>true</tt> if an element was removed as a result of this call
279 * @throws ClassCastException if the type of the specified element
280 * is incompatible with this collection (optional)
281 * @throws NullPointerException if the specified element is null and this
282 * collection does not permit null elements (optional)
283 * @throws UnsupportedOperationException if the <tt>remove</tt> operation
284 * is not supported by this collection
285 */
286 boolean remove(Object o);
287
288
289 // Bulk Operations
290
291 /**
292 * Returns <tt>true</tt> if this collection contains all of the elements
293 * in the specified collection.
294 *
295 * @param c collection to be checked for containment in this collection
296 * @return <tt>true</tt> if this collection contains all of the elements
297 * in the specified collection
298 * @throws ClassCastException if the types of one or more elements
299 * in the specified collection are incompatible with this
300 * collection (optional)
301 * @throws NullPointerException if the specified collection contains one
302 * or more null elements and this collection does not permit null
303 * elements (optional), or if the specified collection is null
304 * @see #contains(Object)
305 */
306 boolean containsAll(Collection<?> c);
307
308 /**
309 * Adds all of the elements in the specified collection to this collection
310 * (optional operation). The behavior of this operation is undefined if
311 * the specified collection is modified while the operation is in progress.
312 * (This implies that the behavior of this call is undefined if the
313 * specified collection is this collection, and this collection is
314 * nonempty.)
315 *
316 * @param c collection containing elements to be added to this collection
317 * @return <tt>true</tt> if this collection changed as a result of the call
318 * @throws UnsupportedOperationException if the <tt>addAll</tt> operation
319 * is not supported by this collection
320 * @throws ClassCastException if the class of an element of the specified
321 * collection prevents it from being added to this collection
322 * @throws NullPointerException if the specified collection contains a
323 * null element and this collection does not permit null elements,
324 * or if the specified collection is null
325 * @throws IllegalArgumentException if some property of an element of the
326 * specified collection prevents it from being added to this
327 * collection
328 * @throws IllegalStateException if not all the elements can be added at
329 * this time due to insertion restrictions
330 * @see #add(Object)
331 */
332 boolean addAll(Collection<? extends E> c);
333
334 /**
335 * Removes all of this collection's elements that are also contained in the
336 * specified collection (optional operation). After this call returns,
337 * this collection will contain no elements in common with the specified
338 * collection.
339 *
340 * @param c collection containing elements to be removed from this collection
341 * @return <tt>true</tt> if this collection changed as a result of the
342 * call
343 * @throws UnsupportedOperationException if the <tt>removeAll</tt> method
344 * is not supported by this collection
345 * @throws ClassCastException if the types of one or more elements
346 * in this collection are incompatible with the specified
347 * collection (optional)
348 * @throws NullPointerException if this collection contains one or more
349 * null elements and the specified collection does not support
350 * null elements (optional), or if the specified collection is null
351 * @see #remove(Object)
352 * @see #contains(Object)
353 */
354 boolean removeAll(Collection<?> c);
355
356 /**
357 * Retains only the elements in this collection that are contained in the
358 * specified collection (optional operation). In other words, removes from
359 * this collection all of its elements that are not contained in the
360 * specified collection.
361 *
362 * @param c collection containing elements to be retained in this collection
363 * @return <tt>true</tt> if this collection changed as a result of the call
364 * @throws UnsupportedOperationException if the <tt>retainAll</tt> operation
365 * is not supported by this collection
366 * @throws ClassCastException if the types of one or more elements
367 * in this collection are incompatible with the specified
368 * collection (optional)
369 * @throws NullPointerException if this collection contains one or more
370 * null elements and the specified collection does not permit null
371 * elements (optional), or if the specified collection is null
372 * @see #remove(Object)
373 * @see #contains(Object)
374 */
375 boolean retainAll(Collection<?> c);
376
377 /**
378 * Removes all of the elements from this collection (optional operation).
379 * The collection will be empty after this method returns.
380 *
381 * @throws UnsupportedOperationException if the <tt>clear</tt> operation
382 * is not supported by this collection
383 */
384 void clear();
385
386
387 // Comparison and hashing
388
389 /**
390 * Compares the specified object with this collection for equality. <p>
391 *
392 * While the <tt>Collection</tt> interface adds no stipulations to the
393 * general contract for the <tt>Object.equals</tt>, programmers who
394 * implement the <tt>Collection</tt> interface "directly" (in other words,
395 * create a class that is a <tt>Collection</tt> but is not a <tt>Set</tt>
396 * or a <tt>List</tt>) must exercise care if they choose to override the
397 * <tt>Object.equals</tt>. It is not necessary to do so, and the simplest
398 * course of action is to rely on <tt>Object</tt>'s implementation, but
399 * the implementor may wish to implement a "value comparison" in place of
400 * the default "reference comparison." (The <tt>List</tt> and
401 * <tt>Set</tt> interfaces mandate such value comparisons.)<p>
402 *
403 * The general contract for the <tt>Object.equals</tt> method states that
404 * equals must be symmetric (in other words, <tt>a.equals(b)</tt> if and
405 * only if <tt>b.equals(a)</tt>). The contracts for <tt>List.equals</tt>
406 * and <tt>Set.equals</tt> state that lists are only equal to other lists,
407 * and sets to other sets. Thus, a custom <tt>equals</tt> method for a
408 * collection class that implements neither the <tt>List</tt> nor
409 * <tt>Set</tt> interface must return <tt>false</tt> when this collection
410 * is compared to any list or set. (By the same logic, it is not possible
411 * to write a class that correctly implements both the <tt>Set</tt> and
412 * <tt>List</tt> interfaces.)
413 *
414 * @param o object to be compared for equality with this collection
415 * @return <tt>true</tt> if the specified object is equal to this
416 * collection
417 *
418 * @see Object#equals(Object)
419 * @see Set#equals(Object)
420 * @see List#equals(Object)
421 */
422 boolean equals(Object o);
423
424 /**
425 * Returns the hash code value for this collection. While the
426 * <tt>Collection</tt> interface adds no stipulations to the general
427 * contract for the <tt>Object.hashCode</tt> method, programmers should
428 * take note that any class that overrides the <tt>Object.equals</tt>
429 * method must also override the <tt>Object.hashCode</tt> method in order
430 * to satisfy the general contract for the <tt>Object.hashCode</tt> method.
431 * In particular, <tt>c1.equals(c2)</tt> implies that
432 * <tt>c1.hashCode()==c2.hashCode()</tt>.
433 *
434 * @return the hash code value for this collection
435 *
436 * @see Object#hashCode()
437 * @see Object#equals(Object)
438 */
439 int hashCode();
440 }