001 /*
002 * Copyright 1997-2006 Sun Microsystems, Inc. All Rights Reserved.
003 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
004 *
005 * This code is free software; you can redistribute it and/or modify it
006 * under the terms of the GNU General Public License version 2 only, as
007 * published by the Free Software Foundation. Sun designates this
008 * particular file as subject to the "Classpath" exception as provided
009 * by Sun in the LICENSE file that accompanied this code.
010 *
011 * This code is distributed in the hope that it will be useful, but WITHOUT
012 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
013 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
014 * version 2 for more details (a copy is included in the LICENSE file that
015 * accompanied this code).
016 *
017 * You should have received a copy of the GNU General Public License version
018 * 2 along with this work; if not, write to the Free Software Foundation,
019 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
020 *
021 * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
022 * CA 95054 USA or visit www.sun.com if you need additional information or
023 * have any questions.
024 */
025
026 package java.util;
027
028 /**
029 * The root interface in the <i>collection hierarchy</i>. A collection
030 * represents a group of objects, known as its <i>elements</i>. Some
031 * collections allow duplicate elements and others do not. Some are ordered
032 * and others unordered. The JDK does not provide any <i>direct</i>
033 * implementations of this interface: it provides implementations of more
034 * specific subinterfaces like <tt>Set</tt> and <tt>List</tt>. This interface
035 * is typically used to pass collections around and manipulate them where
036 * maximum generality is desired.
037 *
038 * <p><i>Bags</i> or <i>multisets</i> (unordered collections that may contain
039 * duplicate elements) should implement this interface directly.
040 *
041 * <p>All general-purpose <tt>Collection</tt> implementation classes (which
042 * typically implement <tt>Collection</tt> indirectly through one of its
043 * subinterfaces) should provide two "standard" constructors: a void (no
044 * arguments) constructor, which creates an empty collection, and a
045 * constructor with a single argument of type <tt>Collection</tt>, which
046 * creates a new collection with the same elements as its argument. In
047 * effect, the latter constructor allows the user to copy any collection,
048 * producing an equivalent collection of the desired implementation type.
049 * There is no way to enforce this convention (as interfaces cannot contain
050 * constructors) but all of the general-purpose <tt>Collection</tt>
051 * implementations in the Java platform libraries comply.
052 *
053 * <p>The "destructive" methods contained in this interface, that is, the
054 * methods that modify the collection on which they operate, are specified to
055 * throw <tt>UnsupportedOperationException</tt> if this collection does not
056 * support the operation. If this is the case, these methods may, but are not
057 * required to, throw an <tt>UnsupportedOperationException</tt> if the
058 * invocation would have no effect on the collection. For example, invoking
059 * the {@link #addAll(Collection)} method on an unmodifiable collection may,
060 * but is not required to, throw the exception if the collection to be added
061 * is empty.
062 *
063 * <p>Some collection implementations have restrictions on the elements that
064 * they may contain. For example, some implementations prohibit null elements,
065 * and some have restrictions on the types of their elements. Attempting to
066 * add an ineligible element throws an unchecked exception, typically
067 * <tt>NullPointerException</tt> or <tt>ClassCastException</tt>. Attempting
068 * to query the presence of an ineligible element may throw an exception,
069 * or it may simply return false; some implementations will exhibit the former
070 * behavior and some will exhibit the latter. More generally, attempting an
071 * operation on an ineligible element whose completion would not result in
072 * the insertion of an ineligible element into the collection may throw an
073 * exception or it may succeed, at the option of the implementation.
074 * Such exceptions are marked as "optional" in the specification for this
075 * interface.
076 *
077 * <p>It is up to each collection to determine its own synchronization
078 * policy. In the absence of a stronger guarantee by the
079 * implementation, undefined behavior may result from the invocation
080 * of any method on a collection that is being mutated by another
081 * thread; this includes direct invocations, passing the collection to
082 * a method that might perform invocations, and using an existing
083 * iterator to examine the collection.
084 *
085 * <p>Many methods in Collections Framework interfaces are defined in
086 * terms of the {@link Object#equals(Object) equals} method. For example,
087 * the specification for the {@link #contains(Object) contains(Object o)}
088 * method says: "returns <tt>true</tt> if and only if this collection
089 * contains at least one element <tt>e</tt> such that
090 * <tt>(o==null ? e==null : o.equals(e))</tt>." This specification should
091 * <i>not</i> be construed to imply that invoking <tt>Collection.contains</tt>
092 * with a non-null argument <tt>o</tt> will cause <tt>o.equals(e)</tt> to be
093 * invoked for any element <tt>e</tt>. Implementations are free to implement
094 * optimizations whereby the <tt>equals</tt> invocation is avoided, for
095 * example, by first comparing the hash codes of the two elements. (The
096 * {@link Object#hashCode()} specification guarantees that two objects with
097 * unequal hash codes cannot be equal.) More generally, implementations of
098 * the various Collections Framework interfaces are free to take advantage of
099 * 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 * @version 1.61, 05/05/07
109 * @see Set
110 * @see List
111 * @see Map
112 * @see SortedSet
113 * @see SortedMap
114 * @see HashSet
115 * @see TreeSet
116 * @see ArrayList
117 * @see LinkedList
118 * @see Vector
119 * @see Collections
120 * @see Arrays
121 * @see AbstractCollection
122 * @since 1.2
123 */
124
125 public interface Collection<E> extends Iterable<E> {
126 // Query Operations
127
128 /**
129 * Returns the number of elements in this collection. If this collection
130 * contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
131 * <tt>Integer.MAX_VALUE</tt>.
132 *
133 * @return the number of elements in this collection
134 */
135 int size();
136
137 /**
138 * Returns <tt>true</tt> if this collection contains no elements.
139 *
140 * @return <tt>true</tt> if this collection contains no elements
141 */
142 boolean isEmpty();
143
144 /**
145 * Returns <tt>true</tt> if this collection contains the specified element.
146 * More formally, returns <tt>true</tt> if and only if this collection
147 * contains at least one element <tt>e</tt> such that
148 * <tt>(o==null ? e==null : o.equals(e))</tt>.
149 *
150 * @param o element whose presence in this collection is to be tested
151 * @return <tt>true</tt> if this collection contains the specified
152 * element
153 * @throws ClassCastException if the type of the specified element
154 * is incompatible with this collection (optional)
155 * @throws NullPointerException if the specified element is null and this
156 * collection does not permit null elements (optional)
157 */
158 boolean contains(Object o);
159
160 /**
161 * Returns an iterator over the elements in this collection. There are no
162 * guarantees concerning the order in which the elements are returned
163 * (unless this collection is an instance of some class that provides a
164 * guarantee).
165 *
166 * @return an <tt>Iterator</tt> over the elements in this collection
167 */
168 Iterator<E> iterator();
169
170 /**
171 * Returns an array containing all of the elements in this collection.
172 * If this collection makes any guarantees as to what order its elements
173 * are returned by its iterator, this method must return the elements in
174 * the same order.
175 *
176 * <p>The returned array will be "safe" in that no references to it are
177 * maintained by this collection. (In other words, this method must
178 * allocate a new array even if this collection is backed by an array).
179 * The caller is thus free to modify the returned array.
180 *
181 * <p>This method acts as bridge between array-based and collection-based
182 * APIs.
183 *
184 * @return an array containing all of the elements in this collection
185 */
186 Object[] toArray();
187
188 /**
189 * Returns an array containing all of the elements in this collection;
190 * the runtime type of the returned array is that of the specified array.
191 * If the collection fits in the specified array, it is returned therein.
192 * Otherwise, a new array is allocated with the runtime type of the
193 * specified array and the size of this collection.
194 *
195 * <p>If this collection fits in the specified array with room to spare
196 * (i.e., the array has more elements than this collection), the element
197 * in the array immediately following the end of the collection is set to
198 * <tt>null</tt>. (This is useful in determining the length of this
199 * collection <i>only</i> if the caller knows that this collection does
200 * not contain any <tt>null</tt> elements.)
201 *
202 * <p>If this collection makes any guarantees as to what order its elements
203 * are returned by its iterator, this method must return the elements in
204 * the same order.
205 *
206 * <p>Like the {@link #toArray()} method, this method acts as bridge between
207 * array-based and collection-based APIs. Further, this method allows
208 * precise control over the runtime type of the output array, and may,
209 * under certain circumstances, be used to save allocation costs.
210 *
211 * <p>Suppose <tt>x</tt> is a collection known to contain only strings.
212 * The following code can be used to dump the collection into a newly
213 * allocated array of <tt>String</tt>:
214 *
215 * <pre>
216 * String[] y = x.toArray(new String[0]);</pre>
217 *
218 * Note that <tt>toArray(new Object[0])</tt> is identical in function to
219 * <tt>toArray()</tt>.
220 *
221 * @param a the array into which the elements of this collection are to be
222 * stored, if it is big enough; otherwise, a new array of the same
223 * runtime type is allocated for this purpose.
224 * @return an array containing all of the elements in this collection
225 * @throws ArrayStoreException if the runtime type of the specified array
226 * is not a supertype of the runtime type of every element in
227 * this collection
228 * @throws NullPointerException if the specified array is null
229 */
230 <T> T[] toArray(T[] a);
231
232 // Modification Operations
233
234 /**
235 * Ensures that this collection contains the specified element (optional
236 * operation). Returns <tt>true</tt> if this collection changed as a
237 * result of the call. (Returns <tt>false</tt> if this collection does
238 * not permit duplicates and already contains the specified element.)<p>
239 *
240 * Collections that support this operation may place limitations on what
241 * elements may be added to this collection. In particular, some
242 * collections will refuse to add <tt>null</tt> elements, and others will
243 * impose restrictions on the type of elements that may be added.
244 * Collection classes should clearly specify in their documentation any
245 * restrictions on what elements may be added.<p>
246 *
247 * If a collection refuses to add a particular element for any reason
248 * other than that it already contains the element, it <i>must</i> throw
249 * an exception (rather than returning <tt>false</tt>). This preserves
250 * the invariant that a collection always contains the specified element
251 * after this call returns.
252 *
253 * @param e element whose presence in this collection is to be ensured
254 * @return <tt>true</tt> if this collection changed as a result of the
255 * call
256 * @throws UnsupportedOperationException if the <tt>add</tt> operation
257 * is not supported by this collection
258 * @throws ClassCastException if the class of the specified element
259 * prevents it from being added to this collection
260 * @throws NullPointerException if the specified element is null and this
261 * collection does not permit null elements
262 * @throws IllegalArgumentException if some property of the element
263 * prevents it from being added to this collection
264 * @throws IllegalStateException if the element cannot be added at this
265 * time due to insertion restrictions
266 */
267 boolean add(E e);
268
269 /**
270 * Removes a single instance of the specified element from this
271 * collection, if it is present (optional operation). More formally,
272 * removes an element <tt>e</tt> such that
273 * <tt>(o==null ? e==null : o.equals(e))</tt>, if
274 * this collection contains one or more such elements. Returns
275 * <tt>true</tt> if this collection contained the specified element (or
276 * equivalently, if this collection changed as a result of the call).
277 *
278 * @param o element to be removed from this collection, if present
279 * @return <tt>true</tt> if an element was removed as a result of this call
280 * @throws ClassCastException if the type of the specified element
281 * is incompatible with this collection (optional)
282 * @throws NullPointerException if the specified element is null and this
283 * collection does not permit null elements (optional)
284 * @throws UnsupportedOperationException if the <tt>remove</tt> operation
285 * is not supported by this collection
286 */
287 boolean remove(Object o);
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 // Comparison and hashing
387
388 /**
389 * Compares the specified object with this collection for equality. <p>
390 *
391 * While the <tt>Collection</tt> interface adds no stipulations to the
392 * general contract for the <tt>Object.equals</tt>, programmers who
393 * implement the <tt>Collection</tt> interface "directly" (in other words,
394 * create a class that is a <tt>Collection</tt> but is not a <tt>Set</tt>
395 * or a <tt>List</tt>) must exercise care if they choose to override the
396 * <tt>Object.equals</tt>. It is not necessary to do so, and the simplest
397 * course of action is to rely on <tt>Object</tt>'s implementation, but
398 * the implementor may wish to implement a "value comparison" in place of
399 * the default "reference comparison." (The <tt>List</tt> and
400 * <tt>Set</tt> interfaces mandate such value comparisons.)<p>
401 *
402 * The general contract for the <tt>Object.equals</tt> method states that
403 * equals must be symmetric (in other words, <tt>a.equals(b)</tt> if and
404 * only if <tt>b.equals(a)</tt>). The contracts for <tt>List.equals</tt>
405 * and <tt>Set.equals</tt> state that lists are only equal to other lists,
406 * and sets to other sets. Thus, a custom <tt>equals</tt> method for a
407 * collection class that implements neither the <tt>List</tt> nor
408 * <tt>Set</tt> interface must return <tt>false</tt> when this collection
409 * is compared to any list or set. (By the same logic, it is not possible
410 * to write a class that correctly implements both the <tt>Set</tt> and
411 * <tt>List</tt> interfaces.)
412 *
413 * @param o object to be compared for equality with this collection
414 * @return <tt>true</tt> if the specified object is equal to this
415 * collection
416 *
417 * @see Object#equals(Object)
418 * @see Set#equals(Object)
419 * @see List#equals(Object)
420 */
421 boolean equals(Object o);
422
423 /**
424 * Returns the hash code value for this collection. While the
425 * <tt>Collection</tt> interface adds no stipulations to the general
426 * contract for the <tt>Object.hashCode</tt> method, programmers should
427 * take note that any class that overrides the <tt>Object.equals</tt>
428 * method must also override the <tt>Object.hashCode</tt> method in order
429 * to satisfy the general contract for the <tt>Object.hashCode</tt>method.
430 * In particular, <tt>c1.equals(c2)</tt> implies that
431 * <tt>c1.hashCode()==c2.hashCode()</tt>.
432 *
433 * @return the hash code value for this collection
434 *
435 * @see Object#hashCode()
436 * @see Object#equals(Object)
437 */
438 int hashCode();
439 }
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