1 /*
2 * Copyright (c) 1997, 2006, 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 *
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 Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22 * or visit www.oracle.com if you need additional information or have any
23 * questions.
24 */
25
26 package java.util;
27
28 /**
29 * A collection that contains no duplicate elements. More formally, sets
30 * contain no pair of elements <code>e1</code> and <code>e2</code> such that
31 * <code>e1.equals(e2)</code>, and at most one null element. As implied by
32 * its name, this interface models the mathematical <i>set</i> abstraction.
33 *
34 * <p>The <tt>Set</tt> interface places additional stipulations, beyond those
35 * inherited from the <tt>Collection</tt> interface, on the contracts of all
36 * constructors and on the contracts of the <tt>add</tt>, <tt>equals</tt> and
37 * <tt>hashCode</tt> methods. Declarations for other inherited methods are
38 * also included here for convenience. (The specifications accompanying these
39 * declarations have been tailored to the <tt>Set</tt> interface, but they do
40 * not contain any additional stipulations.)
41 *
42 * <p>The additional stipulation on constructors is, not surprisingly,
43 * that all constructors must create a set that contains no duplicate elements
44 * (as defined above).
45 *
46 * <p>Note: Great care must be exercised if mutable objects are used as set
47 * elements. The behavior of a set is not specified if the value of an object
48 * is changed in a manner that affects <tt>equals</tt> comparisons while the
49 * object is an element in the set. A special case of this prohibition is
50 * that it is not permissible for a set to contain itself as an element.
51 *
52 * <p>Some set implementations have restrictions on the elements that
53 * they may contain. For example, some implementations prohibit null elements,
54 * and some have restrictions on the types of their elements. Attempting to
55 * add an ineligible element throws an unchecked exception, typically
56 * <tt>NullPointerException</tt> or <tt>ClassCastException</tt>. Attempting
57 * to query the presence of an ineligible element may throw an exception,
58 * or it may simply return false; some implementations will exhibit the former
59 * behavior and some will exhibit the latter. More generally, attempting an
60 * operation on an ineligible element whose completion would not result in
61 * the insertion of an ineligible element into the set may throw an
62 * exception or it may succeed, at the option of the implementation.
63 * Such exceptions are marked as "optional" in the specification for this
64 * interface.
65 *
66 * <p>This interface is a member of the
67 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
68 * Java Collections Framework</a>.
69 *
70 * @param <E> the type of elements maintained by this set
71 *
72 * @author Josh Bloch
73 * @author Neal Gafter
74 * @see Collection
75 * @see List
76 * @see SortedSet
77 * @see HashSet
78 * @see TreeSet
79 * @see AbstractSet
80 * @see Collections#singleton(java.lang.Object)
81 * @see Collections#EMPTY_SET
82 * @since 1.2
83 */
84
85 public interface Set<E> extends Collection<E> {
86 // Query Operations
87
88 /**
89 * Returns the number of elements in this set (its cardinality). If this
90 * set contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
91 * <tt>Integer.MAX_VALUE</tt>.
92 *
93 * @return the number of elements in this set (its cardinality)
94 */
95 int size();
96
97 /**
98 * Returns <tt>true</tt> if this set contains no elements.
99 *
100 * @return <tt>true</tt> if this set contains no elements
101 */
102 boolean isEmpty();
103
104 /**
105 * Returns <tt>true</tt> if this set contains the specified element.
106 * More formally, returns <tt>true</tt> if and only if this set
107 * contains an element <tt>e</tt> such that
108 * <tt>(o==null ? e==null : o.equals(e))</tt>.
109 *
110 * @param o element whose presence in this set is to be tested
111 * @return <tt>true</tt> if this set contains the specified element
112 * @throws ClassCastException if the type of the specified element
113 * is incompatible with this set
114 * (<a href="Collection.html#optional-restrictions">optional</a>)
115 * @throws NullPointerException if the specified element is null and this
116 * set does not permit null elements
117 * (<a href="Collection.html#optional-restrictions">optional</a>)
118 */
119 boolean contains(Object o);
120
121 /**
122 * Returns an iterator over the elements in this set. The elements are
123 * returned in no particular order (unless this set is an instance of some
124 * class that provides a guarantee).
125 *
126 * @return an iterator over the elements in this set
127 */
128 Iterator<E> iterator();
129
130 /**
131 * Returns an array containing all of the elements in this set.
132 * If this set makes any guarantees as to what order its elements
133 * are returned by its iterator, this method must return the
134 * elements in the same order.
135 *
136 * <p>The returned array will be "safe" in that no references to it
137 * are maintained by this set. (In other words, this method must
138 * allocate a new array even if this set is backed by an array).
139 * The caller is thus free to modify the returned array.
140 *
141 * <p>This method acts as bridge between array-based and collection-based
142 * APIs.
143 *
144 * @return an array containing all the elements in this set
145 */
146 Object[] toArray();
147
148 /**
149 * Returns an array containing all of the elements in this set; the
150 * runtime type of the returned array is that of the specified array.
151 * If the set fits in the specified array, it is returned therein.
152 * Otherwise, a new array is allocated with the runtime type of the
153 * specified array and the size of this set.
154 *
155 * <p>If this set fits in the specified array with room to spare
156 * (i.e., the array has more elements than this set), the element in
157 * the array immediately following the end of the set is set to
158 * <tt>null</tt>. (This is useful in determining the length of this
159 * set <i>only</i> if the caller knows that this set does not contain
160 * any null elements.)
161 *
162 * <p>If this set makes any guarantees as to what order its elements
163 * are returned by its iterator, this method must return the elements
164 * in the same order.
165 *
166 * <p>Like the {@link #toArray()} method, this method acts as bridge between
167 * array-based and collection-based APIs. Further, this method allows
168 * precise control over the runtime type of the output array, and may,
169 * under certain circumstances, be used to save allocation costs.
170 *
171 * <p>Suppose <tt>x</tt> is a set known to contain only strings.
172 * The following code can be used to dump the set into a newly allocated
173 * array of <tt>String</tt>:
174 *
175 * <pre>
176 * String[] y = x.toArray(new String[0]);</pre>
177 *
178 * Note that <tt>toArray(new Object[0])</tt> is identical in function to
179 * <tt>toArray()</tt>.
180 *
181 * @param a the array into which the elements of this set are to be
182 * stored, if it is big enough; otherwise, a new array of the same
183 * runtime type is allocated for this purpose.
184 * @return an array containing all the elements in this set
185 * @throws ArrayStoreException if the runtime type of the specified array
186 * is not a supertype of the runtime type of every element in this
187 * set
188 * @throws NullPointerException if the specified array is null
189 */
190 <T> T[] toArray(T[] a);
191
192
193 // Modification Operations
194
195 /**
196 * Adds the specified element to this set if it is not already present
197 * (optional operation). More formally, adds the specified element
198 * <tt>e</tt> to this set if the set contains no element <tt>e2</tt>
199 * such that
200 * <tt>(e==null ? e2==null : e.equals(e2))</tt>.
201 * If this set already contains the element, the call leaves the set
202 * unchanged and returns <tt>false</tt>. In combination with the
203 * restriction on constructors, this ensures that sets never contain
204 * duplicate elements.
205 *
206 * <p>The stipulation above does not imply that sets must accept all
207 * elements; sets may refuse to add any particular element, including
208 * <tt>null</tt>, and throw an exception, as described in the
209 * specification for {@link Collection#add Collection.add}.
210 * Individual set implementations should clearly document any
211 * restrictions on the elements that they may contain.
212 *
213 * @param e element to be added to this set
214 * @return <tt>true</tt> if this set did not already contain the specified
215 * element
216 * @throws UnsupportedOperationException if the <tt>add</tt> operation
217 * is not supported by this set
218 * @throws ClassCastException if the class of the specified element
219 * prevents it from being added to this set
220 * @throws NullPointerException if the specified element is null and this
221 * set does not permit null elements
222 * @throws IllegalArgumentException if some property of the specified element
223 * prevents it from being added to this set
224 */
225 boolean add(E e);
226
227
228 /**
229 * Removes the specified element from this set if it is present
230 * (optional operation). More formally, removes an element <tt>e</tt>
231 * such that
232 * <tt>(o==null ? e==null : o.equals(e))</tt>, if
233 * this set contains such an element. Returns <tt>true</tt> if this set
234 * contained the element (or equivalently, if this set changed as a
235 * result of the call). (This set will not contain the element once the
236 * call returns.)
237 *
238 * @param o object to be removed from this set, if present
239 * @return <tt>true</tt> if this set contained the specified element
240 * @throws ClassCastException if the type of the specified element
241 * is incompatible with this set
242 * (<a href="Collection.html#optional-restrictions">optional</a>)
243 * @throws NullPointerException if the specified element is null and this
244 * set does not permit null elements
245 * (<a href="Collection.html#optional-restrictions">optional</a>)
246 * @throws UnsupportedOperationException if the <tt>remove</tt> operation
247 * is not supported by this set
248 */
249 boolean remove(Object o);
250
251
252 // Bulk Operations
253
254 /**
255 * Returns <tt>true</tt> if this set contains all of the elements of the
256 * specified collection. If the specified collection is also a set, this
257 * method returns <tt>true</tt> if it is a <i>subset</i> of this set.
258 *
259 * @param c collection to be checked for containment in this set
260 * @return <tt>true</tt> if this set contains all of the elements of the
261 * specified collection
262 * @throws ClassCastException if the types of one or more elements
263 * in the specified collection are incompatible with this
264 * set
265 * (<a href="Collection.html#optional-restrictions">optional</a>)
266 * @throws NullPointerException if the specified collection contains one
267 * or more null elements and this set does not permit null
268 * elements
269 * (<a href="Collection.html#optional-restrictions">optional</a>),
270 * or if the specified collection is null
271 * @see #contains(Object)
272 */
273 boolean containsAll(Collection<?> c);
274
275 /**
276 * Adds all of the elements in the specified collection to this set if
277 * they're not already present (optional operation). If the specified
278 * collection is also a set, the <tt>addAll</tt> operation effectively
279 * modifies this set so that its value is the <i>union</i> of the two
280 * sets. The behavior of this operation is undefined if the specified
281 * collection is modified while the operation is in progress.
282 *
283 * @param c collection containing elements to be added to this set
284 * @return <tt>true</tt> if this set changed as a result of the call
285 *
286 * @throws UnsupportedOperationException if the <tt>addAll</tt> operation
287 * is not supported by this set
288 * @throws ClassCastException if the class of an element of the
289 * specified collection prevents it from being added to this set
290 * @throws NullPointerException if the specified collection contains one
291 * or more null elements and this set does not permit null
292 * elements, or if the specified collection is null
293 * @throws IllegalArgumentException if some property of an element of the
294 * specified collection prevents it from being added to this set
295 * @see #add(Object)
296 */
297 boolean addAll(Collection<? extends E> c);
298
299 /**
300 * Retains only the elements in this set that are contained in the
301 * specified collection (optional operation). In other words, removes
302 * from this set all of its elements that are not contained in the
303 * specified collection. If the specified collection is also a set, this
304 * operation effectively modifies this set so that its value is the
305 * <i>intersection</i> of the two sets.
306 *
307 * @param c collection containing elements to be retained in this set
308 * @return <tt>true</tt> if this set changed as a result of the call
309 * @throws UnsupportedOperationException if the <tt>retainAll</tt> operation
310 * is not supported by this set
311 * @throws ClassCastException if the class of an element of this set
312 * is incompatible with the specified collection
313 * (<a href="Collection.html#optional-restrictions">optional</a>)
314 * @throws NullPointerException if this set contains a null element and the
315 * specified collection does not permit null elements
316 * (<a href="Collection.html#optional-restrictions">optional</a>),
317 * or if the specified collection is null
318 * @see #remove(Object)
319 */
320 boolean retainAll(Collection<?> c);
321
322 /**
323 * Removes from this set all of its elements that are contained in the
324 * specified collection (optional operation). If the specified
325 * collection is also a set, this operation effectively modifies this
326 * set so that its value is the <i>asymmetric set difference</i> of
327 * the two sets.
328 *
329 * @param c collection containing elements to be removed from this set
330 * @return <tt>true</tt> if this set changed as a result of the call
331 * @throws UnsupportedOperationException if the <tt>removeAll</tt> operation
332 * is not supported by this set
333 * @throws ClassCastException if the class of an element of this set
334 * is incompatible with the specified collection
335 * (<a href="Collection.html#optional-restrictions">optional</a>)
336 * @throws NullPointerException if this set contains a null element and the
337 * specified collection does not permit null elements
338 * (<a href="Collection.html#optional-restrictions">optional</a>),
339 * or if the specified collection is null
340 * @see #remove(Object)
341 * @see #contains(Object)
342 */
343 boolean removeAll(Collection<?> c);
344
345 /**
346 * Removes all of the elements from this set (optional operation).
347 * The set will be empty after this call returns.
348 *
349 * @throws UnsupportedOperationException if the <tt>clear</tt> method
350 * is not supported by this set
351 */
352 void clear();
353
354
355 // Comparison and hashing
356
357 /**
358 * Compares the specified object with this set for equality. Returns
359 * <tt>true</tt> if the specified object is also a set, the two sets
360 * have the same size, and every member of the specified set is
361 * contained in this set (or equivalently, every member of this set is
362 * contained in the specified set). This definition ensures that the
363 * equals method works properly across different implementations of the
364 * set interface.
365 *
366 * @param o object to be compared for equality with this set
367 * @return <tt>true</tt> if the specified object is equal to this set
368 */
369 boolean equals(Object o);
370
371 /**
372 * Returns the hash code value for this set. The hash code of a set is
373 * defined to be the sum of the hash codes of the elements in the set,
374 * where the hash code of a <tt>null</tt> element is defined to be zero.
375 * This ensures that <tt>s1.equals(s2)</tt> implies that
376 * <tt>s1.hashCode()==s2.hashCode()</tt> for any two sets <tt>s1</tt>
377 * and <tt>s2</tt>, as required by the general contract of
378 * {@link Object#hashCode}.
379 *
380 * @return the hash code value for this set
381 * @see Object#equals(Object)
382 * @see Set#equals(Object)
383 */
384 int hashCode();
385 }