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.
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10 *
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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).
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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 (optional)
114 * @throws NullPointerException if the specified element is null and this
115 * set does not permit null elements (optional)
116 */
117 boolean contains(Object o);
118
119 /**
120 * Returns an iterator over the elements in this set. The elements are
121 * returned in no particular order (unless this set is an instance of some
122 * class that provides a guarantee).
123 *
124 * @return an iterator over the elements in this set
125 */
126 Iterator<E> iterator();
127
128 /**
129 * Returns an array containing all of the elements in this set.
130 * If this set makes any guarantees as to what order its elements
131 * are returned by its iterator, this method must return the
132 * elements in the same order.
133 *
134 * <p>The returned array will be "safe" in that no references to it
135 * are maintained by this set. (In other words, this method must
136 * allocate a new array even if this set is backed by an array).
137 * The caller is thus free to modify the returned array.
138 *
139 * <p>This method acts as bridge between array-based and collection-based
140 * APIs.
141 *
142 * @return an array containing all the elements in this set
143 */
144 Object[] toArray();
145
146 /**
147 * Returns an array containing all of the elements in this set; the
148 * runtime type of the returned array is that of the specified array.
149 * If the set fits in the specified array, it is returned therein.
150 * Otherwise, a new array is allocated with the runtime type of the
151 * specified array and the size of this set.
152 *
153 * <p>If this set fits in the specified array with room to spare
154 * (i.e., the array has more elements than this set), the element in
155 * the array immediately following the end of the set is set to
156 * <tt>null</tt>. (This is useful in determining the length of this
157 * set <i>only</i> if the caller knows that this set does not contain
158 * any null elements.)
159 *
160 * <p>If this set makes any guarantees as to what order its elements
161 * are returned by its iterator, this method must return the elements
162 * in the same order.
163 *
164 * <p>Like the {@link #toArray()} method, this method acts as bridge between
165 * array-based and collection-based APIs. Further, this method allows
166 * precise control over the runtime type of the output array, and may,
167 * under certain circumstances, be used to save allocation costs.
168 *
169 * <p>Suppose <tt>x</tt> is a set known to contain only strings.
170 * The following code can be used to dump the set into a newly allocated
171 * array of <tt>String</tt>:
172 *
173 * <pre>
174 * String[] y = x.toArray(new String[0]);</pre>
175 *
176 * Note that <tt>toArray(new Object[0])</tt> is identical in function to
177 * <tt>toArray()</tt>.
178 *
179 * @param a the array into which the elements of this set are to be
180 * stored, if it is big enough; otherwise, a new array of the same
181 * runtime type is allocated for this purpose.
182 * @return an array containing all the elements in this set
183 * @throws ArrayStoreException if the runtime type of the specified array
184 * is not a supertype of the runtime type of every element in this
185 * set
186 * @throws NullPointerException if the specified array is null
187 */
188 <T> T[] toArray(T[] a);
189
190
191 // Modification Operations
192
193 /**
194 * Adds the specified element to this set if it is not already present
195 * (optional operation). More formally, adds the specified element
196 * <tt>e</tt> to this set if the set contains no element <tt>e2</tt>
197 * such that
198 * <tt>(e==null ? e2==null : e.equals(e2))</tt>.
199 * If this set already contains the element, the call leaves the set
200 * unchanged and returns <tt>false</tt>. In combination with the
201 * restriction on constructors, this ensures that sets never contain
202 * duplicate elements.
203 *
204 * <p>The stipulation above does not imply that sets must accept all
205 * elements; sets may refuse to add any particular element, including
206 * <tt>null</tt>, and throw an exception, as described in the
207 * specification for {@link Collection#add Collection.add}.
208 * Individual set implementations should clearly document any
209 * restrictions on the elements that they may contain.
210 *
211 * @param e element to be added to this set
212 * @return <tt>true</tt> if this set did not already contain the specified
213 * element
214 * @throws UnsupportedOperationException if the <tt>add</tt> operation
215 * is not supported by this set
216 * @throws ClassCastException if the class of the specified element
217 * prevents it from being added to this set
218 * @throws NullPointerException if the specified element is null and this
219 * set does not permit null elements
220 * @throws IllegalArgumentException if some property of the specified element
221 * prevents it from being added to this set
222 */
223 boolean add(E e);
224
225
226 /**
227 * Removes the specified element from this set if it is present
228 * (optional operation). More formally, removes an element <tt>e</tt>
229 * such that
230 * <tt>(o==null ? e==null : o.equals(e))</tt>, if
231 * this set contains such an element. Returns <tt>true</tt> if this set
232 * contained the element (or equivalently, if this set changed as a
233 * result of the call). (This set will not contain the element once the
234 * call returns.)
235 *
236 * @param o object to be removed from this set, if present
237 * @return <tt>true</tt> if this set contained the specified element
238 * @throws ClassCastException if the type of the specified element
239 * is incompatible with this set (optional)
240 * @throws NullPointerException if the specified element is null and this
241 * set does not permit null elements (optional)
242 * @throws UnsupportedOperationException if the <tt>remove</tt> operation
243 * is not supported by this set
244 */
245 boolean remove(Object o);
246
247
248 // Bulk Operations
249
250 /**
251 * Returns <tt>true</tt> if this set contains all of the elements of the
252 * specified collection. If the specified collection is also a set, this
253 * method returns <tt>true</tt> if it is a <i>subset</i> of this set.
254 *
255 * @param c collection to be checked for containment in this set
256 * @return <tt>true</tt> if this set contains all of the elements of the
257 * specified collection
258 * @throws ClassCastException if the types of one or more elements
259 * in the specified collection are incompatible with this
260 * set (optional)
261 * @throws NullPointerException if the specified collection contains one
262 * or more null elements and this set does not permit null
263 * elements (optional), or if the specified collection is null
264 * @see #contains(Object)
265 */
266 boolean containsAll(Collection<?> c);
267
268 /**
269 * Adds all of the elements in the specified collection to this set if
270 * they're not already present (optional operation). If the specified
271 * collection is also a set, the <tt>addAll</tt> operation effectively
272 * modifies this set so that its value is the <i>union</i> of the two
273 * sets. The behavior of this operation is undefined if the specified
274 * collection is modified while the operation is in progress.
275 *
276 * @param c collection containing elements to be added to this set
277 * @return <tt>true</tt> if this set changed as a result of the call
278 *
279 * @throws UnsupportedOperationException if the <tt>addAll</tt> operation
280 * is not supported by this set
281 * @throws ClassCastException if the class of an element of the
282 * specified collection prevents it from being added to this set
283 * @throws NullPointerException if the specified collection contains one
284 * or more null elements and this set does not permit null
285 * elements, or if the specified collection is null
286 * @throws IllegalArgumentException if some property of an element of the
287 * specified collection prevents it from being added to this set
288 * @see #add(Object)
289 */
290 boolean addAll(Collection<? extends E> c);
291
292 /**
293 * Retains only the elements in this set that are contained in the
294 * specified collection (optional operation). In other words, removes
295 * from this set all of its elements that are not contained in the
296 * specified collection. If the specified collection is also a set, this
297 * operation effectively modifies this set so that its value is the
298 * <i>intersection</i> of the two sets.
299 *
300 * @param c collection containing elements to be retained in this set
301 * @return <tt>true</tt> if this set changed as a result of the call
302 * @throws UnsupportedOperationException if the <tt>retainAll</tt> operation
303 * is not supported by this set
304 * @throws ClassCastException if the class of an element of this set
305 * is incompatible with the specified collection (optional)
306 * @throws NullPointerException if this set contains a null element and the
307 * specified collection does not permit null elements (optional),
308 * or if the specified collection is null
309 * @see #remove(Object)
310 */
311 boolean retainAll(Collection<?> c);
312
313 /**
314 * Removes from this set all of its elements that are contained in the
315 * specified collection (optional operation). If the specified
316 * collection is also a set, this operation effectively modifies this
317 * set so that its value is the <i>asymmetric set difference</i> of
318 * the two sets.
319 *
320 * @param c collection containing elements to be removed from this set
321 * @return <tt>true</tt> if this set changed as a result of the call
322 * @throws UnsupportedOperationException if the <tt>removeAll</tt> operation
323 * is not supported by this set
324 * @throws ClassCastException if the class of an element of this set
325 * is incompatible with the specified collection (optional)
326 * @throws NullPointerException if this set contains a null element and the
327 * specified collection does not permit null elements (optional),
328 * or if the specified collection is null
329 * @see #remove(Object)
330 * @see #contains(Object)
331 */
332 boolean removeAll(Collection<?> c);
333
334 /**
335 * Removes all of the elements from this set (optional operation).
336 * The set will be empty after this call returns.
337 *
338 * @throws UnsupportedOperationException if the <tt>clear</tt> method
339 * is not supported by this set
340 */
341 void clear();
342
343
344 // Comparison and hashing
345
346 /**
347 * Compares the specified object with this set for equality. Returns
348 * <tt>true</tt> if the specified object is also a set, the two sets
349 * have the same size, and every member of the specified set is
350 * contained in this set (or equivalently, every member of this set is
351 * contained in the specified set). This definition ensures that the
352 * equals method works properly across different implementations of the
353 * set interface.
354 *
355 * @param o object to be compared for equality with this set
356 * @return <tt>true</tt> if the specified object is equal to this set
357 */
358 boolean equals(Object o);
359
360 /**
361 * Returns the hash code value for this set. The hash code of a set is
362 * defined to be the sum of the hash codes of the elements in the set,
363 * where the hash code of a <tt>null</tt> element is defined to be zero.
364 * This ensures that <tt>s1.equals(s2)</tt> implies that
365 * <tt>s1.hashCode()==s2.hashCode()</tt> for any two sets <tt>s1</tt>
366 * and <tt>s2</tt>, as required by the general contract of
367 * {@link Object#hashCode}.
368 *
369 * @return the hash code value for this set
370 * @see Object#equals(Object)
371 * @see Set#equals(Object)
372 */
373 int hashCode();
374 }