| Set.java |
1 /*
2 * %W% %E%
3 *
4 * Copyright (c) 2006, Oracle and/or its affiliates. All rights reserved.
5 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
6 */
7
8 package java.util;
9
10 /**
11 * A collection that contains no duplicate elements. More formally, sets
12 * contain no pair of elements <code>e1</code> and <code>e2</code> such that
13 * <code>e1.equals(e2)</code>, and at most one null element. As implied by
14 * its name, this interface models the mathematical <i>set</i> abstraction.
15 *
16 * <p>The <tt>Set</tt> interface places additional stipulations, beyond those
17 * inherited from the <tt>Collection</tt> interface, on the contracts of all
18 * constructors and on the contracts of the <tt>add</tt>, <tt>equals</tt> and
19 * <tt>hashCode</tt> methods. Declarations for other inherited methods are
20 * also included here for convenience. (The specifications accompanying these
21 * declarations have been tailored to the <tt>Set</tt> interface, but they do
22 * not contain any additional stipulations.)
23 *
24 * <p>The additional stipulation on constructors is, not surprisingly,
25 * that all constructors must create a set that contains no duplicate elements
26 * (as defined above).
27 *
28 * <p>Note: Great care must be exercised if mutable objects are used as set
29 * elements. The behavior of a set is not specified if the value of an object
30 * is changed in a manner that affects <tt>equals</tt> comparisons while the
31 * object is an element in the set. A special case of this prohibition is
32 * that it is not permissible for a set to contain itself as an element.
33 *
34 * <p>Some set implementations have restrictions on the elements that
35 * they may contain. For example, some implementations prohibit null elements,
36 * and some have restrictions on the types of their elements. Attempting to
37 * add an ineligible element throws an unchecked exception, typically
38 * <tt>NullPointerException</tt> or <tt>ClassCastException</tt>. Attempting
39 * to query the presence of an ineligible element may throw an exception,
40 * or it may simply return false; some implementations will exhibit the former
41 * behavior and some will exhibit the latter. More generally, attempting an
42 * operation on an ineligible element whose completion would not result in
43 * the insertion of an ineligible element into the set may throw an
44 * exception or it may succeed, at the option of the implementation.
45 * Such exceptions are marked as "optional" in the specification for this
46 * interface.
47 *
48 * <p>This interface is a member of the
49 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
50 * Java Collections Framework</a>.
51 *
52 * @param <E> the type of elements maintained by this set
53 *
54 * @author Josh Bloch
55 * @author Neal Gafter
56 * @version %I%, %G%
57 * @see Collection
58 * @see List
59 * @see SortedSet
60 * @see HashSet
61 * @see TreeSet
62 * @see AbstractSet
63 * @see Collections#singleton(java.lang.Object)
64 * @see Collections#EMPTY_SET
65 * @since 1.2
66 */
67
68 public interface Set<E> extends Collection<E> {
69 // Query Operations
70
71 /**
72 * Returns the number of elements in this set (its cardinality). If this
73 * set contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
74 * <tt>Integer.MAX_VALUE</tt>.
75 *
76 * @return the number of elements in this set (its cardinality)
77 */
78 int size();
79
80 /**
81 * Returns <tt>true</tt> if this set contains no elements.
82 *
83 * @return <tt>true</tt> if this set contains no elements
84 */
85 boolean isEmpty();
86
87 /**
88 * Returns <tt>true</tt> if this set contains the specified element.
89 * More formally, returns <tt>true</tt> if and only if this set
90 * contains an element <tt>e</tt> such that
91 * <tt>(o==null ? e==null : o.equals(e))</tt>.
92 *
93 * @param o element whose presence in this set is to be tested
94 * @return <tt>true</tt> if this set contains the specified element
95 * @throws ClassCastException if the type of the specified element
96 * is incompatible with this set (optional)
97 * @throws NullPointerException if the specified element is null and this
98 * set does not permit null elements (optional)
99 */
100 boolean contains(Object o);
101
102 /**
103 * Returns an iterator over the elements in this set. The elements are
104 * returned in no particular order (unless this set is an instance of some
105 * class that provides a guarantee).
106 *
107 * @return an iterator over the elements in this set
108 */
109 Iterator<E> iterator();
110
111 /**
112 * Returns an array containing all of the elements in this set.
113 * If this set makes any guarantees as to what order its elements
114 * are returned by its iterator, this method must return the
115 * elements in the same order.
116 *
117 * <p>The returned array will be "safe" in that no references to it
118 * are maintained by this set. (In other words, this method must
119 * allocate a new array even if this set is backed by an array).
120 * The caller is thus free to modify the returned array.
121 *
122 * <p>This method acts as bridge between array-based and collection-based
123 * APIs.
124 *
125 * @return an array containing all the elements in this set
126 */
127 Object[] toArray();
128
129 /**
130 * Returns an array containing all of the elements in this set; the
131 * runtime type of the returned array is that of the specified array.
132 * If the set fits in the specified array, it is returned therein.
133 * Otherwise, a new array is allocated with the runtime type of the
134 * specified array and the size of this set.
135 *
136 * <p>If this set fits in the specified array with room to spare
137 * (i.e., the array has more elements than this set), the element in
138 * the array immediately following the end of the set is set to
139 * <tt>null</tt>. (This is useful in determining the length of this
140 * set <i>only</i> if the caller knows that this set does not contain
141 * any null elements.)
142 *
143 * <p>If this set makes any guarantees as to what order its elements
144 * are returned by its iterator, this method must return the elements
145 * in the same order.
146 *
147 * <p>Like the {@link #toArray()} method, this method acts as bridge between
148 * array-based and collection-based APIs. Further, this method allows
149 * precise control over the runtime type of the output array, and may,
150 * under certain circumstances, be used to save allocation costs.
151 *
152 * <p>Suppose <tt>x</tt> is a set known to contain only strings.
153 * The following code can be used to dump the set into a newly allocated
154 * array of <tt>String</tt>:
155 *
156 * <pre>
157 * String[] y = x.toArray(new String[0]);</pre>
158 *
159 * Note that <tt>toArray(new Object[0])</tt> is identical in function to
160 * <tt>toArray()</tt>.
161 *
162 * @param a the array into which the elements of this set are to be
163 * stored, if it is big enough; otherwise, a new array of the same
164 * runtime type is allocated for this purpose.
165 * @return an array containing all the elements in this set
166 * @throws ArrayStoreException if the runtime type of the specified array
167 * is not a supertype of the runtime type of every element in this
168 * set
169 * @throws NullPointerException if the specified array is null
170 */
171 <T> T[] toArray(T[] a);
172
173
174 // Modification Operations
175
176 /**
177 * Adds the specified element to this set if it is not already present
178 * (optional operation). More formally, adds the specified element
179 * <tt>e</tt> to this set if the set contains no element <tt>e2</tt>
180 * such that
181 * <tt>(e==null ? e2==null : e.equals(e2))</tt>.
182 * If this set already contains the element, the call leaves the set
183 * unchanged and returns <tt>false</tt>. In combination with the
184 * restriction on constructors, this ensures that sets never contain
185 * duplicate elements.
186 *
187 * <p>The stipulation above does not imply that sets must accept all
188 * elements; sets may refuse to add any particular element, including
189 * <tt>null</tt>, and throw an exception, as described in the
190 * specification for {@link Collection#add Collection.add}.
191 * Individual set implementations should clearly document any
192 * restrictions on the elements that they may contain.
193 *
194 * @param e element to be added to this set
195 * @return <tt>true</tt> if this set did not already contain the specified
196 * element
197 * @throws UnsupportedOperationException if the <tt>add</tt> operation
198 * is not supported by this set
199 * @throws ClassCastException if the class of the specified element
200 * prevents it from being added to this set
201 * @throws NullPointerException if the specified element is null and this
202 * set does not permit null elements
203 * @throws IllegalArgumentException if some property of the specified element
204 * prevents it from being added to this set
205 */
206 boolean add(E e);
207
208
209 /**
210 * Removes the specified element from this set if it is present
211 * (optional operation). More formally, removes an element <tt>e</tt>
212 * such that
213 * <tt>(o==null ? e==null : o.equals(e))</tt>, if
214 * this set contains such an element. Returns <tt>true</tt> if this set
215 * contained the element (or equivalently, if this set changed as a
216 * result of the call). (This set will not contain the element once the
217 * call returns.)
218 *
219 * @param o object to be removed from this set, if present
220 * @return <tt>true</tt> if this set contained the specified element
221 * @throws ClassCastException if the type of the specified element
222 * is incompatible with this set (optional)
223 * @throws NullPointerException if the specified element is null and this
224 * set does not permit null elements (optional)
225 * @throws UnsupportedOperationException if the <tt>remove</tt> operation
226 * is not supported by this set
227 */
228 boolean remove(Object o);
229
230
231 // Bulk Operations
232
233 /**
234 * Returns <tt>true</tt> if this set contains all of the elements of the
235 * specified collection. If the specified collection is also a set, this
236 * method returns <tt>true</tt> if it is a <i>subset</i> of this set.
237 *
238 * @param c collection to be checked for containment in this set
239 * @return <tt>true</tt> if this set contains all of the elements of the
240 * specified collection
241 * @throws ClassCastException if the types of one or more elements
242 * in the specified collection are incompatible with this
243 * set (optional)
244 * @throws NullPointerException if the specified collection contains one
245 * or more null elements and this set does not permit null
246 * elements (optional), or if the specified collection is null
247 * @see #contains(Object)
248 */
249 boolean containsAll(Collection<?> c);
250
251 /**
252 * Adds all of the elements in the specified collection to this set if
253 * they're not already present (optional operation). If the specified
254 * collection is also a set, the <tt>addAll</tt> operation effectively
255 * modifies this set so that its value is the <i>union</i> of the two
256 * sets. The behavior of this operation is undefined if the specified
257 * collection is modified while the operation is in progress.
258 *
259 * @param c collection containing elements to be added to this set
260 * @return <tt>true</tt> if this set changed as a result of the call
261 *
262 * @throws UnsupportedOperationException if the <tt>addAll</tt> operation
263 * is not supported by this set
264 * @throws ClassCastException if the class of an element of the
265 * specified collection prevents it from being added to this set
266 * @throws NullPointerException if the specified collection contains one
267 * or more null elements and this set does not permit null
268 * elements, or if the specified collection is null
269 * @throws IllegalArgumentException if some property of an element of the
270 * specified collection prevents it from being added to this set
271 * @see #add(Object)
272 */
273 boolean addAll(Collection<? extends E> c);
274
275 /**
276 * Retains only the elements in this set that are contained in the
277 * specified collection (optional operation). In other words, removes
278 * from this set all of its elements that are not contained in the
279 * specified collection. If the specified collection is also a set, this
280 * operation effectively modifies this set so that its value is the
281 * <i>intersection</i> of the two sets.
282 *
283 * @param c collection containing elements to be retained in this set
284 * @return <tt>true</tt> if this set changed as a result of the call
285 * @throws UnsupportedOperationException if the <tt>retainAll</tt> operation
286 * is not supported by this set
287 * @throws ClassCastException if the class of an element of this set
288 * is incompatible with the specified collection (optional)
289 * @throws NullPointerException if this set contains a null element and the
290 * specified collection does not permit null elements (optional),
291 * or if the specified collection is null
292 * @see #remove(Object)
293 */
294 boolean retainAll(Collection<?> c);
295
296 /**
297 * Removes from this set all of its elements that are contained in the
298 * specified collection (optional operation). If the specified
299 * collection is also a set, this operation effectively modifies this
300 * set so that its value is the <i>asymmetric set difference</i> of
301 * the two sets.
302 *
303 * @param c collection containing elements to be removed from this set
304 * @return <tt>true</tt> if this set changed as a result of the call
305 * @throws UnsupportedOperationException if the <tt>removeAll</tt> operation
306 * is not supported by this set
307 * @throws ClassCastException if the class of an element of this set
308 * is incompatible with the specified collection (optional)
309 * @throws NullPointerException if this set contains a null element and the
310 * specified collection does not permit null elements (optional),
311 * or if the specified collection is null
312 * @see #remove(Object)
313 * @see #contains(Object)
314 */
315 boolean removeAll(Collection<?> c);
316
317 /**
318 * Removes all of the elements from this set (optional operation).
319 * The set will be empty after this call returns.
320 *
321 * @throws UnsupportedOperationException if the <tt>clear</tt> method
322 * is not supported by this set
323 */
324 void clear();
325
326
327 // Comparison and hashing
328
329 /**
330 * Compares the specified object with this set for equality. Returns
331 * <tt>true</tt> if the specified object is also a set, the two sets
332 * have the same size, and every member of the specified set is
333 * contained in this set (or equivalently, every member of this set is
334 * contained in the specified set). This definition ensures that the
335 * equals method works properly across different implementations of the
336 * set interface.
337 *
338 * @param o object to be compared for equality with this set
339 * @return <tt>true</tt> if the specified object is equal to this set
340 */
341 boolean equals(Object o);
342
343 /**
344 * Returns the hash code value for this set. The hash code of a set is
345 * defined to be the sum of the hash codes of the elements in the set,
346 * where the hash code of a <tt>null</tt> element is defined to be zero.
347 * This ensures that <tt>s1.equals(s2)</tt> implies that
348 * <tt>s1.hashCode()==s2.hashCode()</tt> for any two sets <tt>s1</tt>
349 * and <tt>s2</tt>, as required by the general contract of
350 * {@link Object#hashCode}.
351 *
352 * @return the hash code value for this set
353 * @see Object#equals(Object)
354 * @see Set#equals(Object)
355 */
356 int hashCode();
357 }
358