| HashSet.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 * This class implements the <tt>Set</tt> interface, backed by a hash table
12 * (actually a <tt>HashMap</tt> instance). It makes no guarantees as to the
13 * iteration order of the set; in particular, it does not guarantee that the
14 * order will remain constant over time. This class permits the <tt>null</tt>
15 * element.
16 *
17 * <p>This class offers constant time performance for the basic operations
18 * (<tt>add</tt>, <tt>remove</tt>, <tt>contains</tt> and <tt>size</tt>),
19 * assuming the hash function disperses the elements properly among the
20 * buckets. Iterating over this set requires time proportional to the sum of
21 * the <tt>HashSet</tt> instance's size (the number of elements) plus the
22 * "capacity" of the backing <tt>HashMap</tt> instance (the number of
23 * buckets). Thus, it's very important not to set the initial capacity too
24 * high (or the load factor too low) if iteration performance is important.
25 *
26 * <p><strong>Note that this implementation is not synchronized.</strong>
27 * If multiple threads access a hash set concurrently, and at least one of
28 * the threads modifies the set, it <i>must</i> be synchronized externally.
29 * This is typically accomplished by synchronizing on some object that
30 * naturally encapsulates the set.
31 *
32 * If no such object exists, the set should be "wrapped" using the
33 * {@link Collections#synchronizedSet Collections.synchronizedSet}
34 * method. This is best done at creation time, to prevent accidental
35 * unsynchronized access to the set:<pre>
36 * Set s = Collections.synchronizedSet(new HashSet(...));</pre>
37 *
38 * <p>The iterators returned by this class's <tt>iterator</tt> method are
39 * <i>fail-fast</i>: if the set is modified at any time after the iterator is
40 * created, in any way except through the iterator's own <tt>remove</tt>
41 * method, the Iterator throws a {@link ConcurrentModificationException}.
42 * Thus, in the face of concurrent modification, the iterator fails quickly
43 * and cleanly, rather than risking arbitrary, non-deterministic behavior at
44 * an undetermined time in the future.
45 *
46 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
47 * as it is, generally speaking, impossible to make any hard guarantees in the
48 * presence of unsynchronized concurrent modification. Fail-fast iterators
49 * throw <tt>ConcurrentModificationException</tt> on a best-effort basis.
50 * Therefore, it would be wrong to write a program that depended on this
51 * exception for its correctness: <i>the fail-fast behavior of iterators
52 * should be used only to detect bugs.</i>
53 *
54 * <p>This class is a member of the
55 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
56 * Java Collections Framework</a>.
57 *
58 * @param <E> the type of elements maintained by this set
59 *
60 * @author Josh Bloch
61 * @author Neal Gafter
62 * @version %I%, %G%
63 * @see Collection
64 * @see Set
65 * @see TreeSet
66 * @see HashMap
67 * @since 1.2
68 */
69
70 public class HashSet<E>
71 extends AbstractSet<E>
72 implements Set<E>, Cloneable, java.io.Serializable
73 {
74 static final long serialVersionUID = -5024744406713321676L;
75
76 private transient HashMap<E,Object> map;
77
78 // Dummy value to associate with an Object in the backing Map
79 private static final Object PRESENT = new Object();
80
81 /**
82 * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
83 * default initial capacity (16) and load factor (0.75).
84 */
85 public HashSet() {
86 map = new HashMap<E,Object>();
87 }
88
89 /**
90 * Constructs a new set containing the elements in the specified
91 * collection. The <tt>HashMap</tt> is created with default load factor
92 * (0.75) and an initial capacity sufficient to contain the elements in
93 * the specified collection.
94 *
95 * @param c the collection whose elements are to be placed into this set
96 * @throws NullPointerException if the specified collection is null
97 */
98 public HashSet(Collection<? extends E> c) {
99 map = new HashMap<E,Object>(Math.max((int) (c.size()/.75f) + 1, 16));
100 addAll(c);
101 }
102
103 /**
104 * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
105 * the specified initial capacity and the specified load factor.
106 *
107 * @param initialCapacity the initial capacity of the hash map
108 * @param loadFactor the load factor of the hash map
109 * @throws IllegalArgumentException if the initial capacity is less
110 * than zero, or if the load factor is nonpositive
111 */
112 public HashSet(int initialCapacity, float loadFactor) {
113 map = new HashMap<E,Object>(initialCapacity, loadFactor);
114 }
115
116 /**
117 * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
118 * the specified initial capacity and default load factor (0.75).
119 *
120 * @param initialCapacity the initial capacity of the hash table
121 * @throws IllegalArgumentException if the initial capacity is less
122 * than zero
123 */
124 public HashSet(int initialCapacity) {
125 map = new HashMap<E,Object>(initialCapacity);
126 }
127
128 /**
129 * Constructs a new, empty linked hash set. (This package private
130 * constructor is only used by LinkedHashSet.) The backing
131 * HashMap instance is a LinkedHashMap with the specified initial
132 * capacity and the specified load factor.
133 *
134 * @param initialCapacity the initial capacity of the hash map
135 * @param loadFactor the load factor of the hash map
136 * @param dummy ignored (distinguishes this
137 * constructor from other int, float constructor.)
138 * @throws IllegalArgumentException if the initial capacity is less
139 * than zero, or if the load factor is nonpositive
140 */
141 HashSet(int initialCapacity, float loadFactor, boolean dummy) {
142 map = new LinkedHashMap<E,Object>(initialCapacity, loadFactor);
143 }
144
145 /**
146 * Returns an iterator over the elements in this set. The elements
147 * are returned in no particular order.
148 *
149 * @return an Iterator over the elements in this set
150 * @see ConcurrentModificationException
151 */
152 public Iterator<E> iterator() {
153 return map.keySet().iterator();
154 }
155
156 /**
157 * Returns the number of elements in this set (its cardinality).
158 *
159 * @return the number of elements in this set (its cardinality)
160 */
161 public int size() {
162 return map.size();
163 }
164
165 /**
166 * Returns <tt>true</tt> if this set contains no elements.
167 *
168 * @return <tt>true</tt> if this set contains no elements
169 */
170 public boolean isEmpty() {
171 return map.isEmpty();
172 }
173
174 /**
175 * Returns <tt>true</tt> if this set contains the specified element.
176 * More formally, returns <tt>true</tt> if and only if this set
177 * contains an element <tt>e</tt> such that
178 * <tt>(o==null ? e==null : o.equals(e))</tt>.
179 *
180 * @param o element whose presence in this set is to be tested
181 * @return <tt>true</tt> if this set contains the specified element
182 */
183 public boolean contains(Object o) {
184 return map.containsKey(o);
185 }
186
187 /**
188 * Adds the specified element to this set if it is not already present.
189 * More formally, adds the specified element <tt>e</tt> to this set if
190 * this set contains no element <tt>e2</tt> such that
191 * <tt>(e==null ? e2==null : e.equals(e2))</tt>.
192 * If this set already contains the element, the call leaves the set
193 * unchanged and returns <tt>false</tt>.
194 *
195 * @param e element to be added to this set
196 * @return <tt>true</tt> if this set did not already contain the specified
197 * element
198 */
199 public boolean add(E e) {
200 return map.put(e, PRESENT)==null;
201 }
202
203 /**
204 * Removes the specified element from this set if it is present.
205 * More formally, removes an element <tt>e</tt> such that
206 * <tt>(o==null ? e==null : o.equals(e))</tt>,
207 * if this set contains such an element. Returns <tt>true</tt> if
208 * this set contained the element (or equivalently, if this set
209 * changed as a result of the call). (This set will not contain the
210 * element once the call returns.)
211 *
212 * @param o object to be removed from this set, if present
213 * @return <tt>true</tt> if the set contained the specified element
214 */
215 public boolean remove(Object o) {
216 return map.remove(o)==PRESENT;
217 }
218
219 /**
220 * Removes all of the elements from this set.
221 * The set will be empty after this call returns.
222 */
223 public void clear() {
224 map.clear();
225 }
226
227 /**
228 * Returns a shallow copy of this <tt>HashSet</tt> instance: the elements
229 * themselves are not cloned.
230 *
231 * @return a shallow copy of this set
232 */
233 public Object clone() {
234 try {
235 HashSet<E> newSet = (HashSet<E>) super.clone();
236 newSet.map = (HashMap<E, Object>) map.clone();
237 return newSet;
238 } catch (CloneNotSupportedException e) {
239 throw new InternalError();
240 }
241 }
242
243 /**
244 * Save the state of this <tt>HashSet</tt> instance to a stream (that is,
245 * serialize it).
246 *
247 * @serialData The capacity of the backing <tt>HashMap</tt> instance
248 * (int), and its load factor (float) are emitted, followed by
249 * the size of the set (the number of elements it contains)
250 * (int), followed by all of its elements (each an Object) in
251 * no particular order.
252 */
253 private void writeObject(java.io.ObjectOutputStream s)
254 throws java.io.IOException {
255 // Write out any hidden serialization magic
256 s.defaultWriteObject();
257
258 // Write out HashMap capacity and load factor
259 s.writeInt(map.capacity());
260 s.writeFloat(map.loadFactor());
261
262 // Write out size
263 s.writeInt(map.size());
264
265 // Write out all elements in the proper order.
266 for (Iterator i=map.keySet().iterator(); i.hasNext(); )
267 s.writeObject(i.next());
268 }
269
270 /**
271 * Reconstitute the <tt>HashSet</tt> instance from a stream (that is,
272 * deserialize it).
273 */
274 private void readObject(java.io.ObjectInputStream s)
275 throws java.io.IOException, ClassNotFoundException {
276 // Read in any hidden serialization magic
277 s.defaultReadObject();
278
279 // Read in HashMap capacity and load factor and create backing HashMap
280 int capacity = s.readInt();
281 float loadFactor = s.readFloat();
282 map = (((HashSet)this) instanceof LinkedHashSet ?
283 new LinkedHashMap<E,Object>(capacity, loadFactor) :
284 new HashMap<E,Object>(capacity, loadFactor));
285
286 // Read in size
287 int size = s.readInt();
288
289 // Read in all elements in the proper order.
290 for (int i=0; i<size; i++) {
291 E e = (E) s.readObject();
292 map.put(e, PRESENT);
293 }
294 }
295 }
296