1 /*
2 * Copyright (c) 1997, 2019, 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 import java.io.InvalidObjectException;
29 import jdk.internal.access.SharedSecrets;
30
31 /**
32 * This class implements the {@code Set} interface, backed by a hash table
33 * (actually a {@code HashMap} instance). It makes no guarantees as to the
34 * iteration order of the set; in particular, it does not guarantee that the
35 * order will remain constant over time. This class permits the {@code null}
36 * element.
37 *
38 * <p>This class offers constant time performance for the basic operations
39 * ({@code add}, {@code remove}, {@code contains} and {@code size}),
40 * assuming the hash function disperses the elements properly among the
41 * buckets. Iterating over this set requires time proportional to the sum of
42 * the {@code HashSet} instance's size (the number of elements) plus the
43 * "capacity" of the backing {@code HashMap} instance (the number of
44 * buckets). Thus, it's very important not to set the initial capacity too
45 * high (or the load factor too low) if iteration performance is important.
46 *
47 * <p><strong>Note that this implementation is not synchronized.</strong>
48 * If multiple threads access a hash set concurrently, and at least one of
49 * the threads modifies the set, it <i>must</i> be synchronized externally.
50 * This is typically accomplished by synchronizing on some object that
51 * naturally encapsulates the set.
52 *
53 * If no such object exists, the set should be "wrapped" using the
54 * {@link Collections#synchronizedSet Collections.synchronizedSet}
55 * method. This is best done at creation time, to prevent accidental
56 * unsynchronized access to the set:<pre>
57 * Set s = Collections.synchronizedSet(new HashSet(...));</pre>
58 *
59 * <p>The iterators returned by this class's {@code iterator} method are
60 * <i>fail-fast</i>: if the set is modified at any time after the iterator is
61 * created, in any way except through the iterator's own {@code remove}
62 * method, the Iterator throws a {@link ConcurrentModificationException}.
63 * Thus, in the face of concurrent modification, the iterator fails quickly
64 * and cleanly, rather than risking arbitrary, non-deterministic behavior at
65 * an undetermined time in the future.
66 *
67 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
68 * as it is, generally speaking, impossible to make any hard guarantees in the
69 * presence of unsynchronized concurrent modification. Fail-fast iterators
70 * throw {@code ConcurrentModificationException} on a best-effort basis.
71 * Therefore, it would be wrong to write a program that depended on this
72 * exception for its correctness: <i>the fail-fast behavior of iterators
73 * should be used only to detect bugs.</i>
74 *
75 * <p>This class is a member of the
76 * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
77 * Java Collections Framework</a>.
78 *
79 * @param <E> the type of elements maintained by this set
80 *
81 * @author Josh Bloch
82 * @author Neal Gafter
83 * @see Collection
84 * @see Set
85 * @see TreeSet
86 * @see HashMap
87 * @since 1.2
88 */
89
90 public class HashSet<E>
91 extends AbstractSet<E>
92 implements Set<E>, Cloneable, java.io.Serializable
93 {
94 @java.io.Serial
95 static final long serialVersionUID = -5024744406713321676L;
96
97 private transient HashMap<E,Object> map;
98
99 // Dummy value to associate with an Object in the backing Map
100 private static final Object PRESENT = new Object();
101
102 /**
103 * Constructs a new, empty set; the backing {@code HashMap} instance has
104 * default initial capacity (16) and load factor (0.75).
105 */
106 public HashSet() {
107 map = new HashMap<>();
108 }
109
110 /**
111 * Constructs a new set containing the elements in the specified
112 * collection. The {@code HashMap} is created with default load factor
113 * (0.75) and an initial capacity sufficient to contain the elements in
114 * the specified collection.
115 *
116 * @param c the collection whose elements are to be placed into this set
117 * @throws NullPointerException if the specified collection is null
118 */
119 public HashSet(Collection<? extends E> c) {
120 map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
121 addAll(c);
122 }
123
124 /**
125 * Constructs a new, empty set; the backing {@code HashMap} instance has
126 * the specified initial capacity and the specified load factor.
127 *
128 * @param initialCapacity the initial capacity of the hash map
129 * @param loadFactor the load factor of the hash map
130 * @throws IllegalArgumentException if the initial capacity is less
131 * than zero, or if the load factor is nonpositive
132 */
133 public HashSet(int initialCapacity, float loadFactor) {
134 map = new HashMap<>(initialCapacity, loadFactor);
135 }
136
137 /**
138 * Constructs a new, empty set; the backing {@code HashMap} instance has
139 * the specified initial capacity and default load factor (0.75).
140 *
141 * @param initialCapacity the initial capacity of the hash table
142 * @throws IllegalArgumentException if the initial capacity is less
143 * than zero
144 */
145 public HashSet(int initialCapacity) {
146 map = new HashMap<>(initialCapacity);
147 }
148
149 /**
150 * Constructs a new, empty linked hash set. (This package private
151 * constructor is only used by LinkedHashSet.) The backing
152 * HashMap instance is a LinkedHashMap with the specified initial
153 * capacity and the specified load factor.
154 *
155 * @param initialCapacity the initial capacity of the hash map
156 * @param loadFactor the load factor of the hash map
157 * @param dummy ignored (distinguishes this
158 * constructor from other int, float constructor.)
159 * @throws IllegalArgumentException if the initial capacity is less
160 * than zero, or if the load factor is nonpositive
161 */
162 HashSet(int initialCapacity, float loadFactor, boolean dummy) {
163 map = new LinkedHashMap<>(initialCapacity, loadFactor);
164 }
165
166 /**
167 * Returns an iterator over the elements in this set. The elements
168 * are returned in no particular order.
169 *
170 * @return an Iterator over the elements in this set
171 * @see ConcurrentModificationException
172 */
173 public Iterator<E> iterator() {
174 return map.keySet().iterator();
175 }
176
177 /**
178 * Returns the number of elements in this set (its cardinality).
179 *
180 * @return the number of elements in this set (its cardinality)
181 */
182 public int size() {
183 return map.size();
184 }
185
186 /**
187 * Returns {@code true} if this set contains no elements.
188 *
189 * @return {@code true} if this set contains no elements
190 */
191 public boolean isEmpty() {
192 return map.isEmpty();
193 }
194
195 /**
196 * Returns {@code true} if this set contains the specified element.
197 * More formally, returns {@code true} if and only if this set
198 * contains an element {@code e} such that
199 * {@code Objects.equals(o, e)}.
200 *
201 * @param o element whose presence in this set is to be tested
202 * @return {@code true} if this set contains the specified element
203 */
204 public boolean contains(Object o) {
205 return map.containsKey(o);
206 }
207
208 /**
209 * Adds the specified element to this set if it is not already present.
210 * More formally, adds the specified element {@code e} to this set if
211 * this set contains no element {@code e2} such that
212 * {@code Objects.equals(e, e2)}.
213 * If this set already contains the element, the call leaves the set
214 * unchanged and returns {@code false}.
215 *
216 * @param e element to be added to this set
217 * @return {@code true} if this set did not already contain the specified
218 * element
219 */
220 public boolean add(E e) {
221 return map.put(e, PRESENT)==null;
222 }
223
224 /**
225 * Removes the specified element from this set if it is present.
226 * More formally, removes an element {@code e} such that
227 * {@code Objects.equals(o, e)},
228 * if this set contains such an element. Returns {@code true} if
229 * this set contained the element (or equivalently, if this set
230 * changed as a result of the call). (This set will not contain the
231 * element once the call returns.)
232 *
233 * @param o object to be removed from this set, if present
234 * @return {@code true} if the set contained the specified element
235 */
236 public boolean remove(Object o) {
237 return map.remove(o)==PRESENT;
238 }
239
240 /**
241 * Removes all of the elements from this set.
242 * The set will be empty after this call returns.
243 */
244 public void clear() {
245 map.clear();
246 }
247
248 /**
249 * Returns a shallow copy of this {@code HashSet} instance: the elements
250 * themselves are not cloned.
251 *
252 * @return a shallow copy of this set
253 */
254 @SuppressWarnings("unchecked")
255 public Object clone() {
256 try {
257 HashSet<E> newSet = (HashSet<E>) super.clone();
258 newSet.map = (HashMap<E, Object>) map.clone();
259 return newSet;
260 } catch (CloneNotSupportedException e) {
261 throw new InternalError(e);
262 }
263 }
264
265 /**
266 * Save the state of this {@code HashSet} instance to a stream (that is,
267 * serialize it).
268 *
269 * @serialData The capacity of the backing {@code HashMap} instance
270 * (int), and its load factor (float) are emitted, followed by
271 * the size of the set (the number of elements it contains)
272 * (int), followed by all of its elements (each an Object) in
273 * no particular order.
274 */
275 @java.io.Serial
276 private void writeObject(java.io.ObjectOutputStream s)
277 throws java.io.IOException {
278 // Write out any hidden serialization magic
279 s.defaultWriteObject();
280
281 // Write out HashMap capacity and load factor
282 s.writeInt(map.capacity());
283 s.writeFloat(map.loadFactor());
284
285 // Write out size
286 s.writeInt(map.size());
287
288 // Write out all elements in the proper order.
289 for (E e : map.keySet())
290 s.writeObject(e);
291 }
292
293 /**
294 * Reconstitute the {@code HashSet} instance from a stream (that is,
295 * deserialize it).
296 */
297 @java.io.Serial
298 private void readObject(java.io.ObjectInputStream s)
299 throws java.io.IOException, ClassNotFoundException {
300 // Read in any hidden serialization magic
301 s.defaultReadObject();
302
303 // Read capacity and verify non-negative.
304 int capacity = s.readInt();
305 if (capacity < 0) {
306 throw new InvalidObjectException("Illegal capacity: " +
307 capacity);
308 }
309
310 // Read load factor and verify positive and non NaN.
311 float loadFactor = s.readFloat();
312 if (loadFactor <= 0 || Float.isNaN(loadFactor)) {
313 throw new InvalidObjectException("Illegal load factor: " +
314 loadFactor);
315 }
316
317 // Read size and verify non-negative.
318 int size = s.readInt();
319 if (size < 0) {
320 throw new InvalidObjectException("Illegal size: " +
321 size);
322 }
323
324 // Set the capacity according to the size and load factor ensuring that
325 // the HashMap is at least 25% full but clamping to maximum capacity.
326 capacity = (int) Math.min(size * Math.min(1 / loadFactor, 4.0f),
327 HashMap.MAXIMUM_CAPACITY);
328
329 // Constructing the backing map will lazily create an array when the first element is
330 // added, so check it before construction. Call HashMap.tableSizeFor to compute the
331 // actual allocation size. Check Map.Entry[].class since it's the nearest public type to
332 // what is actually created.
333 SharedSecrets.getJavaObjectInputStreamAccess()
334 .checkArray(s, Map.Entry[].class, HashMap.tableSizeFor(capacity));
335
336 // Create backing HashMap
337 map = (((HashSet<?>)this) instanceof LinkedHashSet ?
338 new LinkedHashMap<>(capacity, loadFactor) :
339 new HashMap<>(capacity, loadFactor));
340
341 // Read in all elements in the proper order.
342 for (int i=0; i<size; i++) {
343 @SuppressWarnings("unchecked")
344 E e = (E) s.readObject();
345 map.put(e, PRESENT);
346 }
347 }
348
349 /**
350 * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
351 * and <em>fail-fast</em> {@link Spliterator} over the elements in this
352 * set.
353 *
354 * <p>The {@code Spliterator} reports {@link Spliterator#SIZED} and
355 * {@link Spliterator#DISTINCT}. Overriding implementations should document
356 * the reporting of additional characteristic values.
357 *
358 * @return a {@code Spliterator} over the elements in this set
359 * @since 1.8
360 */
361 public Spliterator<E> spliterator() {
362 return new HashMap.KeySpliterator<>(map, 0, -1, 0, 0);
363 }
364
365 @Override
366 public Object[] toArray() {
367 return map.keysToArray(new Object[map.size()]);
368 }
369
370 @Override
371 public <T> T[] toArray(T[] a) {
372 return map.keysToArray(map.prepareArray(a));
373 }
374 }