1 /*
2 * Copyright (c) 1994, 2011, 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 import java.io.*;
28
29 /**
30 * This class implements a hash table, which maps keys to values. Any
31 * non-<code>null</code> object can be used as a key or as a value. <p>
32 *
33 * To successfully store and retrieve objects from a hashtable, the
34 * objects used as keys must implement the <code>hashCode</code>
35 * method and the <code>equals</code> method. <p>
36 *
37 * An instance of <code>Hashtable</code> has two parameters that affect its
38 * performance: <i>initial capacity</i> and <i>load factor</i>. The
39 * <i>capacity</i> is the number of <i>buckets</i> in the hash table, and the
40 * <i>initial capacity</i> is simply the capacity at the time the hash table
41 * is created. Note that the hash table is <i>open</i>: in the case of a "hash
42 * collision", a single bucket stores multiple entries, which must be searched
43 * sequentially. The <i>load factor</i> is a measure of how full the hash
44 * table is allowed to get before its capacity is automatically increased.
45 * The initial capacity and load factor parameters are merely hints to
46 * the implementation. The exact details as to when and whether the rehash
47 * method is invoked are implementation-dependent.<p>
48 *
49 * Generally, the default load factor (.75) offers a good tradeoff between
50 * time and space costs. Higher values decrease the space overhead but
51 * increase the time cost to look up an entry (which is reflected in most
52 * <tt>Hashtable</tt> operations, including <tt>get</tt> and <tt>put</tt>).<p>
53 *
54 * The initial capacity controls a tradeoff between wasted space and the
55 * need for <code>rehash</code> operations, which are time-consuming.
56 * No <code>rehash</code> operations will <i>ever</i> occur if the initial
57 * capacity is greater than the maximum number of entries the
58 * <tt>Hashtable</tt> will contain divided by its load factor. However,
59 * setting the initial capacity too high can waste space.<p>
60 *
61 * If many entries are to be made into a <code>Hashtable</code>,
62 * creating it with a sufficiently large capacity may allow the
63 * entries to be inserted more efficiently than letting it perform
64 * automatic rehashing as needed to grow the table. <p>
65 *
66 * This example creates a hashtable of numbers. It uses the names of
67 * the numbers as keys:
68 * <pre> {@code
69 * Hashtable<String, Integer> numbers
70 * = new Hashtable<String, Integer>();
71 * numbers.put("one", 1);
72 * numbers.put("two", 2);
73 * numbers.put("three", 3);}</pre>
74 *
75 * <p>To retrieve a number, use the following code:
76 * <pre> {@code
77 * Integer n = numbers.get("two");
78 * if (n != null) {
79 * System.out.println("two = " + n);
80 * }}</pre>
81 *
82 * <p>The iterators returned by the <tt>iterator</tt> method of the collections
83 * returned by all of this class's "collection view methods" are
84 * <em>fail-fast</em>: if the Hashtable is structurally modified at any time
85 * after the iterator is created, in any way except through the iterator's own
86 * <tt>remove</tt> method, the iterator will throw a {@link
87 * ConcurrentModificationException}. Thus, in the face of concurrent
88 * modification, the iterator fails quickly and cleanly, rather than risking
89 * arbitrary, non-deterministic behavior at an undetermined time in the future.
90 * The Enumerations returned by Hashtable's keys and elements methods are
91 * <em>not</em> fail-fast.
92 *
93 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
94 * as it is, generally speaking, impossible to make any hard guarantees in the
95 * presence of unsynchronized concurrent modification. Fail-fast iterators
96 * throw <tt>ConcurrentModificationException</tt> on a best-effort basis.
97 * Therefore, it would be wrong to write a program that depended on this
98 * exception for its correctness: <i>the fail-fast behavior of iterators
99 * should be used only to detect bugs.</i>
100 *
101 * <p>As of the Java 2 platform v1.2, this class was retrofitted to
102 * implement the {@link Map} interface, making it a member of the
103 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
104 *
105 * Java Collections Framework</a>. Unlike the new collection
106 * implementations, {@code Hashtable} is synchronized. If a
107 * thread-safe implementation is not needed, it is recommended to use
108 * {@link HashMap} in place of {@code Hashtable}. If a thread-safe
109 * highly-concurrent implementation is desired, then it is recommended
110 * to use {@link java.util.concurrent.ConcurrentHashMap} in place of
111 * {@code Hashtable}.
112 *
113 * @author Arthur van Hoff
114 * @author Josh Bloch
115 * @author Neal Gafter
116 * @see Object#equals(java.lang.Object)
117 * @see Object#hashCode()
118 * @see Hashtable#rehash()
119 * @see Collection
120 * @see Map
121 * @see HashMap
122 * @see TreeMap
123 * @since JDK1.0
124 */
125 public class Hashtable<K,V>
126 extends Dictionary<K,V>
127 implements Map<K,V>, Cloneable, java.io.Serializable {
128
129 /**
130 * The hash table data.
131 */
132 private transient Entry<?,?>[] table;
133
134 /**
135 * The total number of entries in the hash table.
136 */
137 private transient int count;
138
139 /**
140 * The table is rehashed when its size exceeds this threshold. (The
141 * value of this field is (int)(capacity * loadFactor).)
142 *
143 * @serial
144 */
145 private int threshold;
146
147 /**
148 * The load factor for the hashtable.
149 *
150 * @serial
151 */
152 private float loadFactor;
153
154 /**
155 * The number of times this Hashtable has been structurally modified
156 * Structural modifications are those that change the number of entries in
157 * the Hashtable or otherwise modify its internal structure (e.g.,
158 * rehash). This field is used to make iterators on Collection-views of
159 * the Hashtable fail-fast. (See ConcurrentModificationException).
160 */
161 private transient int modCount = 0;
162
163 /** use serialVersionUID from JDK 1.0.2 for interoperability */
164 private static final long serialVersionUID = 1421746759512286392L;
165
166 private static class Holder {
167 // Unsafe mechanics
168 /**
169 *
170 */
171 static final sun.misc.Unsafe UNSAFE;
172
173 /**
174 * Offset of "final" hashSeed field we must set in
175 * readObject() method.
176 */
177 static final long HASHSEED_OFFSET;
178
179 static {
180 try {
181 UNSAFE = sun.misc.Unsafe.getUnsafe();
182 HASHSEED_OFFSET = UNSAFE.objectFieldOffset(
183 Hashtable.class.getDeclaredField("hashSeed"));
184 } catch (NoSuchFieldException | SecurityException e) {
185 throw new InternalError("Failed to record hashSeed offset", e);
186 }
187 }
188 }
189
190 /**
191 * A randomizing value associated with this instance that is applied to
192 * hash code of keys to make hash collisions harder to find.
193 */
194 transient final int hashSeed = sun.misc.Hashing.randomHashSeed(this);
195
196 private int hash(Object k) {
197 int h = hashSeed;
198
199 if (k instanceof String) {
200 return h ^ ((String)k).hash32();
201 } else {
202 h ^= k.hashCode();
203
204 // This function ensures that hashCodes that differ only by
205 // constant multiples at each bit position have a bounded
206 // number of collisions (approximately 8 at default load factor).
207 h ^= (h >>> 20) ^ (h >>> 12);
208 return h ^ (h >>> 7) ^ (h >>> 4);
209 }
210 }
211
212 /**
213 * Constructs a new, empty hashtable with the specified initial
214 * capacity and the specified load factor.
215 *
216 * @param initialCapacity the initial capacity of the hashtable.
217 * @param loadFactor the load factor of the hashtable.
218 * @exception IllegalArgumentException if the initial capacity is less
219 * than zero, or if the load factor is nonpositive.
220 */
221 public Hashtable(int initialCapacity, float loadFactor) {
222 if (initialCapacity < 0)
223 throw new IllegalArgumentException("Illegal Capacity: "+
224 initialCapacity);
225 if (loadFactor <= 0 || Float.isNaN(loadFactor))
226 throw new IllegalArgumentException("Illegal Load: "+loadFactor);
227
228 if (initialCapacity==0)
229 initialCapacity = 1;
230 this.loadFactor = loadFactor;
231 table = new Entry<?,?>[initialCapacity];
232 threshold = (int)Math.min(initialCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
233 }
234
235 /**
236 * Constructs a new, empty hashtable with the specified initial capacity
237 * and default load factor (0.75).
238 *
239 * @param initialCapacity the initial capacity of the hashtable.
240 * @exception IllegalArgumentException if the initial capacity is less
241 * than zero.
242 */
243 public Hashtable(int initialCapacity) {
244 this(initialCapacity, 0.75f);
245 }
246
247 /**
248 * Constructs a new, empty hashtable with a default initial capacity (11)
249 * and load factor (0.75).
250 */
251 public Hashtable() {
252 this(11, 0.75f);
253 }
254
255 /**
256 * Constructs a new hashtable with the same mappings as the given
257 * Map. The hashtable is created with an initial capacity sufficient to
258 * hold the mappings in the given Map and a default load factor (0.75).
259 *
260 * @param t the map whose mappings are to be placed in this map.
261 * @throws NullPointerException if the specified map is null.
262 * @since 1.2
263 */
264 public Hashtable(Map<? extends K, ? extends V> t) {
265 this(Math.max(2*t.size(), 11), 0.75f);
266 putAll(t);
267 }
268
269 /**
270 * Returns the number of keys in this hashtable.
271 *
272 * @return the number of keys in this hashtable.
273 */
274 public synchronized int size() {
275 return count;
276 }
277
278 /**
279 * Tests if this hashtable maps no keys to values.
280 *
281 * @return <code>true</code> if this hashtable maps no keys to values;
282 * <code>false</code> otherwise.
283 */
284 public synchronized boolean isEmpty() {
285 return count == 0;
286 }
287
288 /**
289 * Returns an enumeration of the keys in this hashtable.
290 *
291 * @return an enumeration of the keys in this hashtable.
292 * @see Enumeration
293 * @see #elements()
294 * @see #keySet()
295 * @see Map
296 */
297 public synchronized Enumeration<K> keys() {
298 return this.<K>getEnumeration(KEYS);
299 }
300
301 /**
302 * Returns an enumeration of the values in this hashtable.
303 * Use the Enumeration methods on the returned object to fetch the elements
304 * sequentially.
305 *
306 * @return an enumeration of the values in this hashtable.
307 * @see java.util.Enumeration
308 * @see #keys()
309 * @see #values()
310 * @see Map
311 */
312 public synchronized Enumeration<V> elements() {
313 return this.<V>getEnumeration(VALUES);
314 }
315
316 /**
317 * Tests if some key maps into the specified value in this hashtable.
318 * This operation is more expensive than the {@link #containsKey
319 * containsKey} method.
320 *
321 * <p>Note that this method is identical in functionality to
322 * {@link #containsValue containsValue}, (which is part of the
323 * {@link Map} interface in the collections framework).
324 *
325 * @param value a value to search for
326 * @return <code>true</code> if and only if some key maps to the
327 * <code>value</code> argument in this hashtable as
328 * determined by the <tt>equals</tt> method;
329 * <code>false</code> otherwise.
330 * @exception NullPointerException if the value is <code>null</code>
331 */
332 public synchronized boolean contains(Object value) {
333 if (value == null) {
334 throw new NullPointerException();
335 }
336
337 Entry<?,?> tab[] = table;
338 for (int i = tab.length ; i-- > 0 ;) {
339 for (Entry<?,?> e = tab[i] ; e != null ; e = e.next) {
340 if (e.value.equals(value)) {
341 return true;
342 }
343 }
344 }
345 return false;
346 }
347
348 /**
349 * Returns true if this hashtable maps one or more keys to this value.
350 *
351 * <p>Note that this method is identical in functionality to {@link
352 * #contains contains} (which predates the {@link Map} interface).
353 *
354 * @param value value whose presence in this hashtable is to be tested
355 * @return <tt>true</tt> if this map maps one or more keys to the
356 * specified value
357 * @throws NullPointerException if the value is <code>null</code>
358 * @since 1.2
359 */
360 public boolean containsValue(Object value) {
361 return contains(value);
362 }
363
364 /**
365 * Tests if the specified object is a key in this hashtable.
366 *
367 * @param key possible key
368 * @return <code>true</code> if and only if the specified object
369 * is a key in this hashtable, as determined by the
370 * <tt>equals</tt> method; <code>false</code> otherwise.
371 * @throws NullPointerException if the key is <code>null</code>
372 * @see #contains(Object)
373 */
374 public synchronized boolean containsKey(Object key) {
375 Entry<?,?> tab[] = table;
376 int hash = hash(key);
377 int index = (hash & 0x7FFFFFFF) % tab.length;
378 for (Entry<?,?> e = tab[index] ; e != null ; e = e.next) {
379 if ((e.hash == hash) && e.key.equals(key)) {
380 return true;
381 }
382 }
383 return false;
384 }
385
386 /**
387 * Returns the value to which the specified key is mapped,
388 * or {@code null} if this map contains no mapping for the key.
389 *
390 * <p>More formally, if this map contains a mapping from a key
391 * {@code k} to a value {@code v} such that {@code (key.equals(k))},
392 * then this method returns {@code v}; otherwise it returns
393 * {@code null}. (There can be at most one such mapping.)
394 *
395 * @param key the key whose associated value is to be returned
396 * @return the value to which the specified key is mapped, or
397 * {@code null} if this map contains no mapping for the key
398 * @throws NullPointerException if the specified key is null
399 * @see #put(Object, Object)
400 */
401 @SuppressWarnings("unchecked")
402 public synchronized V get(Object key) {
403 Entry<?,?> tab[] = table;
404 int hash = hash(key);
405 int index = (hash & 0x7FFFFFFF) % tab.length;
406 for (Entry<?,?> e = tab[index] ; e != null ; e = e.next) {
407 if ((e.hash == hash) && e.key.equals(key)) {
408 return (V)e.value;
409 }
410 }
411 return null;
412 }
413
414 /**
415 * The maximum size of array to allocate.
416 * Some VMs reserve some header words in an array.
417 * Attempts to allocate larger arrays may result in
418 * OutOfMemoryError: Requested array size exceeds VM limit
419 */
420 private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
421
422 /**
423 * Increases the capacity of and internally reorganizes this
424 * hashtable, in order to accommodate and access its entries more
425 * efficiently. This method is called automatically when the
426 * number of keys in the hashtable exceeds this hashtable's capacity
427 * and load factor.
428 */
429 @SuppressWarnings("unchecked")
430 protected void rehash() {
431 int oldCapacity = table.length;
432 Entry<?,?>[] oldMap = table;
433
434 // overflow-conscious code
435 int newCapacity = (oldCapacity << 1) + 1;
436 if (newCapacity - MAX_ARRAY_SIZE > 0) {
437 if (oldCapacity == MAX_ARRAY_SIZE)
438 // Keep running with MAX_ARRAY_SIZE buckets
439 return;
440 newCapacity = MAX_ARRAY_SIZE;
441 }
442 Entry<?,?>[] newMap = new Entry<?,?>[newCapacity];
443
444 modCount++;
445 threshold = (int)Math.min(newCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
446 table = newMap;
447
448 for (int i = oldCapacity ; i-- > 0 ;) {
449 for (Entry<K,V> old = (Entry<K,V>)oldMap[i] ; old != null ; ) {
450 Entry<K,V> e = old;
451 old = old.next;
452
453 int index = (e.hash & 0x7FFFFFFF) % newCapacity;
454 e.next = (Entry<K,V>)newMap[index];
455 newMap[index] = e;
456 }
457 }
458 }
459
460 /**
461 * Maps the specified <code>key</code> to the specified
462 * <code>value</code> in this hashtable. Neither the key nor the
463 * value can be <code>null</code>. <p>
464 *
465 * The value can be retrieved by calling the <code>get</code> method
466 * with a key that is equal to the original key.
467 *
468 * @param key the hashtable key
469 * @param value the value
470 * @return the previous value of the specified key in this hashtable,
471 * or <code>null</code> if it did not have one
472 * @exception NullPointerException if the key or value is
473 * <code>null</code>
474 * @see Object#equals(Object)
475 * @see #get(Object)
476 */
477 public synchronized V put(K key, V value) {
478 // Make sure the value is not null
479 if (value == null) {
480 throw new NullPointerException();
481 }
482
483 // Makes sure the key is not already in the hashtable.
484 Entry<?,?> tab[] = table;
485 int hash = hash(key);
486 int index = (hash & 0x7FFFFFFF) % tab.length;
487 @SuppressWarnings("unchecked")
488 Entry<K,V> entry = (Entry<K,V>)tab[index];
489 for(; entry != null ; entry = entry.next) {
490 if ((entry.hash == hash) && entry.key.equals(key)) {
491 V old = entry.value;
492 entry.value = value;
493 return old;
494 }
495 }
496
497 modCount++;
498 if (count >= threshold) {
499 // Rehash the table if the threshold is exceeded
500 rehash();
501
502 tab = table;
503 hash = hash(key);
504 index = (hash & 0x7FFFFFFF) % tab.length;
505 }
506
507 // Creates the new entry.
508 @SuppressWarnings("unchecked")
509 Entry<K,V> e = (Entry<K,V>)tab[index];
510 tab[index] = new Entry<>(hash, key, value, e);
511 count++;
512 return null;
513 }
514
515 /**
516 * Removes the key (and its corresponding value) from this
517 * hashtable. This method does nothing if the key is not in the hashtable.
518 *
519 * @param key the key that needs to be removed
520 * @return the value to which the key had been mapped in this hashtable,
521 * or <code>null</code> if the key did not have a mapping
522 * @throws NullPointerException if the key is <code>null</code>
523 */
524 public synchronized V remove(Object key) {
525 Entry<?,?> tab[] = table;
526 int hash = hash(key);
527 int index = (hash & 0x7FFFFFFF) % tab.length;
528 @SuppressWarnings("unchecked")
529 Entry<K,V> e = (Entry<K,V>)tab[index];
530 for(Entry<K,V> prev = null ; e != null ; prev = e, e = e.next) {
531 if ((e.hash == hash) && e.key.equals(key)) {
532 modCount++;
533 if (prev != null) {
534 prev.next = e.next;
535 } else {
536 tab[index] = e.next;
537 }
538 count--;
539 V oldValue = e.value;
540 e.value = null;
541 return oldValue;
542 }
543 }
544 return null;
545 }
546
547 /**
548 * Copies all of the mappings from the specified map to this hashtable.
549 * These mappings will replace any mappings that this hashtable had for any
550 * of the keys currently in the specified map.
551 *
552 * @param t mappings to be stored in this map
553 * @throws NullPointerException if the specified map is null
554 * @since 1.2
555 */
556 public synchronized void putAll(Map<? extends K, ? extends V> t) {
557 for (Map.Entry<? extends K, ? extends V> e : t.entrySet())
558 put(e.getKey(), e.getValue());
559 }
560
561 /**
562 * Clears this hashtable so that it contains no keys.
563 */
564 public synchronized void clear() {
565 Entry<?,?> tab[] = table;
566 modCount++;
567 for (int index = tab.length; --index >= 0; )
568 tab[index] = null;
569 count = 0;
570 }
571
572 /**
573 * Creates a shallow copy of this hashtable. All the structure of the
574 * hashtable itself is copied, but the keys and values are not cloned.
575 * This is a relatively expensive operation.
576 *
577 * @return a clone of the hashtable
578 */
579 public synchronized Object clone() {
580 try {
581 Hashtable<?,?> t = (Hashtable<?,?>)super.clone();
582 t.table = new Entry<?,?>[table.length];
583 for (int i = table.length ; i-- > 0 ; ) {
584 t.table[i] = (table[i] != null)
585 ? (Entry<?,?>) table[i].clone() : null;
586 }
587 t.keySet = null;
588 t.entrySet = null;
589 t.values = null;
590 t.modCount = 0;
591 return t;
592 } catch (CloneNotSupportedException e) {
593 // this shouldn't happen, since we are Cloneable
594 throw new InternalError(e);
595 }
596 }
597
598 /**
599 * Returns a string representation of this <tt>Hashtable</tt> object
600 * in the form of a set of entries, enclosed in braces and separated
601 * by the ASCII characters "<tt>, </tt>" (comma and space). Each
602 * entry is rendered as the key, an equals sign <tt>=</tt>, and the
603 * associated element, where the <tt>toString</tt> method is used to
604 * convert the key and element to strings.
605 *
606 * @return a string representation of this hashtable
607 */
608 public synchronized String toString() {
609 int max = size() - 1;
610 if (max == -1)
611 return "{}";
612
613 StringBuilder sb = new StringBuilder();
614 Iterator<Map.Entry<K,V>> it = entrySet().iterator();
615
616 sb.append('{');
617 for (int i = 0; ; i++) {
618 Map.Entry<K,V> e = it.next();
619 K key = e.getKey();
620 V value = e.getValue();
621 sb.append(key == this ? "(this Map)" : key.toString());
622 sb.append('=');
623 sb.append(value == this ? "(this Map)" : value.toString());
624
625 if (i == max)
626 return sb.append('}').toString();
627 sb.append(", ");
628 }
629 }
630
631
632 private <T> Enumeration<T> getEnumeration(int type) {
633 if (count == 0) {
634 return Collections.emptyEnumeration();
635 } else {
636 return new Enumerator<>(type, false);
637 }
638 }
639
640 private <T> Iterator<T> getIterator(int type) {
641 if (count == 0) {
642 return Collections.emptyIterator();
643 } else {
644 return new Enumerator<>(type, true);
645 }
646 }
647
648 // Views
649
650 /**
651 * Each of these fields are initialized to contain an instance of the
652 * appropriate view the first time this view is requested. The views are
653 * stateless, so there's no reason to create more than one of each.
654 */
655 private transient volatile Set<K> keySet = null;
656 private transient volatile Set<Map.Entry<K,V>> entrySet = null;
657 private transient volatile Collection<V> values = null;
658
659 /**
660 * Returns a {@link Set} view of the keys contained in this map.
661 * The set is backed by the map, so changes to the map are
662 * reflected in the set, and vice-versa. If the map is modified
663 * while an iteration over the set is in progress (except through
664 * the iterator's own <tt>remove</tt> operation), the results of
665 * the iteration are undefined. The set supports element removal,
666 * which removes the corresponding mapping from the map, via the
667 * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
668 * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
669 * operations. It does not support the <tt>add</tt> or <tt>addAll</tt>
670 * operations.
671 *
672 * @since 1.2
673 */
674 public Set<K> keySet() {
675 if (keySet == null)
676 keySet = Collections.synchronizedSet(new KeySet(), this);
677 return keySet;
678 }
679
680 private class KeySet extends AbstractSet<K> {
681 public Iterator<K> iterator() {
682 return getIterator(KEYS);
683 }
684 public int size() {
685 return count;
686 }
687 public boolean contains(Object o) {
688 return containsKey(o);
689 }
690 public boolean remove(Object o) {
691 return Hashtable.this.remove(o) != null;
692 }
693 public void clear() {
694 Hashtable.this.clear();
695 }
696 }
697
698 /**
699 * Returns a {@link Set} view of the mappings contained in this map.
700 * The set is backed by the map, so changes to the map are
701 * reflected in the set, and vice-versa. If the map is modified
702 * while an iteration over the set is in progress (except through
703 * the iterator's own <tt>remove</tt> operation, or through the
704 * <tt>setValue</tt> operation on a map entry returned by the
705 * iterator) the results of the iteration are undefined. The set
706 * supports element removal, which removes the corresponding
707 * mapping from the map, via the <tt>Iterator.remove</tt>,
708 * <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and
709 * <tt>clear</tt> operations. It does not support the
710 * <tt>add</tt> or <tt>addAll</tt> operations.
711 *
712 * @since 1.2
713 */
714 public Set<Map.Entry<K,V>> entrySet() {
715 if (entrySet==null)
716 entrySet = Collections.synchronizedSet(new EntrySet(), this);
717 return entrySet;
718 }
719
720 private class EntrySet extends AbstractSet<Map.Entry<K,V>> {
721 public Iterator<Map.Entry<K,V>> iterator() {
722 return getIterator(ENTRIES);
723 }
724
725 public boolean add(Map.Entry<K,V> o) {
726 return super.add(o);
727 }
728
729 public boolean contains(Object o) {
730 if (!(o instanceof Map.Entry))
731 return false;
732 Map.Entry<?,?> entry = (Map.Entry<?,?>)o;
733 Object key = entry.getKey();
734 Entry<?,?>[] tab = table;
735 int hash = hash(key);
736 int index = (hash & 0x7FFFFFFF) % tab.length;
737
738 for (Entry<?,?> e = tab[index]; e != null; e = e.next)
739 if (e.hash==hash && e.equals(entry))
740 return true;
741 return false;
742 }
743
744 public boolean remove(Object o) {
745 if (!(o instanceof Map.Entry))
746 return false;
747 Map.Entry<?,?> entry = (Map.Entry<?,?>) o;
748 Object key = entry.getKey();
749 Entry<?,?>[] tab = table;
750 int hash = hash(key);
751 int index = (hash & 0x7FFFFFFF) % tab.length;
752
753 @SuppressWarnings("unchecked")
754 Entry<K,V> e = (Entry<K,V>)tab[index];
755 for(Entry<K,V> prev = null; e != null; prev = e, e = e.next) {
756 if (e.hash==hash && e.equals(entry)) {
757 modCount++;
758 if (prev != null)
759 prev.next = e.next;
760 else
761 tab[index] = e.next;
762
763 count--;
764 e.value = null;
765 return true;
766 }
767 }
768 return false;
769 }
770
771 public int size() {
772 return count;
773 }
774
775 public void clear() {
776 Hashtable.this.clear();
777 }
778 }
779
780 /**
781 * Returns a {@link Collection} view of the values contained in this map.
782 * The collection is backed by the map, so changes to the map are
783 * reflected in the collection, and vice-versa. If the map is
784 * modified while an iteration over the collection is in progress
785 * (except through the iterator's own <tt>remove</tt> operation),
786 * the results of the iteration are undefined. The collection
787 * supports element removal, which removes the corresponding
788 * mapping from the map, via the <tt>Iterator.remove</tt>,
789 * <tt>Collection.remove</tt>, <tt>removeAll</tt>,
790 * <tt>retainAll</tt> and <tt>clear</tt> operations. It does not
791 * support the <tt>add</tt> or <tt>addAll</tt> operations.
792 *
793 * @since 1.2
794 */
795 public Collection<V> values() {
796 if (values==null)
797 values = Collections.synchronizedCollection(new ValueCollection(),
798 this);
799 return values;
800 }
801
802 private class ValueCollection extends AbstractCollection<V> {
803 public Iterator<V> iterator() {
804 return getIterator(VALUES);
805 }
806 public int size() {
807 return count;
808 }
809 public boolean contains(Object o) {
810 return containsValue(o);
811 }
812 public void clear() {
813 Hashtable.this.clear();
814 }
815 }
816
817 // Comparison and hashing
818
819 /**
820 * Compares the specified Object with this Map for equality,
821 * as per the definition in the Map interface.
822 *
823 * @param o object to be compared for equality with this hashtable
824 * @return true if the specified Object is equal to this Map
825 * @see Map#equals(Object)
826 * @since 1.2
827 */
828 public synchronized boolean equals(Object o) {
829 if (o == this)
830 return true;
831
832 if (!(o instanceof Map))
833 return false;
834 Map<?,?> t = (Map<?,?>) o;
835 if (t.size() != size())
836 return false;
837
838 try {
839 Iterator<Map.Entry<K,V>> i = entrySet().iterator();
840 while (i.hasNext()) {
841 Map.Entry<K,V> e = i.next();
842 K key = e.getKey();
843 V value = e.getValue();
844 if (value == null) {
845 if (!(t.get(key)==null && t.containsKey(key)))
846 return false;
847 } else {
848 if (!value.equals(t.get(key)))
849 return false;
850 }
851 }
852 } catch (ClassCastException unused) {
853 return false;
854 } catch (NullPointerException unused) {
855 return false;
856 }
857
858 return true;
859 }
860
861 /**
862 * Returns the hash code value for this Map as per the definition in the
863 * Map interface.
864 *
865 * @see Map#hashCode()
866 * @since 1.2
867 */
868 public synchronized int hashCode() {
869 /*
870 * This code detects the recursion caused by computing the hash code
871 * of a self-referential hash table and prevents the stack overflow
872 * that would otherwise result. This allows certain 1.1-era
873 * applets with self-referential hash tables to work. This code
874 * abuses the loadFactor field to do double-duty as a hashCode
875 * in progress flag, so as not to worsen the space performance.
876 * A negative load factor indicates that hash code computation is
877 * in progress.
878 */
879 int h = 0;
880 if (count == 0 || loadFactor < 0)
881 return h; // Returns zero
882
883 loadFactor = -loadFactor; // Mark hashCode computation in progress
884 Entry<?,?>[] tab = table;
885 for (Entry<?,?> entry : tab) {
886 while (entry != null) {
887 h += entry.hashCode();
888 entry = entry.next;
889 }
890 }
891
892 loadFactor = -loadFactor; // Mark hashCode computation complete
893
894 return h;
895 }
896
897 /**
898 * Save the state of the Hashtable to a stream (i.e., serialize it).
899 *
900 * @serialData The <i>capacity</i> of the Hashtable (the length of the
901 * bucket array) is emitted (int), followed by the
902 * <i>size</i> of the Hashtable (the number of key-value
903 * mappings), followed by the key (Object) and value (Object)
904 * for each key-value mapping represented by the Hashtable
905 * The key-value mappings are emitted in no particular order.
906 */
907 private void writeObject(java.io.ObjectOutputStream s)
908 throws IOException {
909 Entry<Object, Object> entryStack = null;
910
911 synchronized (this) {
912 // Write out the length, threshold, loadfactor
913 s.defaultWriteObject();
914
915 // Write out length, count of elements
916 s.writeInt(table.length);
917 s.writeInt(count);
918
919 // Stack copies of the entries in the table
920 for (int index = 0; index < table.length; index++) {
921 Entry<?,?> entry = table[index];
922
923 while (entry != null) {
924 entryStack =
925 new Entry<>(0, entry.key, entry.value, entryStack);
926 entry = entry.next;
927 }
928 }
929 }
930
931 // Write out the key/value objects from the stacked entries
932 while (entryStack != null) {
933 s.writeObject(entryStack.key);
934 s.writeObject(entryStack.value);
935 entryStack = entryStack.next;
936 }
937 }
938
939 /**
940 * Reconstitute the Hashtable from a stream (i.e., deserialize it).
941 */
942 private void readObject(java.io.ObjectInputStream s)
943 throws IOException, ClassNotFoundException
944 {
945 // Read in the length, threshold, and loadfactor
946 s.defaultReadObject();
947
948 // set hashMask
949 Holder.UNSAFE.putIntVolatile(this, Holder.HASHSEED_OFFSET,
950 sun.misc.Hashing.randomHashSeed(this));
951
952 // Read the original length of the array and number of elements
953 int origlength = s.readInt();
954 int elements = s.readInt();
955
956 // Compute new size with a bit of room 5% to grow but
957 // no larger than the original size. Make the length
958 // odd if it's large enough, this helps distribute the entries.
959 // Guard against the length ending up zero, that's not valid.
960 int length = (int)(elements * loadFactor) + (elements / 20) + 3;
961 if (length > elements && (length & 1) == 0)
962 length--;
963 if (origlength > 0 && length > origlength)
964 length = origlength;
965 table = new Entry<?,?>[length];
966 threshold = (int)Math.min(length * loadFactor, MAX_ARRAY_SIZE + 1);
967 count = 0;
968
969 // Read the number of elements and then all the key/value objects
970 for (; elements > 0; elements--) {
971 @SuppressWarnings("unchecked")
972 K key = (K)s.readObject();
973 @SuppressWarnings("unchecked")
974 V value = (V)s.readObject();
975 // synch could be eliminated for performance
976 reconstitutionPut(table, key, value);
977 }
978 }
979
980 /**
981 * The put method used by readObject. This is provided because put
982 * is overridable and should not be called in readObject since the
983 * subclass will not yet be initialized.
984 *
985 * <p>This differs from the regular put method in several ways. No
986 * checking for rehashing is necessary since the number of elements
987 * initially in the table is known. The modCount is not incremented
988 * because we are creating a new instance. Also, no return value
989 * is needed.
990 */
991 private void reconstitutionPut(Entry<?,?>[] tab, K key, V value)
992 throws StreamCorruptedException
993 {
994 if (value == null) {
995 throw new java.io.StreamCorruptedException();
996 }
997 // Makes sure the key is not already in the hashtable.
998 // This should not happen in deserialized version.
999 int hash = hash(key);
1000 int index = (hash & 0x7FFFFFFF) % tab.length;
1001 for (Entry<?,?> e = tab[index] ; e != null ; e = e.next) {
1002 if ((e.hash == hash) && e.key.equals(key)) {
1003 throw new java.io.StreamCorruptedException();
1004 }
1005 }
1006 // Creates the new entry.
1007 @SuppressWarnings("unchecked")
1008 Entry<K,V> e = (Entry<K,V>)tab[index];
1009 tab[index] = new Entry<>(hash, key, value, e);
1010 count++;
1011 }
1012
1013 /**
1014 * Hashtable bucket collision list entry
1015 */
1016 private static class Entry<K,V> implements Map.Entry<K,V> {
1017 final int hash;
1018 K key;
1019 V value;
1020 Entry<K,V> next;
1021
1022 protected Entry(int hash, K key, V value, Entry<K,V> next) {
1023 this.hash = hash;
1024 this.key = key;
1025 this.value = value;
1026 this.next = next;
1027 }
1028
1029 @SuppressWarnings("unchecked")
1030 protected Object clone() {
1031 return new Entry<>(hash, key, value,
1032 (next==null ? null : (Entry<K,V>) next.clone()));
1033 }
1034
1035 // Map.Entry Ops
1036
1037 public K getKey() {
1038 return key;
1039 }
1040
1041 public V getValue() {
1042 return value;
1043 }
1044
1045 public V setValue(V value) {
1046 if (value == null)
1047 throw new NullPointerException();
1048
1049 V oldValue = this.value;
1050 this.value = value;
1051 return oldValue;
1052 }
1053
1054 public boolean equals(Object o) {
1055 if (!(o instanceof Map.Entry))
1056 return false;
1057 Map.Entry<?,?> e = (Map.Entry<?,?>)o;
1058
1059 return (key==null ? e.getKey()==null : key.equals(e.getKey())) &&
1060 (value==null ? e.getValue()==null : value.equals(e.getValue()));
1061 }
1062
1063 public int hashCode() {
1064 return hash ^ (value==null ? 0 : value.hashCode());
1065 }
1066
1067 public String toString() {
1068 return key.toString()+"="+value.toString();
1069 }
1070 }
1071
1072 // Types of Enumerations/Iterations
1073 private static final int KEYS = 0;
1074 private static final int VALUES = 1;
1075 private static final int ENTRIES = 2;
1076
1077 /**
1078 * A hashtable enumerator class. This class implements both the
1079 * Enumeration and Iterator interfaces, but individual instances
1080 * can be created with the Iterator methods disabled. This is necessary
1081 * to avoid unintentionally increasing the capabilities granted a user
1082 * by passing an Enumeration.
1083 */
1084 private class Enumerator<T> implements Enumeration<T>, Iterator<T> {
1085 Entry<?,?>[] table = Hashtable.this.table;
1086 int index = table.length;
1087 Entry<?,?> entry = null;
1088 Entry<?,?> lastReturned = null;
1089 int type;
1090
1091 /**
1092 * Indicates whether this Enumerator is serving as an Iterator
1093 * or an Enumeration. (true -> Iterator).
1094 */
1095 boolean iterator;
1096
1097 /**
1098 * The modCount value that the iterator believes that the backing
1099 * Hashtable should have. If this expectation is violated, the iterator
1100 * has detected concurrent modification.
1101 */
1102 protected int expectedModCount = modCount;
1103
1104 Enumerator(int type, boolean iterator) {
1105 this.type = type;
1106 this.iterator = iterator;
1107 }
1108
1109 public boolean hasMoreElements() {
1110 Entry<?,?> e = entry;
1111 int i = index;
1112 Entry<?,?>[] t = table;
1113 /* Use locals for faster loop iteration */
1114 while (e == null && i > 0) {
1115 e = t[--i];
1116 }
1117 entry = e;
1118 index = i;
1119 return e != null;
1120 }
1121
1122 @SuppressWarnings("unchecked")
1123 public T nextElement() {
1124 Entry<?,?> et = entry;
1125 int i = index;
1126 Entry<?,?>[] t = table;
1127 /* Use locals for faster loop iteration */
1128 while (et == null && i > 0) {
1129 et = t[--i];
1130 }
1131 entry = et;
1132 index = i;
1133 if (et != null) {
1134 Entry<?,?> e = lastReturned = entry;
1135 entry = e.next;
1136 return type == KEYS ? (T)e.key : (type == VALUES ? (T)e.value : (T)e);
1137 }
1138 throw new NoSuchElementException("Hashtable Enumerator");
1139 }
1140
1141 // Iterator methods
1142 public boolean hasNext() {
1143 return hasMoreElements();
1144 }
1145
1146 public T next() {
1147 if (modCount != expectedModCount)
1148 throw new ConcurrentModificationException();
1149 return nextElement();
1150 }
1151
1152 public void remove() {
1153 if (!iterator)
1154 throw new UnsupportedOperationException();
1155 if (lastReturned == null)
1156 throw new IllegalStateException("Hashtable Enumerator");
1157 if (modCount != expectedModCount)
1158 throw new ConcurrentModificationException();
1159
1160 synchronized(Hashtable.this) {
1161 Entry<?,?>[] tab = Hashtable.this.table;
1162 int index = (lastReturned.hash & 0x7FFFFFFF) % tab.length;
1163
1164 @SuppressWarnings("unchecked")
1165 Entry<K,V> e = (Entry<K,V>)tab[index];
1166 for(Entry<K,V> prev = null; e != null; prev = e, e = e.next) {
1167 if (e == lastReturned) {
1168 modCount++;
1169 expectedModCount++;
1170 if (prev == null)
1171 tab[index] = e.next;
1172 else
1173 prev.next = e.next;
1174 count--;
1175 lastReturned = null;
1176 return;
1177 }
1178 }
1179 throw new ConcurrentModificationException();
1180 }
1181 }
1182 }
1183 }