1 /*
2 * Copyright (c) 1997, 2010, Oracle and/or its affiliates. All rights reserved.
3 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
4 *
5 *
6 *
7 *
8 *
9 *
10 *
11 *
12 *
13 *
14 *
15 *
16 *
17 *
18 *
19 *
20 *
21 *
22 *
23 *
24 */
25
26 package java.util;
27 import java.io.*;
28
29 /**
30 * Hash table based implementation of the <tt>Map</tt> interface. This
31 * implementation provides all of the optional map operations, and permits
32 * <tt>null</tt> values and the <tt>null</tt> key. (The <tt>HashMap</tt>
33 * class is roughly equivalent to <tt>Hashtable</tt>, except that it is
34 * unsynchronized and permits nulls.) This class makes no guarantees as to
35 * the order of the map; in particular, it does not guarantee that the order
36 * will remain constant over time.
37 *
38 * <p>This implementation provides constant-time performance for the basic
39 * operations (<tt>get</tt> and <tt>put</tt>), assuming the hash function
40 * disperses the elements properly among the buckets. Iteration over
41 * collection views requires time proportional to the "capacity" of the
42 * <tt>HashMap</tt> instance (the number of buckets) plus its size (the number
43 * of key-value mappings). Thus, it's very important not to set the initial
44 * capacity too high (or the load factor too low) if iteration performance is
45 * important.
46 *
47 * <p>An instance of <tt>HashMap</tt> has two parameters that affect its
48 * performance: <i>initial capacity</i> and <i>load factor</i>. The
49 * <i>capacity</i> is the number of buckets in the hash table, and the initial
50 * capacity is simply the capacity at the time the hash table is created. The
51 * <i>load factor</i> is a measure of how full the hash table is allowed to
52 * get before its capacity is automatically increased. When the number of
53 * entries in the hash table exceeds the product of the load factor and the
54 * current capacity, the hash table is <i>rehashed</i> (that is, internal data
55 * structures are rebuilt) so that the hash table has approximately twice the
56 * number of buckets.
57 *
58 * <p>As a general rule, the default load factor (.75) offers a good tradeoff
59 * between time and space costs. Higher values decrease the space overhead
60 * but increase the lookup cost (reflected in most of the operations of the
61 * <tt>HashMap</tt> class, including <tt>get</tt> and <tt>put</tt>). The
62 * expected number of entries in the map and its load factor should be taken
63 * into account when setting its initial capacity, so as to minimize the
64 * number of rehash operations. If the initial capacity is greater
65 * than the maximum number of entries divided by the load factor, no
66 * rehash operations will ever occur.
67 *
68 * <p>If many mappings are to be stored in a <tt>HashMap</tt> instance,
69 * creating it with a sufficiently large capacity will allow the mappings to
70 * be stored more efficiently than letting it perform automatic rehashing as
71 * needed to grow the table.
72 *
73 * <p><strong>Note that this implementation is not synchronized.</strong>
74 * If multiple threads access a hash map concurrently, and at least one of
75 * the threads modifies the map structurally, it <i>must</i> be
76 * synchronized externally. (A structural modification is any operation
77 * that adds or deletes one or more mappings; merely changing the value
78 * associated with a key that an instance already contains is not a
79 * structural modification.) This is typically accomplished by
80 * synchronizing on some object that naturally encapsulates the map.
81 *
82 * If no such object exists, the map should be "wrapped" using the
83 * {@link Collections#synchronizedMap Collections.synchronizedMap}
84 * method. This is best done at creation time, to prevent accidental
85 * unsynchronized access to the map:<pre>
86 * Map m = Collections.synchronizedMap(new HashMap(...));</pre>
87 *
88 * <p>The iterators returned by all of this class's "collection view methods"
89 * are <i>fail-fast</i>: if the map is structurally modified at any time after
90 * the iterator is created, in any way except through the iterator's own
91 * <tt>remove</tt> method, the iterator will throw a
92 * {@link ConcurrentModificationException}. Thus, in the face of concurrent
93 * modification, the iterator fails quickly and cleanly, rather than risking
94 * arbitrary, non-deterministic behavior at an undetermined time in the
95 * future.
96 *
97 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
98 * as it is, generally speaking, impossible to make any hard guarantees in the
99 * presence of unsynchronized concurrent modification. Fail-fast iterators
100 * throw <tt>ConcurrentModificationException</tt> on a best-effort basis.
101 * Therefore, it would be wrong to write a program that depended on this
102 * exception for its correctness: <i>the fail-fast behavior of iterators
103 * should be used only to detect bugs.</i>
104 *
105 * <p>This class is a member of the
106 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
107 * Java Collections Framework</a>.
108 *
109 * @param <K> the type of keys maintained by this map
110 * @param <V> the type of mapped values
111 *
112 * @author Doug Lea
113 * @author Josh Bloch
114 * @author Arthur van Hoff
115 * @author Neal Gafter
116 * @see Object#hashCode()
117 * @see Collection
118 * @see Map
119 * @see TreeMap
120 * @see Hashtable
121 * @since 1.2
122 */
123
124 public class HashMap<K,V>
125 extends AbstractMap<K,V>
126 implements Map<K,V>, Cloneable, Serializable
127 {
128
129 /**
130 * The default initial capacity - MUST be a power of two.
131 */
132 static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
133
134 /**
135 * The maximum capacity, used if a higher value is implicitly specified
136 * by either of the constructors with arguments.
137 * MUST be a power of two <= 1<<30.
138 */
139 static final int MAXIMUM_CAPACITY = 1 << 30;
140
141 /**
142 * The load factor used when none specified in constructor.
143 */
144 static final float DEFAULT_LOAD_FACTOR = 0.75f;
145
146 /**
147 * An empty table instance to share when the table is not inflated.
148 */
149 static final Entry<?,?>[] EMPTY_TABLE = {};
150
151 /**
152 * The table, resized as necessary. Length MUST Always be a power of two.
153 */
154 transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE;
155
156 /**
157 * The number of key-value mappings contained in this map.
158 */
159 transient int size;
160
161 /**
162 * The next size value at which to resize (capacity * load factor).
163 * @serial
164 */
165 // If table == EMPTY_TABLE then this is the initial capacity at which the
166 // table will be created when inflated.
167 int threshold;
168
169 /**
170 * The load factor for the hash table.
171 *
172 * @serial
173 */
174 final float loadFactor;
175
176 /**
177 * The number of times this HashMap has been structurally modified
178 * Structural modifications are those that change the number of mappings in
179 * the HashMap or otherwise modify its internal structure (e.g.,
180 * rehash). This field is used to make iterators on Collection-views of
181 * the HashMap fail-fast. (See ConcurrentModificationException).
182 */
183 transient int modCount;
184
185 /**
186 * The default threshold of map capacity above which alternative hashing is
187 * used for String keys. Alternative hashing reduces the incidence of
188 * collisions due to weak hash code calculation for String keys.
189 * <p/>
190 * This value may be overridden by defining the system property
191 * {@code jdk.map.althashing.threshold}. A property value of {@code 1}
192 * forces alternative hashing to be used at all times whereas
193 * {@code -1} value ensures that alternative hashing is never used.
194 */
195 static final int ALTERNATIVE_HASHING_THRESHOLD_DEFAULT = Integer.MAX_VALUE;
196
197 /**
198 * holds values which can't be initialized until after VM is booted.
199 */
200 private static class Holder {
201
202 /**
203 * Table capacity above which to switch to use alternative hashing.
204 */
205 static final int ALTERNATIVE_HASHING_THRESHOLD;
206
207 static {
208 String altThreshold = java.security.AccessController.doPrivileged(
209 new sun.security.action.GetPropertyAction(
210 "jdk.map.althashing.threshold"));
211
212 int threshold;
213 try {
214 threshold = (null != altThreshold)
215 ? Integer.parseInt(altThreshold)
216 : ALTERNATIVE_HASHING_THRESHOLD_DEFAULT;
217
218 // disable alternative hashing if -1
219 if (threshold == -1) {
220 threshold = Integer.MAX_VALUE;
221 }
222
223 if (threshold < 0) {
224 throw new IllegalArgumentException("value must be positive integer.");
225 }
226 } catch(IllegalArgumentException failed) {
227 throw new Error("Illegal value for 'jdk.map.althashing.threshold'", failed);
228 }
229
230 ALTERNATIVE_HASHING_THRESHOLD = threshold;
231 }
232 }
233
234 /**
235 * A randomizing value associated with this instance that is applied to
236 * hash code of keys to make hash collisions harder to find. If 0 then
237 * alternative hashing is disabled.
238 */
239 transient int hashSeed = 0;
240
241 /**
242 * Constructs an empty <tt>HashMap</tt> with the specified initial
243 * capacity and load factor.
244 *
245 * @param initialCapacity the initial capacity
246 * @param loadFactor the load factor
247 * @throws IllegalArgumentException if the initial capacity is negative
248 * or the load factor is nonpositive
249 */
250 public HashMap(int initialCapacity, float loadFactor) {
251 if (initialCapacity < 0)
252 throw new IllegalArgumentException("Illegal initial capacity: " +
253 initialCapacity);
254 if (initialCapacity > MAXIMUM_CAPACITY)
255 initialCapacity = MAXIMUM_CAPACITY;
256 if (loadFactor <= 0 || Float.isNaN(loadFactor))
257 throw new IllegalArgumentException("Illegal load factor: " +
258 loadFactor);
259
260 this.loadFactor = loadFactor;
261 threshold = initialCapacity;
262 init();
263 }
264
265 /**
266 * Constructs an empty <tt>HashMap</tt> with the specified initial
267 * capacity and the default load factor (0.75).
268 *
269 * @param initialCapacity the initial capacity.
270 * @throws IllegalArgumentException if the initial capacity is negative.
271 */
272 public HashMap(int initialCapacity) {
273 this(initialCapacity, DEFAULT_LOAD_FACTOR);
274 }
275
276 /**
277 * Constructs an empty <tt>HashMap</tt> with the default initial capacity
278 * (16) and the default load factor (0.75).
279 */
280 public HashMap() {
281 this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
282 }
283
284 /**
285 * Constructs a new <tt>HashMap</tt> with the same mappings as the
286 * specified <tt>Map</tt>. The <tt>HashMap</tt> is created with
287 * default load factor (0.75) and an initial capacity sufficient to
288 * hold the mappings in the specified <tt>Map</tt>.
289 *
290 * @param m the map whose mappings are to be placed in this map
291 * @throws NullPointerException if the specified map is null
292 */
293 public HashMap(Map<? extends K, ? extends V> m) {
294 this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,
295 DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR);
296 inflateTable(threshold);
297
298 putAllForCreate(m);
299 }
300
301 private static int roundUpToPowerOf2(int number) {
302 // assert number >= 0 : "number must be non-negative";
303 int rounded = number >= MAXIMUM_CAPACITY
304 ? MAXIMUM_CAPACITY
305 : (rounded = Integer.highestOneBit(number)) != 0
306 ? (Integer.bitCount(number) > 1) ? rounded << 1 : rounded
307 : 1;
308
309 return rounded;
310 }
311
312 /**
313 * Inflates the table.
314 */
315 private void inflateTable(int toSize) {
316 // Find a power of 2 >= toSize
317 int capacity = roundUpToPowerOf2(toSize);
318
319 threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);
320 table = new Entry[capacity];
321 initHashSeedAsNeeded(capacity);
322 }
323
324 // internal utilities
325
326 /**
327 * Initialization hook for subclasses. This method is called
328 * in all constructors and pseudo-constructors (clone, readObject)
329 * after HashMap has been initialized but before any entries have
330 * been inserted. (In the absence of this method, readObject would
331 * require explicit knowledge of subclasses.)
332 */
333 void init() {
334 }
335
336 /**
337 * Initialize the hashing mask value. We defer initialization until we
338 * really need it.
339 */
340 final boolean initHashSeedAsNeeded(int capacity) {
341 boolean currentAltHashing = hashSeed != 0;
342 boolean useAltHashing = sun.misc.VM.isBooted() &&
343 (capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);
344 boolean switching = currentAltHashing ^ useAltHashing;
345 if (switching) {
346 hashSeed = useAltHashing
347 ? sun.misc.Hashing.randomHashSeed(this)
348 : 0;
349 }
350 return switching;
351 }
352
353 /**
354 * Retrieve object hash code and applies a supplemental hash function to the
355 * result hash, which defends against poor quality hash functions. This is
356 * critical because HashMap uses power-of-two length hash tables, that
357 * otherwise encounter collisions for hashCodes that do not differ
358 * in lower bits. Note: Null keys always map to hash 0, thus index 0.
359 */
360 final int hash(Object k) {
361 int h = hashSeed;
362 if (0 != h && k instanceof String) {
363 return sun.misc.Hashing.stringHash32((String) k);
364 }
365
366 h ^= k.hashCode();
367
368 // This function ensures that hashCodes that differ only by
369 // constant multiples at each bit position have a bounded
370 // number of collisions (approximately 8 at default load factor).
371 h ^= (h >>> 20) ^ (h >>> 12);
372 return h ^ (h >>> 7) ^ (h >>> 4);
373 }
374
375 /**
376 * Returns index for hash code h.
377 */
378 static int indexFor(int h, int length) {
379 // assert Integer.bitCount(length) == 1 : "length must be a non-zero power of 2";
380 return h & (length-1);
381 }
382
383 /**
384 * Returns the number of key-value mappings in this map.
385 *
386 * @return the number of key-value mappings in this map
387 */
388 public int size() {
389 return size;
390 }
391
392 /**
393 * Returns <tt>true</tt> if this map contains no key-value mappings.
394 *
395 * @return <tt>true</tt> if this map contains no key-value mappings
396 */
397 public boolean isEmpty() {
398 return size == 0;
399 }
400
401 /**
402 * Returns the value to which the specified key is mapped,
403 * or {@code null} if this map contains no mapping for the key.
404 *
405 * <p>More formally, if this map contains a mapping from a key
406 * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
407 * key.equals(k))}, then this method returns {@code v}; otherwise
408 * it returns {@code null}. (There can be at most one such mapping.)
409 *
410 * <p>A return value of {@code null} does not <i>necessarily</i>
411 * indicate that the map contains no mapping for the key; it's also
412 * possible that the map explicitly maps the key to {@code null}.
413 * The {@link #containsKey containsKey} operation may be used to
414 * distinguish these two cases.
415 *
416 * @see #put(Object, Object)
417 */
418 public V get(Object key) {
419 if (key == null)
420 return getForNullKey();
421 Entry<K,V> entry = getEntry(key);
422
423 return null == entry ? null : entry.getValue();
424 }
425
426 /**
427 * Offloaded version of get() to look up null keys. Null keys map
428 * to index 0. This null case is split out into separate methods
429 * for the sake of performance in the two most commonly used
430 * operations (get and put), but incorporated with conditionals in
431 * others.
432 */
433 private V getForNullKey() {
434 if (size == 0) {
435 return null;
436 }
437 for (Entry<K,V> e = table[0]; e != null; e = e.next) {
438 if (e.key == null)
439 return e.value;
440 }
441 return null;
442 }
443
444 /**
445 * Returns <tt>true</tt> if this map contains a mapping for the
446 * specified key.
447 *
448 * @param key The key whose presence in this map is to be tested
449 * @return <tt>true</tt> if this map contains a mapping for the specified
450 * key.
451 */
452 public boolean containsKey(Object key) {
453 return getEntry(key) != null;
454 }
455
456 /**
457 * Returns the entry associated with the specified key in the
458 * HashMap. Returns null if the HashMap contains no mapping
459 * for the key.
460 */
461 final Entry<K,V> getEntry(Object key) {
462 if (size == 0) {
463 return null;
464 }
465
466 int hash = (key == null) ? 0 : hash(key);
467 for (Entry<K,V> e = table[indexFor(hash, table.length)];
468 e != null;
469 e = e.next) {
470 Object k;
471 if (e.hash == hash &&
472 ((k = e.key) == key || (key != null && key.equals(k))))
473 return e;
474 }
475 return null;
476 }
477
478 /**
479 * Associates the specified value with the specified key in this map.
480 * If the map previously contained a mapping for the key, the old
481 * value is replaced.
482 *
483 * @param key key with which the specified value is to be associated
484 * @param value value to be associated with the specified key
485 * @return the previous value associated with <tt>key</tt>, or
486 * <tt>null</tt> if there was no mapping for <tt>key</tt>.
487 * (A <tt>null</tt> return can also indicate that the map
488 * previously associated <tt>null</tt> with <tt>key</tt>.)
489 */
490 public V put(K key, V value) {
491 if (table == EMPTY_TABLE) {
492 inflateTable(threshold);
493 }
494 if (key == null)
495 return putForNullKey(value);
496 int hash = hash(key);
497 int i = indexFor(hash, table.length);
498 for (Entry<K,V> e = table[i]; e != null; e = e.next) {
499 Object k;
500 if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
501 V oldValue = e.value;
502 e.value = value;
503 e.recordAccess(this);
504 return oldValue;
505 }
506 }
507
508 modCount++;
509 addEntry(hash, key, value, i);
510 return null;
511 }
512
513 /**
514 * Offloaded version of put for null keys
515 */
516 private V putForNullKey(V value) {
517 for (Entry<K,V> e = table[0]; e != null; e = e.next) {
518 if (e.key == null) {
519 V oldValue = e.value;
520 e.value = value;
521 e.recordAccess(this);
522 return oldValue;
523 }
524 }
525 modCount++;
526 addEntry(0, null, value, 0);
527 return null;
528 }
529
530 /**
531 * This method is used instead of put by constructors and
532 * pseudoconstructors (clone, readObject). It does not resize the table,
533 * check for comodification, etc. It calls createEntry rather than
534 * addEntry.
535 */
536 private void putForCreate(K key, V value) {
537 int hash = null == key ? 0 : hash(key);
538 int i = indexFor(hash, table.length);
539
540 /**
541 * Look for preexisting entry for key. This will never happen for
542 * clone or deserialize. It will only happen for construction if the
543 * input Map is a sorted map whose ordering is inconsistent w/ equals.
544 */
545 for (Entry<K,V> e = table[i]; e != null; e = e.next) {
546 Object k;
547 if (e.hash == hash &&
548 ((k = e.key) == key || (key != null && key.equals(k)))) {
549 e.value = value;
550 return;
551 }
552 }
553
554 createEntry(hash, key, value, i);
555 }
556
557 private void putAllForCreate(Map<? extends K, ? extends V> m) {
558 for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
559 putForCreate(e.getKey(), e.getValue());
560 }
561
562 /**
563 * Rehashes the contents of this map into a new array with a
564 * larger capacity. This method is called automatically when the
565 * number of keys in this map reaches its threshold.
566 *
567 * If current capacity is MAXIMUM_CAPACITY, this method does not
568 * resize the map, but sets threshold to Integer.MAX_VALUE.
569 * This has the effect of preventing future calls.
570 *
571 * @param newCapacity the new capacity, MUST be a power of two;
572 * must be greater than current capacity unless current
573 * capacity is MAXIMUM_CAPACITY (in which case value
574 * is irrelevant).
575 */
576 void resize(int newCapacity) {
577 Entry[] oldTable = table;
578 int oldCapacity = oldTable.length;
579 if (oldCapacity == MAXIMUM_CAPACITY) {
580 threshold = Integer.MAX_VALUE;
581 return;
582 }
583
584 Entry[] newTable = new Entry[newCapacity];
585 transfer(newTable, initHashSeedAsNeeded(newCapacity));
586 table = newTable;
587 threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);
588 }
589
590 /**
591 * Transfers all entries from current table to newTable.
592 */
593 void transfer(Entry[] newTable, boolean rehash) {
594 int newCapacity = newTable.length;
595 for (Entry<K,V> e : table) {
596 while(null != e) {
597 Entry<K,V> next = e.next;
598 if (rehash) {
599 e.hash = null == e.key ? 0 : hash(e.key);
600 }
601 int i = indexFor(e.hash, newCapacity);
602 e.next = newTable[i];
603 newTable[i] = e;
604 e = next;
605 }
606 }
607 }
608
609 /**
610 * Copies all of the mappings from the specified map to this map.
611 * These mappings will replace any mappings that this map had for
612 * any of the keys currently in the specified map.
613 *
614 * @param m mappings to be stored in this map
615 * @throws NullPointerException if the specified map is null
616 */
617 public void putAll(Map<? extends K, ? extends V> m) {
618 int numKeysToBeAdded = m.size();
619 if (numKeysToBeAdded == 0)
620 return;
621
622 if (table == EMPTY_TABLE) {
623 inflateTable((int) Math.max(numKeysToBeAdded * loadFactor, threshold));
624 }
625
626 /*
627 * Expand the map if the map if the number of mappings to be added
628 * is greater than or equal to threshold. This is conservative; the
629 * obvious condition is (m.size() + size) >= threshold, but this
630 * condition could result in a map with twice the appropriate capacity,
631 * if the keys to be added overlap with the keys already in this map.
632 * By using the conservative calculation, we subject ourself
633 * to at most one extra resize.
634 */
635 if (numKeysToBeAdded > threshold) {
636 int targetCapacity = (int)(numKeysToBeAdded / loadFactor + 1);
637 if (targetCapacity > MAXIMUM_CAPACITY)
638 targetCapacity = MAXIMUM_CAPACITY;
639 int newCapacity = table.length;
640 while (newCapacity < targetCapacity)
641 newCapacity <<= 1;
642 if (newCapacity > table.length)
643 resize(newCapacity);
644 }
645
646 for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
647 put(e.getKey(), e.getValue());
648 }
649
650 /**
651 * Removes the mapping for the specified key from this map if present.
652 *
653 * @param key key whose mapping is to be removed from the map
654 * @return the previous value associated with <tt>key</tt>, or
655 * <tt>null</tt> if there was no mapping for <tt>key</tt>.
656 * (A <tt>null</tt> return can also indicate that the map
657 * previously associated <tt>null</tt> with <tt>key</tt>.)
658 */
659 public V remove(Object key) {
660 Entry<K,V> e = removeEntryForKey(key);
661 return (e == null ? null : e.value);
662 }
663
664 /**
665 * Removes and returns the entry associated with the specified key
666 * in the HashMap. Returns null if the HashMap contains no mapping
667 * for this key.
668 */
669 final Entry<K,V> removeEntryForKey(Object key) {
670 if (size == 0) {
671 return null;
672 }
673 int hash = (key == null) ? 0 : hash(key);
674 int i = indexFor(hash, table.length);
675 Entry<K,V> prev = table[i];
676 Entry<K,V> e = prev;
677
678 while (e != null) {
679 Entry<K,V> next = e.next;
680 Object k;
681 if (e.hash == hash &&
682 ((k = e.key) == key || (key != null && key.equals(k)))) {
683 modCount++;
684 size--;
685 if (prev == e)
686 table[i] = next;
687 else
688 prev.next = next;
689 e.recordRemoval(this);
690 return e;
691 }
692 prev = e;
693 e = next;
694 }
695
696 return e;
697 }
698
699 /**
700 * Special version of remove for EntrySet using {@code Map.Entry.equals()}
701 * for matching.
702 */
703 final Entry<K,V> removeMapping(Object o) {
704 if (size == 0 || !(o instanceof Map.Entry))
705 return null;
706
707 Map.Entry<K,V> entry = (Map.Entry<K,V>) o;
708 Object key = entry.getKey();
709 int hash = (key == null) ? 0 : hash(key);
710 int i = indexFor(hash, table.length);
711 Entry<K,V> prev = table[i];
712 Entry<K,V> e = prev;
713
714 while (e != null) {
715 Entry<K,V> next = e.next;
716 if (e.hash == hash && e.equals(entry)) {
717 modCount++;
718 size--;
719 if (prev == e)
720 table[i] = next;
721 else
722 prev.next = next;
723 e.recordRemoval(this);
724 return e;
725 }
726 prev = e;
727 e = next;
728 }
729
730 return e;
731 }
732
733 /**
734 * Removes all of the mappings from this map.
735 * The map will be empty after this call returns.
736 */
737 public void clear() {
738 modCount++;
739 Arrays.fill(table, null);
740 size = 0;
741 }
742
743 /**
744 * Returns <tt>true</tt> if this map maps one or more keys to the
745 * specified value.
746 *
747 * @param value value whose presence in this map is to be tested
748 * @return <tt>true</tt> if this map maps one or more keys to the
749 * specified value
750 */
751 public boolean containsValue(Object value) {
752 if (value == null)
753 return containsNullValue();
754
755 Entry[] tab = table;
756 for (int i = 0; i < tab.length ; i++)
757 for (Entry e = tab[i] ; e != null ; e = e.next)
758 if (value.equals(e.value))
759 return true;
760 return false;
761 }
762
763 /**
764 * Special-case code for containsValue with null argument
765 */
766 private boolean containsNullValue() {
767 Entry[] tab = table;
768 for (int i = 0; i < tab.length ; i++)
769 for (Entry e = tab[i] ; e != null ; e = e.next)
770 if (e.value == null)
771 return true;
772 return false;
773 }
774
775 /**
776 * Returns a shallow copy of this <tt>HashMap</tt> instance: the keys and
777 * values themselves are not cloned.
778 *
779 * @return a shallow copy of this map
780 */
781 public Object clone() {
782 HashMap<K,V> result = null;
783 try {
784 result = (HashMap<K,V>)super.clone();
785 } catch (CloneNotSupportedException e) {
786 // assert false;
787 }
788 if (result.table != EMPTY_TABLE) {
789 result.inflateTable(Math.min(
790 (int) Math.min(
791 size * Math.min(1 / loadFactor, 4.0f),
792 // we have limits...
793 HashMap.MAXIMUM_CAPACITY),
794 table.length));
795 }
796 result.entrySet = null;
797 result.modCount = 0;
798 result.size = 0;
799 result.init();
800 result.putAllForCreate(this);
801
802 return result;
803 }
804
805 static class Entry<K,V> implements Map.Entry<K,V> {
806 final K key;
807 V value;
808 Entry<K,V> next;
809 int hash;
810
811 /**
812 * Creates new entry.
813 */
814 Entry(int h, K k, V v, Entry<K,V> n) {
815 value = v;
816 next = n;
817 key = k;
818 hash = h;
819 }
820
821 public final K getKey() {
822 return key;
823 }
824
825 public final V getValue() {
826 return value;
827 }
828
829 public final V setValue(V newValue) {
830 V oldValue = value;
831 value = newValue;
832 return oldValue;
833 }
834
835 public final boolean equals(Object o) {
836 if (!(o instanceof Map.Entry))
837 return false;
838 Map.Entry e = (Map.Entry)o;
839 Object k1 = getKey();
840 Object k2 = e.getKey();
841 if (k1 == k2 || (k1 != null && k1.equals(k2))) {
842 Object v1 = getValue();
843 Object v2 = e.getValue();
844 if (v1 == v2 || (v1 != null && v1.equals(v2)))
845 return true;
846 }
847 return false;
848 }
849
850 public final int hashCode() {
851 return Objects.hashCode(getKey()) ^ Objects.hashCode(getValue());
852 }
853
854 public final String toString() {
855 return getKey() + "=" + getValue();
856 }
857
858 /**
859 * This method is invoked whenever the value in an entry is
860 * overwritten by an invocation of put(k,v) for a key k that's already
861 * in the HashMap.
862 */
863 void recordAccess(HashMap<K,V> m) {
864 }
865
866 /**
867 * This method is invoked whenever the entry is
868 * removed from the table.
869 */
870 void recordRemoval(HashMap<K,V> m) {
871 }
872 }
873
874 /**
875 * Adds a new entry with the specified key, value and hash code to
876 * the specified bucket. It is the responsibility of this
877 * method to resize the table if appropriate.
878 *
879 * Subclass overrides this to alter the behavior of put method.
880 */
881 void addEntry(int hash, K key, V value, int bucketIndex) {
882 if ((size >= threshold) && (null != table[bucketIndex])) {
883 resize(2 * table.length);
884 hash = (null != key) ? hash(key) : 0;
885 bucketIndex = indexFor(hash, table.length);
886 }
887
888 createEntry(hash, key, value, bucketIndex);
889 }
890
891 /**
892 * Like addEntry except that this version is used when creating entries
893 * as part of Map construction or "pseudo-construction" (cloning,
894 * deserialization). This version needn't worry about resizing the table.
895 *
896 * Subclass overrides this to alter the behavior of HashMap(Map),
897 * clone, and readObject.
898 */
899 void createEntry(int hash, K key, V value, int bucketIndex) {
900 Entry<K,V> e = table[bucketIndex];
901 table[bucketIndex] = new Entry<>(hash, key, value, e);
902 size++;
903 }
904
905 private abstract class HashIterator<E> implements Iterator<E> {
906 Entry<K,V> next; // next entry to return
907 int expectedModCount; // For fast-fail
908 int index; // current slot
909 Entry<K,V> current; // current entry
910
911 HashIterator() {
912 expectedModCount = modCount;
913 if (size > 0) { // advance to first entry
914 Entry[] t = table;
915 while (index < t.length && (next = t[index++]) == null)
916 ;
917 }
918 }
919
920 public final boolean hasNext() {
921 return next != null;
922 }
923
924 final Entry<K,V> nextEntry() {
925 if (modCount != expectedModCount)
926 throw new ConcurrentModificationException();
927 Entry<K,V> e = next;
928 if (e == null)
929 throw new NoSuchElementException();
930
931 if ((next = e.next) == null) {
932 Entry[] t = table;
933 while (index < t.length && (next = t[index++]) == null)
934 ;
935 }
936 current = e;
937 return e;
938 }
939
940 public void remove() {
941 if (current == null)
942 throw new IllegalStateException();
943 if (modCount != expectedModCount)
944 throw new ConcurrentModificationException();
945 Object k = current.key;
946 current = null;
947 HashMap.this.removeEntryForKey(k);
948 expectedModCount = modCount;
949 }
950 }
951
952 private final class ValueIterator extends HashIterator<V> {
953 public V next() {
954 return nextEntry().value;
955 }
956 }
957
958 private final class KeyIterator extends HashIterator<K> {
959 public K next() {
960 return nextEntry().getKey();
961 }
962 }
963
964 private final class EntryIterator extends HashIterator<Map.Entry<K,V>> {
965 public Map.Entry<K,V> next() {
966 return nextEntry();
967 }
968 }
969
970 // Subclass overrides these to alter behavior of views' iterator() method
971 Iterator<K> newKeyIterator() {
972 return new KeyIterator();
973 }
974 Iterator<V> newValueIterator() {
975 return new ValueIterator();
976 }
977 Iterator<Map.Entry<K,V>> newEntryIterator() {
978 return new EntryIterator();
979 }
980
981
982 // Views
983
984 private transient Set<Map.Entry<K,V>> entrySet = null;
985
986 /**
987 * Returns a {@link Set} view of the keys contained in this map.
988 * The set is backed by the map, so changes to the map are
989 * reflected in the set, and vice-versa. If the map is modified
990 * while an iteration over the set is in progress (except through
991 * the iterator's own <tt>remove</tt> operation), the results of
992 * the iteration are undefined. The set supports element removal,
993 * which removes the corresponding mapping from the map, via the
994 * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
995 * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
996 * operations. It does not support the <tt>add</tt> or <tt>addAll</tt>
997 * operations.
998 */
999 public Set<K> keySet() {
1000 Set<K> ks = keySet;
1001 return (ks != null ? ks : (keySet = new KeySet()));
1002 }
1003
1004 private final class KeySet extends AbstractSet<K> {
1005 public Iterator<K> iterator() {
1006 return newKeyIterator();
1007 }
1008 public int size() {
1009 return size;
1010 }
1011 public boolean contains(Object o) {
1012 return containsKey(o);
1013 }
1014 public boolean remove(Object o) {
1015 return HashMap.this.removeEntryForKey(o) != null;
1016 }
1017 public void clear() {
1018 HashMap.this.clear();
1019 }
1020 }
1021
1022 /**
1023 * Returns a {@link Collection} view of the values contained in this map.
1024 * The collection is backed by the map, so changes to the map are
1025 * reflected in the collection, and vice-versa. If the map is
1026 * modified while an iteration over the collection is in progress
1027 * (except through the iterator's own <tt>remove</tt> operation),
1028 * the results of the iteration are undefined. The collection
1029 * supports element removal, which removes the corresponding
1030 * mapping from the map, via the <tt>Iterator.remove</tt>,
1031 * <tt>Collection.remove</tt>, <tt>removeAll</tt>,
1032 * <tt>retainAll</tt> and <tt>clear</tt> operations. It does not
1033 * support the <tt>add</tt> or <tt>addAll</tt> operations.
1034 */
1035 public Collection<V> values() {
1036 Collection<V> vs = values;
1037 return (vs != null ? vs : (values = new Values()));
1038 }
1039
1040 private final class Values extends AbstractCollection<V> {
1041 public Iterator<V> iterator() {
1042 return newValueIterator();
1043 }
1044 public int size() {
1045 return size;
1046 }
1047 public boolean contains(Object o) {
1048 return containsValue(o);
1049 }
1050 public void clear() {
1051 HashMap.this.clear();
1052 }
1053 }
1054
1055 /**
1056 * Returns a {@link Set} view of the mappings contained in this map.
1057 * The set is backed by the map, so changes to the map are
1058 * reflected in the set, and vice-versa. If the map is modified
1059 * while an iteration over the set is in progress (except through
1060 * the iterator's own <tt>remove</tt> operation, or through the
1061 * <tt>setValue</tt> operation on a map entry returned by the
1062 * iterator) the results of the iteration are undefined. The set
1063 * supports element removal, which removes the corresponding
1064 * mapping from the map, via the <tt>Iterator.remove</tt>,
1065 * <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and
1066 * <tt>clear</tt> operations. It does not support the
1067 * <tt>add</tt> or <tt>addAll</tt> operations.
1068 *
1069 * @return a set view of the mappings contained in this map
1070 */
1071 public Set<Map.Entry<K,V>> entrySet() {
1072 return entrySet0();
1073 }
1074
1075 private Set<Map.Entry<K,V>> entrySet0() {
1076 Set<Map.Entry<K,V>> es = entrySet;
1077 return es != null ? es : (entrySet = new EntrySet());
1078 }
1079
1080 private final class EntrySet extends AbstractSet<Map.Entry<K,V>> {
1081 public Iterator<Map.Entry<K,V>> iterator() {
1082 return newEntryIterator();
1083 }
1084 public boolean contains(Object o) {
1085 if (!(o instanceof Map.Entry))
1086 return false;
1087 Map.Entry<K,V> e = (Map.Entry<K,V>) o;
1088 Entry<K,V> candidate = getEntry(e.getKey());
1089 return candidate != null && candidate.equals(e);
1090 }
1091 public boolean remove(Object o) {
1092 return removeMapping(o) != null;
1093 }
1094 public int size() {
1095 return size;
1096 }
1097 public void clear() {
1098 HashMap.this.clear();
1099 }
1100 }
1101
1102 /**
1103 * Save the state of the <tt>HashMap</tt> instance to a stream (i.e.,
1104 * serialize it).
1105 *
1106 * @serialData The <i>capacity</i> of the HashMap (the length of the
1107 * bucket array) is emitted (int), followed by the
1108 * <i>size</i> (an int, the number of key-value
1109 * mappings), followed by the key (Object) and value (Object)
1110 * for each key-value mapping. The key-value mappings are
1111 * emitted in no particular order.
1112 */
1113 private void writeObject(java.io.ObjectOutputStream s)
1114 throws IOException
1115 {
1116 // Write out the threshold, loadfactor, and any hidden stuff
1117 s.defaultWriteObject();
1118
1119 // Write out number of buckets
1120 if (table==EMPTY_TABLE) {
1121 s.writeInt(roundUpToPowerOf2(threshold));
1122 } else {
1123 s.writeInt(table.length);
1124 }
1125
1126 // Write out size (number of Mappings)
1127 s.writeInt(size);
1128
1129 // Write out keys and values (alternating)
1130 if (size > 0) {
1131 for(Map.Entry<K,V> e : entrySet0()) {
1132 s.writeObject(e.getKey());
1133 s.writeObject(e.getValue());
1134 }
1135 }
1136 }
1137
1138 private static final long serialVersionUID = 362498820763181265L;
1139
1140 /**
1141 * Reconstitute the {@code HashMap} instance from a stream (i.e.,
1142 * deserialize it).
1143 */
1144 private void readObject(java.io.ObjectInputStream s)
1145 throws IOException, ClassNotFoundException
1146 {
1147 // Read in the threshold (ignored), loadfactor, and any hidden stuff
1148 s.defaultReadObject();
1149 if (loadFactor <= 0 || Float.isNaN(loadFactor)) {
1150 throw new InvalidObjectException("Illegal load factor: " +
1151 loadFactor);
1152 }
1153
1154 // set other fields that need values
1155 table = (Entry<K,V>[]) EMPTY_TABLE;
1156
1157 // Read in number of buckets
1158 s.readInt(); // ignored.
1159
1160 // Read number of mappings
1161 int mappings = s.readInt();
1162 if (mappings < 0)
1163 throw new InvalidObjectException("Illegal mappings count: " +
1164 mappings);
1165
1166 // capacity chosen by number of mappings and desired load (if >= 0.25)
1167 int capacity = (int) Math.min(
1168 mappings * Math.min(1 / loadFactor, 4.0f),
1169 // we have limits...
1170 HashMap.MAXIMUM_CAPACITY);
1171
1172 // allocate the bucket array;
1173 if (mappings > 0) {
1174 inflateTable(capacity);
1175 } else {
1176 threshold = capacity;
1177 }
1178
1179 init(); // Give subclass a chance to do its thing.
1180
1181 // Read the keys and values, and put the mappings in the HashMap
1182 for (int i = 0; i < mappings; i++) {
1183 K key = (K) s.readObject();
1184 V value = (V) s.readObject();
1185 putForCreate(key, value);
1186 }
1187 }
1188
1189 // These methods are used when serializing HashSets
1190 int capacity() { return table.length; }
1191 float loadFactor() { return loadFactor; }
1192 }
1 /*
2 * Copyright (c) 1997, 2010, Oracle and/or its affiliates. All rights reserved.
3 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
4 *
5 *
6 *
7 *
8 *
9 *
10 *
11 *
12 *
13 *
14 *
15 *
16 *
17 *
18 *
19 *
20 *
21 *
22 *
23 *
24 */
25
26 package java.util;
27 import java.io.*;
28
29 /**
30 * Hash table based implementation of the <tt>Map</tt> interface. This
31 * implementation provides all of the optional map operations, and permits
32 * <tt>null</tt> values and the <tt>null</tt> key. (The <tt>HashMap</tt>
33 * class is roughly(粗糙地；概略地) equivalent(相等的，相当的) to <tt>Hashtable</tt>, except that it is
34 * unsynchronized(非同步的) and permits nulls.) This class makes no guarantees as to
35 * the order of the map; in particular(尤其，特别), it does not guarantee(保证) that the order
36 * will remain constant over time.
37 *
38 * <p>This implementation provides constant-time performance for the basic
39 * operations (<tt>get</tt> and <tt>put</tt>), assuming the hash function
40 * disperses(散开，驱散) the elements properly among the buckets(桶). Iteration over
41 * collection views requires time proportional to the "capacity" of the
42 * <tt>HashMap</tt> instance (the number of buckets) plus its size (the number
43 * of key-value mappings). Thus, it's very important not to set the initial
44 * capacity too high (or the load factor too low) if iteration performance is
45 * important.
46 *
47 * <p>An instance(例子，实例) of <tt>HashMap</tt> has two parameters that affect its
48 * performance: <i>initial capacity</i> and <i>load factor</i>. The
49 * <i>capacity</i> is the number of buckets in the hash table, and the initial
50 * capacity is simply the capacity at the time the hash table is created. The
51 * <i>load factor</i> is a measure of how full the hash table is allowed to
52 * get before its capacity is automatically increased. When the number of
53 * entries in the hash table exceeds(超过；超越) the product of the load factor and the
54 * current capacity, the hash table is <i>rehashed</i> (that is, internal data
55 * structures are rebuilt) so that the hash table has approximately(近似地，大约) twice the
56 * number of buckets.
57 *
58 * <p>As a general rule, the default load factor (.75) offers a good tradeoff(权衡，折中)
59 * between time and space costs. Higher values decrease the space overhead
60 * but increase the lookup cost (reflected in most of the operations of the
61 * <tt>HashMap</tt> class, including <tt>get</tt> and <tt>put</tt>). The
62 * expected number of entries in the map and its load factor should be taken
63 * into account when setting its initial capacity, so as to minimize the
64 * number of rehash operations. If the initial capacity is greater
65 * than the maximum number of entries divided by the load factor, no
66 * rehash operations will ever occur.
67 *
68 * <p>If many mappings are to be stored in a <tt>HashMap</tt> instance,
69 * creating it with a sufficiently large capacity will allow the mappings to
70 * be stored more efficiently than letting it perform automatic rehashing as
71 * needed to grow the table.
72 *
73 * <p><strong>Note that this implementation is not synchronized.</strong>
74 * If multiple threads access a hash map concurrently, and at least one of
75 * the threads modifies the map structurally, it <i>must</i> be
76 * synchronized externally. (A structural modification is any operation
77 * that adds or deletes one or more mappings; merely changing the value
78 * associated with a key that an instance already contains is not a
79 * structural modification.) This is typically accomplished by
80 * synchronizing on some object that naturally encapsulates(总结；扼要概括) the map.
81 *
82 * If no such object exists, the map should be "wrapped" using the
83 * {@link Collections#synchronizedMap Collections.synchronizedMap}
84 * method. This is best done at creation time, to prevent accidental
85 * unsynchronized access to the map:<pre>
86 * Map m = Collections.synchronizedMap(new HashMap(...));</pre>
87 *
88 * <p>The iterators returned by all of this class's "collection view methods"
89 * are <i>fail-fast</i>: if the map is structurally modified at any time after
90 * the iterator is created, in any way except through the iterator's own
91 * <tt>remove</tt> method, the iterator will throw a
92 * {@link ConcurrentModificationException}. Thus, in the face of concurrent
93 * modification, the iterator fails quickly and cleanly, rather than risking
94 * arbitrary, non-deterministic behavior at an undetermined time in the
95 * future.
96 *
97 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
98 * as it is, generally speaking, impossible to make any hard guarantees in the
99 * presence of unsynchronized concurrent modification. Fail-fast iterators
100 * throw <tt>ConcurrentModificationException</tt> on a best-effort basis.
101 * Therefore, it would be wrong to write a program that depended on this
102 * exception for its correctness: <i>the fail-fast behavior of iterators
103 * should be used only to detect bugs.</i>
104 *
105 * <p>This class is a member of the
106 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
107 * Java Collections Framework</a>.
108 *
109 * @param <K> the type of keys maintained by this map
110 * @param <V> the type of mapped values
111 *
112 * @author Doug Lea
113 * @author Josh Bloch
114 * @author Arthur van Hoff
115 * @author Neal Gafter
116 * @see Object#hashCode()
117 * @see Collection
118 * @see Map
119 * @see TreeMap
120 * @see Hashtable
121 * @since 1.2
122 */
123
124 public class HashMap<K,V>
125 extends AbstractMap<K,V>
126 implements Map<K,V>, Cloneable, Serializable
127 {
128
129 /**
130 * The default initial capacity - MUST be a power of two.
131 */
132 static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
133
134 /**
135 * The maximum capacity, used if a higher value is implicitly specified
136 * by either of the constructors with arguments.
137 * MUST be a power of two <= 1<<30.
138 */
139 static final int MAXIMUM_CAPACITY = 1 << 30;
140
141 /**
142 * The load factor used when none specified in constructor.
143 */
144 static final float DEFAULT_LOAD_FACTOR = 0.75f;
145
146 /**
147 * An empty table instance to share when the table is not inflated.
148 */
149 static final Entry<?,?>[] EMPTY_TABLE = {};
150
151 /**
152 * The table, resized as necessary. Length MUST Always be a power of two.
153 */
154 transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE;
155
156 /**
157 * The number of key-value mappings contained in this map.
158 */
159 transient int size;
160
161 /**
162 * The next size value at which to resize (capacity * load factor).
163 * @serial
164 */
165 // If table == EMPTY_TABLE then this is the initial capacity at which the
166 // table will be created when inflated.
167 int threshold;
168
169 /**
170 * The load factor for the hash table.
171 *
172 * @serial
173 */
174 final float loadFactor;
175
176 /**
177 * The number of times this HashMap has been structurally modified
178 * Structural modifications are those that change the number of mappings in
179 * the HashMap or otherwise modify its internal structure (e.g.,
180 * rehash). This field is used to make iterators on Collection-views of
181 * the HashMap fail-fast. (See ConcurrentModificationException).
182 */
183 transient int modCount;
184
185 /**
186 * The default threshold of map capacity above which alternative hashing is
187 * used for String keys. Alternative hashing reduces the incidence of
188 * collisions due to weak hash code calculation for String keys.
189 * <p/>
190 * This value may be overridden by defining the system property
191 * {@code jdk.map.althashing.threshold}. A property value of {@code 1}
192 * forces alternative hashing to be used at all times whereas
193 * {@code -1} value ensures that alternative hashing is never used.
194 */
195 static final int ALTERNATIVE_HASHING_THRESHOLD_DEFAULT = Integer.MAX_VALUE;
196
197 /**
198 * holds values which can't be initialized until after VM is booted.
199 */
200 private static class Holder {
201
202 /**
203 * Table capacity above which to switch to use alternative hashing.
204 */
205 static final int ALTERNATIVE_HASHING_THRESHOLD;
206
207 static {
208 String altThreshold = java.security.AccessController.doPrivileged(
209 new sun.security.action.GetPropertyAction(
210 "jdk.map.althashing.threshold"));
211
212 int threshold;
213 try {
214 threshold = (null != altThreshold)
215 ? Integer.parseInt(altThreshold)
216 : ALTERNATIVE_HASHING_THRESHOLD_DEFAULT;
217
218 // disable alternative hashing if -1
219 if (threshold == -1) {
220 threshold = Integer.MAX_VALUE;
221 }
222
223 if (threshold < 0) {
224 throw new IllegalArgumentException("value must be positive integer.");
225 }
226 } catch(IllegalArgumentException failed) {
227 throw new Error("Illegal value for 'jdk.map.althashing.threshold'", failed);
228 }
229
230 ALTERNATIVE_HASHING_THRESHOLD = threshold;
231 }
232 }
233
234 /**
235 * A randomizing value associated with this instance that is applied to
236 * hash code of keys to make hash collisions harder to find. If 0 then
237 * alternative hashing is disabled.
238 */
239 transient int hashSeed = 0;
240
241 /**
242 * Constructs an empty <tt>HashMap</tt> with the specified initial
243 * capacity and load factor.
244 *
245 * @param initialCapacity the initial capacity
246 * @param loadFactor the load factor
247 * @throws IllegalArgumentException if the initial capacity is negative
248 * or the load factor is nonpositive
249 */
250 public HashMap(int initialCapacity, float loadFactor) {
251 if (initialCapacity < 0)
252 throw new IllegalArgumentException("Illegal initial capacity: " +
253 initialCapacity);
254 if (initialCapacity > MAXIMUM_CAPACITY)
255 initialCapacity = MAXIMUM_CAPACITY;
256 if (loadFactor <= 0 || Float.isNaN(loadFactor))
257 throw new IllegalArgumentException("Illegal load factor: " +
258 loadFactor);
259
260 this.loadFactor = loadFactor;
261 threshold = initialCapacity;
262 init();
263 }
264
265 /**
266 * Constructs an empty <tt>HashMap</tt> with the specified initial
267 * capacity and the default load factor (0.75).
268 *
269 * @param initialCapacity the initial capacity.
270 * @throws IllegalArgumentException if the initial capacity is negative.
271 */
272 public HashMap(int initialCapacity) {
273 this(initialCapacity, DEFAULT_LOAD_FACTOR);
274 }
275
276 /**
277 * Constructs an empty <tt>HashMap</tt> with the default initial capacity
278 * (16) and the default load factor (0.75).
279 */
280 public HashMap() {
281 this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
282 }
283
284 /**
285 * Constructs a new <tt>HashMap</tt> with the same mappings as the
286 * specified <tt>Map</tt>. The <tt>HashMap</tt> is created with
287 * default load factor (0.75) and an initial capacity sufficient to
288 * hold the mappings in the specified <tt>Map</tt>.
289 *
290 * @param m the map whose mappings are to be placed in this map
291 * @throws NullPointerException if the specified map is null
292 */
293 public HashMap(Map<? extends K, ? extends V> m) {
294 this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,
295 DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR);
296 inflateTable(threshold);
297
298 putAllForCreate(m);
299 }
300
301 private static int roundUpToPowerOf2(int number) {
302 // assert number >= 0 : "number must be non-negative";
303 int rounded = number >= MAXIMUM_CAPACITY
304 ? MAXIMUM_CAPACITY
305 : (rounded = Integer.highestOneBit(number)) != 0
306 ? (Integer.bitCount(number) > 1) ? rounded << 1 : rounded
307 : 1;
308
309 return rounded;
310 }
311
312 /**
313 * Inflates the table.
314 */
315 private void inflateTable(int toSize) {
316 // Find a power of 2 >= toSize
317 int capacity = roundUpToPowerOf2(toSize);
318
319 threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);
320 table = new Entry[capacity];
321 initHashSeedAsNeeded(capacity);
322 }
323
324 // internal utilities
325
326 /**
327 * Initialization hook for subclasses. This method is called
328 * in all constructors and pseudo-constructors (clone, readObject)
329 * after HashMap has been initialized but before any entries have
330 * been inserted. (In the absence of this method, readObject would
331 * require explicit knowledge of subclasses.)
332 */
333 void init() {
334 }
335
336 /**
337 * Initialize the hashing mask value. We defer initialization until we
338 * really need it.
339 */
340 final boolean initHashSeedAsNeeded(int capacity) {
341 boolean currentAltHashing = hashSeed != 0;
342 boolean useAltHashing = sun.misc.VM.isBooted() &&
343 (capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);
344 boolean switching = currentAltHashing ^ useAltHashing;
345 if (switching) {
346 hashSeed = useAltHashing
347 ? sun.misc.Hashing.randomHashSeed(this)
348 : 0;
349 }
350 return switching;
351 }
352
353 /**
354 * Retrieve object hash code and applies a supplemental hash function to the
355 * result hash, which defends against poor quality hash functions. This is
356 * critical because HashMap uses power-of-two length hash tables, that
357 * otherwise encounter collisions for hashCodes that do not differ
358 * in lower bits. Note: Null keys always map to hash 0, thus index 0.
359 */
360 final int hash(Object k) {
361 int h = hashSeed;
362 if (0 != h && k instanceof String) {
363 return sun.misc.Hashing.stringHash32((String) k);
364 }
365
366 h ^= k.hashCode();
367
368 // This function ensures that hashCodes that differ only by
369 // constant multiples at each bit position have a bounded
370 // number of collisions (approximately 8 at default load factor).
371 h ^= (h >>> 20) ^ (h >>> 12);
372 return h ^ (h >>> 7) ^ (h >>> 4);
373 }
374
375 /**
376 * Returns index for hash code h.
377 */
378 static int indexFor(int h, int length) {
379 // assert Integer.bitCount(length) == 1 : "length must be a non-zero power of 2";
380 return h & (length-1);
381 }
382
383 /**
384 * Returns the number of key-value mappings in this map.
385 *
386 * @return the number of key-value mappings in this map
387 */
388 public int size() {
389 return size;
390 }
391
392 /**
393 * Returns <tt>true</tt> if this map contains no key-value mappings.
394 *
395 * @return <tt>true</tt> if this map contains no key-value mappings
396 */
397 public boolean isEmpty() {
398 return size == 0;
399 }
400
401 /**
402 * Returns the value to which the specified key is mapped,
403 * or {@code null} if this map contains no mapping for the key.
404 *
405 * <p>More formally, if this map contains a mapping from a key
406 * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
407 * key.equals(k))}, then this method returns {@code v}; otherwise
408 * it returns {@code null}. (There can be at most one such mapping.)
409 *
410 * <p>A return value of {@code null} does not <i>necessarily</i>
411 * indicate that the map contains no mapping for the key; it's also
412 * possible that the map explicitly maps the key to {@code null}.
413 * The {@link #containsKey containsKey} operation may be used to
414 * distinguish these two cases.
415 *
416 * @see #put(Object, Object)
417 */
418 public V get(Object key) {
419 if (key == null)
420 return getForNullKey();
421 Entry<K,V> entry = getEntry(key);
422
423 return null == entry ? null : entry.getValue();
424 }
425
426 /**
427 * Offloaded version of get() to look up null keys. Null keys map
428 * to index 0. This null case is split out into separate methods
429 * for the sake of performance in the two most commonly used
430 * operations (get and put), but incorporated with conditionals in
431 * others.
432 */
433 private V getForNullKey() {
434 if (size == 0) {
435 return null;
436 }
437 for (Entry<K,V> e = table[0]; e != null; e = e.next) {
438 if (e.key == null)
439 return e.value;
440 }
441 return null;
442 }
443
444 /**
445 * Returns <tt>true</tt> if this map contains a mapping for the
446 * specified key.
447 *
448 * @param key The key whose presence in this map is to be tested
449 * @return <tt>true</tt> if this map contains a mapping for the specified
450 * key.
451 */
452 public boolean containsKey(Object key) {
453 return getEntry(key) != null;
454 }
455
456 /**
457 * Returns the entry associated with the specified key in the
458 * HashMap. Returns null if the HashMap contains no mapping
459 * for the key.
460 */
461 final Entry<K,V> getEntry(Object key) {
462 if (size == 0) {
463 return null;
464 }
465
466 int hash = (key == null) ? 0 : hash(key);
467 for (Entry<K,V> e = table[indexFor(hash, table.length)];
468 e != null;
469 e = e.next) {
470 Object k;
471 if (e.hash == hash &&
472 ((k = e.key) == key || (key != null && key.equals(k))))
473 return e;
474 }
475 return null;
476 }
477
478 /**
479 * Associates the specified value with the specified key in this map.
480 * If the map previously contained a mapping for the key, the old
481 * value is replaced.
482 *
483 * @param key key with which the specified value is to be associated
484 * @param value value to be associated with the specified key
485 * @return the previous value associated with <tt>key</tt>, or
486 * <tt>null</tt> if there was no mapping for <tt>key</tt>.
487 * (A <tt>null</tt> return can also indicate that the map
488 * previously associated <tt>null</tt> with <tt>key</tt>.)
489 */
490 public V put(K key, V value) {
491 if (table == EMPTY_TABLE) {
492 inflateTable(threshold);
493 }
494 if (key == null)
495 return putForNullKey(value);
496 int hash = hash(key);
497 int i = indexFor(hash, table.length);
498 for (Entry<K,V> e = table[i]; e != null; e = e.next) {
499 Object k;
500 if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
501 V oldValue = e.value;
502 e.value = value;
503 e.recordAccess(this);
504 return oldValue;
505 }
506 }
507
508 modCount++;
509 addEntry(hash, key, value, i);
510 return null;
511 }
512
513 /**
514 * Offloaded version of put for null keys
515 */
516 private V putForNullKey(V value) {
517 for (Entry<K,V> e = table[0]; e != null; e = e.next) {
518 if (e.key == null) {
519 V oldValue = e.value;
520 e.value = value;
521 e.recordAccess(this);
522 return oldValue;
523 }
524 }
525 modCount++;
526 addEntry(0, null, value, 0);
527 return null;
528 }
529
530 /**
531 * This method is used instead of put by constructors and
532 * pseudoconstructors (clone, readObject). It does not resize the table,
533 * check for comodification, etc. It calls createEntry rather than
534 * addEntry.
535 */
536 private void putForCreate(K key, V value) {
537 int hash = null == key ? 0 : hash(key);
538 int i = indexFor(hash, table.length);
539
540 /**
541 * Look for preexisting entry for key. This will never happen for
542 * clone or deserialize. It will only happen for construction if the
543 * input Map is a sorted map whose ordering is inconsistent w/ equals.
544 */
545 for (Entry<K,V> e = table[i]; e != null; e = e.next) {
546 Object k;
547 if (e.hash == hash &&
548 ((k = e.key) == key || (key != null && key.equals(k)))) {
549 e.value = value;
550 return;
551 }
552 }
553
554 createEntry(hash, key, value, i);
555 }
556
557 private void putAllForCreate(Map<? extends K, ? extends V> m) {
558 for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
559 putForCreate(e.getKey(), e.getValue());
560 }
561
562 /**
563 * Rehashes the contents of this map into a new array with a
564 * larger capacity. This method is called automatically when the
565 * number of keys in this map reaches its threshold.
566 *
567 * If current capacity is MAXIMUM_CAPACITY, this method does not
568 * resize the map, but sets threshold to Integer.MAX_VALUE.
569 * This has the effect of preventing future calls.
570 *
571 * @param newCapacity the new capacity, MUST be a power of two;
572 * must be greater than current capacity unless current
573 * capacity is MAXIMUM_CAPACITY (in which case value
574 * is irrelevant).
575 */
576 void resize(int newCapacity) {
577 Entry[] oldTable = table;
578 int oldCapacity = oldTable.length;
579 if (oldCapacity == MAXIMUM_CAPACITY) {
580 threshold = Integer.MAX_VALUE;
581 return;
582 }
583
584 Entry[] newTable = new Entry[newCapacity];
585 transfer(newTable, initHashSeedAsNeeded(newCapacity));
586 table = newTable;
587 threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);
588 }
589
590 /**
591 * Transfers all entries from current table to newTable.
592 */
593 void transfer(Entry[] newTable, boolean rehash) {
594 int newCapacity = newTable.length;
595 for (Entry<K,V> e : table) {
596 while(null != e) {
597 Entry<K,V> next = e.next;
598 if (rehash) {
599 e.hash = null == e.key ? 0 : hash(e.key);
600 }
601 int i = indexFor(e.hash, newCapacity);
602 e.next = newTable[i];
603 newTable[i] = e;
604 e = next;
605 }
606 }
607 }
608
609 /**
610 * Copies all of the mappings from the specified map to this map.
611 * These mappings will replace any mappings that this map had for
612 * any of the keys currently in the specified map.
613 *
614 * @param m mappings to be stored in this map
615 * @throws NullPointerException if the specified map is null
616 */
617 public void putAll(Map<? extends K, ? extends V> m) {
618 int numKeysToBeAdded = m.size();
619 if (numKeysToBeAdded == 0)
620 return;
621
622 if (table == EMPTY_TABLE) {
623 inflateTable((int) Math.max(numKeysToBeAdded * loadFactor, threshold));
624 }
625
626 /*
627 * Expand the map if the map if the number of mappings to be added
628 * is greater than or equal to threshold. This is conservative; the
629 * obvious condition is (m.size() + size) >= threshold, but this
630 * condition could result in a map with twice the appropriate capacity,
631 * if the keys to be added overlap with the keys already in this map.
632 * By using the conservative calculation, we subject ourself
633 * to at most one extra resize.
634 */
635 if (numKeysToBeAdded > threshold) {
636 int targetCapacity = (int)(numKeysToBeAdded / loadFactor + 1);
637 if (targetCapacity > MAXIMUM_CAPACITY)
638 targetCapacity = MAXIMUM_CAPACITY;
639 int newCapacity = table.length;
640 while (newCapacity < targetCapacity)
641 newCapacity <<= 1;
642 if (newCapacity > table.length)
643 resize(newCapacity);
644 }
645
646 for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
647 put(e.getKey(), e.getValue());
648 }
649
650 /**
651 * Removes the mapping for the specified key from this map if present.
652 *
653 * @param key key whose mapping is to be removed from the map
654 * @return the previous value associated with <tt>key</tt>, or
655 * <tt>null</tt> if there was no mapping for <tt>key</tt>.
656 * (A <tt>null</tt> return can also indicate that the map
657 * previously associated <tt>null</tt> with <tt>key</tt>.)
658 */
659 public V remove(Object key) {
660 Entry<K,V> e = removeEntryForKey(key);
661 return (e == null ? null : e.value);
662 }
663
664 /**
665 * Removes and returns the entry associated with the specified key
666 * in the HashMap. Returns null if the HashMap contains no mapping
667 * for this key.
668 */
669 final Entry<K,V> removeEntryForKey(Object key) {
670 if (size == 0) {
671 return null;
672 }
673 int hash = (key == null) ? 0 : hash(key);
674 int i = indexFor(hash, table.length);
675 Entry<K,V> prev = table[i];
676 Entry<K,V> e = prev;
677
678 while (e != null) {
679 Entry<K,V> next = e.next;
680 Object k;
681 if (e.hash == hash &&
682 ((k = e.key) == key || (key != null && key.equals(k)))) {
683 modCount++;
684 size--;
685 if (prev == e)
686 table[i] = next;
687 else
688 prev.next = next;
689 e.recordRemoval(this);
690 return e;
691 }
692 prev = e;
693 e = next;
694 }
695
696 return e;
697 }
698
699 /**
700 * Special version of remove for EntrySet using {@code Map.Entry.equals()}
701 * for matching.
702 */
703 final Entry<K,V> removeMapping(Object o) {
704 if (size == 0 || !(o instanceof Map.Entry))
705 return null;
706
707 Map.Entry<K,V> entry = (Map.Entry<K,V>) o;
708 Object key = entry.getKey();
709 int hash = (key == null) ? 0 : hash(key);
710 int i = indexFor(hash, table.length);
711 Entry<K,V> prev = table[i];
712 Entry<K,V> e = prev;
713
714 while (e != null) {
715 Entry<K,V> next = e.next;
716 if (e.hash == hash && e.equals(entry)) {
717 modCount++;
718 size--;
719 if (prev == e)
720 table[i] = next;
721 else
722 prev.next = next;
723 e.recordRemoval(this);
724 return e;
725 }
726 prev = e;
727 e = next;
728 }
729
730 return e;
731 }
732
733 /**
734 * Removes all of the mappings from this map.
735 * The map will be empty after this call returns.
736 */
737 public void clear() {
738 modCount++;
739 Arrays.fill(table, null);
740 size = 0;
741 }
742
743 /**
744 * Returns <tt>true</tt> if this map maps one or more keys to the
745 * specified value.
746 *
747 * @param value value whose presence in this map is to be tested
748 * @return <tt>true</tt> if this map maps one or more keys to the
749 * specified value
750 */
751 public boolean containsValue(Object value) {
752 if (value == null)
753 return containsNullValue();
754
755 Entry[] tab = table;
756 for (int i = 0; i < tab.length ; i++)
757 for (Entry e = tab[i] ; e != null ; e = e.next)
758 if (value.equals(e.value))
759 return true;
760 return false;
761 }
762
763 /**
764 * Special-case code for containsValue with null argument
765 */
766 private boolean containsNullValue() {
767 Entry[] tab = table;
768 for (int i = 0; i < tab.length ; i++)
769 for (Entry e = tab[i] ; e != null ; e = e.next)
770 if (e.value == null)
771 return true;
772 return false;
773 }
774
775 /**
776 * Returns a shallow copy of this <tt>HashMap</tt> instance: the keys and
777 * values themselves are not cloned.
778 *
779 * @return a shallow copy of this map
780 */
781 public Object clone() {
782 HashMap<K,V> result = null;
783 try {
784 result = (HashMap<K,V>)super.clone();
785 } catch (CloneNotSupportedException e) {
786 // assert false;
787 }
788 if (result.table != EMPTY_TABLE) {
789 result.inflateTable(Math.min(
790 (int) Math.min(
791 size * Math.min(1 / loadFactor, 4.0f),
792 // we have limits...
793 HashMap.MAXIMUM_CAPACITY),
794 table.length));
795 }
796 result.entrySet = null;
797 result.modCount = 0;
798 result.size = 0;
799 result.init();
800 result.putAllForCreate(this);
801
802 return result;
803 }
804
805 static class Entry<K,V> implements Map.Entry<K,V> {
806 final K key;
807 V value;
808 Entry<K,V> next;
809 int hash;
810
811 /**
812 * Creates new entry.
813 */
814 Entry(int h, K k, V v, Entry<K,V> n) {
815 value = v;
816 next = n;
817 key = k;
818 hash = h;
819 }
820
821 public final K getKey() {
822 return key;
823 }
824
825 public final V getValue() {
826 return value;
827 }
828
829 public final V setValue(V newValue) {
830 V oldValue = value;
831 value = newValue;
832 return oldValue;
833 }
834
835 public final boolean equals(Object o) {
836 if (!(o instanceof Map.Entry))
837 return false;
838 Map.Entry e = (Map.Entry)o;
839 Object k1 = getKey();
840 Object k2 = e.getKey();
841 if (k1 == k2 || (k1 != null && k1.equals(k2))) {
842 Object v1 = getValue();
843 Object v2 = e.getValue();
844 if (v1 == v2 || (v1 != null && v1.equals(v2)))
845 return true;
846 }
847 return false;
848 }
849
850 public final int hashCode() {
851 return Objects.hashCode(getKey()) ^ Objects.hashCode(getValue());
852 }
853
854 public final String toString() {
855 return getKey() + "=" + getValue();
856 }
857
858 /**
859 * This method is invoked whenever the value in an entry is
860 * overwritten by an invocation of put(k,v) for a key k that's already
861 * in the HashMap.
862 */
863 void recordAccess(HashMap<K,V> m) {
864 }
865
866 /**
867 * This method is invoked whenever the entry is
868 * removed from the table.
869 */
870 void recordRemoval(HashMap<K,V> m) {
871 }
872 }
873
874 /**
875 * Adds a new entry with the specified key, value and hash code to
876 * the specified bucket. It is the responsibility of this
877 * method to resize the table if appropriate.
878 *
879 * Subclass overrides this to alter the behavior of put method.
880 */
881 void addEntry(int hash, K key, V value, int bucketIndex) {
882 if ((size >= threshold) && (null != table[bucketIndex])) {
883 resize(2 * table.length);
884 hash = (null != key) ? hash(key) : 0;
885 bucketIndex = indexFor(hash, table.length);
886 }
887
888 createEntry(hash, key, value, bucketIndex);
889 }
890
891 /**
892 * Like addEntry except that this version is used when creating entries
893 * as part of Map construction or "pseudo-construction" (cloning,
894 * deserialization). This version needn't worry about resizing the table.
895 *
896 * Subclass overrides this to alter the behavior of HashMap(Map),
897 * clone, and readObject.
898 */
899 void createEntry(int hash, K key, V value, int bucketIndex) {
900 Entry<K,V> e = table[bucketIndex];
901 table[bucketIndex] = new Entry<>(hash, key, value, e);
902 size++;
903 }
904
905 private abstract class HashIterator<E> implements Iterator<E> {
906 Entry<K,V> next; // next entry to return
907 int expectedModCount; // For fast-fail
908 int index; // current slot
909 Entry<K,V> current; // current entry
910
911 HashIterator() {
912 expectedModCount = modCount;
913 if (size > 0) { // advance to first entry
914 Entry[] t = table;
915 while (index < t.length && (next = t[index++]) == null)
916 ;
917 }
918 }
919
920 public final boolean hasNext() {
921 return next != null;
922 }
923
924 final Entry<K,V> nextEntry() {
925 if (modCount != expectedModCount)
926 throw new ConcurrentModificationException();
927 Entry<K,V> e = next;
928 if (e == null)
929 throw new NoSuchElementException();
930
931 if ((next = e.next) == null) {
932 Entry[] t = table;
933 while (index < t.length && (next = t[index++]) == null)
934 ;
935 }
936 current = e;
937 return e;
938 }
939
940 public void remove() {
941 if (current == null)
942 throw new IllegalStateException();
943 if (modCount != expectedModCount)
944 throw new ConcurrentModificationException();
945 Object k = current.key;
946 current = null;
947 HashMap.this.removeEntryForKey(k);
948 expectedModCount = modCount;
949 }
950 }
951
952 private final class ValueIterator extends HashIterator<V> {
953 public V next() {
954 return nextEntry().value;
955 }
956 }
957
958 private final class KeyIterator extends HashIterator<K> {
959 public K next() {
960 return nextEntry().getKey();
961 }
962 }
963
964 private final class EntryIterator extends HashIterator<Map.Entry<K,V>> {
965 public Map.Entry<K,V> next() {
966 return nextEntry();
967 }
968 }
969
970 // Subclass overrides these to alter behavior of views' iterator() method
971 Iterator<K> newKeyIterator() {
972 return new KeyIterator();
973 }
974 Iterator<V> newValueIterator() {
975 return new ValueIterator();
976 }
977 Iterator<Map.Entry<K,V>> newEntryIterator() {
978 return new EntryIterator();
979 }
980
981
982 // Views
983
984 private transient Set<Map.Entry<K,V>> entrySet = null;
985
986 /**
987 * Returns a {@link Set} view of the keys contained in this map.
988 * The set is backed by the map, so changes to the map are
989 * reflected in the set, and vice-versa. If the map is modified
990 * while an iteration over the set is in progress (except through
991 * the iterator's own <tt>remove</tt> operation), the results of
992 * the iteration are undefined. The set supports element removal,
993 * which removes the corresponding mapping from the map, via the
994 * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
995 * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
996 * operations. It does not support the <tt>add</tt> or <tt>addAll</tt>
997 * operations.
998 */
999 public Set<K> keySet() {
1000 Set<K> ks = keySet;
1001 return (ks != null ? ks : (keySet = new KeySet()));
1002 }
1003
1004 private final class KeySet extends AbstractSet<K> {
1005 public Iterator<K> iterator() {
1006 return newKeyIterator();
1007 }
1008 public int size() {
1009 return size;
1010 }
1011 public boolean contains(Object o) {
1012 return containsKey(o);
1013 }
1014 public boolean remove(Object o) {
1015 return HashMap.this.removeEntryForKey(o) != null;
1016 }
1017 public void clear() {
1018 HashMap.this.clear();
1019 }
1020 }
1021
1022 /**
1023 * Returns a {@link Collection} view of the values contained in this map.
1024 * The collection is backed by the map, so changes to the map are
1025 * reflected in the collection, and vice-versa. If the map is
1026 * modified while an iteration over the collection is in progress
1027 * (except through the iterator's own <tt>remove</tt> operation),
1028 * the results of the iteration are undefined. The collection
1029 * supports element removal, which removes the corresponding
1030 * mapping from the map, via the <tt>Iterator.remove</tt>,
1031 * <tt>Collection.remove</tt>, <tt>removeAll</tt>,
1032 * <tt>retainAll</tt> and <tt>clear</tt> operations. It does not
1033 * support the <tt>add</tt> or <tt>addAll</tt> operations.
1034 */
1035 public Collection<V> values() {
1036 Collection<V> vs = values;
1037 return (vs != null ? vs : (values = new Values()));
1038 }
1039
1040 private final class Values extends AbstractCollection<V> {
1041 public Iterator<V> iterator() {
1042 return newValueIterator();
1043 }
1044 public int size() {
1045 return size;
1046 }
1047 public boolean contains(Object o) {
1048 return containsValue(o);
1049 }
1050 public void clear() {
1051 HashMap.this.clear();
1052 }
1053 }
1054
1055 /**
1056 * Returns a {@link Set} view of the mappings contained in this map.
1057 * The set is backed by the map, so changes to the map are
1058 * reflected in the set, and vice-versa. If the map is modified
1059 * while an iteration over the set is in progress (except through
1060 * the iterator's own <tt>remove</tt> operation, or through the
1061 * <tt>setValue</tt> operation on a map entry returned by the
1062 * iterator) the results of the iteration are undefined. The set
1063 * supports element removal, which removes the corresponding
1064 * mapping from the map, via the <tt>Iterator.remove</tt>,
1065 * <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and
1066 * <tt>clear</tt> operations. It does not support the
1067 * <tt>add</tt> or <tt>addAll</tt> operations.
1068 *
1069 * @return a set view of the mappings contained in this map
1070 */
1071 public Set<Map.Entry<K,V>> entrySet() {
1072 return entrySet0();
1073 }
1074
1075 private Set<Map.Entry<K,V>> entrySet0() {
1076 Set<Map.Entry<K,V>> es = entrySet;
1077 return es != null ? es : (entrySet = new EntrySet());
1078 }
1079
1080 private final class EntrySet extends AbstractSet<Map.Entry<K,V>> {
1081 public Iterator<Map.Entry<K,V>> iterator() {
1082 return newEntryIterator();
1083 }
1084 public boolean contains(Object o) {
1085 if (!(o instanceof Map.Entry))
1086 return false;
1087 Map.Entry<K,V> e = (Map.Entry<K,V>) o;
1088 Entry<K,V> candidate = getEntry(e.getKey());
1089 return candidate != null && candidate.equals(e);
1090 }
1091 public boolean remove(Object o) {
1092 return removeMapping(o) != null;
1093 }
1094 public int size() {
1095 return size;
1096 }
1097 public void clear() {
1098 HashMap.this.clear();
1099 }
1100 }
1101
1102 /**
1103 * Save the state of the <tt>HashMap</tt> instance to a stream (i.e.,
1104 * serialize it).
1105 *
1106 * @serialData The <i>capacity</i> of the HashMap (the length of the
1107 * bucket array) is emitted (int), followed by the
1108 * <i>size</i> (an int, the number of key-value
1109 * mappings), followed by the key (Object) and value (Object)
1110 * for each key-value mapping. The key-value mappings are
1111 * emitted in no particular order.
1112 */
1113 private void writeObject(java.io.ObjectOutputStream s)
1114 throws IOException
1115 {
1116 // Write out the threshold, loadfactor, and any hidden stuff
1117 s.defaultWriteObject();
1118
1119 // Write out number of buckets
1120 if (table==EMPTY_TABLE) {
1121 s.writeInt(roundUpToPowerOf2(threshold));
1122 } else {
1123 s.writeInt(table.length);
1124 }
1125
1126 // Write out size (number of Mappings)
1127 s.writeInt(size);
1128
1129 // Write out keys and values (alternating)
1130 if (size > 0) {
1131 for(Map.Entry<K,V> e : entrySet0()) {
1132 s.writeObject(e.getKey());
1133 s.writeObject(e.getValue());
1134 }
1135 }
1136 }
1137
1138 private static final long serialVersionUID = 362498820763181265L;
1139
1140 /**
1141 * Reconstitute the {@code HashMap} instance from a stream (i.e.,
1142 * deserialize it).
1143 */
1144 private void readObject(java.io.ObjectInputStream s)
1145 throws IOException, ClassNotFoundException
1146 {
1147 // Read in the threshold (ignored), loadfactor, and any hidden stuff
1148 s.defaultReadObject();
1149 if (loadFactor <= 0 || Float.isNaN(loadFactor)) {
1150 throw new InvalidObjectException("Illegal load factor: " +
1151 loadFactor);
1152 }
1153
1154 // set other fields that need values
1155 table = (Entry<K,V>[]) EMPTY_TABLE;
1156
1157 // Read in number of buckets
1158 s.readInt(); // ignored.
1159
1160 // Read number of mappings
1161 int mappings = s.readInt();
1162 if (mappings < 0)
1163 throw new InvalidObjectException("Illegal mappings count: " +
1164 mappings);
1165
1166 // capacity chosen by number of mappings and desired load (if >= 0.25)
1167 int capacity = (int) Math.min(
1168 mappings * Math.min(1 / loadFactor, 4.0f),
1169 // we have limits...
1170 HashMap.MAXIMUM_CAPACITY);
1171
1172 // allocate the bucket array;
1173 if (mappings > 0) {
1174 inflateTable(capacity);
1175 } else {
1176 threshold = capacity;
1177 }
1178
1179 init(); // Give subclass a chance to do its thing.
1180
1181 // Read the keys and values, and put the mappings in the HashMap
1182 for (int i = 0; i < mappings; i++) {
1183 K key = (K) s.readObject();
1184 V value = (V) s.readObject();
1185 putForCreate(key, value);
1186 }
1187 }
1188
1189 // These methods are used when serializing HashSets
1190 int capacity() { return table.length; }
1191 float loadFactor() { return loadFactor; }
1192 }
来源：https://www.cnblogs.com/LideAiYaner-1wn/p/5752295.html