87 *
88 *

The iterators returned by all of this class's "collection view methods"
89 * are fail-fast: 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 * remove 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 *

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 ConcurrentModificationException on a best-effort basis.
101 * Therefore, it would be wrong to write a program that depended on this
102 * exception for its correctness: the fail-fast behavior of iterators
103 * should be used only to detect bugs.
104 *
105 *

This class is a member of the
106 *
107 * Java Collections Framework.
108 *
109 * @param the type of keys maintained by this map
110 * @param 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
125 extends AbstractMap
126 implements Map, Cloneable, Serializable
127 {
128
129 /**
130 * The default initial capacity - MUST be a power of two.
131 */
132 static final int DEFAULT_INITIAL_CAPACITY = 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 * The table, resized as necessary. Length MUST Always be a power of two.
148 */
149 transient Entry[] table;
150
151 /**
152 * The number of key-value mappings contained in this map.
153 */
154 transient int size;
155
156 /**
157 * The next size value at which to resize (capacity * load factor).
158 * @serial
159 */
160 int threshold;
161
162 /**
163 * The load factor for the hash table.
164 *
165 * @serial
166 */
167 final float loadFactor;
168
169 /**
170 * The number of times this HashMap has been structurally modified
171 * Structural modifications are those that change the number of mappings in
172 * the HashMap or otherwise modify its internal structure (e.g.,
173 * rehash). This field is used to make iterators on Collection-views of
174 * the HashMap fail-fast. (See ConcurrentModificationException).
175 */
176 transient int modCount;
177
178 /**
179 * Constructs an empty HashMap with the specified initial
180 * capacity and load factor.
181 *
182 * @param initialCapacity the initial capacity
183 * @param loadFactor the load factor
184 * @throws IllegalArgumentException if the initial capacity is negative
185 * or the load factor is nonpositive
186 */
187 public HashMap(int initialCapacity, float loadFactor) {
188 if (initialCapacity < 0)
189 throw new IllegalArgumentException("Illegal initial capacity: " +
190 initialCapacity);
191 if (initialCapacity > MAXIMUM_CAPACITY)
192 initialCapacity = MAXIMUM_CAPACITY;
193 if (loadFactor <= 0 || Float.isNaN(loadFactor))
194 throw new IllegalArgumentException("Illegal load factor: " +
195 loadFactor);
196
197 // Find a power of 2 >= initialCapacity
198 int capacity = 1;
199 while (capacity < initialCapacity)
200 capacity <<= 1;
201
202 this.loadFactor = loadFactor;
203 threshold = (int)(capacity * loadFactor);
204 table = new Entry[capacity];
205 init();
206 }
207
208 /**
209 * Constructs an empty HashMap with the specified initial
210 * capacity and the default load factor (0.75).
211 *
212 * @param initialCapacity the initial capacity.
213 * @throws IllegalArgumentException if the initial capacity is negative.
214 */
215 public HashMap(int initialCapacity) {
216 this(initialCapacity, DEFAULT_LOAD_FACTOR);
217 }
218
219 /**
220 * Constructs an empty HashMap with the default initial capacity
221 * (16) and the default load factor (0.75).
222 */
223 public HashMap() {
224 this.loadFactor = DEFAULT_LOAD_FACTOR;
225 threshold = (int)(DEFAULT_INITIAL_CAPACITY * DEFAULT_LOAD_FACTOR);
226 table = new Entry[DEFAULT_INITIAL_CAPACITY];
227 init();
228 }
229
230 /**
231 * Constructs a new HashMap with the same mappings as the
232 * specified Map. The HashMap is created with
233 * default load factor (0.75) and an initial capacity sufficient to
234 * hold the mappings in the specified Map.
235 *
236 * @param m the map whose mappings are to be placed in this map
237 * @throws NullPointerException if the specified map is null
238 */
239 public HashMap(Map m) {
240 this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,
241 DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR);
242 putAllForCreate(m);
243 }
244
245 // internal utilities
246
247 /**
248 * Initialization hook for subclasses. This method is called
249 * in all constructors and pseudo-constructors (clone, readObject)
250 * after HashMap has been initialized but before any entries have
251 * been inserted. (In the absence of this method, readObject would
252 * require explicit knowledge of subclasses.)
253 */
254 void init() {
255 }
256
257 /**
258 * Applies a supplemental hash function to a given hashCode, which
259 * defends against poor quality hash functions. This is critical
260 * because HashMap uses power-of-two length hash tables, that
261 * otherwise encounter collisions for hashCodes that do not differ
262 * in lower bits. Note: Null keys always map to hash 0, thus index 0.
263 */
264 static int hash(int h) {
265 // This function ensures that hashCodes that differ only by
266 // constant multiples at each bit position have a bounded
267 // number of collisions (approximately 8 at default load factor).
268 h ^= (h >>> 20) ^ (h >>> 12);
269 return h ^ (h >>> 7) ^ (h >>> 4);
270 }
271
272 /**
273 * Returns index for hash code h.
274 */
275 static int indexFor(int h, int length) {
276 return h & (length-1);
277 }
278
279 /**
280 * Returns the number of key-value mappings in this map.
281 *
282 * @return the number of key-value mappings in this map
283 */
284 public int size() {
285 return size;
286 }
287
288 /**
289 * Returns true if this map contains no key-value mappings.
290 *
291 * @return true if this map contains no key-value mappings
292 */
293 public boolean isEmpty() {
294 return size == 0;
295 }
296
297 /**
298 * Returns the value to which the specified key is mapped,
299 * or {@code null} if this map contains no mapping for the key.
300 *
301 *

More formally, if this map contains a mapping from a key
302 * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
303 * key.equals(k))}, then this method returns {@code v}; otherwise
304 * it returns {@code null}. (There can be at most one such mapping.)
305 *
306 *

A return value of {@code null} does not necessarily
307 * indicate that the map contains no mapping for the key; it's also
308 * possible that the map explicitly maps the key to {@code null}.
309 * The {@link #containsKey containsKey} operation may be used to
310 * distinguish these two cases.
311 *
312 * @see #put(Object, Object)
313 */
314 public V get(Object key) {
315 if (key == null)
316 return getForNullKey();
317 int hash = hash(key.hashCode());
318 for (Entry e = table[indexFor(hash, table.length)];
319 e != null;
320 e = e.next) {
321 Object k;
322 if (e.hash == hash && ((k = e.key) == key || key.equals(k)))
323 return e.value;
324 }
325 return null;
326 }
327
328 /**
329 * Offloaded version of get() to look up null keys. Null keys map
330 * to index 0. This null case is split out into separate methods
331 * for the sake of performance in the two most commonly used
332 * operations (get and put), but incorporated with conditionals in
333 * others.
334 */
335 private V getForNullKey() {
336 for (Entry e = table[0]; e != null; e = e.next) {
337 if (e.key == null)
338 return e.value;
339 }
340 return null;
341 }
342
343 /**
344 * Returns true if this map contains a mapping for the
345 * specified key.
346 *
347 * @param key The key whose presence in this map is to be tested
348 * @return true if this map contains a mapping for the specified
349 * key.
350 */
351 public boolean containsKey(Object key) {
352 return getEntry(key) != null;
353 }
354
355 /**
356 * Returns the entry associated with the specified key in the
357 * HashMap. Returns null if the HashMap contains no mapping
358 * for the key.
359 */
360 final Entry getEntry(Object key) {
361 int hash = (key == null) ? 0 : hash(key.hashCode());
362 for (Entry e = table[indexFor(hash, table.length)];
363 e != null;
364 e = e.next) {
365 Object k;
366 if (e.hash == hash &&
367 ((k = e.key) == key || (key != null && key.equals(k))))
368 return e;
369 }
370 return null;
371 }
372
373
374 /**
375 * Associates the specified value with the specified key in this map.
376 * If the map previously contained a mapping for the key, the old
377 * value is replaced.
378 *
379 * @param key key with which the specified value is to be associated
380 * @param value value to be associated with the specified key
381 * @return the previous value associated with key, or
382 * null if there was no mapping for key.
383 * (A null return can also indicate that the map
384 * previously associated null with key.)
385 */
386 public V put(K key, V value) {
387 if (key == null)
388 return putForNullKey(value);
389 int hash = hash(key.hashCode());
390 int i = indexFor(hash, table.length);
391 for (Entry e = table[i]; e != null; e = e.next) {
392 Object k;
393 if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
394 V oldValue = e.value;
395 e.value = value;
396 e.recordAccess(this);
397 return oldValue;
398 }
399 }
400
401 modCount++;
402 addEntry(hash, key, value, i);
403 return null;
404 }
405
406 /**
407 * Offloaded version of put for null keys
408 */
409 private V putForNullKey(V value) {
410 for (Entry e = table[0]; e != null; e = e.next) {
411 if (e.key == null) {
412 V oldValue = e.value;
413 e.value = value;
414 e.recordAccess(this);
415 return oldValue;
416 }
417 }
418 modCount++;
419 addEntry(0, null, value, 0);
420 return null;
421 }
422
423 /**
424 * This method is used instead of put by constructors and
425 * pseudoconstructors (clone, readObject). It does not resize the table,
426 * check for comodification, etc. It calls createEntry rather than
427 * addEntry.
428 */
429 private void putForCreate(K key, V value) {
430 int hash = (key == null) ? 0 : hash(key.hashCode());
431 int i = indexFor(hash, table.length);
432
433 /**
434 * Look for preexisting entry for key. This will never happen for
435 * clone or deserialize. It will only happen for construction if the
436 * input Map is a sorted map whose ordering is inconsistent w/ equals.
437 */
438 for (Entry e = table[i]; e != null; e = e.next) {
439 Object k;
440 if (e.hash == hash &&
441 ((k = e.key) == key || (key != null && key.equals(k)))) {
442 e.value = value;
443 return;
444 }
445 }
446
447 createEntry(hash, key, value, i);
448 }
449
450 private void putAllForCreate(Map m) {
451 for (Map.Entry e : m.entrySet())
452 putForCreate(e.getKey(), e.getValue());
453 }
454
455 /**
456 * Rehashes the contents of this map into a new array with a
457 * larger capacity. This method is called automatically when the
458 * number of keys in this map reaches its threshold.
459 *
460 * If current capacity is MAXIMUM_CAPACITY, this method does not
461 * resize the map, but sets threshold to Integer.MAX_VALUE.
462 * This has the effect of preventing future calls.
463 *
464 * @param newCapacity the new capacity, MUST be a power of two;
465 * must be greater than current capacity unless current
466 * capacity is MAXIMUM_CAPACITY (in which case value
467 * is irrelevant).
468 */
469 void resize(int newCapacity) {
470 Entry[] oldTable = table;
471 int oldCapacity = oldTable.length;
472 if (oldCapacity == MAXIMUM_CAPACITY) {
473 threshold = Integer.MAX_VALUE;
474 return;
475 }
476
477 Entry[] newTable = new Entry[newCapacity];
478 transfer(newTable);
479 table = newTable;
480 threshold = (int)(newCapacity * loadFactor);
481 }
482
483 /**
484 * Transfers all entries from current table to newTable.
485 */
486 void transfer(Entry[] newTable) {
487 Entry[] src = table;
488 int newCapacity = newTable.length;
489 for (int j = 0; j < src.length; j++) {
490 Entry e = src[j];
491 if (e != null) {
492 src[j] = null;
493 do {
494 Entry next = e.next;
495 int i = indexFor(e.hash, newCapacity);
496 e.next = newTable[i];
497 newTable[i] = e;
498 e = next;
499 } while (e != null);
500 }
501 }
502 }
503
504 /**
505 * Copies all of the mappings from the specified map to this map.
506 * These mappings will replace any mappings that this map had for
507 * any of the keys currently in the specified map.
508 *
509 * @param m mappings to be stored in this map
510 * @throws NullPointerException if the specified map is null
511 */
512 public void putAll(Map m) {
513 int numKeysToBeAdded = m.size();
514 if (numKeysToBeAdded == 0)
515 return;
516
517 /*
518 * Expand the map if the map if the number of mappings to be added
519 * is greater than or equal to threshold. This is conservative; the
520 * obvious condition is (m.size() + size) >= threshold, but this
521 * condition could result in a map with twice the appropriate capacity,
522 * if the keys to be added overlap with the keys already in this map.
523 * By using the conservative calculation, we subject ourself
524 * to at most one extra resize.
525 */
526 if (numKeysToBeAdded > threshold) {
527 int targetCapacity = (int)(numKeysToBeAdded / loadFactor + 1);
528 if (targetCapacity > MAXIMUM_CAPACITY)
529 targetCapacity = MAXIMUM_CAPACITY;
530 int newCapacity = table.length;
531 while (newCapacity < targetCapacity)
532 newCapacity <<= 1;
533 if (newCapacity > table.length)
534 resize(newCapacity);
535 }
536
537 for (Map.Entry e : m.entrySet())
538 put(e.getKey(), e.getValue());
539 }
540
541 /**
542 * Removes the mapping for the specified key from this map if present.
543 *
544 * @param key key whose mapping is to be removed from the map
545 * @return the previous value associated with key, or
546 * null if there was no mapping for key.
547 * (A null return can also indicate that the map
548 * previously associated null with key.)
549 */
550 public V remove(Object key) {
551 Entry e = removeEntryForKey(key);
552 return (e == null ? null : e.value);
553 }
554
555 /**
556 * Removes and returns the entry associated with the specified key
557 * in the HashMap. Returns null if the HashMap contains no mapping
558 * for this key.
559 */
560 final Entry removeEntryForKey(Object key) {
561 int hash = (key == null) ? 0 : hash(key.hashCode());
562 int i = indexFor(hash, table.length);
563 Entry prev = table[i];
564 Entry e = prev;
565
566 while (e != null) {
567 Entry next = e.next;
568 Object k;
569 if (e.hash == hash &&
570 ((k = e.key) == key || (key != null && key.equals(k)))) {
571 modCount++;
572 size--;
573 if (prev == e)
574 table[i] = next;
575 else
576 prev.next = next;
577 e.recordRemoval(this);
578 return e;
579 }
580 prev = e;
581 e = next;
582 }
583
584 return e;
585 }
586
587 /**
588 * Special version of remove for EntrySet.
589 */
590 final Entry removeMapping(Object o) {
591 if (!(o instanceof Map.Entry))
592 return null;
593
594 Map.Entry entry = (Map.Entry) o;
595 Object key = entry.getKey();
596 int hash = (key == null) ? 0 : hash(key.hashCode());
597 int i = indexFor(hash, table.length);
598 Entry prev = table[i];
599 Entry e = prev;
600
601 while (e != null) {
602 Entry next = e.next;
603 if (e.hash == hash && e.equals(entry)) {
604 modCount++;
605 size--;
606 if (prev == e)
607 table[i] = next;
608 else
609 prev.next = next;
610 e.recordRemoval(this);
611 return e;
612 }
613 prev = e;
614 e = next;
615 }
616
617 return e;
618 }
619
620 /**
621 * Removes all of the mappings from this map.
622 * The map will be empty after this call returns.
623 */
624 public void clear() {
625 modCount++;
626 Entry[] tab = table;
627 for (int i = 0; i < tab.length; i++)
628 tab[i] = null;
629 size = 0;
630 }
631
632 /**
633 * Returns true if this map maps one or more keys to the
634 * specified value.
635 *
636 * @param value value whose presence in this map is to be tested
637 * @return true if this map maps one or more keys to the
638 * specified value
639 */
640 public boolean containsValue(Object value) {
641 if (value == null)
642 return containsNullValue();
643
644 Entry[] tab = table;
645 for (int i = 0; i < tab.length ; i++)
646 for (Entry e = tab[i] ; e != null ; e = e.next)
647 if (value.equals(e.value))
648 return true;
649 return false;
650 }
651
652 /**
653 * Special-case code for containsValue with null argument
654 */
655 private boolean containsNullValue() {
656 Entry[] tab = table;
657 for (int i = 0; i < tab.length ; i++)
658 for (Entry e = tab[i] ; e != null ; e = e.next)
659 if (e.value == null)
660 return true;
661 return false;
662 }
663
664 /**
665 * Returns a shallow copy of this HashMap instance: the keys and
666 * values themselves are not cloned.
667 *
668 * @return a shallow copy of this map
669 */
670 public Object clone() {
671 HashMap result = null;
672 try {
673 result = (HashMap)super.clone();
674 } catch (CloneNotSupportedException e) {
675 // assert false;
676 }
677 result.table = new Entry[table.length];
678 result.entrySet = null;
679 result.modCount = 0;
680 result.size = 0;
681 result.init();
682 result.putAllForCreate(this);
683
684 return result;
685 }
686
687 static class Entry implements Map.Entry {
688 final K key;
689 V value;
690 Entry next;
691 final int hash;
692
693 /**
694 * Creates new entry.
695 */
696 Entry(int h, K k, V v, Entry n) {
697 value = v;
698 next = n;
699 key = k;
700 hash = h;
701 }
702
703 public final K getKey() {
704 return key;
705 }
706
707 public final V getValue() {
708 return value;
709 }
710
711 public final V setValue(V newValue) {
712 V oldValue = value;
713 value = newValue;
714 return oldValue;
715 }
716
717 public final boolean equals(Object o) {
718 if (!(o instanceof Map.Entry))
719 return false;
720 Map.Entry e = (Map.Entry)o;
721 Object k1 = getKey();
722 Object k2 = e.getKey();
723 if (k1 == k2 || (k1 != null && k1.equals(k2))) {
724 Object v1 = getValue();
725 Object v2 = e.getValue();
726 if (v1 == v2 || (v1 != null && v1.equals(v2)))
727 return true;
728 }
729 return false;
730 }
731
732 public final int hashCode() {
733 return (key==null ? 0 : key.hashCode()) ^
734 (value==null ? 0 : value.hashCode());
735 }
736
737 public final String toString() {
738 return getKey() + "=" + getValue();
739 }
740
741 /**
742 * This method is invoked whenever the value in an entry is
743 * overwritten by an invocation of put(k,v) for a key k that's already
744 * in the HashMap.
745 */
746 void recordAccess(HashMap m) {
747 }
748
749 /**
750 * This method is invoked whenever the entry is
751 * removed from the table.
752 */
753 void recordRemoval(HashMap m) {
754 }
755 }
756
757 /**
758 * Adds a new entry with the specified key, value and hash code to
759 * the specified bucket. It is the responsibility of this
760 * method to resize the table if appropriate.
761 *
762 * Subclass overrides this to alter the behavior of put method.
763 */
764 void addEntry(int hash, K key, V value, int bucketIndex) {
765 Entry e = table[bucketIndex];
766 table[bucketIndex] = new Entry<>(hash, key, value, e);
767 if (size++ >= threshold)
768 resize(2 * table.length);
769 }
770
771 /**
772 * Like addEntry except that this version is used when creating entries
773 * as part of Map construction or "pseudo-construction" (cloning,
774 * deserialization). This version needn't worry about resizing the table.
775 *
776 * Subclass overrides this to alter the behavior of HashMap(Map),
777 * clone, and readObject.
778 */
779 void createEntry(int hash, K key, V value, int bucketIndex) {
780 Entry e = table[bucketIndex];
781 table[bucketIndex] = new Entry<>(hash, key, value, e);
782 size++;
783 }
784
785 private abstract class HashIterator implements Iterator {
786 Entry next; // next entry to return
787 int expectedModCount; // For fast-fail
788 int index; // current slot
789 Entry current; // current entry
790
791 HashIterator() {
792 expectedModCount = modCount;
793 if (size > 0) { // advance to first entry
794 Entry[] t = table;
795 while (index < t.length && (next = t[index++]) == null)
796 ;
797 }
798 }
799
800 public final boolean hasNext() {
801 return next != null;
802 }
803
804 final Entry nextEntry() {
805 if (modCount != expectedModCount)
806 throw new ConcurrentModificationException();
807 Entry e = next;
808 if (e == null)
809 throw new NoSuchElementException();
810
811 if ((next = e.next) == null) {
812 Entry[] t = table;
813 while (index < t.length && (next = t[index++]) == null)
814 ;
815 }
816 current = e;
817 return e;
818 }
819
820 public void remove() {
821 if (current == null)
822 throw new IllegalStateException();
823 if (modCount != expectedModCount)
824 throw new ConcurrentModificationException();
825 Object k = current.key;
826 current = null;
827 HashMap.this.removeEntryForKey(k);
828 expectedModCount = modCount;
829 }
830
831 }
832
833 private final class ValueIterator extends HashIterator {
834 public V next() {
835 return nextEntry().value;
836 }
837 }
838
839 private final class KeyIterator extends HashIterator {
840 public K next() {
841 return nextEntry().getKey();
842 }
843 }
844
845 private final class EntryIterator extends HashIterator> {
846 public Map.Entry next() {
847 return nextEntry();
848 }
849 }
850
851 // Subclass overrides these to alter behavior of views' iterator() method
852 Iterator newKeyIterator() {
853 return new KeyIterator();
854 }
855 Iterator newValueIterator() {
856 return new ValueIterator();
857 }
858 Iterator> newEntryIterator() {
859 return new EntryIterator();
860 }
861
862
863 // Views
864
865 private transient Set> entrySet = null;
866
867 /**
868 * Returns a {@link Set} view of the keys contained in this map.
869 * The set is backed by the map, so changes to the map are
870 * reflected in the set, and vice-versa. If the map is modified
871 * while an iteration over the set is in progress (except through
872 * the iterator's own remove operation), the results of
873 * the iteration are undefined. The set supports element removal,
874 * which removes the corresponding mapping from the map, via the
875 * Iterator.remove, Set.remove,
876 * removeAll, retainAll, and clear
877 * operations. It does not support the add or addAll
878 * operations.
879 */
880 public Set keySet() {
881 Set ks = keySet;
882 return (ks != null ? ks : (keySet = new KeySet()));
883 }
884
885 private final class KeySet extends AbstractSet {
886 public Iterator iterator() {
887 return newKeyIterator();
888 }
889 public int size() {
890 return size;
891 }
892 public boolean contains(Object o) {
893 return containsKey(o);
894 }
895 public boolean remove(Object o) {
896 return HashMap.this.removeEntryForKey(o) != null;
897 }
898 public void clear() {
899 HashMap.this.clear();
900 }
901 }
902
903 /**
904 * Returns a {@link Collection} view of the values contained in this map.
905 * The collection is backed by the map, so changes to the map are
906 * reflected in the collection, and vice-versa. If the map is
907 * modified while an iteration over the collection is in progress
908 * (except through the iterator's own remove operation),
909 * the results of the iteration are undefined. The collection
910 * supports element removal, which removes the corresponding
911 * mapping from the map, via the Iterator.remove,
912 * Collection.remove, removeAll,
913 * retainAll and clear operations. It does not
914 * support the add or addAll operations.
915 */
916 public Collection values() {
917 Collection vs = values;
918 return (vs != null ? vs : (values = new Values()));
919 }
920
921 private final class Values extends AbstractCollection {
922 public Iterator iterator() {
923 return newValueIterator();
924 }
925 public int size() {
926 return size;
927 }
928 public boolean contains(Object o) {
929 return containsValue(o);
930 }
931 public void clear() {
932 HashMap.this.clear();
933 }
934 }
935
936 /**
937 * Returns a {@link Set} view of the mappings contained in this map.
938 * The set is backed by the map, so changes to the map are
939 * reflected in the set, and vice-versa. If the map is modified
940 * while an iteration over the set is in progress (except through
941 * the iterator's own remove operation, or through the
942 * setValue operation on a map entry returned by the
943 * iterator) the results of the iteration are undefined. The set
944 * supports element removal, which removes the corresponding
945 * mapping from the map, via the Iterator.remove,
946 * Set.remove, removeAll, retainAll and
947 * clear operations. It does not support the
948 * add or addAll operations.
949 *
950 * @return a set view of the mappings contained in this map
951 */
952 public Set> entrySet() {
953 return entrySet0();
954 }
955
956 private Set> entrySet0() {
957 Set> es = entrySet;
958 return es != null ? es : (entrySet = new EntrySet());
959 }
960
961 private final class EntrySet extends AbstractSet> {
962 public Iterator> iterator() {
963 return newEntryIterator();
964 }
965 public boolean contains(Object o) {
966 if (!(o instanceof Map.Entry))
967 return false;
968 Map.Entry e = (Map.Entry) o;
969 Entry candidate = getEntry(e.getKey());
970 return candidate != null && candidate.equals(e);
971 }
972 public boolean remove(Object o) {
973 return removeMapping(o) != null;
974 }
975 public int size() {
976 return size;
977 }
978 public void clear() {
979 HashMap.this.clear();
980 }
981 }
982
983 /**
984 * Save the state of the HashMap instance to a stream (i.e.,
985 * serialize it).
986 *
987 * @serialData The capacity of the HashMap (the length of the
988 * bucket array) is emitted (int), followed by the
989 * size (an int, the number of key-value
990 * mappings), followed by the key (Object) and value (Object)
991 * for each key-value mapping. The key-value mappings are
992 * emitted in no particular order.
993 */
994 private void writeObject(java.io.ObjectOutputStream s)
995 throws IOException
996 {
997 Iterator> i =
998 (size > 0) ? entrySet0().iterator() : null;
999
1000 // Write out the threshold, loadfactor, and any hidden stuff
1001 s.defaultWriteObject();
1002
1003 // Write out number of buckets
1004 s.writeInt(table.length);
1005
1006 // Write out size (number of Mappings)
1007 s.writeInt(size);
1008
1009 // Write out keys and values (alternating)
1010 if (i != null) {
1011 while (i.hasNext()) {
1012 Map.Entry e = i.next();
1013 s.writeObject(e.getKey());
1014 s.writeObject(e.getValue());
1015 }
1016 }
1017 }
1018
1019 private static final long serialVersionUID = 362498820763181265L;
1020
1021 /**
1022 * Reconstitute the HashMap instance from a stream (i.e.,
1023 * deserialize it).
1024 */
1025 private void readObject(java.io.ObjectInputStream s)
1026 throws IOException, ClassNotFoundException
1027 {
1028 // Read in the threshold, loadfactor, and any hidden stuff
1029 s.defaultReadObject();
1030
1031 // Read in number of buckets and allocate the bucket array;
1032 int numBuckets = s.readInt();
1033 table = new Entry[numBuckets];
1034
1035 init(); // Give subclass a chance to do its thing.
1036
1037 // Read in size (number of Mappings)
1038 int size = s.readInt();
1039
1040 // Read the keys and values, and put the mappings in the HashMap
1041 for (int i=0; i 1042 K key = (K) s.readObject();
1043 V value = (V) s.readObject();
1044 putForCreate(key, value);
1045 }
1046 }
1047
1048 // These methods are used when serializing HashSets
1049 int capacity() { return table.length; }
1050 float loadFactor() { return loadFactor; }
1051 }

public class RemoveDuplicatesFromArrayList {

public static void main(String[] args) {
ArrayList i = new ArrayList();
i.add(1);
i.add(3);
i.add(1);
i.add(3);
i.add(2);
Set s = removeDups(i);
for (Object object : s) {
System.out.println(object.toString()
);
}
System.out.println("end");
System.out.println();
}

public static Set removeDups(ArrayList al)
{
Set hs = new LinkedHashSet();
hs.addAll(al);
al.clear();
al.addAll(hs);
return hs;