001 /* HashMap.java -- a class providing a basic hashtable data structure,
002 mapping Object --> Object
003 Copyright (C) 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005 Free Software Foundation, Inc.
004
005 This file is part of GNU Classpath.
006
007 GNU Classpath is free software; you can redistribute it and/or modify
008 it under the terms of the GNU General Public License as published by
009 the Free Software Foundation; either version 2, or (at your option)
010 any later version.
011
012 GNU Classpath is distributed in the hope that it will be useful, but
013 WITHOUT ANY WARRANTY; without even the implied warranty of
014 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
015 General Public License for more details.
016
017 You should have received a copy of the GNU General Public License
018 along with GNU Classpath; see the file COPYING. If not, write to the
019 Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
020 02110-1301 USA.
021
022 Linking this library statically or dynamically with other modules is
023 making a combined work based on this library. Thus, the terms and
024 conditions of the GNU General Public License cover the whole
025 combination.
026
027 As a special exception, the copyright holders of this library give you
028 permission to link this library with independent modules to produce an
029 executable, regardless of the license terms of these independent
030 modules, and to copy and distribute the resulting executable under
031 terms of your choice, provided that you also meet, for each linked
032 independent module, the terms and conditions of the license of that
033 module. An independent module is a module which is not derived from
034 or based on this library. If you modify this library, you may extend
035 this exception to your version of the library, but you are not
036 obligated to do so. If you do not wish to do so, delete this
037 exception statement from your version. */
038
039
040 package java.util;
041
042 import java.io.IOException;
043 import java.io.ObjectInputStream;
044 import java.io.ObjectOutputStream;
045 import java.io.Serializable;
046
047 // NOTE: This implementation is very similar to that of Hashtable. If you fix
048 // a bug in here, chances are you should make a similar change to the Hashtable
049 // code.
050
051 // NOTE: This implementation has some nasty coding style in order to
052 // support LinkedHashMap, which extends this.
053
054 /**
055 * This class provides a hashtable-backed implementation of the
056 * Map interface.
057 * <p>
058 *
059 * It uses a hash-bucket approach; that is, hash collisions are handled
060 * by linking the new node off of the pre-existing node (or list of
061 * nodes). In this manner, techniques such as linear probing (which
062 * can cause primary clustering) and rehashing (which does not fit very
063 * well with Java's method of precomputing hash codes) are avoided.
064 * <p>
065 *
066 * Under ideal circumstances (no collisions), HashMap offers O(1)
067 * performance on most operations (<code>containsValue()</code> is,
068 * of course, O(n)). In the worst case (all keys map to the same
069 * hash code -- very unlikely), most operations are O(n).
070 * <p>
071 *
072 * HashMap is part of the JDK1.2 Collections API. It differs from
073 * Hashtable in that it accepts the null key and null values, and it
074 * does not support "Enumeration views." Also, it is not synchronized;
075 * if you plan to use it in multiple threads, consider using:<br>
076 * <code>Map m = Collections.synchronizedMap(new HashMap(...));</code>
077 * <p>
078 *
079 * The iterators are <i>fail-fast</i>, meaning that any structural
080 * modification, except for <code>remove()</code> called on the iterator
081 * itself, cause the iterator to throw a
082 * <code>ConcurrentModificationException</code> rather than exhibit
083 * non-deterministic behavior.
084 *
085 * @author Jon Zeppieri
086 * @author Jochen Hoenicke
087 * @author Bryce McKinlay
088 * @author Eric Blake (ebb9@email.byu.edu)
089 * @see Object#hashCode()
090 * @see Collection
091 * @see Map
092 * @see TreeMap
093 * @see LinkedHashMap
094 * @see IdentityHashMap
095 * @see Hashtable
096 * @since 1.2
097 * @status updated to 1.4
098 */
099 public class HashMap<K, V> extends AbstractMap<K, V>
100 implements Map<K, V>, Cloneable, Serializable
101 {
102 /**
103 * Default number of buckets. This is the value the JDK 1.3 uses. Some
104 * early documentation specified this value as 101. That is incorrect.
105 * Package visible for use by HashSet.
106 */
107 static final int DEFAULT_CAPACITY = 11;
108
109 /**
110 * The default load factor; this is explicitly specified by the spec.
111 * Package visible for use by HashSet.
112 */
113 static final float DEFAULT_LOAD_FACTOR = 0.75f;
114
115 /**
116 * Compatible with JDK 1.2.
117 */
118 private static final long serialVersionUID = 362498820763181265L;
119
120 /**
121 * The rounded product of the capacity and the load factor; when the number
122 * of elements exceeds the threshold, the HashMap calls
123 * <code>rehash()</code>.
124 * @serial the threshold for rehashing
125 */
126 private int threshold;
127
128 /**
129 * Load factor of this HashMap: used in computing the threshold.
130 * Package visible for use by HashSet.
131 * @serial the load factor
132 */
133 final float loadFactor;
134
135 /**
136 * Array containing the actual key-value mappings.
137 * Package visible for use by nested and subclasses.
138 */
139 transient HashEntry<K, V>[] buckets;
140
141 /**
142 * Counts the number of modifications this HashMap has undergone, used
143 * by Iterators to know when to throw ConcurrentModificationExceptions.
144 * Package visible for use by nested and subclasses.
145 */
146 transient int modCount;
147
148 /**
149 * The size of this HashMap: denotes the number of key-value pairs.
150 * Package visible for use by nested and subclasses.
151 */
152 transient int size;
153
154 /**
155 * The cache for {@link #entrySet()}.
156 */
157 private transient Set<Map.Entry<K, V>> entries;
158
159 /**
160 * Class to represent an entry in the hash table. Holds a single key-value
161 * pair. Package visible for use by subclass.
162 *
163 * @author Eric Blake (ebb9@email.byu.edu)
164 */
165 static class HashEntry<K, V> extends AbstractMap.SimpleEntry<K, V>
166 {
167 /**
168 * The next entry in the linked list. Package visible for use by subclass.
169 */
170 HashEntry<K, V> next;
171
172 /**
173 * Simple constructor.
174 * @param key the key
175 * @param value the value
176 */
177 HashEntry(K key, V value)
178 {
179 super(key, value);
180 }
181
182 /**
183 * Called when this entry is accessed via {@link #put(Object, Object)}.
184 * This version does nothing, but in LinkedHashMap, it must do some
185 * bookkeeping for access-traversal mode.
186 */
187 void access()
188 {
189 }
190
191 /**
192 * Called when this entry is removed from the map. This version simply
193 * returns the value, but in LinkedHashMap, it must also do bookkeeping.
194 *
195 * @return the value of this key as it is removed
196 */
197 V cleanup()
198 {
199 return value;
200 }
201 }
202
203 /**
204 * Construct a new HashMap with the default capacity (11) and the default
205 * load factor (0.75).
206 */
207 public HashMap()
208 {
209 this(DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR);
210 }
211
212 /**
213 * Construct a new HashMap from the given Map, with initial capacity
214 * the greater of the size of <code>m</code> or the default of 11.
215 * <p>
216 *
217 * Every element in Map m will be put into this new HashMap.
218 *
219 * @param m a Map whose key / value pairs will be put into the new HashMap.
220 * <b>NOTE: key / value pairs are not cloned in this constructor.</b>
221 * @throws NullPointerException if m is null
222 */
223 public HashMap(Map<? extends K, ? extends V> m)
224 {
225 this(Math.max(m.size() * 2, DEFAULT_CAPACITY), DEFAULT_LOAD_FACTOR);
226 putAll(m);
227 }
228
229 /**
230 * Construct a new HashMap with a specific inital capacity and
231 * default load factor of 0.75.
232 *
233 * @param initialCapacity the initial capacity of this HashMap (>=0)
234 * @throws IllegalArgumentException if (initialCapacity < 0)
235 */
236 public HashMap(int initialCapacity)
237 {
238 this(initialCapacity, DEFAULT_LOAD_FACTOR);
239 }
240
241 /**
242 * Construct a new HashMap with a specific inital capacity and load factor.
243 *
244 * @param initialCapacity the initial capacity (>=0)
245 * @param loadFactor the load factor (> 0, not NaN)
246 * @throws IllegalArgumentException if (initialCapacity < 0) ||
247 * ! (loadFactor > 0.0)
248 */
249 public HashMap(int initialCapacity, float loadFactor)
250 {
251 if (initialCapacity < 0)
252 throw new IllegalArgumentException("Illegal Capacity: "
253 + initialCapacity);
254 if (! (loadFactor > 0)) // check for NaN too
255 throw new IllegalArgumentException("Illegal Load: " + loadFactor);
256
257 if (initialCapacity == 0)
258 initialCapacity = 1;
259 buckets = (HashEntry<K, V>[]) new HashEntry[initialCapacity];
260 this.loadFactor = loadFactor;
261 threshold = (int) (initialCapacity * loadFactor);
262 }
263
264 /**
265 * Returns the number of kay-value mappings currently in this Map.
266 *
267 * @return the size
268 */
269 public int size()
270 {
271 return size;
272 }
273
274 /**
275 * Returns true if there are no key-value mappings currently in this Map.
276 *
277 * @return <code>size() == 0</code>
278 */
279 public boolean isEmpty()
280 {
281 return size == 0;
282 }
283
284 /**
285 * Return the value in this HashMap associated with the supplied key,
286 * or <code>null</code> if the key maps to nothing. NOTE: Since the value
287 * could also be null, you must use containsKey to see if this key
288 * actually maps to something.
289 *
290 * @param key the key for which to fetch an associated value
291 * @return what the key maps to, if present
292 * @see #put(Object, Object)
293 * @see #containsKey(Object)
294 */
295 public V get(Object key)
296 {
297 int idx = hash(key);
298 HashEntry<K, V> e = buckets[idx];
299 while (e != null)
300 {
301 if (equals(key, e.key))
302 return e.value;
303 e = e.next;
304 }
305 return null;
306 }
307
308 /**
309 * Returns true if the supplied object <code>equals()</code> a key
310 * in this HashMap.
311 *
312 * @param key the key to search for in this HashMap
313 * @return true if the key is in the table
314 * @see #containsValue(Object)
315 */
316 public boolean containsKey(Object key)
317 {
318 int idx = hash(key);
319 HashEntry<K, V> e = buckets[idx];
320 while (e != null)
321 {
322 if (equals(key, e.key))
323 return true;
324 e = e.next;
325 }
326 return false;
327 }
328
329 /**
330 * Puts the supplied value into the Map, mapped by the supplied key.
331 * The value may be retrieved by any object which <code>equals()</code>
332 * this key. NOTE: Since the prior value could also be null, you must
333 * first use containsKey if you want to see if you are replacing the
334 * key's mapping.
335 *
336 * @param key the key used to locate the value
337 * @param value the value to be stored in the HashMap
338 * @return the prior mapping of the key, or null if there was none
339 * @see #get(Object)
340 * @see Object#equals(Object)
341 */
342 public V put(K key, V value)
343 {
344 int idx = hash(key);
345 HashEntry<K, V> e = buckets[idx];
346
347 while (e != null)
348 {
349 if (equals(key, e.key))
350 {
351 e.access(); // Must call this for bookkeeping in LinkedHashMap.
352 V r = e.value;
353 e.value = value;
354 return r;
355 }
356 else
357 e = e.next;
358 }
359
360 // At this point, we know we need to add a new entry.
361 modCount++;
362 if (++size > threshold)
363 {
364 rehash();
365 // Need a new hash value to suit the bigger table.
366 idx = hash(key);
367 }
368
369 // LinkedHashMap cannot override put(), hence this call.
370 addEntry(key, value, idx, true);
371 return null;
372 }
373
374 /**
375 * Copies all elements of the given map into this hashtable. If this table
376 * already has a mapping for a key, the new mapping replaces the current
377 * one.
378 *
379 * @param m the map to be hashed into this
380 */
381 public void putAll(Map<? extends K, ? extends V> m)
382 {
383 final Map<K,V> addMap = (Map<K,V>) m;
384 final Iterator<Map.Entry<K,V>> it = addMap.entrySet().iterator();
385 while (it.hasNext())
386 {
387 final Map.Entry<K,V> e = it.next();
388 // Optimize in case the Entry is one of our own.
389 if (e instanceof AbstractMap.SimpleEntry)
390 {
391 AbstractMap.SimpleEntry<? extends K, ? extends V> entry
392 = (AbstractMap.SimpleEntry<? extends K, ? extends V>) e;
393 put(entry.key, entry.value);
394 }
395 else
396 put(e.getKey(), e.getValue());
397 }
398 }
399
400 /**
401 * Removes from the HashMap and returns the value which is mapped by the
402 * supplied key. If the key maps to nothing, then the HashMap remains
403 * unchanged, and <code>null</code> is returned. NOTE: Since the value
404 * could also be null, you must use containsKey to see if you are
405 * actually removing a mapping.
406 *
407 * @param key the key used to locate the value to remove
408 * @return whatever the key mapped to, if present
409 */
410 public V remove(Object key)
411 {
412 int idx = hash(key);
413 HashEntry<K, V> e = buckets[idx];
414 HashEntry<K, V> last = null;
415
416 while (e != null)
417 {
418 if (equals(key, e.key))
419 {
420 modCount++;
421 if (last == null)
422 buckets[idx] = e.next;
423 else
424 last.next = e.next;
425 size--;
426 // Method call necessary for LinkedHashMap to work correctly.
427 return e.cleanup();
428 }
429 last = e;
430 e = e.next;
431 }
432 return null;
433 }
434
435 /**
436 * Clears the Map so it has no keys. This is O(1).
437 */
438 public void clear()
439 {
440 if (size != 0)
441 {
442 modCount++;
443 Arrays.fill(buckets, null);
444 size = 0;
445 }
446 }
447
448 /**
449 * Returns true if this HashMap contains a value <code>o</code>, such that
450 * <code>o.equals(value)</code>.
451 *
452 * @param value the value to search for in this HashMap
453 * @return true if at least one key maps to the value
454 * @see #containsKey(Object)
455 */
456 public boolean containsValue(Object value)
457 {
458 for (int i = buckets.length - 1; i >= 0; i--)
459 {
460 HashEntry<K, V> e = buckets[i];
461 while (e != null)
462 {
463 if (equals(value, e.value))
464 return true;
465 e = e.next;
466 }
467 }
468 return false;
469 }
470
471 /**
472 * Returns a shallow clone of this HashMap. The Map itself is cloned,
473 * but its contents are not. This is O(n).
474 *
475 * @return the clone
476 */
477 public Object clone()
478 {
479 HashMap<K, V> copy = null;
480 try
481 {
482 copy = (HashMap<K, V>) super.clone();
483 }
484 catch (CloneNotSupportedException x)
485 {
486 // This is impossible.
487 }
488 copy.buckets = (HashEntry<K, V>[]) new HashEntry[buckets.length];
489 copy.putAllInternal(this);
490 // Clear the entry cache. AbstractMap.clone() does the others.
491 copy.entries = null;
492 return copy;
493 }
494
495 /**
496 * Returns a "set view" of this HashMap's keys. The set is backed by the
497 * HashMap, so changes in one show up in the other. The set supports
498 * element removal, but not element addition.
499 *
500 * @return a set view of the keys
501 * @see #values()
502 * @see #entrySet()
503 */
504 public Set<K> keySet()
505 {
506 if (keys == null)
507 // Create an AbstractSet with custom implementations of those methods
508 // that can be overridden easily and efficiently.
509 keys = new AbstractSet<K>()
510 {
511 public int size()
512 {
513 return size;
514 }
515
516 public Iterator<K> iterator()
517 {
518 // Cannot create the iterator directly, because of LinkedHashMap.
519 return HashMap.this.iterator(KEYS);
520 }
521
522 public void clear()
523 {
524 HashMap.this.clear();
525 }
526
527 public boolean contains(Object o)
528 {
529 return containsKey(o);
530 }
531
532 public boolean remove(Object o)
533 {
534 // Test against the size of the HashMap to determine if anything
535 // really got removed. This is necessary because the return value
536 // of HashMap.remove() is ambiguous in the null case.
537 int oldsize = size;
538 HashMap.this.remove(o);
539 return oldsize != size;
540 }
541 };
542 return keys;
543 }
544
545 /**
546 * Returns a "collection view" (or "bag view") of this HashMap's values.
547 * The collection is backed by the HashMap, so changes in one show up
548 * in the other. The collection supports element removal, but not element
549 * addition.
550 *
551 * @return a bag view of the values
552 * @see #keySet()
553 * @see #entrySet()
554 */
555 public Collection<V> values()
556 {
557 if (values == null)
558 // We don't bother overriding many of the optional methods, as doing so
559 // wouldn't provide any significant performance advantage.
560 values = new AbstractCollection<V>()
561 {
562 public int size()
563 {
564 return size;
565 }
566
567 public Iterator<V> iterator()
568 {
569 // Cannot create the iterator directly, because of LinkedHashMap.
570 return HashMap.this.iterator(VALUES);
571 }
572
573 public void clear()
574 {
575 HashMap.this.clear();
576 }
577 };
578 return values;
579 }
580
581 /**
582 * Returns a "set view" of this HashMap's entries. The set is backed by
583 * the HashMap, so changes in one show up in the other. The set supports
584 * element removal, but not element addition.<p>
585 *
586 * Note that the iterators for all three views, from keySet(), entrySet(),
587 * and values(), traverse the HashMap in the same sequence.
588 *
589 * @return a set view of the entries
590 * @see #keySet()
591 * @see #values()
592 * @see Map.Entry
593 */
594 public Set<Map.Entry<K, V>> entrySet()
595 {
596 if (entries == null)
597 // Create an AbstractSet with custom implementations of those methods
598 // that can be overridden easily and efficiently.
599 entries = new AbstractSet<Map.Entry<K, V>>()
600 {
601 public int size()
602 {
603 return size;
604 }
605
606 public Iterator<Map.Entry<K, V>> iterator()
607 {
608 // Cannot create the iterator directly, because of LinkedHashMap.
609 return HashMap.this.iterator(ENTRIES);
610 }
611
612 public void clear()
613 {
614 HashMap.this.clear();
615 }
616
617 public boolean contains(Object o)
618 {
619 return getEntry(o) != null;
620 }
621
622 public boolean remove(Object o)
623 {
624 HashEntry<K, V> e = getEntry(o);
625 if (e != null)
626 {
627 HashMap.this.remove(e.key);
628 return true;
629 }
630 return false;
631 }
632 };
633 return entries;
634 }
635
636 /**
637 * Helper method for put, that creates and adds a new Entry. This is
638 * overridden in LinkedHashMap for bookkeeping purposes.
639 *
640 * @param key the key of the new Entry
641 * @param value the value
642 * @param idx the index in buckets where the new Entry belongs
643 * @param callRemove whether to call the removeEldestEntry method
644 * @see #put(Object, Object)
645 */
646 void addEntry(K key, V value, int idx, boolean callRemove)
647 {
648 HashEntry<K, V> e = new HashEntry<K, V>(key, value);
649 e.next = buckets[idx];
650 buckets[idx] = e;
651 }
652
653 /**
654 * Helper method for entrySet(), which matches both key and value
655 * simultaneously.
656 *
657 * @param o the entry to match
658 * @return the matching entry, if found, or null
659 * @see #entrySet()
660 */
661 // Package visible, for use in nested classes.
662 final HashEntry<K, V> getEntry(Object o)
663 {
664 if (! (o instanceof Map.Entry))
665 return null;
666 Map.Entry<K, V> me = (Map.Entry<K, V>) o;
667 K key = me.getKey();
668 int idx = hash(key);
669 HashEntry<K, V> e = buckets[idx];
670 while (e != null)
671 {
672 if (equals(e.key, key))
673 return equals(e.value, me.getValue()) ? e : null;
674 e = e.next;
675 }
676 return null;
677 }
678
679 /**
680 * Helper method that returns an index in the buckets array for `key'
681 * based on its hashCode(). Package visible for use by subclasses.
682 *
683 * @param key the key
684 * @return the bucket number
685 */
686 final int hash(Object key)
687 {
688 return key == null ? 0 : Math.abs(key.hashCode() % buckets.length);
689 }
690
691 /**
692 * Generates a parameterized iterator. Must be overrideable, since
693 * LinkedHashMap iterates in a different order.
694 *
695 * @param type {@link #KEYS}, {@link #VALUES}, or {@link #ENTRIES}
696 * @return the appropriate iterator
697 */
698 <T> Iterator<T> iterator(int type)
699 {
700 // FIXME: bogus cast here.
701 return new HashIterator<T>(type);
702 }
703
704 /**
705 * A simplified, more efficient internal implementation of putAll(). clone()
706 * should not call putAll or put, in order to be compatible with the JDK
707 * implementation with respect to subclasses.
708 *
709 * @param m the map to initialize this from
710 */
711 void putAllInternal(Map<? extends K, ? extends V> m)
712 {
713 final Map<K,V> addMap = (Map<K,V>) m;
714 final Iterator<Map.Entry<K,V>> it = addMap.entrySet().iterator();
715 size = 0;
716 while (it.hasNext())
717 {
718 final Map.Entry<K,V> e = it.next();
719 size++;
720 K key = e.getKey();
721 int idx = hash(key);
722 addEntry(key, e.getValue(), idx, false);
723 }
724 }
725
726 /**
727 * Increases the size of the HashMap and rehashes all keys to new
728 * array indices; this is called when the addition of a new value
729 * would cause size() > threshold. Note that the existing Entry
730 * objects are reused in the new hash table.
731 *
732 * <p>This is not specified, but the new size is twice the current size
733 * plus one; this number is not always prime, unfortunately.
734 */
735 private void rehash()
736 {
737 HashEntry<K, V>[] oldBuckets = buckets;
738
739 int newcapacity = (buckets.length * 2) + 1;
740 threshold = (int) (newcapacity * loadFactor);
741 buckets = (HashEntry<K, V>[]) new HashEntry[newcapacity];
742
743 for (int i = oldBuckets.length - 1; i >= 0; i--)
744 {
745 HashEntry<K, V> e = oldBuckets[i];
746 while (e != null)
747 {
748 int idx = hash(e.key);
749 HashEntry<K, V> dest = buckets[idx];
750 HashEntry<K, V> next = e.next;
751 e.next = buckets[idx];
752 buckets[idx] = e;
753 e = next;
754 }
755 }
756 }
757
758 /**
759 * Serializes this object to the given stream.
760 *
761 * @param s the stream to write to
762 * @throws IOException if the underlying stream fails
763 * @serialData the <i>capacity</i>(int) that is the length of the
764 * bucket array, the <i>size</i>(int) of the hash map
765 * are emitted first. They are followed by size entries,
766 * each consisting of a key (Object) and a value (Object).
767 */
768 private void writeObject(ObjectOutputStream s) throws IOException
769 {
770 // Write the threshold and loadFactor fields.
771 s.defaultWriteObject();
772
773 s.writeInt(buckets.length);
774 s.writeInt(size);
775 // Avoid creating a wasted Set by creating the iterator directly.
776 Iterator<HashEntry<K, V>> it = iterator(ENTRIES);
777 while (it.hasNext())
778 {
779 HashEntry<K, V> entry = it.next();
780 s.writeObject(entry.key);
781 s.writeObject(entry.value);
782 }
783 }
784
785 /**
786 * Deserializes this object from the given stream.
787 *
788 * @param s the stream to read from
789 * @throws ClassNotFoundException if the underlying stream fails
790 * @throws IOException if the underlying stream fails
791 * @serialData the <i>capacity</i>(int) that is the length of the
792 * bucket array, the <i>size</i>(int) of the hash map
793 * are emitted first. They are followed by size entries,
794 * each consisting of a key (Object) and a value (Object).
795 */
796 private void readObject(ObjectInputStream s)
797 throws IOException, ClassNotFoundException
798 {
799 // Read the threshold and loadFactor fields.
800 s.defaultReadObject();
801
802 // Read and use capacity, followed by key/value pairs.
803 buckets = (HashEntry<K, V>[]) new HashEntry[s.readInt()];
804 int len = s.readInt();
805 size = len;
806 while (len-- > 0)
807 {
808 Object key = s.readObject();
809 addEntry((K) key, (V) s.readObject(), hash(key), false);
810 }
811 }
812
813 /**
814 * Iterate over HashMap's entries.
815 * This implementation is parameterized to give a sequential view of
816 * keys, values, or entries.
817 *
818 * @author Jon Zeppieri
819 */
820 private final class HashIterator<T> implements Iterator<T>
821 {
822 /**
823 * The type of this Iterator: {@link #KEYS}, {@link #VALUES},
824 * or {@link #ENTRIES}.
825 */
826 private final int type;
827 /**
828 * The number of modifications to the backing HashMap that we know about.
829 */
830 private int knownMod = modCount;
831 /** The number of elements remaining to be returned by next(). */
832 private int count = size;
833 /** Current index in the physical hash table. */
834 private int idx = buckets.length;
835 /** The last Entry returned by a next() call. */
836 private HashEntry last;
837 /**
838 * The next entry that should be returned by next(). It is set to something
839 * if we're iterating through a bucket that contains multiple linked
840 * entries. It is null if next() needs to find a new bucket.
841 */
842 private HashEntry next;
843
844 /**
845 * Construct a new HashIterator with the supplied type.
846 * @param type {@link #KEYS}, {@link #VALUES}, or {@link #ENTRIES}
847 */
848 HashIterator(int type)
849 {
850 this.type = type;
851 }
852
853 /**
854 * Returns true if the Iterator has more elements.
855 * @return true if there are more elements
856 */
857 public boolean hasNext()
858 {
859 return count > 0;
860 }
861
862 /**
863 * Returns the next element in the Iterator's sequential view.
864 * @return the next element
865 * @throws ConcurrentModificationException if the HashMap was modified
866 * @throws NoSuchElementException if there is none
867 */
868 public T next()
869 {
870 if (knownMod != modCount)
871 throw new ConcurrentModificationException();
872 if (count == 0)
873 throw new NoSuchElementException();
874 count--;
875 HashEntry e = next;
876
877 while (e == null)
878 e = buckets[--idx];
879
880 next = e.next;
881 last = e;
882 if (type == VALUES)
883 return (T) e.value;
884 if (type == KEYS)
885 return (T) e.key;
886 return (T) e;
887 }
888
889 /**
890 * Removes from the backing HashMap the last element which was fetched
891 * with the <code>next()</code> method.
892 * @throws ConcurrentModificationException if the HashMap was modified
893 * @throws IllegalStateException if called when there is no last element
894 */
895 public void remove()
896 {
897 if (knownMod != modCount)
898 throw new ConcurrentModificationException();
899 if (last == null)
900 throw new IllegalStateException();
901
902 HashMap.this.remove(last.key);
903 last = null;
904 knownMod++;
905 }
906 }
907 }