1 /* HashMap.java -- a class providing a basic hashtable data structure,
2 mapping Object --> Object
3 Copyright (C) 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005 Free Software Foundation, Inc.
5 This file is part of GNU Classpath.
7 GNU Classpath is free software; you can redistribute it and/or modify
8 it under the terms of the GNU General Public License as published by
9 the Free Software Foundation; either version 2, or (at your option)
12 GNU Classpath is distributed in the hope that it will be useful, but
13 WITHOUT ANY WARRANTY; without even the implied warranty of
14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
15 General Public License for more details.
17 You should have received a copy of the GNU General Public License
18 along with GNU Classpath; see the file COPYING. If not, write to the
19 Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
22 Linking this library statically or dynamically with other modules is
23 making a combined work based on this library. Thus, the terms and
24 conditions of the GNU General Public License cover the whole
27 As a special exception, the copyright holders of this library give you
28 permission to link this library with independent modules to produce an
29 executable, regardless of the license terms of these independent
30 modules, and to copy and distribute the resulting executable under
31 terms of your choice, provided that you also meet, for each linked
32 independent module, the terms and conditions of the license of that
33 module. An independent module is a module which is not derived from
34 or based on this library. If you modify this library, you may extend
35 this exception to your version of the library, but you are not
36 obligated to do so. If you do not wish to do so, delete this
37 exception statement from your version. */
42 import java.io.IOException;
43 import java.io.ObjectInputStream;
44 import java.io.ObjectOutputStream;
45 import java.io.Serializable;
47 // NOTE: This implementation is very similar to that of Hashtable. If you fix
48 // a bug in here, chances are you should make a similar change to the Hashtable
51 // NOTE: This implementation has some nasty coding style in order to
52 // support LinkedHashMap, which extends this.
55 * This class provides a hashtable-backed implementation of the
59 * It uses a hash-bucket approach; that is, hash collisions are handled
60 * by linking the new node off of the pre-existing node (or list of
61 * nodes). In this manner, techniques such as linear probing (which
62 * can cause primary clustering) and rehashing (which does not fit very
63 * well with Java's method of precomputing hash codes) are avoided.
66 * Under ideal circumstances (no collisions), HashMap offers O(1)
67 * performance on most operations (<code>containsValue()</code> is,
68 * of course, O(n)). In the worst case (all keys map to the same
69 * hash code -- very unlikely), most operations are O(n).
72 * HashMap is part of the JDK1.2 Collections API. It differs from
73 * Hashtable in that it accepts the null key and null values, and it
74 * does not support "Enumeration views." Also, it is not synchronized;
75 * if you plan to use it in multiple threads, consider using:<br>
76 * <code>Map m = Collections.synchronizedMap(new HashMap(...));</code>
79 * The iterators are <i>fail-fast</i>, meaning that any structural
80 * modification, except for <code>remove()</code> called on the iterator
81 * itself, cause the iterator to throw a
82 * <code>ConcurrentModificationException</code> rather than exhibit
83 * non-deterministic behavior.
85 * @author Jon Zeppieri
86 * @author Jochen Hoenicke
87 * @author Bryce McKinlay
88 * @author Eric Blake (ebb9@email.byu.edu)
89 * @see Object#hashCode()
94 * @see IdentityHashMap
97 * @status updated to 1.4
99 public class HashMap<K, V> extends AbstractMap<K, V>
100 implements Map<K, V>, Cloneable, Serializable
103 * Default number of buckets; this is currently set to 16.
104 * Package visible for use by HashSet.
106 static final int DEFAULT_CAPACITY = 16;
109 * The default load factor; this is explicitly specified by the spec.
110 * Package visible for use by HashSet.
112 static final float DEFAULT_LOAD_FACTOR = 0.75f;
115 * Compatible with JDK 1.2.
117 private static final long serialVersionUID = 362498820763181265L;
120 * The rounded product of the capacity and the load factor; when the number
121 * of elements exceeds the threshold, the HashMap calls
122 * <code>rehash()</code>.
123 * @serial the threshold for rehashing
125 private int threshold;
128 * Load factor of this HashMap: used in computing the threshold.
129 * Package visible for use by HashSet.
130 * @serial the load factor
132 final float loadFactor;
135 * Array containing the actual key-value mappings.
136 * Package visible for use by nested and subclasses.
138 transient HashEntry<K, V>[] buckets;
141 * Counts the number of modifications this HashMap has undergone, used
142 * by Iterators to know when to throw ConcurrentModificationExceptions.
143 * Package visible for use by nested and subclasses.
145 transient int modCount;
148 * The size of this HashMap: denotes the number of key-value pairs.
149 * Package visible for use by nested and subclasses.
154 * The cache for {@link #entrySet()}.
156 private transient Set<Map.Entry<K, V>> entries;
159 * Class to represent an entry in the hash table. Holds a single key-value
160 * pair. Package visible for use by subclass.
162 * @author Eric Blake (ebb9@email.byu.edu)
164 static class HashEntry<K, V> extends AbstractMap.SimpleEntry<K, V>
167 * The next entry in the linked list. Package visible for use by subclass.
169 HashEntry<K, V> next;
172 * Simple constructor.
174 * @param value the value
176 HashEntry(K key, V value)
182 * Called when this entry is accessed via {@link #put(Object, Object)}.
183 * This version does nothing, but in LinkedHashMap, it must do some
184 * bookkeeping for access-traversal mode.
191 * Called when this entry is removed from the map. This version simply
192 * returns the value, but in LinkedHashMap, it must also do bookkeeping.
194 * @return the value of this key as it is removed
203 * Construct a new HashMap with the default capacity (11) and the default
204 * load factor (0.75).
208 this(DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR);
212 * Construct a new HashMap from the given Map, with initial capacity
213 * the greater of the size of <code>m</code> or the default of 11.
216 * Every element in Map m will be put into this new HashMap.
218 * @param m a Map whose key / value pairs will be put into the new HashMap.
219 * <b>NOTE: key / value pairs are not cloned in this constructor.</b>
220 * @throws NullPointerException if m is null
222 public HashMap(Map<? extends K, ? extends V> m)
224 this(Math.max(m.size() * 2, DEFAULT_CAPACITY), DEFAULT_LOAD_FACTOR);
229 * Construct a new HashMap with a specific inital capacity and
230 * default load factor of 0.75.
232 * @param initialCapacity the initial capacity of this HashMap (>=0)
233 * @throws IllegalArgumentException if (initialCapacity < 0)
235 public HashMap(int initialCapacity)
237 this(initialCapacity, DEFAULT_LOAD_FACTOR);
241 * Construct a new HashMap with a specific inital capacity and load factor.
243 * @param initialCapacity the initial capacity (>=0)
244 * @param loadFactor the load factor (> 0, not NaN)
245 * @throws IllegalArgumentException if (initialCapacity < 0) ||
246 * ! (loadFactor > 0.0)
248 public HashMap(int initialCapacity, float loadFactor)
250 if (initialCapacity < 0)
251 throw new IllegalArgumentException("Illegal Capacity: "
253 if (! (loadFactor > 0)) // check for NaN too
254 throw new IllegalArgumentException("Illegal Load: " + loadFactor);
256 if (initialCapacity == 0)
258 buckets = (HashEntry<K, V>[]) new HashEntry[initialCapacity];
259 this.loadFactor = loadFactor;
260 threshold = (int) (initialCapacity * loadFactor);
264 * Returns the number of kay-value mappings currently in this Map.
274 * Returns true if there are no key-value mappings currently in this Map.
276 * @return <code>size() == 0</code>
278 public boolean isEmpty()
284 * Return the value in this HashMap associated with the supplied key,
285 * or <code>null</code> if the key maps to nothing. NOTE: Since the value
286 * could also be null, you must use containsKey to see if this key
287 * actually maps to something.
289 * @param key the key for which to fetch an associated value
290 * @return what the key maps to, if present
291 * @see #put(Object, Object)
292 * @see #containsKey(Object)
294 public V get(Object key)
297 HashEntry<K, V> e = buckets[idx];
300 if (equals(key, e.key))
308 * Returns true if the supplied object <code>equals()</code> a key
311 * @param key the key to search for in this HashMap
312 * @return true if the key is in the table
313 * @see #containsValue(Object)
315 public boolean containsKey(Object key)
318 HashEntry<K, V> e = buckets[idx];
321 if (equals(key, e.key))
329 * Puts the supplied value into the Map, mapped by the supplied key.
330 * The value may be retrieved by any object which <code>equals()</code>
331 * this key. NOTE: Since the prior value could also be null, you must
332 * first use containsKey if you want to see if you are replacing the
335 * @param key the key used to locate the value
336 * @param value the value to be stored in the HashMap
337 * @return the prior mapping of the key, or null if there was none
339 * @see Object#equals(Object)
341 public V put(K key, V value)
344 HashEntry<K, V> e = buckets[idx];
346 int hash1 = key == null ? 0 : key.hashCode();
349 int hash2 = e.key == null ? 0 : e.key.hashCode();
351 if ((hash1 == hash2) && equals(key, e.key))
353 e.access(); // Must call this for bookkeeping in LinkedHashMap.
362 // At this point, we know we need to add a new entry.
364 if (++size > threshold)
367 // Need a new hash value to suit the bigger table.
371 // LinkedHashMap cannot override put(), hence this call.
372 addEntry(key, value, idx, true);
377 * Copies all elements of the given map into this hashtable. If this table
378 * already has a mapping for a key, the new mapping replaces the current
381 * @param m the map to be hashed into this
383 public void putAll(Map<? extends K, ? extends V> m)
385 final Map<K,V> addMap = (Map<K,V>) m;
386 final Iterator<Map.Entry<K,V>> it = addMap.entrySet().iterator();
389 final Map.Entry<K,V> e = it.next();
390 // Optimize in case the Entry is one of our own.
391 if (e instanceof AbstractMap.SimpleEntry)
393 AbstractMap.SimpleEntry<? extends K, ? extends V> entry
394 = (AbstractMap.SimpleEntry<? extends K, ? extends V>) e;
395 put(entry.key, entry.value);
398 put(e.getKey(), e.getValue());
403 * Removes from the HashMap and returns the value which is mapped by the
404 * supplied key. If the key maps to nothing, then the HashMap remains
405 * unchanged, and <code>null</code> is returned. NOTE: Since the value
406 * could also be null, you must use containsKey to see if you are
407 * actually removing a mapping.
409 * @param key the key used to locate the value to remove
410 * @return whatever the key mapped to, if present
412 public V remove(Object key)
415 HashEntry<K, V> e = buckets[idx];
416 HashEntry<K, V> last = null;
420 if (equals(key, e.key))
424 buckets[idx] = e.next;
428 // Method call necessary for LinkedHashMap to work correctly.
438 * Clears the Map so it has no keys. This is O(1).
445 Arrays.fill(buckets, null);
451 * Returns true if this HashMap contains a value <code>o</code>, such that
452 * <code>o.equals(value)</code>.
454 * @param value the value to search for in this HashMap
455 * @return true if at least one key maps to the value
456 * @see #containsKey(Object)
458 public boolean containsValue(Object value)
460 for (int i = buckets.length - 1; i >= 0; i--)
462 HashEntry<K, V> e = buckets[i];
465 if (equals(value, e.value))
474 * Returns a shallow clone of this HashMap. The Map itself is cloned,
475 * but its contents are not. This is O(n).
479 public Object clone()
481 HashMap<K, V> copy = null;
484 copy = (HashMap<K, V>) super.clone();
486 catch (CloneNotSupportedException x)
488 // This is impossible.
490 copy.buckets = (HashEntry<K, V>[]) new HashEntry[buckets.length];
491 copy.putAllInternal(this);
492 // Clear the entry cache. AbstractMap.clone() does the others.
498 * Returns a "set view" of this HashMap's keys. The set is backed by the
499 * HashMap, so changes in one show up in the other. The set supports
500 * element removal, but not element addition.
502 * @return a set view of the keys
506 public Set<K> keySet()
509 // Create an AbstractSet with custom implementations of those methods
510 // that can be overridden easily and efficiently.
511 keys = new AbstractSet<K>()
518 public Iterator<K> iterator()
520 // Cannot create the iterator directly, because of LinkedHashMap.
521 return HashMap.this.iterator(KEYS);
526 HashMap.this.clear();
529 public boolean contains(Object o)
531 return containsKey(o);
534 public boolean remove(Object o)
536 // Test against the size of the HashMap to determine if anything
537 // really got removed. This is necessary because the return value
538 // of HashMap.remove() is ambiguous in the null case.
540 HashMap.this.remove(o);
541 return oldsize != size;
548 * Returns a "collection view" (or "bag view") of this HashMap's values.
549 * The collection is backed by the HashMap, so changes in one show up
550 * in the other. The collection supports element removal, but not element
553 * @return a bag view of the values
557 public Collection<V> values()
560 // We don't bother overriding many of the optional methods, as doing so
561 // wouldn't provide any significant performance advantage.
562 values = new AbstractCollection<V>()
569 public Iterator<V> iterator()
571 // Cannot create the iterator directly, because of LinkedHashMap.
572 return HashMap.this.iterator(VALUES);
577 HashMap.this.clear();
584 * Returns a "set view" of this HashMap's entries. The set is backed by
585 * the HashMap, so changes in one show up in the other. The set supports
586 * element removal, but not element addition.<p>
588 * Note that the iterators for all three views, from keySet(), entrySet(),
589 * and values(), traverse the HashMap in the same sequence.
591 * @return a set view of the entries
596 public Set<Map.Entry<K, V>> entrySet()
599 // Create an AbstractSet with custom implementations of those methods
600 // that can be overridden easily and efficiently.
601 entries = new AbstractSet<Map.Entry<K, V>>()
608 public Iterator<Map.Entry<K, V>> iterator()
610 // Cannot create the iterator directly, because of LinkedHashMap.
611 return HashMap.this.iterator(ENTRIES);
616 HashMap.this.clear();
619 public boolean contains(Object o)
621 return getEntry(o) != null;
624 public boolean remove(Object o)
626 HashEntry<K, V> e = getEntry(o);
629 HashMap.this.remove(e.key);
639 * Helper method for put, that creates and adds a new Entry. This is
640 * overridden in LinkedHashMap for bookkeeping purposes.
642 * @param key the key of the new Entry
643 * @param value the value
644 * @param idx the index in buckets where the new Entry belongs
645 * @param callRemove whether to call the removeEldestEntry method
646 * @see #put(Object, Object)
648 void addEntry(K key, V value, int idx, boolean callRemove)
650 HashEntry<K, V> e = new HashEntry<K, V>(key, value);
651 e.next = buckets[idx];
656 * Helper method for entrySet(), which matches both key and value
659 * @param o the entry to match
660 * @return the matching entry, if found, or null
663 // Package visible, for use in nested classes.
664 final HashEntry<K, V> getEntry(Object o)
666 if (! (o instanceof Map.Entry))
668 Map.Entry<K, V> me = (Map.Entry<K, V>) o;
671 HashEntry<K, V> e = buckets[idx];
674 if (equals(e.key, key))
675 return equals(e.value, me.getValue()) ? e : null;
682 * Helper method that returns an index in the buckets array for `key'
683 * based on its hashCode(). Package visible for use by subclasses.
686 * @return the bucket number
688 final int hash(Object key)
690 return key == null ? 0 : Math.abs(key.hashCode() % buckets.length);
694 * Generates a parameterized iterator. Must be overrideable, since
695 * LinkedHashMap iterates in a different order.
697 * @param type {@link #KEYS}, {@link #VALUES}, or {@link #ENTRIES}
698 * @return the appropriate iterator
700 <T> Iterator<T> iterator(int type)
702 // FIXME: bogus cast here.
703 return new HashIterator<T>(type);
707 * A simplified, more efficient internal implementation of putAll(). clone()
708 * should not call putAll or put, in order to be compatible with the JDK
709 * implementation with respect to subclasses.
711 * @param m the map to initialize this from
713 void putAllInternal(Map<? extends K, ? extends V> m)
715 final Map<K,V> addMap = (Map<K,V>) m;
716 final Iterator<Map.Entry<K,V>> it = addMap.entrySet().iterator();
720 final Map.Entry<K,V> e = it.next();
724 addEntry(key, e.getValue(), idx, false);
729 * Increases the size of the HashMap and rehashes all keys to new
730 * array indices; this is called when the addition of a new value
731 * would cause size() > threshold. Note that the existing Entry
732 * objects are reused in the new hash table.
734 * <p>This is not specified, but the new size is twice the current size
735 * plus one; this number is not always prime, unfortunately.
737 private void rehash()
739 HashEntry<K, V>[] oldBuckets = buckets;
741 int newcapacity = (buckets.length * 2) + 1;
742 threshold = (int) (newcapacity * loadFactor);
743 buckets = (HashEntry<K, V>[]) new HashEntry[newcapacity];
745 for (int i = oldBuckets.length - 1; i >= 0; i--)
747 HashEntry<K, V> e = oldBuckets[i];
750 int idx = hash(e.key);
751 HashEntry<K, V> dest = buckets[idx];
752 HashEntry<K, V> next = e.next;
753 e.next = buckets[idx];
761 * Serializes this object to the given stream.
763 * @param s the stream to write to
764 * @throws IOException if the underlying stream fails
765 * @serialData the <i>capacity</i>(int) that is the length of the
766 * bucket array, the <i>size</i>(int) of the hash map
767 * are emitted first. They are followed by size entries,
768 * each consisting of a key (Object) and a value (Object).
770 private void writeObject(ObjectOutputStream s) throws IOException
772 // Write the threshold and loadFactor fields.
773 s.defaultWriteObject();
775 s.writeInt(buckets.length);
777 // Avoid creating a wasted Set by creating the iterator directly.
778 Iterator<HashEntry<K, V>> it = iterator(ENTRIES);
781 HashEntry<K, V> entry = it.next();
782 s.writeObject(entry.key);
783 s.writeObject(entry.value);
788 * Deserializes this object from the given stream.
790 * @param s the stream to read from
791 * @throws ClassNotFoundException if the underlying stream fails
792 * @throws IOException if the underlying stream fails
793 * @serialData the <i>capacity</i>(int) that is the length of the
794 * bucket array, the <i>size</i>(int) of the hash map
795 * are emitted first. They are followed by size entries,
796 * each consisting of a key (Object) and a value (Object).
798 private void readObject(ObjectInputStream s)
799 throws IOException, ClassNotFoundException
801 // Read the threshold and loadFactor fields.
802 s.defaultReadObject();
804 // Read and use capacity, followed by key/value pairs.
805 buckets = (HashEntry<K, V>[]) new HashEntry[s.readInt()];
806 int len = s.readInt();
810 Object key = s.readObject();
811 addEntry((K) key, (V) s.readObject(), hash(key), false);
816 * Iterate over HashMap's entries.
817 * This implementation is parameterized to give a sequential view of
818 * keys, values, or entries.
820 * @author Jon Zeppieri
822 private final class HashIterator<T> implements Iterator<T>
825 * The type of this Iterator: {@link #KEYS}, {@link #VALUES},
826 * or {@link #ENTRIES}.
828 private final int type;
830 * The number of modifications to the backing HashMap that we know about.
832 private int knownMod = modCount;
833 /** The number of elements remaining to be returned by next(). */
834 private int count = size;
835 /** Current index in the physical hash table. */
836 private int idx = buckets.length;
837 /** The last Entry returned by a next() call. */
838 private HashEntry last;
840 * The next entry that should be returned by next(). It is set to something
841 * if we're iterating through a bucket that contains multiple linked
842 * entries. It is null if next() needs to find a new bucket.
844 private HashEntry next;
847 * Construct a new HashIterator with the supplied type.
848 * @param type {@link #KEYS}, {@link #VALUES}, or {@link #ENTRIES}
850 HashIterator(int type)
856 * Returns true if the Iterator has more elements.
857 * @return true if there are more elements
859 public boolean hasNext()
865 * Returns the next element in the Iterator's sequential view.
866 * @return the next element
867 * @throws ConcurrentModificationException if the HashMap was modified
868 * @throws NoSuchElementException if there is none
872 if (knownMod != modCount)
873 throw new ConcurrentModificationException();
875 throw new NoSuchElementException();
892 * Removes from the backing HashMap the last element which was fetched
893 * with the <code>next()</code> method.
894 * @throws ConcurrentModificationException if the HashMap was modified
895 * @throws IllegalStateException if called when there is no last element
899 if (knownMod != modCount)
900 throw new ConcurrentModificationException();
902 throw new IllegalStateException();
904 HashMap.this.remove(last.key);