HashMap<K,V> (GNU Classpath 0.94-pre Documentation)

This class provides a hashtable-backed implementation of the Map interface.

It uses a hash-bucket approach; that is, hash collisions are handled by linking the new node off of the pre-existing node (or list of nodes). In this manner, techniques such as linear probing (which can cause primary clustering) and rehashing (which does not fit very well with Java's method of precomputing hash codes) are avoided.

Under ideal circumstances (no collisions), HashMap offers O(1) performance on most operations (containsValue() is, of course, O(n)). In the worst case (all keys map to the same hash code -- very unlikely), most operations are O(n).

HashMap is part of the JDK1.2 Collections API. It differs from Hashtable in that it accepts the null key and null values, and it does not support "Enumeration views." Also, it is not synchronized; if you plan to use it in multiple threads, consider using:
Map m = Collections.synchronizedMap(new HashMap(...));

The iterators are fail-fast, meaning that any structural modification, except for remove() called on the iterator itself, cause the iterator to throw a ConcurrentModificationException rather than exhibit non-deterministic behavior.

Since: 1.2

UNKNOWN: updated to 1.4

Constructor Summary
	HashMap() Construct a new HashMap with the default capacity (11) and the default load factor (0.75).
	HashMap(Map<? extends K,? extends V> m) Construct a new HashMap from the given Map, with initial capacity the greater of the size of `m` or the default of 11.
	HashMap(int initialCapacity) Construct a new HashMap with a specific inital capacity and default load factor of 0.75.
	HashMap(int initialCapacity, float loadFactor) Construct a new HashMap with a specific inital capacity and load factor.

Method Summary
void	clear() Clears the Map so it has no keys.
Object	clone() Returns a shallow clone of this HashMap.
boolean	containsKey(Object key) Returns true if the supplied object `equals()` a key in this HashMap.
boolean	containsValue(Object value) Returns true if this HashMap contains a value `o`, such that `o.equals(value)`.
Set<Entry<K,V>>	entrySet() Returns a "set view" of this HashMap's entries.
V	get(Object key) Return the value in this HashMap associated with the supplied key, or `null` if the key maps to nothing.
boolean	isEmpty() Returns true if there are no key-value mappings currently in this Map.
Set<K>	keySet() Returns a "set view" of this HashMap's keys.
V	put(K key, V value) Puts the supplied value into the Map, mapped by the supplied key.
void	putAll(Map<? extends K,? extends V> m) Copies all elements of the given map into this hashtable.
V	remove(Object key) Removes from the HashMap and returns the value which is mapped by the supplied key.
int	size() Returns the number of kay-value mappings currently in this Map.
Collection<V>	values() Returns a "collection view" (or "bag view") of this HashMap's values.

Constructor Detail

HashMap

Construct a new HashMap from the given Map, with initial capacity the greater of the size of m or the default of 11.

Every element in Map m will be put into this new HashMap.

Parameters: m a Map whose key / value pairs will be put into the new HashMap. NOTE: key / value pairs are not cloned in this constructor.

Throws: NullPointerException if m is null

Returns true if the supplied object equals() a key in this HashMap.

Parameters: key the key to search for in this HashMap

Returns: true if the key is in the table

Class HashMap<K,V>