java.util

Class HashMap<K,V>

public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable

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

See Also: hashCode Collection Map TreeMap LinkedHashMap IdentityHashMap Hashtable

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
voidclear()
Clears the Map so it has no keys.
Objectclone()
Returns a shallow clone of this HashMap.
booleancontainsKey(Object key)
Returns true if the supplied object equals() a key in this HashMap.
booleancontainsValue(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.
Vget(Object key)
Return the value in this HashMap associated with the supplied key, or null if the key maps to nothing.
booleanisEmpty()
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.
Vput(K key, V value)
Puts the supplied value into the Map, mapped by the supplied key.
voidputAll(Map<? extends K,? extends V> m)
Copies all elements of the given map into this hashtable.
Vremove(Object key)
Removes from the HashMap and returns the value which is mapped by the supplied key.
intsize()
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

public HashMap()
Construct a new HashMap with the default capacity (11) and the default load factor (0.75).

HashMap

public 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.

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

HashMap

public HashMap(int initialCapacity)
Construct a new HashMap with a specific inital capacity and default load factor of 0.75.

Parameters: initialCapacity the initial capacity of this HashMap (>=0)

Throws: IllegalArgumentException if (initialCapacity < 0)

HashMap

public HashMap(int initialCapacity, float loadFactor)
Construct a new HashMap with a specific inital capacity and load factor.

Parameters: initialCapacity the initial capacity (>=0) loadFactor the load factor (> 0, not NaN)

Throws: IllegalArgumentException if (initialCapacity < 0) || ! (loadFactor > 0.0)

Method Detail

clear

public void clear()
Clears the Map so it has no keys. This is O(1).

clone

public Object clone()
Returns a shallow clone of this HashMap. The Map itself is cloned, but its contents are not. This is O(n).

Returns: the clone

containsKey

public boolean containsKey(Object key)
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

See Also: containsValue

containsValue

public boolean containsValue(Object value)
Returns true if this HashMap contains a value o, such that o.equals(value).

Parameters: value the value to search for in this HashMap

Returns: true if at least one key maps to the value

See Also: containsKey

entrySet

public Set<Entry<K,V>> entrySet()
Returns a "set view" of this HashMap's entries. The set is backed by the HashMap, so changes in one show up in the other. The set supports element removal, but not element addition.

Note that the iterators for all three views, from keySet(), entrySet(), and values(), traverse the HashMap in the same sequence.

Returns: a set view of the entries

See Also: keySet values Entry

get

public V get(Object key)
Return the value in this HashMap associated with the supplied key, or null if the key maps to nothing. NOTE: Since the value could also be null, you must use containsKey to see if this key actually maps to something.

Parameters: key the key for which to fetch an associated value

Returns: what the key maps to, if present

See Also: HashMap containsKey

isEmpty

public boolean isEmpty()
Returns true if there are no key-value mappings currently in this Map.

Returns: size() == 0

keySet

public Set<K> keySet()
Returns a "set view" of this HashMap's keys. The set is backed by the HashMap, so changes in one show up in the other. The set supports element removal, but not element addition.

Returns: a set view of the keys

See Also: values entrySet

put

public V put(K key, V value)
Puts the supplied value into the Map, mapped by the supplied key. The value may be retrieved by any object which equals() this key. NOTE: Since the prior value could also be null, you must first use containsKey if you want to see if you are replacing the key's mapping.

Parameters: key the key used to locate the value value the value to be stored in the HashMap

Returns: the prior mapping of the key, or null if there was none

See Also: get equals

putAll

public void putAll(Map<? extends K,? extends V> m)
Copies all elements of the given map into this hashtable. If this table already has a mapping for a key, the new mapping replaces the current one.

Parameters: m the map to be hashed into this

remove

public V remove(Object key)
Removes from the HashMap and returns the value which is mapped by the supplied key. If the key maps to nothing, then the HashMap remains unchanged, and null is returned. NOTE: Since the value could also be null, you must use containsKey to see if you are actually removing a mapping.

Parameters: key the key used to locate the value to remove

Returns: whatever the key mapped to, if present

size

public int size()
Returns the number of kay-value mappings currently in this Map.

Returns: the size

values

public Collection<V> values()
Returns a "collection view" (or "bag view") of this HashMap's values. The collection is backed by the HashMap, so changes in one show up in the other. The collection supports element removal, but not element addition.

Returns: a bag view of the values

See Also: keySet entrySet