java.util
Class HashMap

java.lang.Object
  java.util.AbstractMap
      java.util.HashMap

All Implemented Interfaces:

Map

public class HashMap
extends AbstractMap
implements Map

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:

Object.hashCode(), Collection, Map, TreeMap, LinkedHashMap, IdentityHashMap, Hashtable

Nested Class Summary

Nested classes/interfaces inherited from interface java.util.Map

Map.Entry

Constructor Summary

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

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.

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

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	entrySet() Returns a "set view" of this HashMap's entries.
Object	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	keySet() Returns a "set view" of this HashMap's keys.
Object	put(Object key, Object value) Puts the supplied value into the Map, mapped by the supplied key.
void	putAll(Map m) Copies all elements of the given map into this hashtable.
Object	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	values() Returns a "collection view" (or "bag view") of this HashMap's values.

Methods inherited from class java.util.AbstractMap

equals, hashCode, toString

Methods inherited from class java.lang.Object

finalize, getClass, notify, notifyAll, wait, wait, wait

Methods inherited from interface java.util.Map

equals, hashCode

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 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

size

public int size()

Returns the number of kay-value mappings currently in this Map.

Specified by:

size in interface Map

Overrides:

size in class AbstractMap

Returns:

the size

See Also:

Set.size()

isEmpty

public boolean isEmpty()

Returns true if there are no key-value mappings currently in this Map.

Specified by:

isEmpty in interface Map

Overrides:

isEmpty in class AbstractMap

Returns:

size() == 0

See Also:

AbstractMap.size()

get

public Object 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.

Specified by:

get in interface Map

Overrides:

get in class AbstractMap

Parameters:

key - the key for which to fetch an associated value

Returns:

what the key maps to, if present

See Also:

put(Object, Object), containsKey(Object)

containsKey

public boolean containsKey(Object key)

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

Specified by:

containsKey in interface Map

Overrides:

containsKey in class AbstractMap

Parameters:

key - the key to search for in this HashMap

Returns:

true if the key is in the table

See Also:

containsValue(Object)

put

public Object put(Object key,
                  Object 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.

Specified by:

put in interface Map

Overrides:

put in class AbstractMap

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(Object), Object.equals(Object)

putAll

public void putAll(Map 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.

Specified by:

putAll in interface Map

Overrides:

putAll in class AbstractMap

Parameters:

m - the map to be hashed into this

See Also:

AbstractMap.put(Object, Object)

remove

public Object 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.

Specified by:

remove in interface Map

Overrides:

remove in class AbstractMap

Parameters:

key - the key used to locate the value to remove

Returns:

whatever the key mapped to, if present

See Also:

Iterator.remove()

clear

public void clear()

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

Specified by:

clear in interface Map

Overrides:

clear in class AbstractMap

See Also:

Set.clear()

containsValue

public boolean containsValue(Object value)

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

Specified by:

containsValue in interface Map

Overrides:

containsValue in class AbstractMap

Parameters:

value - the value to search for in this HashMap

Returns:

true if at least one key maps to the value

See Also:

containsKey(Object)

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).

Overrides:

clone in class AbstractMap

Returns:

the clone

See Also:

Cloneable, Object.clone()

keySet

public Set 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.

Specified by:

keySet in interface Map

Overrides:

keySet in class AbstractMap

Returns:

a set view of the keys

See Also:

values(), entrySet()

values

public Collection 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.

Specified by:

values in interface Map

Overrides:

values in class AbstractMap

Returns:

a bag view of the values

See Also:

keySet(), entrySet()

entrySet

public Set 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.

Specified by:

entrySet in interface Map

Specified by:

entrySet in class AbstractMap

Returns:

a set view of the entries

See Also:

keySet(), values(), Map.Entry