java.util

Class HashMap<K,V>

java.lang.Object

java.util.AbstractMap<K,V>

java.util.HashMap<K,V>

All Implemented Interfaces:

Map<K,V>

Direct Known Subclasses:

LinkedHashMap

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

HashMap is an implementation of Map. All optional operations (adding and removing) are supported. Keys and values can be any objects.

Nested Class Summary

Nested classes/interfaces inherited from class java.util.AbstractMap

AbstractMap.SimpleEntry<K,V>, AbstractMap.SimpleImmutableEntry<K,V>

Constructor Summary

Constructors

Constructor and Description
HashMap() Constructs a new empty HashMap instance.
HashMap(int capacity) Constructs a new HashMap instance with the specified capacity.
HashMap(int capacity, float loadFactor) Constructs a new HashMap instance with the specified capacity and load factor.
HashMap(Map<? extends K,? extends V> map) Constructs a new HashMap instance containing the mappings from the specified map.

Method Summary

Modifier and Type	Method and Description
void	clear() Removes all mappings from this hash map, leaving it empty.
boolean	containsKey(Object key) Returns whether this map contains the specified key.
boolean	containsValue(Object value) Returns whether this map contains the specified value.
Set<Map.Entry<K,V>>	entrySet() Returns a set containing all of the mappings in this map.
V	get(Object key) Returns the value of the mapping with the specified key.
boolean	isEmpty() Returns whether this map is empty.
Set<K>	keySet() Returns a set of the keys contained in this map.
V	put(K key, V value) Maps the specified key to the specified value.
void	putAll(Map<? extends K,? extends V> map) Copies all the mappings in the specified map to this map.
V	remove(Object key) Removes the mapping with the specified key from this map.
int	size() Returns the number of elements in this map.
Collection<V>	values() Returns a collection of the values contained in this map.

Methods inherited from class java.util.AbstractMap

equals, hashCode, toString

Methods inherited from class java.lang.Object

clone, getClass, notify, notifyAll, wait, wait, wait

Methods inherited from interface java.util.Map

equals, hashCode

Constructor Detail

HashMap

public HashMap()

Constructs a new empty HashMap instance.

HashMap

public HashMap(int capacity)

Constructs a new HashMap instance with the specified capacity.

Parameters:

capacity - the initial capacity of this hash map.

Throws:

IllegalArgumentException - when the capacity is less than zero.

HashMap

public HashMap(int capacity,
               float loadFactor)

Constructs a new HashMap instance with the specified capacity and load factor.

Parameters:

capacity - the initial capacity of this hash map.

loadFactor - the initial load factor.

Throws:

IllegalArgumentException - when the capacity is less than zero or the load factor is less or equal to zero.

HashMap

public HashMap(Map<? extends K,? extends V> map)

Constructs a new HashMap instance containing the mappings from the specified map.

Parameters:

map - the mappings to add.

Method Detail

clear

public void clear()

Removes all mappings from this hash map, leaving it empty.

Specified by:

clear in interface Map<K,V>

Overrides:

clear in class AbstractMap<K,V>

See Also:

isEmpty(), size()

containsKey

public boolean containsKey(Object key)

Returns whether this map contains the specified key.

Specified by:

containsKey in interface Map<K,V>

Overrides:

containsKey in class AbstractMap<K,V>

Parameters:

key - the key to search for.

Returns:

true if this map contains the specified key, false otherwise.

containsValue

public boolean containsValue(Object value)

Returns whether this map contains the specified value.

Specified by:

containsValue in interface Map<K,V>

Overrides:

containsValue in class AbstractMap<K,V>

Parameters:

value - the value to search for.

Returns:

true if this map contains the specified value, false otherwise.

entrySet

public Set<Map.Entry<K,V>> entrySet()

Returns a set containing all of the mappings in this map. Each mapping is an instance of Map.Entry. As the set is backed by this map, changes in one will be reflected in the other.

Specified by:

entrySet in interface Map<K,V>

Specified by:

entrySet in class AbstractMap<K,V>

Returns:

a set of the mappings.

get

public V get(Object key)

Returns the value of the mapping with the specified key.

Specified by:

get in interface Map<K,V>

Overrides:

get in class AbstractMap<K,V>

Parameters:

key - the key.

Returns:

the value of the mapping with the specified key, or null if no mapping for the specified key is found.

isEmpty

public boolean isEmpty()

Returns whether this map is empty.

Specified by:

isEmpty in interface Map<K,V>

Overrides:

isEmpty in class AbstractMap<K,V>

Returns:

true if this map has no elements, false otherwise.

See Also:

size()

keySet

public Set<K> keySet()

Returns a set of the keys contained in this map. The set is backed by this map so changes to one are reflected by the other. The set does not support adding.

Specified by:

keySet in interface Map<K,V>

Overrides:

keySet in class AbstractMap<K,V>

Returns:

a set of the keys.

put

public V put(K key,
             V value)

Maps the specified key to the specified value.

Specified by:

put in interface Map<K,V>

Overrides:

put in class AbstractMap<K,V>

Parameters:

key - the key.

value - the value.

Returns:

the value of any previous mapping with the specified key or null if there was no such mapping.

putAll

public void putAll(Map<? extends K,? extends V> map)

Copies all the mappings in the specified map to this map. These mappings will replace all mappings that this map had for any of the keys currently in the given map.

Specified by:

putAll in interface Map<K,V>

Overrides:

putAll in class AbstractMap<K,V>

Parameters:

map - the map to copy mappings from.

Throws:

NullPointerException - if map is null.

remove

public V remove(Object key)

Removes the mapping with the specified key from this map.

Specified by:

remove in interface Map<K,V>

Overrides:

remove in class AbstractMap<K,V>

Parameters:

key - the key of the mapping to remove.

Returns:

the value of the removed mapping or null if no mapping for the specified key was found.

size

public int size()

Returns the number of elements in this map.

Specified by:

size in interface Map<K,V>

Overrides:

size in class AbstractMap<K,V>

Returns:

the number of elements in this map.

values

public Collection<V> values()

Returns a collection of the values contained in this map. The collection is backed by this map so changes to one are reflected by the other. The collection supports remove, removeAll, retainAll and clear operations, and it does not support add or addAll operations.

This method returns a collection which is the subclass of AbstractCollection. The iterator method of this subclass returns a "wrapper object" over the iterator of map's entrySet(). The size method wraps the map's size method and the contains method wraps the map's containsValue method.

The collection is created when this method is called for the first time and returned in response to all subsequent calls. This method may return different collections when multiple concurrent calls occur, since no synchronization is performed.

Specified by:

values in interface Map<K,V>

Overrides:

values in class AbstractMap<K,V>

Returns:

a collection of the values contained in this map.