Android APIs
public class

HashMap

extends AbstractMap<K, V>
implements Cloneable Serializable
java.lang.Object
   ↳ java.util.AbstractMap<K, V>
     ↳ java.util.HashMap<K, V>
Known Direct Subclasses

Class Overview

HashMap is an implementation of Map. All optional operations are supported.

All elements are permitted as keys or values, including null.

Note that the iteration order for HashMap is non-deterministic. If you want deterministic iteration, use LinkedHashMap.

Note: the implementation of HashMap is not synchronized. If one thread of several threads accessing an instance modifies the map structurally, access to the map needs to be synchronized. A structural modification is an operation that adds or removes an entry. Changes in the value of an entry are not structural changes.

The Iterator created by calling the iterator method may throw a ConcurrentModificationException if the map is structurally changed while an iterator is used to iterate over the elements. Only the remove method that is provided by the iterator allows for removal of elements during iteration. It is not possible to guarantee that this mechanism works in all cases of unsynchronized concurrent modification. It should only be used for debugging purposes.

Summary

Public Constructors
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.
Public Methods
void clear()
Removes all mappings from this hash map, leaving it empty.
Object clone()
Returns a shallow copy of this map.
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<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.
[Expand]
Inherited Methods
From class java.util.AbstractMap
From class java.lang.Object
From interface java.util.Map

Public Constructors

public HashMap ()

Added in API level 1

Constructs a new empty HashMap instance.

public HashMap (int capacity)

Added in API level 1

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.

public HashMap (int capacity, float loadFactor)

Added in API level 1

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 or NaN.

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

Added in API level 1

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

Parameters
map the mappings to add.

Public Methods

public void clear ()

Added in API level 1

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

See Also

public Object clone ()

Added in API level 1

Returns a shallow copy of this map.

Returns
  • a shallow copy of this map.

public boolean containsKey (Object key)

Added in API level 1

Returns whether this map contains the specified key.

Parameters
key the key to search for.
Returns
  • true if this map contains the specified key, false otherwise.

public boolean containsValue (Object value)

Added in API level 1

Returns whether this map contains the specified value.

Parameters
value the value to search for.
Returns
  • true if this map contains the specified value, false otherwise.

public Set<Entry<K, V>> entrySet ()

Added in API level 1

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.

Returns
  • a set of the mappings.

public V get (Object key)

Added in API level 1

Returns the value of the mapping with the specified key.

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.

public boolean isEmpty ()

Added in API level 1

Returns whether this map is empty.

Returns
  • true if this map has no elements, false otherwise.
See Also

public Set<K> keySet ()

Added in API level 1

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.

Returns
  • a set of the keys.

public V put (K key, V value)

Added in API level 1

Maps the specified key to the specified value.

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.

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

Added in API level 1

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.

Parameters
map the map to copy mappings from.

public V remove (Object key)

Added in API level 1

Removes the mapping with the specified key from this map.

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.

public int size ()

Added in API level 1

Returns the number of elements in this map.

Returns
  • the number of elements in this map.

public Collection<V> values ()

Added in API level 1

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.

Returns
  • a collection of the values contained in this map.