Summary: Ctors | Methods | Inherited Methods | [Expand All]

public class

HashMap

extends AbstractMap<K, V>
implements Serializable Cloneable

java.lang.Object
↳	java.util.AbstractMap<K, V>
	↳	java.util.HashMap<K, V>

Known Direct Subclasses

LinkedHashMap<K, V>

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

void	clear() Removes all elements from this `Map`, leaving it empty. This implementation calls `entrySet().clear()`.
Object	clone() Creates and returns a copy of this `Object`.
boolean	containsKey(Object key) Returns whether this `Map` contains the specified key. This implementation iterates its key set, looking for a key that `key` equals.
boolean	containsValue(Object value) Returns whether this `Map` contains the specified value. This implementation iterates its entry set, looking for an entry with a value that `value` equals.
abstract Set<Entry<K, V>>	entrySet() Returns a `Set` containing all of the mappings in this `Map`.
boolean	equals(Object object) Compares this instance with the specified object and indicates if they are equal. This implementation first checks the structure of `object`.
V	get(Object key) Returns the value of the mapping with the specified key. This implementation iterates its entry set, looking for an entry with a key that `key` equals.
int	hashCode() Returns an integer hash code for this object. This implementation iterates its entry set, summing the hashcodes of its entries.
boolean	isEmpty() Returns whether this map is empty. This implementation compares `size()` to 0.
Set<K>	keySet() Returns a set of the keys contained in this `Map`. This implementation returns a view that calls through this to map.
V	put(K key, V value) Maps the specified key to the specified value. This base implementation throws `UnsupportedOperationException`.
void	putAll(Map<? extends K, ? extends V> map) Copies every mapping in the specified `Map` to this `Map`. This implementation iterates through `map`'s entry set, calling `put()` for each.
V	remove(Object key) Removes a mapping with the specified key from this `Map`. This implementation iterates its entry set, removing the entry with a key that `key` equals.
int	size() Returns the number of mappings in this `Map`. This implementation returns its entry set's size.
String	toString() Returns a string containing a concise, human-readable description of this object. This implementation composes a string by iterating its entry set.
Collection<V>	values() Returns a `Collection` of the values contained in this `Map`. This implementation returns a view that calls through this to map.

From class java.lang.Object

Object	clone() Creates and returns a copy of this `Object`.
boolean	equals(Object o) Compares this instance with the specified object and indicates if they are equal.
void	finalize() Called before the object's memory is reclaimed by the VM.
final Class<? extends Object>	getClass() Returns the unique instance of `Class` that represents this object's class.
int	hashCode() Returns an integer hash code for this object.
final void	notify() Causes a thread which is waiting on this object's monitor (by means of calling one of the `wait()` methods) to be woken up.
final void	notifyAll() Causes all threads which are waiting on this object's monitor (by means of calling one of the `wait()` methods) to be woken up.
String	toString() Returns a string containing a concise, human-readable description of this object.
final void	wait() Causes the calling thread to wait until another thread calls the `notify()` or `notifyAll()` method of this object.
final void	wait(long millis, int nanos) Causes the calling thread to wait until another thread calls the `notify()` or `notifyAll()` method of this object or until the specified timeout expires.
final void	wait(long millis) Causes the calling thread to wait until another thread calls the `notify()` or `notifyAll()` method of this object or until the specified timeout expires.

From interface java.util.Map

abstract void	clear() Removes all elements from this `Map`, leaving it empty.
abstract boolean	containsKey(Object key) Returns whether this `Map` contains the specified key.
abstract boolean	containsValue(Object value) Returns whether this `Map` contains the specified value.
abstract Set<Entry<K, V>>	entrySet() Returns a `Set` containing all of the mappings in this `Map`.
abstract boolean	equals(Object object) Compares the argument to the receiver, and returns `true` if the specified object is a `Map` and both `Map`s contain the same mappings.
abstract V	get(Object key) Returns the value of the mapping with the specified key.
abstract int	hashCode() Returns an integer hash code for the receiver.
abstract boolean	isEmpty() Returns whether this map is empty.
abstract Set<K>	keySet() Returns a set of the keys contained in this `Map`.
abstract V	put(K key, V value) Maps the specified key to the specified value.
abstract void	putAll(Map<? extends K, ? extends V> map) Copies every mapping in the specified `Map` to this `Map`.
abstract V	remove(Object key) Removes a mapping with the specified key from this `Map`.
abstract int	size() Returns the number of mappings in this `Map`.
abstract Collection<V>	values() Returns a `Collection` of the values contained in this `Map`.

Public Constructors

public HashMap ()

Since: API Level 1

Constructs a new empty HashMap instance.

public HashMap (int capacity)

Since: 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)

Since: 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)

Since: 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 ()

Since: API Level 1

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

public Object clone ()

Since: API Level 1

Returns a shallow copy of this map.

Returns

a shallow copy of this map.

public boolean containsKey (Object key)

Since: 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)

Since: 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 ()

Since: 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)

Since: 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 ()

Since: API Level 1

Returns whether this map is empty.

Returns

true if this map has no elements, false otherwise.

public Set<K> keySet ()

Since: 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)

Since: 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)

Since: 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)

Since: 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 ()

Since: API Level 1

Returns the number of elements in this map.

Returns

the number of elements in this map.

public Collection<V> values ()

Since: 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.

Interfaces

Classes

Enums

Exceptions

Errors

HashMap

Class Overview

Summary

Public Constructors

public HashMap ()

public HashMap (int capacity)

Parameters

Throws

public HashMap (int capacity, float loadFactor)

Parameters

Throws

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

Parameters

Public Methods

public void clear ()

See Also

public Object clone ()

Returns

public boolean containsKey (Object key)

Parameters

Returns

public boolean containsValue (Object value)

Parameters

Returns

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

Returns

public V get (Object key)

Parameters

Returns

public boolean isEmpty ()

Returns

See Also

public Set<K> keySet ()

Returns

public V put (K key, V value)

Parameters

Returns

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

Parameters

public V remove (Object key)

Parameters

Returns

public int size ()

Returns

public Collection<V> values ()

Returns