public class HashMap<K, V>

  1. Object
  2. AbstractMap<K, V>
  3. HashMap

ImplementsMap<K, V>

Known subtypesProperties, LinkedHashMap

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

Constructors

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

Methods

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

Inherited nested types

From AbstractMap

AbstractMap.SimpleImmutableEntry, AbstractMap.SimpleEntry

Inherited methods

From AbstractMap

equals, hashCode, toString

From Object

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

From Map

getOrDefault, putIfAbsent, remove, replace, replace, forEach, replaceAll, computeIfAbsent, computeIfPresent, compute, merge

Constructor details

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 int
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 int
the initial capacity of this hash map.
loadFactor float
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 Map<? extends K, ? extends V>
the mappings to add.

Method details

clear

public void clear()
Removes all mappings from this hash map, leaving it empty.

Throws

UnsupportedOperationException
if removing from this map is not supported.

See also

containsKey

public boolean containsKey(Object key)
Returns whether this map contains the specified key.

Parameters

key Object
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.

Parameters

value Object
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.

Returns

a set of the mappings.

get

public V get(Object key)
Returns the value of the mapping with the specified key.

Parameters

key Object
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.

Returns

true if this map has no elements, false otherwise.

See also

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.

Returns

a set of the keys.

put

public V put(K key, V value)
Maps the specified key to the specified value.

Parameters

key K
the key.
value V
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.

Parameters

map Map<? extends K, ? extends V>
the map to copy mappings from.

Throws

NullPointerException
if map is null.
UnsupportedOperationException
if adding to this map is not supported.
ClassCastException
if the class of a key or value is inappropriate for this map.
IllegalArgumentException
if a key or value cannot be added to this map.

remove

public V remove(Object key)
Removes the mapping with the specified key from this map.

Parameters

key Object
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.

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.

Returns

a collection of the values contained in this map.