java.util

Interface Map<K,V>

All Known Subinterfaces:

NavigableMap<K,V>, SortedMap<K,V>

All Known Implementing Classes:

AbstractMap, HashMap, Hashtable, IdentityHashMap, LinkedHashMap, TreeMap

public interface Map<K,V>

A Map is a data structure consisting of a set of keys and values in which each key is mapped to a single value. The class of the objects used as keys is declared when the Map is declared, as is the class of the corresponding values.

A Map provides helper methods to iterate through all of the keys contained in it, as well as various methods to access and update the key/value pairs.

Nested Class Summary

Nested Classes

Modifier and Type	Interface and Description
static interface	Map.Entry<K,V> Map.Entry is a key/value mapping contained in a Map.

Method Summary

Modifier and Type	Method and Description
void	clear() Removes all elements from this 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.
boolean	equals(Object object) Compares the argument to the receiver, and returns true if the specified object is a Map and both Maps contain the same mappings.
V	get(Object key) Returns the value of the mapping with the specified key.
int	hashCode() Returns an integer hash code for the receiver.
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 every mapping in the specified Map to this Map.
V	remove(Object key) Removes a mapping with the specified key from this Map.
int	size() Returns the number of mappings in this Map.
Collection<V>	values() Returns a Collection of the values contained in this Map.

Method Detail

clear

void clear()

Removes all elements from this Map, leaving it empty.

Throws:

UnsupportedOperationException - if removing elements from this Map is not supported.

See Also:

isEmpty(), size()

containsKey

boolean containsKey(Object key)

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.

containsValue

boolean containsValue(Object value)

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.

entrySet

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

equals

boolean equals(Object object)

Compares the argument to the receiver, and returns true if the specified object is a Map and both Maps contain the same mappings.

Overrides:

equals in class Object

Parameters:

object - the Object to compare with this Object.

Returns:

boolean true if the Object is the same as this Object false if it is different from this Object.

See Also:

hashCode(), entrySet()

get

V get(Object key)

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.

hashCode

int hashCode()

Returns an integer hash code for the receiver. Objects which are equal return the same value for this method.

Overrides:

hashCode in class Object

Returns:

the receiver's hash.

See Also:

equals(Object)

isEmpty

boolean isEmpty()

Returns whether this map is empty.

Returns:

true if this map has no elements, false otherwise.

See Also:

size()

keySet

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

V put(K key,
      V value)

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 mapping.

Throws:

UnsupportedOperationException - if adding to this Map is not supported.

ClassCastException - if the class of the key or value is inappropriate for this Map.

IllegalArgumentException - if the key or value cannot be added to this Map.

NullPointerException - if the key or value is null and this Map does not support null keys or values.

putAll

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

Copies every mapping in the specified Map to this Map.

Parameters:

map - the Map to copy mappings from.

Throws:

UnsupportedOperationException - if adding to this Map is not supported.

ClassCastException - if the class of a key or a value of the specified Map is inappropriate for this Map.

IllegalArgumentException - if a key or value cannot be added to this Map.

NullPointerException - if a key or value is null and this Map does not support null keys or values.

remove

V remove(Object key)

Removes a 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.

Throws:

UnsupportedOperationException - if removing from this Map is not supported.

size

int size()

Returns the number of mappings in this Map.

Returns:

the number of mappings in this Map.

values

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 Collection.remove(java.lang.Object), Collection.removeAll(java.util.Collection<?>), Collection.retainAll(java.util.Collection<?>), and Collection.clear() operations, and it does not support Collection.add(E) or Collection.addAll(java.util.Collection<? extends E>) operations.

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

The collection is created when this method is called at first time and returned in response to all subsequent calls. This method may return different Collection when multiple calls to this method, since it has no synchronization performed.

Returns:

a collection of the values contained in this map.