org.apache.commons.collections
Interface MultiMap<K,V>

All Superinterfaces:

Map

All Known Implementing Classes:

MultiHashMap

public interface MultiMap<K,V>
extends Map

Defines a map that holds a collection of values against each key.

A MultiMap is a Map with slightly different semantics. Putting a value into the map will add the value to a Collection at that key. Getting a value will return a Collection, holding all the values put to that key.

For example:

 MultiMap mhm = new MultiHashMap();
 mhm.put(key, "A");
 mhm.put(key, "B");
 mhm.put(key, "C");
 Collection coll = (Collection) mhm.get(key);

coll will be a collection containing "A", "B", "C".

NOTE: Additional methods were added to this interface in Commons Collections 3.1. These were added solely for documentation purposes and do not change the interface as they were defined in the superinterface Map anyway.

Since:

Commons Collections 2.0

Version:

$Revision: 1.1.1.1 $ $Date: 2005/05/23 04:33:15 $

Author:

Christopher Berry, James Strachan, Matt Hall, John Watkinson, Stephen Colebourne

Nested Class Summary

Nested classes/interfaces inherited from interface java.util.Map

Map.Entry<K,V>

Method Summary

boolean	containsValue(Object value) Checks whether the map contains the value specified.
Object	get(Object key) Gets the collection of values associated with the specified key.
V	put(Object key, Object value) Adds the value to the collection associated with the specified key.
V	remove(K key, V item) Removes a specific value from map.
Object	remove(Object key) Removes all values associated with the specified key.
int	size() Gets the number of keys in this map.
Collection<V>	values() Gets a collection containing all the values in the map.

Methods inherited from interface java.util.Map

clear, containsKey, entrySet, equals, hashCode, isEmpty, keySet, putAll

Method Detail

remove

V remove(K key,
         V item)

Removes a specific value from map.

The item is removed from the collection mapped to the specified key. Other values attached to that key are unaffected.

If the last value for a key is removed, implementations typically return null from a subsequant get(Object), however they may choose to return an empty collection.

Parameters:

key - the key to remove from

item - the item to remove

Returns:

the value removed (which was passed in), null if nothing removed

Throws:

UnsupportedOperationException - if the map is unmodifiable

ClassCastException - if the key or value is of an invalid type

NullPointerException - if the key or value is null and null is invalid

size

int size()

Gets the number of keys in this map.

Implementations typically return only the count of keys in the map This cannot be mandated due to backwards compatability of this interface.

Specified by:

size in interface Map

Returns:

the number of key-collection mappings in this map

get

Object get(Object key)

Gets the collection of values associated with the specified key.

The returned value will implement Collection. Implementations are free to declare that they return Collection subclasses such as List or Set.

Implementations typically return null if no values have been mapped to the key, however the implementation may choose to return an empty collection.

Implementations may choose to return a clone of the internal collection.

Specified by:

get in interface Map

Parameters:

key - the key to retrieve

Returns:

the Collection of values, implementations should return null for no mapping, but may return an empty collection

Throws:

ClassCastException - if the key is of an invalid type

NullPointerException - if the key is null and null keys are invalid

containsValue

boolean containsValue(Object value)

Checks whether the map contains the value specified.

Implementations typically check all collections against all keys for the value. This cannot be mandated due to backwards compatability of this interface.

Specified by:

containsValue in interface Map

Parameters:

value - the value to search for

Returns:

true if the map contains the value

Throws:

ClassCastException - if the value is of an invalid type

NullPointerException - if the value is null and null value are invalid

put

V put(Object key,
      Object value)

Adds the value to the collection associated with the specified key.

Unlike a normal Map the previous value is not replaced. Instead the new value is added to the collection stored against the key. The collection may be a List, Set or other collection dependent on implementation.

Specified by:

put in interface Map

Parameters:

key - the key to store against

value - the value to add to the collection at the key

Returns:

typically the value added if the map changed and null if the map did not change

Throws:

UnsupportedOperationException - if the map is unmodifiable

ClassCastException - if the key or value is of an invalid type

NullPointerException - if the key or value is null and null is invalid

IllegalArgumentException - if the key or value is invalid

remove

Object remove(Object key)

Removes all values associated with the specified key.

Implementations typically return null from a subsequant get(Object), however they may choose to return an empty collection.

Specified by:

remove in interface Map

Parameters:

key - the key to remove values from

Returns:

the Collection of values removed, implementations should return null for no mapping found, but may return an empty collection

Throws:

UnsupportedOperationException - if the map is unmodifiable

ClassCastException - if the key is of an invalid type

NullPointerException - if the key is null and null keys are invalid

values

Collection<V> values()

Gets a collection containing all the values in the map.

Inplementations typically return a collection containing the combination of values from all keys. This cannot be mandated due to backwards compatability of this interface.

Specified by:

values in interface Map

Returns:

a collection view of the values contained in this map