public interface

MultiMap

implements Map<K, V>
org.apache.commons.collections.MultiMap
Known Indirect Subclasses

Class Overview

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.

Summary

Public Methods
abstract boolean containsValue(Object value)
Checks whether the map contains the value specified.
abstract Object get(Object key)
Gets the collection of values associated with the specified key.
abstract Object put(Object key, Object value)
Adds the value to the collection associated with the specified key.
abstract Object remove(Object key)
Removes all values associated with the specified key.
abstract Object remove(Object key, Object item)
Removes a specific value from map.
abstract int size()
Gets the number of keys in this map.
abstract Collection values()
Gets a collection containing all the values in the map.
[Expand]
Inherited Methods
From interface java.util.Map

Public Methods

public abstract 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.

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

public abstract 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.

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

public abstract Object 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.

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

public abstract 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.

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

public abstract Object remove (Object key, Object 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

public abstract 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.

Returns
  • the number of key-collection mappings in this map

public abstract Collection 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.

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