org.apache.commons.collections

Class MultiHashMap

Implemented Interfaces:
Map, MultiMap

public class MultiHashMap
extends HashMap
implements MultiMap

MultiHashMap is the default implementation of the MultiMap interface.

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.

This implementation uses an ArrayList as the collection. The internal storage list is made available without cloning via the get(Object) and entrySet() methods. The implementation returns null when there are no values mapped to a key.

For example:

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

list will be a list containing "A", "B", "C".

Version:
$Revision: 1.20 $ $Date: 2004/06/09 22:11:54 $
Authors:
Christopher Berry
James Strachan
Steve Downey
Stephen Colebourne
Julien Buret
Serhiy Yevtushenko
Since:
Commons Collections 2.0

Constructor Summary

MultiHashMap()
Constructor.
MultiHashMap(Map mapToCopy)
Constructor that copies the input map creating an independent copy.
MultiHashMap(int initialCapacity)
Constructor.
MultiHashMap(int initialCapacity, float loadFactor)
Constructor.

Method Summary

void
clear()
Clear the map.
Object
clone()
Clones the map creating an independent copy.
boolean
containsValue(Object value)
Checks whether the map contains the value specified.
boolean
containsValue(Object key, Object value)
Checks whether the collection at the specified key contains the value.
protected Collection
createCollection(Collection coll)
Creates a new instance of the map value Collection container.
Collection
getCollection(Object key)
Gets the collection mapped to the specified key.
Iterator
iterator(Object key)
Gets an iterator for the collection mapped to the specified key.
Object
put(Object key, Object value)
Adds the value to the collection associated with the specified key.
boolean
putAll(Object key, Collection values)
Adds a collection of values to the collection associated with the specified key.
Object
remove(Object key, Object item)
Removes a specific value from map.
int
size(Object key)
Gets the size of the collection mapped to the specified key.
int
totalSize()
Gets the total size of the map by counting all the values.
Collection
values()
Gets a collection containing all the values in the map.

Constructor Details

MultiHashMap

public MultiHashMap()
Constructor.

MultiHashMap

public MultiHashMap(Map mapToCopy)
Constructor that copies the input map creating an independent copy.

This method performs different behaviour depending on whether the map specified is a MultiMap or not. If a MultiMap is specified, each internal collection is also cloned. If the specified map only implements Map, then the values are not cloned.

NOTE: From Commons Collections 3.1 this method correctly copies a MultiMap to form a truly independent new map.

Parameters:
mapToCopy - a Map to copy

MultiHashMap

public MultiHashMap(int initialCapacity)
Constructor.
Parameters:
initialCapacity - the initial map capacity

MultiHashMap

public MultiHashMap(int initialCapacity,
                    float loadFactor)
Constructor.
Parameters:
initialCapacity - the initial map capacity
loadFactor - the amount 0.0-1.0 at which to resize the map

Method Details

clear

public void clear()
Clear the map.

This clears each collection in the map, and so may be slow.


clone

public Object clone()
Clones the map creating an independent copy.

The clone will shallow clone the collections as well as the map.

Returns:
the cloned map

containsValue

public boolean containsValue(Object value)
Checks whether the map contains the value specified.

This checks all collections against all keys for the value, and thus could be slow.

Specified by:
containsValue in interface MultiMap
Parameters:
value - the value to search for
Returns:
true if the map contains the value

containsValue

public boolean containsValue(Object key,
                             Object value)
Checks whether the collection at the specified key contains the value.
Parameters:
value - the value to search for
Returns:
true if the map contains the value
Since:
Commons Collections 3.1

createCollection

protected Collection createCollection(Collection coll)
Creates a new instance of the map value Collection container.

This method can be overridden to use your own collection type.

Parameters:
coll - the collection to copy, may be null
Returns:
the new collection

getCollection

public Collection getCollection(Object key)
Gets the collection mapped to the specified key. This method is a convenience method to typecast the result of get(key).
Parameters:
key - the key to retrieve
Returns:
the collection mapped to the key, null if no mapping
Since:
Commons Collections 3.1

iterator

public Iterator iterator(Object key)
Gets an iterator for the collection mapped to the specified key.
Parameters:
key - the key to get an iterator for
Returns:
the iterator of the collection at the key, empty iterator if key not in map
Since:
Commons Collections 3.1

put

public 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.

Specified by:
put in interface MultiMap
Parameters:
key - the key to store against
value - the value to add to the collection at the key
Returns:
the value added if the map changed and null if the map did not change

putAll

public boolean putAll(Object key,
                      Collection values)
Adds a collection of values to the collection associated with the specified key.
Parameters:
key - the key to store against
values - the values to add to the collection at the key, null ignored
Returns:
true if this map changed
Since:
Commons Collections 3.1

remove

public 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, null will be returned from a subsequant get(key).

Specified by:
remove in interface MultiMap
Parameters:
key - the key to remove from
item - the value to remove
Returns:
the value removed (which was passed in), null if nothing removed

size

public int size(Object key)
Gets the size of the collection mapped to the specified key.
Parameters:
key - the key to get size for
Returns:
the size of the collection at the key, zero if key not in map
Since:
Commons Collections 3.1

totalSize

public int totalSize()
Gets the total size of the map by counting all the values.
Returns:
the total size of the map counting all values
Since:
Commons Collections 3.1

values

public Collection values()
Gets a collection containing all the values in the map.

This returns a collection containing the combination of values from all keys.

Specified by:
values in interface MultiMap
Returns:
a collection view of the values contained in this map

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