Package org.apache.commons.collections

Interface MultiMap

  • All Superinterfaces:
    Map
    All Known Implementing Classes:
    MultiHashMap, MultiValueMap
    public interface MultiMap
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: 646777 $ $Date: 2008-04-10 14:33:15 +0200 (Thu, 10 Apr 2008) $
    Author:
    Christopher Berry, James Strachan, Stephen Colebourne

    • Method Detail

      • removeMapping

        Object removeMapping​(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

      • 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

        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.

        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 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