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package org.apache.commons.collections;

import java.util.Collection;
import java.util.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.

Author:Christopher Berry, James Strachan, Stephen Colebourne
Since:Commons Collections 2.0
Version:$Revision: 646777 $ $Date: 2008-04-10 14:33:15 +0200 (Thu, 10 Apr 2008) $
/** * Defines a map that holds a collection of values against each key. * <p> * A <code>MultiMap</code> 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. * <p> * For example: * <pre> * MultiMap mhm = new MultiHashMap(); * mhm.put(key, "A"); * mhm.put(key, "B"); * mhm.put(key, "C"); * Collection coll = (Collection) mhm.get(key);</pre> * <p> * <code>coll</code> will be a collection containing "A", "B", "C". * <p> * 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 <code>Map</code> anyway. * * @since Commons Collections 2.0 * @version $Revision: 646777 $ $Date: 2008-04-10 14:33:15 +0200 (Thu, 10 Apr 2008) $ * * @author Christopher Berry * @author James Strachan * @author Stephen Colebourne */
public interface MultiMap extends Map {
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.

Params:
  • key – the key to remove from
  • item – the item to remove
Throws:
Returns:the value removed (which was passed in), null if nothing removed
/** * Removes a specific value from map. * <p> * The item is removed from the collection mapped to the specified key. * Other values attached to that key are unaffected. * <p> * If the last value for a key is removed, implementations typically * return <code>null</code> from a subsequant <code>get(Object)</code>, however * they may choose to return an empty collection. * * @param key the key to remove from * @param item the item to remove * @return the value removed (which was passed in), null if nothing removed * @throws UnsupportedOperationException if the map is unmodifiable * @throws ClassCastException if the key or value is of an invalid type * @throws NullPointerException if the key or value is null and null is invalid */
public Object remove(Object key, Object item); //-----------------------------------------------------------------------
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
/** * Gets the number of keys in this map. * <p> * Implementations typically return only the count of keys in the map * This cannot be mandated due to backwards compatability of this interface. * * @return the number of key-collection mappings in this map */
int size();
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.

Params:
  • key – the key to retrieve
Throws:
Returns:the Collection of values, implementations should return null for no mapping, but may return an empty collection
/** * Gets the collection of values associated with the specified key. * <p> * The returned value will implement <code>Collection</code>. Implementations * are free to declare that they return <code>Collection</code> subclasses * such as <code>List</code> or <code>Set</code>. * <p> * Implementations typically return <code>null</code> if no values have * been mapped to the key, however the implementation may choose to * return an empty collection. * <p> * Implementations may choose to return a clone of the internal collection. * * @param key the key to retrieve * @return the <code>Collection</code> of values, implementations should * return <code>null</code> for no mapping, but may return an empty collection * @throws ClassCastException if the key is of an invalid type * @throws NullPointerException if the key is null and null keys are invalid */
Object get(Object key);
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.

Params:
  • value – the value to search for
Throws:
Returns:true if the map contains the value
/** * Checks whether the map contains the value specified. * <p> * Implementations typically check all collections against all keys for the value. * This cannot be mandated due to backwards compatability of this interface. * * @param value the value to search for * @return true if the map contains the value * @throws ClassCastException if the value is of an invalid type * @throws NullPointerException if the value is null and null value are invalid */
boolean containsValue(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.

Params:
  • key – the key to store against
  • value – the value to add to the collection at the key
Throws:
Returns:typically the value added if the map changed and null if the map did not change
/** * Adds the value to the collection associated with the specified key. * <p> * Unlike a normal <code>Map</code> the previous value is not replaced. * Instead the new value is added to the collection stored against the key. * The collection may be a <code>List</code>, <code>Set</code> or other * collection dependent on implementation. * * @param key the key to store against * @param value the value to add to the collection at the key * @return typically the value added if the map changed and null if the map did not change * @throws UnsupportedOperationException if the map is unmodifiable * @throws ClassCastException if the key or value is of an invalid type * @throws NullPointerException if the key or value is null and null is invalid * @throws IllegalArgumentException if the key or value is invalid */
Object put(Object key, Object value);
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.

Params:
  • key – the key to remove values from
Throws:
Returns:the Collection of values removed, implementations should return null for no mapping found, but may return an empty collection
/** * Removes all values associated with the specified key. * <p> * Implementations typically return <code>null</code> from a subsequant * <code>get(Object)</code>, however they may choose to return an empty collection. * * @param key the key to remove values from * @return the <code>Collection</code> of values removed, implementations should * return <code>null</code> for no mapping found, but may return an empty collection * @throws UnsupportedOperationException if the map is unmodifiable * @throws ClassCastException if the key is of an invalid type * @throws NullPointerException if the key is null and null keys are invalid */
Object remove(Object key);
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
/** * Gets a collection containing all the values in the map. * <p> * Inplementations typically return a collection containing the combination * of values from all keys. * This cannot be mandated due to backwards compatability of this interface. * * @return a collection view of the values contained in this map */
Collection values(); }