Jakarta Commons Cookbook

5.10. Storing Multiple Values in a Map

Problem

You need to store multiple values for a single key in a Map. Instead of a one-to-one relationship between keys and values, you need a Map that provides one-to-many relationships between keys and values.

Solution

Use a MultiMap. A MultiMap maintains a collection of values for a given key. When a new key/value pair is added to a MultiMap, the value is added to the collection of values for that key. The MultiMap also provides a way to remove a specific key/value pair. The following example demonstrates the use of MultiMap:

import org.apache.commons.collections.MultiMap;
import org.apache.commons.collections.MultiHashMap;

MultiMap map = new MultiHashMap( );

map.put( "ONE", "TEST" );
map.put( "TWO", "PICNIC" );

map.put( "ONE", "HELLO" );
map.put( "TWO", "TESTIMONY" );

Set keySet = map.keySet( );
Iterator keyIterator = keySet.iterator( );
while( keyIterator.hasNext( ) ) {
    Object key = keyIterator.next( );

    System.out.print( "Key: " + key + ", " );

    Collection values = (Collection) map.get( key );
    Iterator valuesIterator = values.iterator( );
    while( valuesIterator.hasNext( ) ) {
        System.out.print( "Value: " + valuesIterator.next( ) + ". " );
    }

    System.out.print( "\n" );
}

Each key in a MultiMap corresponds to a collection of values. This example produces the following output while iterating through all values for each key:

Key: ONE, Value: TEST. Value: HELLO
Key: TWO, Value: PICNIC. Value: Testimony

Discussion

In a traditional Map, a key is removed ...