Documentation Version: 3.3 - Publication Date : Sep 07, 2014

MultiMap

MultiMap is a specialized map where you can store multiple values under a single key. Just like any other distributed data structure implementation in Hazelcast, MultiMap is distributed and thread-safe.

MultiMap is not an implementation of java.util.Map due to the difference in method signatures. It supports most features of IMap except for indexing, predicates and MapLoader/MapStore. Yet, like map, entries are almost evenly distributed onto all cluster members and when a new member joins to the cluster, the same ownership logic used in distributed map applies.

Sample MultiMap Code

Let's write a code that puts data into a MultiMap.

public class PutMember {
  public static void main( String[] args ) {
    HazelcastInstance hazelcastInstance = Hazelcast.newHazelcastInstance();
    MultiMap <String , String > map = hazelcastInstance.getMultiMap( "map" );

    map.put( "a", "1" );
    map.put( "a", "2" );
    map.put( "b", "3" ); 
    System.out.println( "PutMember:Done" );
  }
}

And, now let's print the entries in this MultiMap.

public class PrintMember {
  public static void main( String[] args ) { 
    HazelcastInstance hazelcastInstance = Hazelcast.newHazelcastInstance();
    MultiMap <String, String > map = hazelcastInstance.getMultiMap( "map" );
    for ( String key : map.keySet() ){
      Collection <String > values = map.get( key );
      System.out.println( "%s -> %s\n",key, values );
    }
  }
}

After you run the first code, run the PrintMember. You will see the key a has two values, as shown below.

b -> [3]

a -> [2, 1]

MultiMap Configuration

When using MultiMap, the collection type of values can be either a Set or a List. This is configured with valueCollectionType parameter. If you choose Set, duplicate and null values are allowed in your collection and ordering is irrelevant. If you choose List, ordering is relevant and your collection can include duplicate and null values.

You can also enable statistics for your MultiMap using the statisticsEnabled parameter. If enabled, statistics can be retrieved with getLocalMultiMapStats() method.

RELATED INFORMATION

Please refer to MultiMapConfig.java for more information on configuration options.