Hazelcast Set is a distributed and concurrent implementation of java.util.Set.

  • Hazelcast Set does not allow duplicate elements.
  • Hazelcast Set does not preserve the order of elements.
  • Hazelcast Set is a non-partitioned data structure--all the data that belongs to a set will live on one single partition in that member.
  • Hazelcast Set cannot be scaled beyond the capacity of a single machine. Since the whole set lives on a single partition, storing a large amount of data on a single set may cause memory pressure. Therefore, you should use multiple sets to store a large amount of data. This way, all the sets will be spread across the cluster, sharing the load.
  • A backup of Hazelcast Set is stored on a partition of another member in the cluster so that data is not lost in the event of a primary member failure.
  • All items are copied to the local member and iteration occurs locally.
  • The equals method implemented in Hazelcast Set uses a serialized byte version of objects, as opposed to java.util.HashSet.

Getting a Set and Putting Items

Use the HazelcastInstance getSet method to get the Set, then use the set put method to put items into the Set.

import com.hazelcast.core.Hazelcast;
import java.util.Set;
import java.util.Iterator;

HazelcastInstance hazelcastInstance = Hazelcast.newHazelcastInstance();

Set<Price> set = hazelcastInstance.getSet( "IBM-Quote-History" );
set.add( new Price( 10, time1 ) );
set.add( new Price( 11, time2 ) );
set.add( new Price( 12, time3 ) );
set.add( new Price( 11, time4 ) );
Iterator<Price> iterator = set.iterator();
while ( iterator.hasNext() ) { 
  Price price = iterator.next(); 

Hazelcast Set uses ItemListener to listen to events that occur when items are added to and removed from the Set. Please refer to the Listening for Item Events section for information on how to create an item listener class and register it.

Configuring Set

The following are the example set configurations.


<set name="default">


Config config = new Config();
CollectionConfig collectionSet = config.getCollectionConfig();
collectionSet.setName( "MySet" ).setBackupCount( "1" )
        .setMaxSize( "10" );

Set configuration has the following elements.

  • statistics-enabled: True (default) if statistics gathering is enabled on the Set, false otherwise.
  • backup-count: Count of synchronous backups. Set is a non-partitioned data structure, so all entries of a Set reside in one partition. When this parameter is '1', it means there will be one backup of that Set in another member in the cluster. When it is '2', two members will have the backup.
  • async-backup-count: Count of asynchronous backups.
  • max-size: The maximum number of entries for this Set.
  • item-listeners: Lets you add listeners (listener classes) for the list items. You can also set the attributes include-value to true if you want the item event to contain the item values, and you can set local to true if you want to listen to the items on the local member.