singleton using enum

Question

I read a lot on stackoverflow regarding the creation of singleton classes using enum. I must have missed something because i can't reach the INSTANCE anywhere.

this is my code:

public class UserActivity {      private DataSource _dataSource;     private JdbcTemplate _jdbcTemplate;      static enum Singleton {         INSTANCE;          private static final UserActivity singleton = new UserActivity();          public UserActivity getSingleton() {             return singleton;         }     }      public UserActivity() {         this._dataSource = MysqlDb.getInstance().getDataSource();         this._jdbcTemplate = new JdbcTemplate(this._dataSource);     }      public void dostuff() {      ...     } } 

and outside I'm trying to do

UserActivity.INSTANCE.getSingleton() 

or

UserActivity.Singleton. 

but eclipse's code completion doesn't find anything

thanks!

Answer 1

The trick is to make the enum itself the singleton. Try this:

public enum UserActivity {     INSTANCE;      private DataSource _dataSource;     private JdbcTemplate _jdbcTemplate;      private UserActivity() {         this._dataSource = MysqlDb.getInstance().getDataSource();         this._jdbcTemplate = new JdbcTemplate(this._dataSource);     }      public void dostuff() {      ...     } }  // use it as ... UserActivity.INSTANCE.doStuff(); 

Answer 2

INSTANCE is a member of Singleton, not of UserActivity - so you'd need:

UserActivity.Singleton.INSTANCE.getSingleton(); 

However, you haven't actually made UserActivity a singleton - normally you'd make the type itself an enum, not embed an enum within the type...