Singleton using enum

Question

Is this the correct implementation of a singleton using enum?

public class Item3 {
public static void main(String[] args) {
 Singleton s=Singleton.Single.INSTANCE.getInstance();
 Singleton s2=Singleton.Single.INSTANCE.getInstance();
 System.out.printf("%b",s==s2);
}
}
class Singleton {
 enum Single{
 INSTANCE;
 Singleton s=new Singleton();
 public Singleton getInstance(){
 if(s==null)
 return new Singleton();
 else return s;
 }
 }
}

Answer 1

No, it is simpler to code than that:

enum Singleton
{
 INSTANCE;
 // instance vars, constructor
 private final Connection connection;
 Singleton()
 {
 // Initialize the connection
 connection = DB.getConnection();
 }
 // Static getter
 public static Singleton getInstance()
 {
 return INSTANCE;
 }
 public Connection getConnection()
 {
 return connection;
 }
}

Then you use can use final Singleton s = Singleton.getInstance(). Note however that since this is an enum you can always access this via Singleton.INSTANCE.

And NEVER do that:

public Singleton getInstance(){
 if(s==null)
 return new Singleton();
 else return s;
}

this is not thread safe! What is more, the new Singleton() value is never assigned to s... (thanks @DaveJarvis for noticing this!)

• Nishant: thanks. if I need to obtain a Database connection using singleton in this fashion then I would write enum Singleton{ INSTANCE; Connection c=DB.getConnection(); } and use Connection con = Singleton.INSTANCE.c in client code? Please correct me if my understanding is not correct
• fge: See my edit. While you could make the connection available as you say, it is something I would not recommend personally.
• SSC: @fge I know that this will help avoiding multiple objects creation from reflection and deserialization and cloning but will it also avoid creating multiple instances from threading?
• Mario: What if you're initialization throws an exception?
• fge: @Mario then try { ... } catch (Whatever e) { throw new ExceptionInInitializererror(e) }

Answer 2

public enum Singleton{
 INSTANCE;
}

This is enough; you can directly use Singleton.INSTANCE to get the instance of the class.

Answer 3

Your response and other samples in Singleton_pattern

public enum Singleton {
 INSTANCE;
 public void execute (String arg) {
 //... perform operation here ...
 }
}