18
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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;
    }
    }
}
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27
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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!)

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6
  • \$\begingroup\$ 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 \$\endgroup\$ – Nishant Jun 12 '13 at 10:30
  • \$\begingroup\$ See my edit. While you could make the connection available as you say, it is something I would not recommend personally. \$\endgroup\$ – fge Jun 12 '13 at 10:37
  • \$\begingroup\$ @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? \$\endgroup\$ – SSC Apr 12 '15 at 15:39
  • \$\begingroup\$ What if you're initialization throws an exception? \$\endgroup\$ – Mario Oct 14 '15 at 20:55
  • \$\begingroup\$ @Mario then try { ... } catch (Whatever e) { throw new ExceptionInInitializererror(e) } \$\endgroup\$ – fge Oct 15 '15 at 6:30
7
\$\begingroup\$ 
public enum Singleton{
   INSTANCE;
}

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

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0
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Your response and other samples in Singleton_pattern 

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