Question

I am trying to create a Singleton class, which will be accessed from two other classes.Can anyone please tell me whats wrong with the following code? I am just not able to figure out!

import java.util.LinkedList;

public class MessageQueue {

    private static final LinkedList<ServerDataEvent> queue = new LinkedList<ServerDataEvent>();;

    private static MessageQueue messageQueue = null;

    /** A private Constructor prevents any other class from instantiating. */
    private MessageQueue() {
    }

    /** Static 'instance' method */
    public static MessageQueue getInstance() {
        if (MessageQueue.messageQueue == null) {
            System.out.println("Creating MessageQueue instance.");
            MessageQueue.messageQueue = new MessageQueue();
        }
        return MessageQueue.messageQueue;
    }

    public Object clone() throws CloneNotSupportedException {
        throw new CloneNotSupportedException();
    }
}

I am accessing the singleton object from other classes like this:

MessageQueue messageQueue = MessageQueue.getInstance();

There are no errors, but

System.out.println("Creating MessageQueue instance.");

is getting executed whenever I do

MessageQueue messageQueue = MessageQueue.getInstance();

EDIT 1

import java.util.LinkedList;

public class MessageQueue {

    private static final LinkedList<ServerDataEvent> queue = new LinkedList<ServerDataEvent>();;

    private static final MessageQueue messageQueue = new MessageQueue();

    /** A private Constructor prevents any other class from instantiating. */
    private MessageQueue() {
        System.out.println("problem...");
    }

    /** Static 'instance' method */
    public static MessageQueue getInstance() {
        return MessageQueue.messageQueue;
    }

    public Object clone() throws CloneNotSupportedException {
        throw new CloneNotSupportedException();
    }
}

Answer 1

It is much better to define and instantiate your singleton object like this:

private static final MessageQueue messageQueue = new MessageQueue();

And then getInstance will be just:

public static MessageQueue getInstance() {
   return MessageQueue.messageQueue;
}

This way your singleton object is instantiated and will be thread safe because it is created by the class loader.