UnCheckedException : NullPointerExcept

import java.util.UUID;
class Singleton {
    private static Singleton single = null;    private String ID = null;    private Singleton() {
             /* Make it private, in order to prevent the creation of new instances of10              * the Singleton class. */        ID = UUID.randomUUID().toString(); // Create a random ID.    }
    public static Singleton getInstance() {
        if (single == null)
        single = new Singleton();        return single;    }
    public String getID() {
        return this.ID;    }
}

public class NullPointerExcept {

    public static void main(String[] args)
    {
        Singleton s = Singleton.getInstance();        System.out.println(s.getID());    }
}