Java Singleton Design Pattern : Questions

Question

I had an interview recently and he asked me about Singleton Design Patterns about how are they implemented and I told him that using static variables and static methods we can i

Answer 1

Query 1:

Different ways of creating Singleton

1. Normal Singleton : static initialization
2. ENUM
3. Lazy Singleton : Double locking Singleton & : Initialization-on-demand_holder_idiom singleton

Have a look at below code:

public final class Singleton{
    private static final Singleton instance = new Singleton();

    public static Singleton getInstance(){
        return instance; 
    }
    public enum EnumSingleton {
        INSTANCE;   
    }   
    public static void main(String args[]){
        System.out.println("Singleton:"+Singleton.getInstance());
        System.out.println("Enum.."+EnumSingleton.INSTANCE);
        System.out.println("Lazy.."+LazySingleton.getInstance());
    }
}
final class LazySingleton {
    private LazySingleton() {}
    public static LazySingleton getInstance() {
        return LazyHolder.INSTANCE;
    }
    private static class LazyHolder {
        private static final LazySingleton INSTANCE = new LazySingleton();
    }
}

Related SE questions:

What is an efficient way to implement a singleton pattern in Java?

Query 2:

One Singleton instance is created per ClassLoader. If you want to avoid creation of Singleton object during Serializaiton, override below method and return same instance.

private Object readResolve()  { 
    return instance; 
}

Query 3:

To achieve a cluster level Singleton among multiple servers, store this Singleton object in a distributed caches like Terracotta, Coherence etc.