Wiki Definition: "In software engineering, the singleton pattern is a design pattern used to implement the mathematical concept of a singleton, by restricting the instantiation of a class to one object."

Singleton.class

// Traditional Simple Way:
public class Singleton {
    private static final Singleton instance = new Singleton();
    private Singleton() {
    }
    public static Singleton getInstance() {
        return instance;
    }
}

// The solution of Bill Pugh:
public class Singleton {
    private Singleton() {
    }
    private static class SingletonHolder {
        public static final Singleton instance = new Singleton();
    }
    public static Singleton getInstance() {
        return SingletonHolder.instance;
    }
}

// The Enum-way:
public enum Singleton {
    INSTANCE;
}