Question

In my coding, I use Singleton class with Singleton Design Pattern. Question is why its sub class does not allowed to use default constructor?

I get compile time error :

Implicit super constructor Singleton() is not visible. Must explicitly invoke another constructor

Singleton.java

public class Singleton {
    private static Singleton singleton;

    private Singleton() {
        System.out.println("I am user class");
    }

    public static Singleton getInstance() {
        if(singleton == null) {
            singleton = new Singleton();
        }
        return singleton;
    }

}

SubClass.java

public class SubClass extends Singleton {
    public SubClass(){
        System.out.println("I am sub class");
    }
}

Answer 1

When you create an instance of SubClass then it automatically invokes the constructor of its SuperClass to initialize its fields, and that further invokes all the superclass constructors in the inheritance hierarchy

Now since your SuperClass constructor is private, so it cannot invoke that. So, you are getting that exception..

But it doesn’t make sense to subclass a singleton class, because in that case, your class will no longer be singleton.

You should re-think about your design and what you are trying to do. And change your design accordingly.