Question

class A {
     A(int i){    
        System.out.println("A(int)");
     }
 }

class B1 extends A{
    public static void main(String args[]){
        A ob=new A(2);  
    }
}

Answer 1

You have declared a 1-arg constructor in class A. So, compiler does not provide a default 0-arg constructor.

Now, in class B1, you have not defined any constructor, so compiler adds a default 0-arg constructor in that class, which looks like this: –

public B1() {
    super();
}

As you can see, Compiler adds a super() call to invoke the 0-arg constructor of the super class, which is class A, in this case.

Now, since your class A, does not have any 0-arg constructor, hence the error.

---

So, either you can add a 0-arg constructor in your class A: –

public A() {

}

This will solve the issue.

---

Or, add a 0-arg constructor in your class B1 explicitly, and add a super() call to the 1-arg constructor of class A: –

public B1() {
    super(0);  // Or any value
}

---

But, the problem in the 2nd solution will be that, from every constructor in your class B1, you would have to invoke the 1-arg constructor of class A explicitly. As soon as you miss one, you will get a compiler error immediately.

So, I would suggest to go with the 1st option. Add a 0-arg constructor in class A. And you are all good.