why does the subclass have to invoke the no args constructor in the super class?

Question

On the piece of code below, I understand super(t) in the subclass is explicitly invoking a no-args constructor in its superclass (class B). What I seem to be having problems understanding is why does the subclass have to invoke the no-args constructor in the superclass? I can't seem to work out the purpose of this?

public class Test {

    public static void main(String[] args) {

       B b = new B(5);

    }
}
class A extends B {
    public A(int t)  {
        super(t);
        System.out.println("A's constructor is invoked");
    }
}

class B  {



    public B(int k)   {
    System.out.println("B's constructor is invoked");
    }
}

Answer 1

Super must be instantiated before sub like parent must exists before child .

I understand super(t) in the subclass is explicitly invoking a no-args constructor in its >superclass (class B)

super(t) actually super(5) will invoke public B(int k) .
super() try to invoke public B() which does not exist.

Note: compiler provides no-arg constructor if there is no constructor available in the class.

Answer 2

super(t) in the subclass is explicitly invoking a no-args constructor in its superclass (class B).

No, super(t) can be rewritten as B(t) . It is invoking the constructor you have shown in the code.

I believe what you are thinking about is

If a constructor does not explicitly invoke a superclass constructor, the Java compiler automatically inserts a call to the no-argument constructor of the superclass.

From http://docs.oracle.com/javase/tutorial/java/IandI/super.html

ie If the code did not do super(t) the default no-args constructor would be called, However

If the super class does not have a no-argument constructor, you will get a compile-time error

Answer 3

Your class B does not have a no-args constructor (that would be one without arguments). If it had one, it would be called automatically by the constructors of A . But in the example you have to call it manually with super(t) in order to specify the argument(s) that should be used.