Why the parameterised constructor of the super class is not called in the following code?

Question

             class demo1
            {
                demo1(int x){
                System.out.println("This is super class constructor!");
                }
            }

             public class demo extends demo1
             {
                int y;
                demo()//or if i do: demo(int y)
                {
                    super(y);
                    System.out.print(4);
                 }
                 public static void main(String args[]){
                     demo d = new demo();
                 }

Following is the error showing up

demo.java:13: error: cannot reference y before supertype constructor has been called super(y); ^ 1 error

Answer 1

The sub-class instance variable y is only initialized after the super class constructor is called. Therefore it can't be referenced in the call to the super class constructor.

Even if it was allowed, the super(y); call would make no sense in your example, since it is not passing any meaningful data to the super class constructor (since you didn't assign anything to the y member). It only makes sense to pass to the super class constructor arguments that were passed to the sub-class constructor, or constant values.

For example, the following will pass compilation:

public class Demo extends Demo1
{
    int y;
    Demo (int y) {
        super(y); // here y is the argument passed to the Demo constructor, not the
                  // instance variable of the same name
        System.out.print(4);
    }
    public static void main(String args[]) {
        Demo d = new Demo(10);
    }
}