Java: how to initialize child when super constructor requires parameter

Question

I have

class A {

    int var;
    public A(int x) {
        var = x;
    }
}


class B extends A {
     int var2;

     public B(int x, int y) {
         super(...);
         var2 = y;
         x = f(y);
     }
 }

For the subclass B, I need to calculate the value x that is used in the constructor of A. If I were free to move super below my x=f(y) then I could pass in the result to the constructor of A (super). But super has to be the first line in the constructor of B.

Is there any way to initialize A with the proper value the first time? What if A.var were final and i couldn't go back and change it after construction?

Sure, I could put super(f(y)), but I could imagine cases where this would become difficult.

Answer 1

Assuming var is private and you need to set the value with the constructor (which seems to be the point of the question, otherwise there are many easy solutions), I would just do it with a static factory-like method.

class B extends A {
     int var2;

     public static B createB(int x, int y) {
         x = f(y);
         return new B(x, y);
     }

     public B(x, y) {
         super(x);
         this.var2 = y;
     }
 }

something like that. You have no choice, as explicit constructor invocation must happen on the first line of the wrapping constructor.

Answer 2

You can do it like this :

class B extends A {
    int var2;

    public B(int x, int y) {
        super(calculateX(y));
        var2 = y;
    }

    private static int calculateX(int y) {
        return y;
    }
}

Calling a static method is the only thing you can do before calling the superclass constructor.