Extending a class such that any parent class can be cast to it, in Java

Question

I have a feeling this is impossible, but if not it would be very useful.

I’m trying to extend a parent class in a way that the child class only has new methods, no new constructors, no new fields. So the underlying data structure of the child class is identical to the parent class. This tends to occur when I want to give added functions to a built in java class (e.g. Vector3d). Given that the underlying data is identical is it possible in any way to downcast an object initialized as the parent class to the child class so I can use the added functionality. As an example of what I mean see below

import javax.vecmath.Vector3d;

public class Vector3dPlus extends Vector3d{
    //same fields, same constructors as Vector3d

    public double extraMethod(){
        return x+y+z;
    }
}

Try to use the new method added to Vector3d

import javax.vecmath.Vector3d;     

public class Test {

    public static void main(String[] args) {
        Vector3d basic=new Vector3d(1,2,3);

        useExtraMethod(basic); //this line correctly raises an exception, but is there a way around that
    }

    public static void useExtraMethod(Vector3dPlus plus){
        System.out.println(plus.extraMethod());
    }
}

Clearly java gets upset with this because usually we can't guarantee that Vector3dPlus methods will work with all Vector3d. But is there anyway I can say to java that the underlying data structures are the same and so allow all downcasting from all Vector3d to Vector3dPlus.

The current way I deal with this is to put all the extra methods in a generic utilities class, but that's obviously a bit horrible

Answer 1

You can achieve it with method overloading and a copy constructor:

public class Vector3dPlus extends Vector3d {
    public Vector3dPlus(Vector3d vector) {
        super( ... ); // parameters from vector
    }

    // rest of your Vector3dPlus code
}

and in your test class, overload the method:

    public class Test {

        public static void useExtraMethod(Vector3d vector) {
            useExtraMethod(new Vector3dPlus(vector));
        }

        public static void useExtraMethod(Vector3dPlus plus){
             System.out.println(plus.extraMethod());
        }

        public static void main(String[] args) {
           Vector3d basic=new Vector3d(1,2,3);
           useExtraMethod(basic); // this line works now
        }
    }