How should I construct a subclass using an instance of the superclass?

Question

Let's say I have the following code

public class SuperClass {
    protected int super_class_value;

    public SuperClass (int value) {
        this.super_class_value = value;
    }
}

public class Subclass extends SuperClass {
    protected int subclass_value;

    public SubClass(int super_class_value, int subclass_value) {
        super(super_class_value);
        this.subclass_value = subclass_value;
    }
}

However, now I want to be able to pass a SuperClass object into the SubClass constructor. How would I do that?

public SubClass(SuperClass super_class, int subclass_value) {
    //What do I do here?
    this.subclass_value = subclass_value;
}

Basically, I'd like to do something like this...

public SubClass(SuperClass super_calss, int subclass_value) {
    super(super_class.super_class_value);
    this.subclass_value = subclass_value;
}

But if SuperClass is more complex, I don't want to add each value to the super() call. Instead, I'd like to simply pass in an object that already exists, and use it as the super class.

I want to do this...

public SubClass(SuperClass super_class, int subclass_value) {
    super(super_class);
    this.subclass_value = subclass_value;
}

But I'm not sure if that's allowed?

---

Seems as though I can do the above, if I add a constructor to the SuperClass definition. But what would that look like?

public class SuperClass {
    protected int super_class_value;

    public SuperClass (int value) {
        this.super_class_value = value;
    }

    public SuperClass (SuperClass super_class_object) {
        //What is done here?
    }
}

Answer 1

Don't you just want to have a copy constructor for your Superclass?

public class SuperClass {
    protected int super_class_value;

    public SuperClass (int value) {
        this.super_class_value = value;
    }
    protected SuperClass (SuperClass super_class) {
        this.super_class_value = super_class.supper_class_value;
    }
}

public class Subclass extends SuperClass {
    protected int subclass_value;

    public SubClass(SuperClass super_class, int subclass_value) {
        super(super_class);
        this.subclass_value = subclass_value;
    }
}

See also a question here on SO about writing copy ctors:

Does Java have a default copy constructor (like in C++)?