Question

I know abstract fields do not exist in java. I also read this question but the solutions proposed won’t solve my problem. Maybe there is no solution, but it’s worth asking 🙂

Problem

I have an abstract class that does an operation in the constructor depending on the value of one of its fields.
The problem is that the value of this field will change depending on the subclass.
How can I do so that the operation is done on the value of the field redefined by the subclass ?

If I just “override” the field in the subclass the operation is done on the value of the field in the abstract class.

I’m open to any solution that would ensure that the operation will be done during the instantiation of the subclass (ie putting the operation in a method called by each subclass in the constructor is not a valid solution, because someone might extend the abstract class and forget to call the method).

Also, I don’t want to give the value of the field as an argument of the constructor.

Is there any solution to do that, or should I just change my design ?

---

Edit:

My subclasses are actually some tools used by my main program, so the constructor has to be public and take exactly the arguments with which they will be called:

tools[0]=new Hand(this);
tools[1]=new Pencil(this);
tools[2]=new AddObject(this);

(the subclasses are Hand, Pencil and AddObject that all extend the abstract class Tool)

That’s why I don’t want to change the constructor.

The solution I’m about to use is to slightly change the above code to:

tools[0]=new Hand(this);
tools[0].init();
tools[1]=new Pencil(this);
tools[1].init();
tools[2]=new AddObject(this);
tools[2].init();

and use an abstract getter to acces the field.

Answer 1

How about abstract getter/setter for field?

abstract class AbstractSuper {
    public AbstractSuper() {
        if (getFldName().equals("abc")) {
            //....
        }
    }

    abstract public void setFldName();
    abstract public String getFldName();
}

class Sub extends AbstractSuper {
    @Override
    public void setFldName() {
        ///....
    }

    @Override
    public String getFldName() {
        return "def";
    }
}