Overriding parent class variable with extended child variable

Question

I am trying to extend class which has a variable that I also need to be extended. Is there a "good" way of achieving this?

I have extended parent class and a variable that is in the parent class. Now I need to use extended variable in child class. Here is pseudo-code:

public class Parent {

    private ParentVariable variableToUse;
    //getters, setters
}

public class ParentVariable {
    //some fields
}

public class ChildVariable extends ParentVariable {
    //more fields
}

public class Child extends Parent {

    private ChildVariable variableToUse;
    //getters, setters
}

How should I override "variableToUse" so that every-time I try to access this variable from "Child" I would access "ChildVariable" instead of "ParentVariable" ?

Answer 1

You will be effectively hiding the parent variable here by including "variableToUse" in the child class.

As variableToUse is of the type ParentVariable anyway, i suggest you leave it out of the ChildClass altogether, and use a getter which ensures type. Eg -

public class Child extends Parent {

    //private ChildVariable variableToUse; -> remove this

    private ChildVariable checkAndGetChildVariable() {
          if(variableToUse instanceof ChildVariable) {
                return (ChildVariable)variableToUse;
          }
          return null;//Or throw exception
    }
}

For further clarity, ensure this in the constructor

public class Child extends Parent {

    //private ChildVariable variableToUse; -> remove this
    public Child(ParentVariable variable) {
          super(checkChildVariable(variable));
    }

    private static ChildVariable checkChildVariable(ParentVariable variable) {
          if(variable instanceof ChildVariable) {
                return (ChildVariable)variable;
          }
          throw UnsupportedOperation("invalid type");
    }
}