We have two stand alone Java classes. Each is working fine now. Similar
like this:

public Sub {
    ChildA person = new ChildA();

    person.doA();
}

public Sub2 {
    ChildB person = new ChildB();

    person.doB();
}

Now, we hope to refactor the code. We see Sub and Sub2 have something in
common and want to extract these common things and put them in an
abstract class Base. But doA() is only available for ChildA, doB() is
only available for ChildB. My boss gave me the layout:

public abstract Base {
    protected Person _person;
    ...
}

public Sub extends Base {
    _person = new ChildA();

    _person.doA();  //WRONG !!!! error message: doA() is unresolved

}

public Sub2 extends Base {
    _person = new ChildB();

    _person.doB();  //also wrong!!!

}

After I finished the code, I just found out the errors. This is what I
am asking for the newsgroup.