Can I make a derived class inherit a derived member from its base class in Java?

Question

I have code that looks like this:

public class A
{
    public void doStuff()
    {
        System.out.print("Stuff successfully done");
    }
}

public class B extends A
{
    public void doStuff()
    {
        System.out.print("Stuff successfully done, but in a different way");
    }

    public void doMoreStuff()
    {

        System.out.print("More advanced stuff successully done");
    }
}

public class AWrapper
{
    public A member;

    public AWrapper(A member)
    {
        this.member = member;
    }

    public void doStuffWithMember()
    {
        a.doStuff();
    }
}

public class BWrapper extends AWrapper
{
    public B member;

    public BWrapper(B member)
    {
        super(member);         //Pointer to member stored in two places:
        this.member = member;  //Not great if one changes, but the other does not

    }

    public void doStuffWithMember()
    {
        member.doMoreStuff();
    }
}

However, there is a problem with this code. I'm storing a reference to the member in two places, but if one changes and the other does not, there could be trouble. I know that in Java, an inherited method can narrow down its return type (and perhaps arguments, but I'm not certain) to a derived class. Is the same true of fields?

Answer 1

You can accomplish this better with generics.

public class AWrapper<T extends A>
{
    public T member;
    public AWrapper(T member)
    {
        this.member = member;
    }

    public void doStuffWithMember()
    {
        a.doStuff();
    }
}    

public class BWrapper extends Wrapper<B>
{
    public BWrapper(B member)
    {
        super(member);
    }

    public void doStuffWithMember()
    {
        member.doMoreStuff();
    }
}

The fact that the subclass wrapper specifies the type of B allows you to access B's functions in the BWrapper, without storing an additional reference.

Answer 2

In your class BWrapper you have to remove the line public B member;. And in the method doMoreStuffWithMember() replace the line with:

((B) member).doMoreStuff();