Re: an inheritance question

From:

"Ingo R. Homann" <ihomann_spam@web.de>

Newsgroups:

comp.lang.java.programmer

Date:

Wed, 23 May 2007 16:19:45 +0200

Message-ID:

<46544d80$0$6392$9b4e6d93@newsspool2.arcor-online.net>

Hi,

www wrote:

Hi,

I am puzzled here by an inheritance question:

I have 3 classes:

public Parent {
    ...
}

public ChildA extends Parent {
    ...
    void doA() {
        ...
    }
}

public ChildB extends Parent {
    ...
    void doB() {
        ...
    }
}

Now, I am trying to use the classes above.

public Base {
    protected _person;
    ...
}

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

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

}

I ran into this problem in my real application. What should I do?

It depends:

(a) doA() only makes sense on Objects of type ChildA. Then use a proper
declaration:
ChildA person=new ChildA();
person.doA();

(b) doA() makes sense on both ChildA and ChildB. Then declare the method
in the superclass "Parent".

(c) doA() makes sense for both classes but should not do anything for
objects of type ChildB. Note that this is a special case of (b) and
should be handled as described above.

(d) Your whole design is broken. Then a quick and dirty solution would
be to do "
> Some

type casting to force _person as ChildA ?

". But of course the question is: What do you except the program to do
if the variable person indeed refers to an object of type ChildB?
Is an abnormal termination of the program OK for you? (That's why your
design is broken!)

Ciao,
Ingo