Calling Generic Java function at the Abstract level

Question

Say I have an Abstract generic class:

public abstract class QuestionObject<T extends Answerable<?>> {   
    public abstract T returnAnswerable();
    public abstract void renderQuestionLayout(T answer);
}

and a companion generic class to go with it:

public interface Answerable<T> {
...
}

I have a bunch of subclasses that extend QuestionObject and have their own particular Answerable. So for example, a TrueFalseQuestion would be declared with a TrueFalseAnswer:

public class TrueFalseQuestion extends QuestionObject<TrueFalseAnswer> {
...
}

Let's saw I want to call the method renderQuestionLayout at the QuestionObject level. Is that possible? Something like:

QuestionObject<?> q = getCurrentQuestion();
Answerable<?>a = getAnswer();
q.renderQuestionLayout(a);

doesn't work.

sandymatt · Accepted Answer · 2013-11-19 06:48:52Z

1

Look at your last code snippet:

QuestionObject<?> q = getCurrentQuestion();
Answerable<?>a = getAnswer();
q.renderQuestionLayout(a);

The ? wildcard does not help the compiler determine which version of renderQuestionLayout to call. Right now, q is an instance your abstract parent class where the method has an abstract declaration.

You'll need to explicitly cast to the appropriate subclass type, or rethink your design a little bit so you don't have casts everywhere :p

answered Nov 19, 2013 at 6:48

sandymatt

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2 Comments

IcedDante Over a year ago

I figured this might be the answer (explicitly casting). Do you think this is an example of bad design? What would a redesign look like? Thx.

sandymatt Over a year ago

Well, if you have too many subclasses, doing the casts everywhere can get pretty nasty. My suggestion would be to use a factory of some kind to get instances of the particular subclass that you need.

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