augment the factory pattern in java

Question

I am trying to use a factory pattern to create a QuestionTypeFactory where the instantiated classes will be like MultipleChoice, TrueFalseQuestion etc.

The factory code

Answer 1

You can probably do that with the register method you've shown, through the Reflection API (that Class thingy).

I am not proficient enough with Java Reflection to write a more helpful answer, but if you look for some getConstructor method or something you'll probably get there.

To call that method you should do something like (note the .class syntax):

QuestionFactory.registerType(QuestionType.TrueFalse, TrueFalseQuestion.class);

EDIT Ah, whatever, I have the time to investigate. Try this:

public class QuestionFactory {
    static final Map> map =
        new HashMap>();

    public static void registerType(QuestionType quesType, Class ques) {
        map.put(quesType, ques.getConstructor());
    }

    public static Question createQuestion(QuestionType quesType) {
        return map.get(quesType).newInstance();
    }
}

I haven't compiled this, but it should work, or at least guide you in the right direction. For this to work the Question implementations must have a constructor without arguments.

Because you're using a static factory (aka, object-oriented global variables) you can make questions register themselves in their static initializer.

public class TrueFalseQuestion implements Question {
    static {
        QuestionFactory.registerType(QuestionType.TrueFalse, TrueFalseQuestion.class);
    }
    // Whatever else goes here
}