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 looks something like this

class QuestionFactory {
    public enum QuestionType {
        TrueFalse,
        MultipleChoice,
        Essay
    }

public static Question createQuestion(QuestionType quesType) {
    switch (quesType) {
        case TrueFalse:
            return new TrueFalseQuestion();
        case MultipleChoice:
            return new MultipleChoiceQuestion();
        case Essay:
            return new EssayQuestion();
    }
    throw new IllegalArgumentException("Not recognized.");
}
}

This works ok for now. If I want to add another question type I will need to modify the factory class and I do not want to do that.

How can I set it up so that each question class registers itself with the Factory so that when I add a new question type, I do not have to change the code for the factory? I am a bit new to java and am not sure how to do this.

Edit

Additional Information

All the question classes implement an IQuestion interface. I am looking for a way to implement a method like

public static void registerType(QuestionType quesType, Class<IQuestion> ques)

so that I can call this method from a static block from my classes so that when I add a new question type, I will not have to change or add any code in the Question Factory. I know I would have to change the current implementation to make it generic. I am not sure the method that I wrote above is correct in terms of its arguments syntactically or not but it shows what I want in concept.

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<QuestionType, Constructor<? extends Question>> map =
        new HashMap<QuestionType, Class<? extends Question>>();

    public static void registerType(QuestionType quesType, Class<? extends Question> 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
}

Answer 2

One possibility:

public enum QuestionType {
    TrueFalse(TrueFalseQuestion.class),
    MultipleChoice(MultipleChoiceQuestion.class),
    Essay(EssayQuestion.class)

    private final Class<? extends Question> implementationType;
    QuestionType(Class<? extends Question> implementationType) {
       this.implementationType = implementationType;
    }

    public Question createQuestion() { 
       return implementationType.newInstance(); 
    }
}

Of course, this gets rid of the factory and assumes all your questions have no-args constructors, but as far as I can tell, it covers all the cases of the code sketch above. If construction for the particularly classes is more complicated, you can always setup something like:

public enum QuestionType {
    TrueFalse { public Question createQuestion() { /* construction logic goes here */ } }
    public abstract Question createQuestion();
}