在java中增加工厂模式

问题描述

我试图使用工厂模式创建一个QuestionTypeFactory，其中实例化的类将像MultipleChoice，TrueFalseQuestion等。

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

出厂代码看起来像这样

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.");
}
}

现在可以正常工作。如果我想添加另一个问题类型，我将需要修改工厂类，我不想这样做。

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.

我如何设置它，以便每个问题类都向Factory注册，这样当我添加一个新的问题类型时，我不必更改代码工厂？我对java有点新鲜感，不知道该怎么做。

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.

附加信息

所有问题类都实现了一个IQuestion界面。我正在寻找一种方法来实现一个方法，如

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)

所以我可以从我的类的静态块调用这个方法，这样当我添加一个新的问题键入，我不必在问题工厂中更改或添加任何代码。我知道我将不得不改变当前的实现，使其成为通用的。我不知道我上面写的方法在语法上是不正确的，但它显示了我想要的概念。

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.

推荐答案

您可以通过Reflection API（即 Class thingy）使用您显示的注册方法来执行此操作。

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

我不太熟练地使用Java Reflection编写更有用的答案，但是如果你寻找一些 getConstructor 方法或者你可能会得到的东西

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.

要调用该方法，您应该执行以下操作（请注意 .class 语法）：

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

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

编辑啊，无论如何，我有时间调查。尝试这样：

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();
    }
}

我没有编译这个，但它应该工作，或者至少指导你正确的方向。为了这个工作，Question实现必须有一个没有参数的构造函数。

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.

因为你使用一个静态工厂（也就是面向对象的全局变量），你可以提出问题注册自己的。

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
}

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