java - 可以在下面的代码中使用多态性代替 if/else

Question

我应该在条件语句上使用多态性，但我可以在下面的情况下使用它吗？

下面的代码根据 jsonObject 的内容返回一个 Screen 对象。

public static Screen getNextScreen(){

        JSONObject jsonObject = RatingsUtils.getCurrentJsonObjectFromServer();

        if(isNextProgramScreen(jsonObject)) {
            ParentRatingsObject parentRatingsObject = JsonBusinessObjectFactory.createParentRatingsObject(jsonObject);
            return new NextProgramScreen(parentRatingsObject);
        } 
        else if(isTimerScreen(jsonObject)) {
            ChildWithParentRatingsObject childWithParentRatingsObject = JsonBusinessObjectFactory.createChildWithParentRatingsObject(jsonObject);
            return new TimerScreen(childWithParentRatingsObject);
        } 
        else if(isNextContestantPreJudgeScreen(jsonObject)) {
            ChildWithParentRatingsObject childWithParentRatingsObject = JsonBusinessObjectFactory.createChildWithParentRatingsObject(jsonObject);
            return new NextContestantPreJudgingScreen(childWithParentRatingsObject);
        } 
        else if(isNextContestantJudgeScreen(jsonObject)) {
            ChildWithParentRatingsObject childWithParentRatingsObject = JsonBusinessObjectFactory.createChildWithParentRatingsObject(jsonObject);
            return new TimerScreen(childWithParentRatingsObject);
        } 
        else {
            return null;
        }
}

Answer 1

当然。我自己更经常地采用多态方法，我真的很喜欢它。由于 Java 语言，这看起来有点臃肿。我们真的需要 lambda 表达式来正确地做到这一点!

private static List<ScreenProvider> screenProviders = screenProviders();

public static Screen getNextScreen(JSONObject jsonObject) {
    for (ScreenProvider screenProvider : screenProviders) {
        if (screenProvider.supports(jsonObject)) {
            return screenProvider.getScreen(jsonObject);
        }
    }
    return null;
}

interface ScreenProvider {
    boolean supports(JSONObject jsonObject);
    Screen getScreen(JSONObject jsonObject);
}

private static List<ScreenProvider> screenProviders() {
    return Arrays.asList(
            new ScreenProvider() {
                public boolean supports(JSONObject jsonObject) {
                    return isNextProgramScreen(jsonObject);
                }

                public Screen getScreen(JSONObject jsonObject) {
                    ParentRatingsObject parentRatingsObject = JsonBusinessObjectFactory.createParentRatingsObject(jsonObject);
                    return new NextProgramScreen(parentRatingsObject);
                }
            },
            new ScreenProvider() {
                public boolean supports(JSONObject jsonObject) {
                    return isTimerScreen(jsonObject);
                }

                public Screen getScreen(JSONObject jsonObject) {
                    ChildWithParentRatingsObject childWithParentRatingsObject = JsonBusinessObjectFactory.createChildWithParentRatingsObject(jsonObject);
                    return new TimerScreen(childWithParentRatingsObject);
                }
            },
            new ScreenProvider() {
                public boolean supports(JSONObject jsonObject) {
                    return isNextContestantPreJudgeScreen(jsonObject);
                }

                public Screen getScreen(JSONObject jsonObject) {
                    ChildWithParentRatingsObject childWithParentRatingsObject = JsonBusinessObjectFactory.createChildWithParentRatingsObject(jsonObject);
                    return new NextContestantPreJudgingScreen(childWithParentRatingsObject);
                }
            },
            new ScreenProvider() {
                public boolean supports(JSONObject jsonObject) {
                    return isNextContestantJudgeScreen(jsonObject);
                }

                public Screen getScreen(JSONObject jsonObject) {
                    ChildWithParentRatingsObject childWithParentRatingsObject = JsonBusinessObjectFactory.createChildWithParentRatingsObject(jsonObject);
                    return new TimerScreen(childWithParentRatingsObject);
                }
            }
    );
}

这种方法的一个巨大好处是 getNextScreen() 方法的简单性。考虑一下，可能可以使 screenProviders() 方法更紧凑——也许可以通过添加一个实现 ScreenProvider 的抽象类来完成一些工作。