子类化 Java Builder 类 [英] Subclassing a Java Builder class

问题描述

提供这篇 Dr Dobbs 文章，特别是 Builder Pattern，我们如何处理 Builder 子类化的情况?以我们想要子类化以添加 GMO 标签的示例的简化版本，一个简单的实现将是:

Give this Dr Dobbs article, and the Builder Pattern in particular, how do we handle the case of subclassing a Builder? Taking a cut-down version of the example where we want to subclass to add GMO labelling, a naive implementation would be:

public class NutritionFacts {                                                                                                    

    private final int calories;                                                                                                  

    public static class Builder {                                                                                                
        private int calories = 0;                                                                                                

        public Builder() {}                                                                                                      

        public Builder calories(int val) { calories = val; return this; }                                                                                                                        

        public NutritionFacts build() { return new NutritionFacts(this); }                                                       
    }                                                                                                                            

    protected NutritionFacts(Builder builder) {                                                                                  
        calories = builder.calories;                                                                                             
    }                                                                                                                            
}

子类:

public class GMOFacts extends NutritionFacts {                                                                                   

    private final boolean hasGMO;                                                                                                

    public static class Builder extends NutritionFacts.Builder {                                                                 

        private boolean hasGMO = false;                                                                                          

        public Builder() {}                                                                                                      

        public Builder GMO(boolean val) { hasGMO = val; return this; }                                                           

        public GMOFacts build() { return new GMOFacts(this); }                                                                   
    }                                                                                                                            

    protected GMOFacts(Builder builder) {                                                                                        
        super(builder);                                                                                                          
        hasGMO = builder.hasGMO;                                                                                                 
    }                                                                                                                            
}

现在，我们可以写这样的代码:

Now, we can write code like this:

GMOFacts.Builder b = new GMOFacts.Builder();
b.GMO(true).calories(100);

但是，如果我们弄错了顺序，一切都会失败:

But, if we get the order wrong, it all fails:

GMOFacts.Builder b = new GMOFacts.Builder();
b.calories(100).GMO(true);

问题当然是NutritionFacts.Builder返回的是NutritionFacts.Builder，而不是GMOFacts.Builder，那么我们如何解决这个问题，或者有更好的模式可以使用吗?

The problem is of course that NutritionFacts.Builder returns a NutritionFacts.Builder, not a GMOFacts.Builder, so how do we solve this problem, or is there a better Pattern to use?

注意:这个对类似问题的回答提供了我上面的课程；我的问题是关于确保构建器调用顺序正确的问题.

Note: this answer to a similar question offers up the classes I have above; my question is regarding the problem of ensuring the builder calls are in the correct order.

推荐答案

您可以使用泛型来解决.我认为这被称为 奇怪的重复通用模式"

You can solve it using generics. I think this is called the "Curiously recurring generic patterns"

将基类构建器方法的返回类型设为通用参数.

Make the return type of the base class builder methods a generic argument.

public class NutritionFacts {

    private final int calories;

    public static class Builder<T extends Builder<T>> {

        private int calories = 0;

        public Builder() {}

        public T calories(int val) {
            calories = val;
            return (T) this;
        }

        public NutritionFacts build() { return new NutritionFacts(this); }
    }

    protected NutritionFacts(Builder<?> builder) {
        calories = builder.calories;
    }
}

现在用派生类构建器作为泛型参数实例化基础构建器.

Now instantiate the base builder with the derived class builder as the generic argument.

public class GMOFacts extends NutritionFacts {

    private final boolean hasGMO;

    public static class Builder extends NutritionFacts.Builder<Builder> {

        private boolean hasGMO = false;

        public Builder() {}

        public Builder GMO(boolean val) {
            hasGMO = val;
            return this;
        }

        public GMOFacts build() { return new GMOFacts(this); }
    }

    protected GMOFacts(Builder builder) {
        super(builder);
        hasGMO = builder.hasGMO;
    }
}

这篇关于子类化 Java Builder 类的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！