标签：builder-pattern java

经典的Builder Pattern要求在要构建的类中声明字段,并在构建器类中声明完全相同的字段.当存在许多字段时,这会导致问题,并且在重构期间,字段类型不会保持同步.这是我的意思的一个例子(我从Joshua Block的一篇文章中借用了这个代码示例)：

// Builder Pattern
public class NutritionFacts {
    private final int servingSize;
    private final int servings;
    private final int calories;
    private final int fat;
    private final int sodium;
    private final int carbohydrate;

    public static class Builder {
        private int servingSize   = 0;
        private int servings      = 0;
        private int calories      = 0;
        private int fat           = 0;
        private int carbohydrate  = 0;
        private int sodium        = 0;

        public Builder servingSize(int val)
            { servingSize = val;   return this; }
        public Builder servings(int val)
            { servings = val;      return this; }
        public Builder calories(int val)
            { calories = val;      return this; }
        public Builder fat(int val)
            { fat = val;           return this; }
        public Builder carbohydrate(int val)
            { carbohydrate = val;  return this; }
        public Builder sodium(int val)
            { sodium = val;        return this; }

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

    private NutritionFacts(Builder builder) {
        servingSize  = builder.servingSize;
        servings     = builder.servings;
        calories     = builder.calories;
        fat          = builder.fat;
        sodium       = builder.sodium;
        carbohydrate = builder.carbohydrate;
    }
}

现在让我们假设serveSize需要从int更改为long,并且这个更改是在NutritonFacts中完成的,但是,不可能在静态Builder中完成.

无可比拟的问题是字段的数量. NutritionFacts有6个领域,因此,Builder也是如此.如果有20或100个字段怎么办？在NutritionFacts和Builder中复制它们将是真正的痛苦.有没有更好的方法可以避免所有重复和类型同步错误的可能性？

解决方法:

您可以使用NutritionFacts对象来存储Builder的状态：

// Builder Pattern
public class NutritionFacts {
    private final int servingSize;
    private final int servings;
    private final int calories;
    private final int fat;
    private final int sodium;
    private final int carbohydrate;

    public static class Builder {
        private NutritionFacts state = new NutritionFacts(0,0,0,0,0,0);

        public Builder servingSize(int val) { 
            state = new NutritionFacts(val, state.servings, state.calories, state.fat, state.sodium, state.carbohydrate);
            return this;
        }
        [...]

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

    private NutritionFacts(Builder builder) {
        servingSize  = builder.state.servingSize;
        servings     = builder.state.servings;
        calories     = builder.state.calories;
        fat          = builder.state.fat;
        sodium       = builder.state.sodium;
        carbohydrate = builder.state.carbohydrate;
    }

    private NutritionFacts(int servingSize, int servings, int calories, int fat, int sodium, int carbohydrate) {
        this.servingSize  = servingSize;
        this.servings     = servings;
        this.calories     = calories;
        this.fat          = fat;
        this.sodium       = sodium;
        this.carbohydrate = carbohydrate;
    }
}  

由于NutritionFacts是不可变的,因此需要为每次更改构建一个新的状态对象,这可能是也可能不值得.

如果你可以使NutritionFacts的内部状态相互作用会更容易,但是使用私有的setter – 根据定义使对象不可变,而不是使用final关键字：

// Builder Pattern
public class NutritionFacts {
    private int servingSize = 0;
    private int servings = 0;
    private int calories = 0;
    private int fat = 0;
    private int sodium = 0;
    private int carbohydrate = 0;

    public static class Builder {
        private NutritionFacts state = new NutritionFacts();

        public Builder servingSize(int val) { 
            state.servingSize = val;
            return this;
        }
        public Builder servings(int val) { 
            state.servings = val;
            return this;
        }
        [...]

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

    private NutritionFacts(Builder builder) {
        servingSize  = builder.state.servingSize;
        servings     = builder.state.servings;
        calories     = builder.state.calories;
        fat          = builder.state.fat;
        sodium       = builder.state.sodium;
        carbohydrate = builder.state.carbohydrate;
    }
}  

标签：builder-pattern,java
来源： https://codeday.me/bug/20190830/1765758.html

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