《Effective Java》读书笔记：用Builder模式代替含多个参数的构造器

当一个对象含有多个成员变量且创建对象时需要初始化时，直接使用构造器来初始化会造成使用上的不便。例如

public class BuilderDemo {
    private final int servingSize;
    private final int servings;
    private final int calories;
    private final int fat;
    private final int sodium;
    private final int carbohydrate;
}

通常的做法是提供重叠构造器，首先提供一个只有必要参数的构造器，然后不断增加包含可选参数的构造器，直至包含所有的可选参数。例如

public BuilderDemo(int servingSize, int servings) {
        this.servingSize = servingSize;
        this.servings = servings;
    }
    
    public BuilderDemo(int servingSize, int servings, int calories) {
        this.servingSize = servingSize;
        this.servings = servings;
        this.calories = calories;
    }

    public BuilderDemo(int servingSize, int servings, int calories, int fat) {
        this.servingSize = servingSize;
        this.servings = servings;
        this.calories = calories;
        this.fat = fat;
    }

    public BuilderDemo(int servingSize, int servings, int calories, int fat, int sodium) {
        this.servingSize = servingSize;
        this.servings = servings;
        this.calories = calories;
        this.fat = fat;
        this.sodium = sodium;
    }

    public BuilderDemo(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;
    }

如果只需要后面的可选参数，但调用的时候必须提供前面的可选参数。而且参数很长时会造成阅读和修改上的不便。

另外的一种做法是提供每个成员变量的set方法，在创建对象之后设置成员变量的内容。但因为构造过程被分到了几个调用中，构造过程中对象可能处于不一致的状态。

最理想的方法是使用Builder模式，通过一个内部静态类来完整的构造对象。代码如下：

public class BuilderDemo {
    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 final int servingSize;
        private final int servings;
        //
        private int calories = 0;
        private int fat = 0;
        private int sodium = 0;
        private int carbohydrate = 0;
        //
        public Builder(int servingSize, int servings) {
            this.servingSize = servingSize;
            this.servings = servings;
        }

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

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

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

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

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

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

    public static void main(String[] args) {
        BuilderDemo demo = new Builder(34, 32).calories(34).carbohydrate(43).build();
    }
}

这样的代码很容易编写和阅读。而且可以对其参数强加约束条件，build方法可以检验这些约束条件，将参数从builder拷贝到对象中，并在对象域中对它们进行检验。

Builder模式的不足：1.创建对象前需要创建构建器，在十分注重性能的情况下不适用。2.Builder比重叠构造器模式更加冗长，因此只有在很多参数时才使用，比如4个以上。