二、Builder模式

构造方法参数过多时，Builder模式比使用伸缩构造方法（telescoping constructors）更容易读写，比JavaBeans模式更安全。

Builder通常是它所构建的类的一个静态成员类(条目24)。以下是它在实践中的示例：

// 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 {
        // Required parameters
        private final int servingSize;
        private final int servings;

        // Optional parameters - initialized to default values
        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 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;
    }
}

NutritionFacts类是不可变的，所有的参数默认值都在一个地方。builder的setter方法返回builder本身，这样调用就可以被链接起来，从而生成一个流畅的API。下面是客户端代码的示例：

NutritionFacts cocaCola = new NutritionFacts.Builder(240, 8)
    .calories(100).sodium(35).carbohydrate(27).build();