在Java中调用多个方法

Question

我发现了一种在Java中调用多个方法的新方法,我并不真正了解背后发生的事情:

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 carbohydrate  = 0;
    private int sodium        = 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 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;
    }

}

现在使用这一行来实例化类,这里让它变得混乱:

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

这一切都有意义,直到NutritionFacts.Build(int,int),之后,究竟发生了什么？为什么在calories,sodium,carbohydrate从生成器类方法需要返回this？那个班级的地址在哪里？

谢谢!

Answer 1

它没有"进入"任何东西.

这些方法返回一个值.在这种情况下,它们返回当前实例this.那个实例有方法,比如calories()和carbohydrates().

foo.calories(12)返回实例,我们可以调用它的方法:foo.calories(12).sodium(35).

它与构造函数中的"返回值"没有什么不同,隐式定义为新实例.在这种情况下,它是正常的方法,仍然返回一个实例 - 当前的实例.

它与此相同:

Builder foo = new Builder(1, 2); // The "return" value of a ctor is the reference, foo
foo.sodium(10);   // Returns foo, but we ignore it
foo.calories(42); // Returns foo, but we ignore it

(foo.sodium(10)).calories(42);
^^^^^^^^^^^^^^^^ foo, with the side effect of setting the sodium value

这是一个SO问题,有一些很好的例子.