Java类的静态成员类

遇到多个构造器参数时要考虑用构建器：

1. 静态工厂和构造器有个共同的局限性，它们都不能很好的扩展到大量的可选参数。
2. JavaBeans模式，在这种模式下调用一个无参数构造器来创建对象，然后调用setter方法来设置每个必要的参数，以及每个 相关的可选参数。
3. Builder模式，既能保证向重载构造器模式那样安全，也能保证像JavaBeans模式那么好的可读性。

静态内部类模式：

package thread;
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) {
    this.servingSize = builder.servingSize;
    this.servings = builder.servings;
    this.calories = builder.calories;
    this.fat = builder.fat;
    this.sodium = builder.sodium;
    this.carbohydrate = builder.carbohydrate;
  }

}

注意：NutritionFacts是不可变的，所有的默认参数都是单独放在一个地方。builder的setter方法返回builder本身，以便可以把调用链接起来。下面是客户端代码：

new NutritionFacts.Builder(20, 6).calories(1).fat(2).carbohydrate(2).sodium(2).build();