第2条，遇到多个构造器参数时要考虑使用构建器

静态工厂和构造器的缺陷：都不能很好地扩展到大量的可选参数。

重叠构造器：第一个构造器只有必要参数；第二个构造器有一个可选参数；第三个构造器有两个可选参数.....

重叠构造器当参数多的时候就会很难写。

生成器模式（builder）：不直接生成想要的对象，而是让客户端利用所有必要的参数调用构造器（或者静态工厂），得到一个builder对象。然后客户端在builder对象上调用类似setter的方法，来设置相关的可选参数。最后客户端调用无参的build方法来生成通常是不可变的对象。这个builder通常是它构建的类的静态成员类。

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 {
		// 必要参数
		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就是返回当前对象的引用
			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;
	}

	public static void main(String[] args) {
		NutritionFacts nf = new NutritionFacts.Builder(1, 2).carbohydrate(100).calories(20).Build();
	}

}