Builder模式创建对象

package builder;

public class NutritionFacts {
	//必选参数
	private final int servingSize;
	private final int servings;
	//可选参数
	private final int carlories;
	private final int fat;
	private final int  sodium;
	private final int carbohydrate;
	
	public static class Builder{
		//必选参数定义为final，强制其在构造函数是进行初始化
		private final int servingSize;
		private final int servings;
		public Builder(int servingSize,int servings){
			this.servingSize=servingSize;
			this.servings=servings;
		}
		//可选参数先给它一个默认的初始值，通过赋值函数可改变它的值
		private int carlories=0;
		private int fat=0;
		private int sodium=0;
		private int carbohydrate=0;
		public Builder carlories(int val){
			this.carlories=val;
			return this;
		}
		public Builder fat(int val){
			this.fat=val;
			return this;
		}
		public Builder sodium(int val){
			this.sodium=val;
			return this;
		}
		public Builder carbohydrate(int val){
			this.carbohydrate=val;
			return this;
		}
		
		//通过build函数来创建NutritionFacts对象
		public NutritionFacts build(){
			return new NutritionFacts(this);
		}
	}
	
	private NutritionFacts(Builder builder){
		servingSize=builder.servingSize;
		servings=builder.servings;
		carlories=builder.carlories;
		fat=builder.fat;
		sodium=builder.fat;
		carbohydrate=builder.carbohydrate;
	}
	
	public static void main(String[] args){
		NutritionFacts inst1=new NutritionFacts.Builder(240, 8).
				carlories(100).sodium(35).carbohydrate(27).build();
		NutritionFacts inst2=new NutritionFacts.Builder(150, 14).fat(50).build();
	}
}

Builder模式十分灵活，可以利用单个Builder构建多个对象，builder参数可以在创建对象时进行调整。且这样的代码非常易于阅读。

如果类的构造器或者静态工厂中具有多个参数，设计这种类时，Builder模式就是不错的选择，特别是当大多数参数都是可选的时候。