创建对象的几种方式

构造器
静态工厂方法
通过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，得到一个builder对象，然后客户端在builder对象上调用类似于setter的方法，来设置每个可选参数，最后在客户端调用无参的build()方法来生成对象。下面是一个简单的示例：

//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
        private final int servingSize;
        private final int servings;
        //optional
        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;
        }
        //build 构造对象
        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;
    }
}
//客户端调用
NutritionFacts cocaCola = new NutritionFacts.Builder(20, 8).calories(100).sodium(35).carbohydrate(27).build()；