java构建器

背景：

当类中属性太多，我们需要通过各种不同参数组合的构造方法来初始化对象，这样写起来很麻烦，而且选择构造方式时容易混淆出错，如果通过javabean的setter方式来初始化， 又会遇到线程安全的问题（因为对象的创建分在了好几步的过程中，不能保证对象状态的一致性）

实例：通过内部类的方式。

public class Test {
    private int servingSize;
    private int servings;
    private int calories;
    private int fat;
    private int sodium;
    private int carbohydrate;
    
    public static class Builder {
        private int servingSize;
        private int servings;
        private int calories;
        private int fat;
        private int sodium;
        private int carbohydrate;
        
        public Builder (int servingSize, int servings) {
            this.servingSize = servingSize;
            this.servings = servings;
        }
        
        public Builder calories(int calories) {
            calories = calories;
            return this;
        }

        public Builder fat(int fat) {
            fat = fat;
            return this;
        }

        public Builder sodium(int sodium) {
            sodium = sodium;
            return this;
        }

        public Builder carbohydrate(int carbohydrate) {
            carbohydrate = carbohydrate;
            return this;
        }
        
        public Test builder () {
            return new Test(this);
        }
    }
    
    private Test(Builder builder) {
        servingSize = builder.servingSize;
        servings = builder.servings;
        calories = builder.calories;
        fat = builder.fat;
        sodium = builder.sodium;
        carbohydrate = builder.carbohydrate;
    }
}

测试:

public static void main(String[] args) {
         Test test = new         Builder(1,2).calories(3).fat(4).sodium(5).carbohydrate(6).builder();
}