2、遇到多个构造器参数的时候要考虑使用构建器

来自《effective Java》

贴上书上代码

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{
        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 setCalories(int val){
            calories=val;
            return this;
        }
        public Builder setFat(int val){
            fat=val;
            return this;
        }
        public Builder setSodium(int val){
            sodium=val;
            return this;
        }
        public Builder setCarbohydrate(int val){
            carbohydrate=val;
            return this;
        }
        public NutritionFacts builder(){
            return new NutritionFacts(this);
        }
    }
    private NutritionFacts(Builder builder){
        fat=builder.fat;
        servings=builder.servings;
        servingSize=builder.servingSize;
        calories=builder.calories;
        sodium=builder.sodium;
        carbohydrate=builder.carbohydrate;
    }

    @Override
    public String toString() {
        return "NutritionFacts{" +
                "servingSize=" + servingSize +
                ", servings=" + servings +
                ", calories=" + calories +
                ", fat=" + fat +
                ", sodium=" + sodium +
                ", carbohydrate=" + carbohydrate +
                '}';
    }


    public static void main(String[] args) {
        NutritionFacts nutritionFacts = new Builder(1, 2)
                .setCalories(3)
                .setCarbohydrate(4)
                .setFat(5)
                .setSodium(6)
                .builder();
        System.out.println(nutritionFacts.toString());
    }
}

类中含有多个变量，但是每次使用的时候需要使用不同的构造函数初始化。

首先需要在类中定义一个静态的内部类，静态内部类还有的变量和外部类相同，

在内部类中定义每个函数的set方法，注意每次set的时候需要返回当前的builder对象以便下次继续执行后面对应函数成员变量的set方法。

最后使用内部类的build()函数，函数返回的是外部类对象，这时候内部类的成员变量都有我们想要初始化的值，所以在内部类的build()函数中new一个外部类的实例并返回，

这时候注意需要重写外部类的构造函数，函数里传递的是内部类对象，使用内部类已经初始化的变量初始化外部类的变量