多个构造器参数时要考虑用构建器

向右看齐 2022-06-12 04:18 141阅读 0赞

当一个类有大量的可选属性是，要考虑用构建器替代构造器，构造器模式会让类的实例化变得太过复杂。

构建器可以见名之意，并且有很大的灵活性。

构建器构造对象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对象上调用类似于setter的方法，来设置每个相关的可选参数。

最后，客户端调用无参的build方法来生成不可变的对象。这个builder是它构建的类的静态成员类。

示例：

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;
    
        /** * 这里必须用静态方法，否则在没有外部类的情况下无法新建内部类 */ public static class Builder {
            //必选属性
            private final int servingSize;
            private final int servings;
            //可选属性
            private int calories = 0;
            private int fat = 0;
            private int carbohydrate = 0;
            private int sodium = 0;
    
            public Builder(int servingSize, int servings) {
                this.servingSize = servingSize;
                this.servings = servings;
            }
    
            public Builder calories(int val) {
                calories = val;
                return this;
            }
    
            public Builder carbohydrate(int val) {
                carbohydrate = val;
                return this;
            }
    
            public Builder sodium(int val) {
                sodium = val;
                return this;
            }
    
            public Builder fat(int val) {
                fat = val;
                return this;
            }
    
            //内部类可以访问外部类的私有成员
            public NutritionFacts build() {
                return new NutritionFacts(this);
            }
        }
    
        /** * 外部类可以访问内部类的私有属性 */ private NutritionFacts(Builder builder) {
            servingSize = builder.servingSize;
            servings = builder.servings;
            calories = builder.calories;
            fat = builder.fat;
            sodium = builder.sodium;
            carbohydrate = builder.carbohydrate;
        }
    
        public String toString() {
            return "[" + servingSize + "," + servings + "," + calories + "," + fat + "," + sodium + "," + carbohydrate + "]";
        }
    }

public class Main {
        public static void main(String[] args){
            /** * 初始化时，能够见名之意，并且可选属性可以连环赋值，避免了像setter方法一样需要很长的代码。 */ NutritionFacts nutritionFacts=new NutritionFacts.Builder(10,20).calories(30).sodium(35).build();
            System.out.println(nutritionFacts.toString());
        }
    }

\[10,20,30,0,35,0\]