Builder模式创建实例

在java中实例化一个类的对象一般有三种模式，重叠构造器模式，javabean模式，builder模式。

重叠构造器模式

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 NutritionFacts(int servingSize, int servings) {
        this(servingSize, servings, 0);
    }

    public NutritionFacts(int servingSize, int servings, int calories) {
        this(servingSize, servings, calories, 0);
    }

    public NutritionFacts(int servingSize, int servings, int calories, int fat) {
        this(servingSize, servings, calories, fat, 0);
    }

    public NutritionFacts(int servingSize, int servings, int calories, int fat, int sodium) {
        this(servingSize, servings, calories, fat, sodium, 0);
    }

    public NutritionFacts(int servingSize, int servings, int calories, int fat, int sodium, int carbohydrate) {
        this.servingSize = servingSize;
        this.servings = servings;
        this.calories = calories;
        this.fat = fat;
        this.sodium = sodium;
        this.carbohydrate = carbohydrate;
    }
}

这种方式在有大量参数的时候代码会变得很不直观，代码可读性太差。

javabean模式

就是给定一个默认构造器，然后对所有属性提供setter，getter方法。
实例化与属性赋值在不同语句中，所以此种模式线程不安全

Builder模式

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 final int servingSize;
        private final int servings;

        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){
            this.calories = 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;
        }

    }

    public NutritionFacts(Builder builder) {
        servingSize = builder.servingSize;
        servings = builder.servings;
        calories = builder.calories;
        fat = builder.fat;
        sodium = builder.sodium;
        carbohydrate = builder.carbohydrate;
    }
}

使用方法：

NutritionFacts.Builder builder = new NutritionFacts.Builder(2, 3)
            .calories(1)
            .carbohydrate(2)
            .fat(4)
            .sodium(3);
NutritionFacts nutritionFacts = builder.build();