DesignPattern-Builder

Builder模式

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

既能保证像重叠构造器模式那样的安全性，也能保证像JavaBeans模式那么好的可读性

public class NutritionFacts_03_Builder {
    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 parameters
        private  final  int servingSize;
        private  final  int servings;

        //Optional parameters = initialized to default values
        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 fat(int val){
            this.fat = fat;
            return  this;
        }
        public  Builder carbohydrate(int val){
            carbohydrate = val;
            return  this;
        }
        public  Builder sodium(int val){
            sodium = val;
            return  this;
        }
        public  NutritionFacts_03_Builder build(){
            return  new NutritionFacts_03_Builder(this);
        }
    }

    private NutritionFacts_03_Builder(Builder builder) {
        this.servingSize = builder.servingSize;
        this.servings = builder.servings;
        this.calories = builder.calories;
        this.fat = builder.fat;
        this.sodium = builder.sodium;
        this.carbohydrate = builder.carbohydrate;
    }

    public static void main(String[] args) {
        NutritionFacts_03_Builder cocaCola = new NutritionFacts_03_Builder.Builder(240,9)
                .calories(100).sodium(35).carbohydrate(27).build();
    }
}

返回的this时怎么替代对象的

对象创建更加灵活

当 isRef 为 true 的时候，arg 表示 String 类型的 refBeanId，type 不需要设置；当 isRef 为 false 的时候，arg、type 都需要设置

public class ConstructorArg {
    private boolean isRef;
    private Class type;
    private Object arg;

    private ConstructorArg(Builder builder) {
        this.isRef = isRef;
        this.type = type;
        this.arg = arg;
    }

    public static class Builder {
        private static boolean isRef;
        private static Class type;
        private static Object arg;

        public Builder(boolean isRef) {
            this.isRef = isRef;
        }

        public Builder setType(Class type) {
            this.type = type;
            return this;
        }

        public Builder setArg(Object arg) {
            this.arg = arg;
            return this;
        }

        public ConstructorArg build() {
            if (isRef) {
                if (!(arg instanceof String)) {
                    //这里refBeanId是ConstructorArg吗？只是判断了下arg类型是否为String
                    throw new IllegalArgumentException("当 isRef 为 true 的时候，arg 表示 String 类型refBeanId");
                }
                if (type != null) {
                    throw new IllegalArgumentException("当 isRef 为 true 的时候，type 不需要设置");
                }
            }
            if (!isRef) {
                if (arg == null || type == null) {
                    throw new IllegalArgumentException("当 isRef 为 false 的时候，arg、type 都需要设置");
                }
            }
            return new ConstructorArg(this);
        }
    }
}