EJ.02 遇到多个构造器参数时要考虑用构建器

• 重叠构造器(telescoping constructor)模式 —— 代码复杂，阅读困难

示例代码： 

// Telescoping constructor pattern - does not scale well!
public class NutritionFacts {
    private final int servingSize;    // (ml)            required
    private final int servings;    // (per container)    required
    private final int calories;    //            optional
    private final int fat;        // (g)            optional
    private final int sodium;    // (mg)            optional
    private final int carbohydrate; // (g)            optional
 
    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;
    }
}

public class Client {
    public static void main(String[] args) {
        NutritionFacts cocaCola = new NutritionFacts(240, 8, 100, 0, 35, 27);
    }
}

 

• JavaBean模式 —— 线程不安全，类可变

示例代码：

// JavaBean Pattern - allows inconsistency, mandates mutability
public class NutritionFacts {
    // Parameters initialized to default values (if any)
    private int servingSize  = -1; // Required; no default value
    private int servings     = -1;
    private int calories     = 0;
    private int fat          = 0;
    private int sodium       = 0;
    private int carbohydrate = 0;
 
    public NutritionFacts() {
    }
 
    // Setters
    public void setServingSize(int servingSize) {
        this.servingSize = servingSize;
    }
 
    public void setServings(int servings) {
        this.servings = servings;
    }
 
    public void setCalories(int calories) {
        this.calories = calories;
    }
 
    public void setFat(int fat) {
        this.fat = fat;
    }
 
    public void setSodium(int sodium) {
        this.sodium = sodium;
    }
 
    public void setCarbohydrate(int carbohydrate) {
        this.carbohydrate = carbohydrate;
    }
}

 

public class Client {
    public static void main(String[] args) {
        NutritionFacts cocaCola = new NutritionFacts();
        cocaCola.setServingSize(240);
        cocaCola.setServings(8);
        cocaCola.setCalories(100);
        cocaCola.setSodium(35);
        cocaCola.setCarbohydrate(27);
    }
}

 

• 构建器模式 —— 适应多参数，安全，方便编写，可维护性高，开销大

示例代码：

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
    {
        // 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)
        {
            fat = val;
            return this;
        }
 
        public Builder carbohydrate(int val)
        {
            carbohydrate = val;
            return this;
        }
 
        public Builder sodium(int val)
        {
            sodium = val;
            return this;
        }
 
        public NutritionFacts build()
        {
            return new NutritionFacts(this);
        }
    }
 
    private NutritionFacts(Builder builder)
    {
        servingSize = builder.servingSize;
        servings = builder.servings;
        calories = builder.calories;
        fat = builder.fat;
        sodium = builder.sodium;
        carbohydrate = builder.carbohydrate;
        //对象域抛出异常
        if(this.fat>this.sodium){
            throw new IllegalArgumentException("The fat must be little than sodium!!");
        }
    }
}

 

public class Client {
    public static void main(String[] args) {
        NutritionFacts cocaCola = new NutritionFacts.Builder(240, 8)
                .calories(100).sodium(35).carbohydrate(27).build();
    }
}

 

 

来自为知笔记(Wiz)