Effective Java 的笔记，代码、英语原文为主，批注、翻译为辅。

Item 2: Consider a builder when faced with many constructor parameters

当构造函数有多个参数时，考虑使用builder模式

1. Telescoping constructor pattern - does not scale well!

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// 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;     // (per serving)   optional
    private final int fat;          // (g/serving)     optional
    private final int sodium;       // (mg/serving)    optional
    private final int carbohydrate; // (g/serving)     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 static void main(String[] args) {
        NutritionFacts cocaCola =
                new NutritionFacts(240, 8, 100, 0, 35, 27);
    }

}

重叠构造器方法：当有很多参数的时候，客户端代码会

  • 很难编写
  • 难以阅读

没记错的话我之前做的项目中写的代码就是这样的，使用了大量不同参数的构造方法😅。

2. JavaBeans Pattern - allows inconsistency, mandates mutability

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// JavaBeans 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; // Required; no default value
    private int calories     = 0;
    private int fat          = 0;
    private int sodium       = 0;
    private int carbohydrate = 0;

    public NutritionFacts() { }
    // Setters
    public void setServingSize(int val)  { servingSize = val; }
    public void setServings(int val)     { servings = val; }
    public void setCalories(int val)     { calories = val; }
    public void setFat(int val)          { fat = val; }
    public void setSodium(int val)       { sodium = val; }
    public void setCarbohydrate(int val) { carbohydrate = val; }

    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);
    }
}

缺点：

Because construction is split across multiple calls, a JavaBean may be in an inconsistent state partway through its construction.

构造过程中， JavaBean可能处于不一致的状态。

A related disadvantage is that the JavaBeans pattern precludes the possibility of making a class immutable and requires added effort on the part of the programmer to ensure thread safety.

JavaBeans 模式会使类可变，导致线程不安全的情况。

没记错的话我之前做的项目中也写过这样的代码，使用了大量setter方法😅。

3. Builder Pattern

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// Builder Pattern
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 sodium = 0;
        private int carbohydrate = 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 sodium(int val) {
            sodium = val;
            return this;
        }

        public Builder carbohydrate(int val) {
            carbohydrate = 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;
    }

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

建造者(Builder)模式

1、优点：

  • 保证像重叠构造器模式一样的安全性
  • 保证像JavaBeans模式一样的可读性

2、步骤：

Instead of making the desired object directly, the client calls a constructor (or static factory) with all of the required parameters and gets a builder object. Then the client calls setter-like methods on the builder object to set each optional parameter of interest. Finally, the client calls a parameterless build method to generate the object, which is typically immutable.

  • 客户端使用所有必需的参数(the required parameters)调用构造函数/静态工厂，得到一个 builder 对象
  • 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(each optional parameter of interest)
  • 客户端调用无参的build方法来生成对象（通常是不可变的）。

3、注意点：

NutritionFacts 不可变。

public static class Builder Builder是类NutritionFacts的静态成员类。

Summary

In summary, the Builder pattern is a good choice when designing classes whose constructors or static factories would have more than a handful of parameters, especially if many of the parameters are optional or of identical type. Client code is much easier to read and write with builders than with telescoping constructors, and builders are much safer than JavaBeans.

所以既然学习了Builder模式，在编码的时候应该灵活使用，而不是再写之前蹩脚的代码了。

设计模式果然是有实际应用的，自己学这门课程的时候，感觉例子枯燥，也没有实际应用。果然还是要看书看书📚，使用设计模式编码设计好的代码。