JavaBuilder模式使用

最近，我看到一些开发人员使用嵌套的构建器类（如

public class User {

    private String firstName;
    private String lastName;

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    public static class UserBuilder {

        private String firstName;
        private String lastName;

        public User build() {
            User user = new User();
            user.firstName = firstName;
            user.lastName = lastName;
            return user;
        }

        public UserBuilder withFirstName(String firstName) {
            this.firstName = firstName;
            return this;
        }

        public UserBuilder withLastName(String lastName) {
            this.firstName = firstName;
            return this;
        }       
    }

}

现在，他们声称这使代码更具可读性。我的观点是，这有以下缺点：

我不能简单地添加字段并期望IDE为我完成代码，因为现在我也需要更新这个内部类

简单POJO携带与VO无关的代码

如果我在这里遗漏了什么，我会寻求任何建议。请随意添加您的想法

修改后的示例代码如下所示：

User user = new User.UserBuilder()
                .withFirstName("Name")
                .withLastName("surName")
                .build();

---

这是约书亚·布洛赫的一篇文章。他很好地解释了为什么、何时以及如何使用建筑商：

这是他在《有效Java》一书中的一个项目。如果您对Java有一点经验，我强烈建议您阅读本书

要点:

当你得到一个有很多属性的类时，有几种方法可以创建一个对象并初始化它

如果你一个接一个地设置每一个属性，它可能会很冗长，并且你的对象在创建后可能会被改变。使用此方法，不可能使类不可变，也无法确保对象处于一致状态

文章中的示例：

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 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 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 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;
    }
}

您可以使用伸缩构造函数。它可以使对象不可变。但是，如果您获得了许多属性，那么编写和读取代码可能会很困难。更详细地说，当您只想使用一个已设置的属性创建时，不幸的是，这是构造函数的最后一个参数，您必须设置所有参数

文章中的示例：

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 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 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 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;
    }
}

生成器允许您的代码更可读、更易于编写。它还允许您使类不可变

文章中的示例：

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 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 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 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;
    }
}

在您的示例中，我不确定为只有两个属性的类创建生成器是否非常有用

---

我希望这将对您有所帮助。

在给定的示例中，使用构建器模式没有任何价值。您可以在不使用生成器的情况下创建

User

对象（因为有所有的setter）。在这种特殊情况下，Builder提供给您的唯一信息是

当存在创建有效对象的各种组合时，应该使用生成器模式。由于这一点，您不必实现许多构造函数或工厂方法

当创建需要许多参数的有效对象时，生成器也很有用

生成器应只负责生成有效的对象。在您的情况下，如果用户需要名和姓，则生成器不允许创建未设置这些属性的用户实例。

从小型不可变对象开始 如果所有属性都是必需的，那么应该只使用构造函数。通过这样做，您可能会创建一个漂亮的不可变的小对象

当您有多个可选字段时，生成器非常有用 如果有多个可选字段和不同的方法来创建对象，则需要多个构造函数

public User (int requiredParameter) { ... }
public User (int reqiredParameter, int optionalParameter) { ... }
public User (int reqiredParameter, int optionalParameter, String optionalParameter2) { ... }
public User (int reqiredParameter, String optionalParameter2) { ... }

它会产生混乱的代码。很难确定应该使用哪个构造函数。

---

在这种情况下，您可以使用嵌套生成器以直观的方式创建对象

当您的构造函数有许多不同的可能性，并且其中许多类型相同时，您通常使用构建器。因此，构建器使一切都更具可读性和可重用性。构建器避免了“伸缩构造函数”反模式，在这种反模式中，所有可能的可选构造函数参数都会组合爆炸。有效Java第二版第2项中有详细讨论。这个链接提供了您应该知道的关于构建器模式使用的所有信息。我认为构建器对于构建不可变对象最有用，否则您必须将所有参数传递给构造函数。生成器模式使构造更具可读性和灵活性。但是在你的例子中，

用户

是可变的，所以你对构建器所能做的事情没有设置器所不能做的。Josh bloch解释了这个用例-请在你的答案中添加至少最重要的几点。完成了，我希望一切正常。