Builder中有效Java模式 [英] Builder Pattern in Effective Java

问题描述

最近我开始阅读Joshua Bloch的有效Java。我发现Builder模式的想法[书中的第2项]非常有趣。我试图在我的项目中实现它，但是存在编译错误。以下是本质上我正试图做的：

具有多个属性的类及其构建器类：

  public class NutritionalFacts {
 private int sodium; 
 private int fat; 
 private int carbo; 
 
 public class Builder {
 private int sodium; 
 private int fat; 
 private int carbo; 
 
 public Builder（int s）{
 this.sodium = s; 
} 
 
 public Builder fat（int f）{
 this.fat = f; 
返回这个; 
} 
 
 public Builder carbo（int c）{
 this.carbo = c; 
返回这个; 
} 
 
 public NutritionalFacts build（）{
 return new NutritionalFacts（this）; 
} 
} 
 
私人营养物品（生成器b）{
 this.sodium = b.sodium; 
 this.fat = b.fat; 
 this.carbo = b.carbo; 
} 
} 
  

我尝试使用上述类的类： / p>

  public class Main {
 public static void main（String args []）{
 NutritionalFacts n = 
 new NutritionalFacts.Builder（10）.carbo（23）.fat（1）.build（）; 
} 
} 
  

我收到以下编译器错误：

包含
effectivejava.BuilderPattern.NutritionalFacts.Builder
的附加实例
NutritionalFacts n = new
NutritionalFacts.Builder（10）.carbo（23）.fat（1）.build（）;

我不了解消息的含义。请解释。上面的代码与Bloch在他的书中提出的例子相似。

解决方案

使构建器成为静态类。然后它会工作。如果它是非静态的，它将需要一个它自己的类的一个实例，而且这个点不是有一个实例，甚至禁止在没有构建器的情况下进行实例。

  public class NutritionFacts {
 public static class Builder {
} 
} 
  

参考：嵌套类

I have recently started to read Effective Java by Joshua Bloch. I found the idea of the Builder pattern [Item 2 in the book] really interesting. I tried to implement it in my project but there were compilation errors. Following is in essence what I was trying to do:

The class with multiple attributes and its builder class:

public class NutritionalFacts {
    private int sodium;
    private int fat;
    private int carbo;

    public class Builder {
        private int sodium;
        private int fat;
        private int carbo;

        public Builder(int s) {
            this.sodium = s;
        }

        public Builder fat(int f) {
            this.fat = f;
            return this;
        }

        public Builder carbo(int c) {
            this.carbo = c;
            return this;
        }

        public NutritionalFacts build() {
            return new NutritionalFacts(this);
        }
    }

    private NutritionalFacts(Builder b) {
        this.sodium = b.sodium;
        this.fat = b.fat;
        this.carbo = b.carbo;
    }
}

Class where I try to use the above class:

public class Main {
    public static void main(String args[]) {
        NutritionalFacts n = 
            new NutritionalFacts.Builder(10).carbo(23).fat(1).build();
    }
}

I am getting the following compiler error:

an enclosing instance that contains effectivejava.BuilderPattern.NutritionalFacts.Builder is required NutritionalFacts n = new NutritionalFacts.Builder(10).carbo(23).fat(1).build();

I do not understand what the message means. Please explain. The above code is similar to the example suggested by Bloch in his book.

解决方案

Make the builder a static class. Then it will work. If it is non-static, it would require an instance of its owning class - and the point is not to have an instance of it, and even to forbid making instances without the builder.

public class NutritionFacts {
    public static class Builder {
    }
}

Reference: Nested classes

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