重载构造函数调用其他构造函数，但不作为第一个语句 [英] Overloaded constructor calling other constructor, but not as first statement

问题描述

我在java中使用多个构造函数时遇到了一些麻烦。

I'm having some trouble using multiple constructors in java.

我想做的是这样的：

public class MyClass {

 // first constructor
 public MyClass(arg1, arg2, arg3) {
  // do some construction
 }

 // second constructor
 public MyClass(arg1) {
      // do some stuff to calculate arg2 and arg3
      this(arg1, arg2, arg3);
    }
}

但我不能，因为第二个构造函数不能调用另一个构造函数，除非它是第一行。

but I can't, since the second constructor cannot call another constructor, unless it is the first line.

这种情况的常见解决方案是什么？
我无法计算arg2和arg3在行。我想也许可以创建一个构造辅助方法，它将进行实际构造，但我不确定它是如此漂亮......

What is the common solution for such situation? I can't calculate arg2 and arg3 "in line". I thought maybe creating a construction helper method, that will do the actual construction, but I'm not sure that's so "pretty"...

编辑：使用辅助方法也存在问题，因为我的一些字段是最终的，我无法使用辅助方法设置它们。

EDIT: Using a helper method is also problematic since some of my fields are final, and I can't set them using a helper method.

推荐答案

通常使用另一种常用方法 - 如你所建议的建筑助手。

Typically use another common method - a "construction helper" as you've suggested.

public class MyClass { 

    // first constructor 
    public MyClass(arg1, arg2, arg3) { 
      init(arg1, arg2, arg3); 
    } 

    // second constructor 
    public MyClass(int arg1) { 
      // do some stuff to calculate arg2 and arg3 
      init(arg1, arg2, arg3); 
    } 

    private init(int arg1, int arg2, int arg3) {
      // do some construction 
    }
} 

替代方案是工厂式方法，其中您有 MyClassFactory 它给你 MyClass 个实例， MyClass 只有一个构造函数：

The alternative is a Factory-style approach in which you have a MyClassFactory that gives you MyClass instances, and MyClass has only the one constructor:

public class MyClass { 

    // constructor 
    public MyClass(arg1, arg2, arg3) { 
      // do some construction 
    } 
} 

public class MyClassFactory { 

    public static MyClass MakeMyClass(arg1, arg2, arg3) { 
      return new MyClass(arg1, arg2, arg3);
    } 

    public static MyClass MakeMyClass(arg1) { 
      // do some stuff to calculate arg2 and arg3 
      return new MyClass(arg1, arg2, arg3);
    } 
} 

我绝对更喜欢第一种选择。

I definitely prefer the first option.

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