如何在BaseClass复制构造函数中复制SubClass对象 [英] how to copy SubClass object in BaseClass copy constructor

问题描述

我想在BaseClass构造函数中复制SubClass对象。我需要以下代码正确执行。

I would like to make copy of SubClass object in BaseClass constructor. I need that the following code execute correctly.

class BaseClass{
    BaseClass() {}
    BaseClass(BaseClass base) {
        //TODO: how to implement?
    }
}

class SubClass extends BaseClass {    
   SubClass() {}
}

public class Test {
    public static void main(String[] args) {

        BaseClass sub = new SubClass();
        BaseClass subCopy = new BaseClass(sub);
        if (subCopy instanceof SubClass) {
            // need to be true
        }
    }
}

有可能吗？如果可以，我该怎么办？其他如何获得类似的效果？

Is it even possible? If yes how can I do it? Else how can I get similar effect?

推荐答案

这是不可能的。类A的构造函数为您提供了A的实例，无法绕开它。为什么不显式实例化子类？

It's not possible. A constructor of class A gives you an instance of A, no way to circumvent this. Why not instantiate the subclass explicitly?

另一种可能性可能涉及静态工厂方法，例如：

Another possibility might involve a static factory method like:

public static BaseClass create(BaseClass input) {
       // return BaseClass or subclass
}

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