复制抽象类的构造函数 [英] Copy constructor for abstract class

问题描述

我有一个名为 AClass 的抽象类。在同一个包中我有 AnotherClass ，其中我有一个 ArrayList AClass 对象。在 AnotherClass 的复制构造函数中，我需要在 ArrayList中复制 AClass 对象。

I have an abstract class named AClass. In the same package I have AnotherClass, in which I have an ArrayList of AClass objects. In the copy constructor of AnotherClass I need to make a duplicate of AClass objects inside the ArrayList.

问题：

我无法在 AClass 因为它是一个抽象类，我不知道将由 AClass 继承的类的名称。实际上，在这个项目中，没有对象会从这个类继承，但是这个项目将被其他项目用作库，这些项目将提供 AClass 的类子项。我的设计中是否有错误或是否有解决此问题的方法？

I cannot create a copy constructor in AClass because it is an abstract class and I cannot know the name of the class which will inherit by AClass. Actually, in this project, no object will inherit from this class, but this project will be used as a library by other projects which will provide a class child of AClass. Is there an error in my design or is there a solution to this problem?

编辑：这里有一些代码：

public class AnotherClass{
    private ArrayList<AClass> list;
...
    /** Copy constructor
    */
    public AnotherClass(AnotherClass another){
        // copy all fields from "another"
        this.list = new ArrayList<AClass>();
        for(int i = 0; i < another.list.size(); i++){
            // Option 1: this.list.add(new AClass(another.list.get(i)));
            // problem: cannot instantiate AClass as it is abstract
            // Option 2: this.list.add(another.list.get(i).someKindOfClone());
            // problem? I'm thinking about it, seems to be what dasblinkenlight is suggesting below
        }
    }
...
}

推荐答案

我无法在 AClass 因为它是一个抽象类，我不知道将继承的类的名称 AClass

I cannot create a copy constructor in AClass because it is an abstract class and I cannot know the name of the class which will inherit by AClass

这通常是正确的。但是，由于您有一个 AClass 的列表，您不需要知道确切的子类型：创建副本的抽象函数就足够了：

This is generally correct. However, since you have a list of AClass, you do not need to know the exact subtype: an abstract function that make a copy would be sufficient:

protected abstract AClass makeCopy();

这类似于 clone（） java.lang.Object 的函数，除了所有子类必须实现它，并且返回类型必须是 AClass 。

This is similar to the clone() function of the java.lang.Object, except all subclasses must implement it, and the return type is required to be AClass.

由于每个子类都知道自己的类型，因此实现 makeCopy（）方法应该没有问题。以下是您的代码中的内容：

Since each subclass knows its own type, they should have no problem implementing makeCopy() method. Here is how this would look in your code:

for (int i = 0 ; i < another.list.size() ; i++) {
    this.list.add(another.list.get(i).makeCopy());
}

注意：此设计称为 原型模式 ，有时非正式地称为虚拟构造函数。

Note: this design is known as the prototype pattern, sometimes informally called the "virtual constructor".

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