复制抽象类的构造函数

Question

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 .

The problem:

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?

Edit : here's some code:

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
        }
    }
...
}

Answer 1

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

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();

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 .

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());
}

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