如何克服访问者实施的类型擦除问题

Question

I am starting to work with Java after some projects in C# and C++.

I wanted to design visitor interfaces like this:

public interface ActionVisitor<A> {
    void visitAction(A action);    
}

public interface MySmallActionVisitor 
extends ActionVisitor<ActionA>,
    ActionVisitor<ActionB>
{

}

public interface MyFullActionVisitor 
extends ActionVisitor<ActionA>,
    ActionVisitor<ActionB>,ActionVisitor<ActionC>,ActionVisitor<ActionD> //....
{

}

Of course this doesn't work because of type erasure. (The reason why I want something like this is that I will have different Visitor interfaces for different groups of Actions that can be visited.)

The only solution that comes to my mind is to declare interfaces

public interface ActionAVisitor {
        void visitAction(ActionA action);    
    }
public interface ActionBVisitor {
        void visitAction(ActionB action);    
    }
//...

and then

public interface MySmallActionVisitor 
extends ActionAVisitor, ActionBVisitor
{

}

This would work, but I wouldn't like the declaration of all the ActionXVisitor-Interfaces which is stupid repetition and lots of files...

Do you have any ideas how to do this better?

Thanks a lot!

Answer 1

I work with a large and complex library in Java which extensively uses the Visitor Pattern in a very clean and neat way. In particular, I came across with the same problem of type erasure and it is solved now.

If you have a chance, please have a look at an article I've written about this .

It's a long article, which explains in detail what the Visitor pattern is about conceptually and, in last part of the article, it is discussed a real life example which involves polymorphism and type erasure.

Cheers

Answer 2

我会使用一个未参数化的访问者界面，然后在访问者方法内部根据类型进行调度。

Answer 3

There's no way to be able to avoid instanceof of inside the method. But you can make it more graceful:

public interface MarkerInterface{}

public interface ActionVisitor<T extends MarkerInterface> {
void visitAction(T action);}

public class A implements MarkerInterface{}

public class B implements MarkerInterface{}

public class MySmallActionVisitor implements ActionVisitor<MarkerInterface>{

@Override
public void visitAction(MarkerInterface action) {
    if(action instanceof A){

    }
    else if(action instanceof B){

    }
}

}