Question

I was doing some homework today with the visitor-pattern, and i made a visitor that looked somewhat like this (edited sample code from wikipedia):

class CarElementPrintVisitor implements CarElementVisitor {
    public void visit(CarElement element) {      
        if (element.getClass() == Wheel.class)
        {
            return visit((Wheel)element);
        }
        else if (element.getClass() == Engine.class)
        {
            return visit((Engine)element);
        }
        else if (element.getClass() == Body.class)
        {
            return visit((Body)element);
        }
        else //if (v.getClass() == Car car.class)
        {
            return visit((Car)element);
        }
    }

    public void visit(Wheel wheel) {      
        System.out.println("Visiting " + wheel.getName() + " wheel");
    }

    public void visit(Engine engine) {
        System.out.println("Visiting engine");
    }

    public void visit(Body body) {
        System.out.println("Visiting body");
    }

    public void visit(Car car) {      
        System.out.println("Visiting car");
    }
}

The “public void visit(CarElement element)” method is kind of ugly (long and needs to be maintained if more CarElements are added) but i want to keep the method, so i tried to do it better.

I ended up trying this out:

    public void visit(CarElement element) {      
        return visit(element.getClass().cast(element));
    }

But that just returns “visit(CarElement element)”, even though element.getClass() returns the correct class, so it ends up in a infinite loop.

Does anyone know how to do, what i’m trying to do? (If it is even possible, i’m not sure).

Answer 1

You missed what makes the beauty of the visitor pattern: the accept(CarElementVisitor) method that must be in the CarElement interface, and which calls the visitor back with itself, using the approriate type. Re-read the wikipedia article.