Question

I’m trying to figure out the correct approach (and pattern) that my current problem requires. Everything seems to lead me towards the visitor pattern, and wikipedia’s example is almost exactly what I need. However, my issue is that I’m retrieving these CarElements from the database and need to create the correct visitor depending on the type of CarElements I asked for. For example, if I retrieved a list of Windshields I want to pass in a CarElementWashVisitor. It feels like I’m going against the pattern here but I’m not sure what the correct approach is without checking the subclass type. I basically need some way to figure out what to do based on the runtime type of the object(s). Here’s a summarized portion of the wikipedia example that I linked to above:

interface CarElementVisitor {
    void visit(Wheel wheel);
    void visit(Engine engine);
    void visit(Body body);
    void visit(Car car);
}

interface CarElement {
    void accept(CarElementVisitor visitor); // CarElements have to provide accept().
}

class CarElementPrintVisitor implements CarElementVisitor { /**/ }

class CarElementDoVisitor implements CarElementVisitor { /**/ }

UPDATE:

Turns out that the issue is actually pretty simple (as illustrated by the accepted answer below). Here’s what I was looking for:

class MyVisitor implements CarElementVisitor {
    private MyService carwashService;

    /* visit(Wheel w), visit(Engine e)... etc */

    void visit(Windshield w) {
        carwashService.wash(w);
    }
}

Answer 1

Would something like this help?

class FindTheRightVisitorVisitor implements CarElementVisitor {
    private CarElementVisitor theVisitor;
    ... getter etc ...
    void visit(Windshield w) {
        if (theVisitors == null) {
            theVisitors = new CarElementWashVisitor();
        }
    }
}

And use here:

CarElement root = ...;
CarElementVisitor findIt = new FindTheRightVisitorVisitor();
root.accept(findIt);
CarElementVisitor theRightVisitor = findIt.getTheVisitor();
root.accept(theRightVisitor);