oop - Avoiding 'instanceof' in Java

Question

I have the following (maybe common) problem and it absolutely puzzles me at the moment:

There are a couple of generated event objects which extends the abstract class Event and I want to divide them to Session Beans, like

public void divideEvent(Event event) {
    if (event instanceof DocumentEvent) {
        documentGenerator.gerenateDocument(event);
    } else if (event instanceof MailEvent) {
        deliveryManager.deliverMail(event);
        ...
    }
    ...

}

But there could be more than two event types in future, so the if-else will be long and maybe unreadable. Additionally I think instanceof is not really "best practice" in this case.

I could add an abstract method to the Event type and have them divide itself but then I have to inject the specific Session Beans within each entity.

Is there any hint to achieve a "pretty" solution for this problem?

Thanks for any help!

Answer 1

The simplest approach is to have the Event provide a method you can call so the Event knows what to do.

interface Event {
    public void onEvent(Context context);
}

class DocumentEvent implements Event {
    public void onEvent(Context context) {
         context.getDocumentGenerator().gerenateDocument(this);
    }
}

class MailEvent implements Event {
    public void onEvent(Context context) {
         context.getDeliveryManager().deliverMail(event);
    }
}


class Context {
    public void divideEvent(Event event) {
        event.onEvent(this);
    }
}