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);
    }
}

Answer 2

Polymorphism is your friend.

class DocumentGenerator {
   public void generate(DocumentEvent ev){}
   public void generate(MainEvent ev){}
   //... and so on
}

Then just

 DocumentGenerator dg = new DocumentGenerator();

 // ....
 dg.generate(event);

Update

A number of people have raised the objection that you "have to know the kinds of event at compile time." And, yes, you clearly have to know what events you're interpreting at compile time of the generator part, when else would you be able to write the generating part?

These competing examples use Command pattern, which is fine, but means Events have to know the details not just of their representation but of how to print their representation. That means each class may have two kinds of requirements changes to which their sensitive: changes in what events represent, and changes in the way the events are represented in print.

Now, consider, for example, needing to internationalize this. In the Command-pattern case, you have to go to n classes for n different Event types and write new do methods. In the polymorphism case, the changes are localized to one class.

Naturally if you need to internationalize once, you may need many languages, which drive you to adding something like a Strategy to each class in the Command-pattern case, requiring now n classes × m languages; again, you need only have one strategy and one class in the polymorphism case.

There are reasons to choose either approach, but to claim the polymorphism approach is wrong is just incorrect.

Answer 3

Each event has a function, say do. Each subclass overrides do, to do (:P) the appropriate action. Dynamic dispatch does everything else afterwards. All you need to do, is call event.do()

Answer 4

I have no commenting rights and i dont know the exact answer. But is it just me or some ppl here suggest using overloading (which happens at compile time and therefore just generate compile error) to solve this problem?

Just an example. As you see, it will not compile.

package com.stackoverflow;

public class Test {
    static abstract class Event {}
    static class MailEvent extends Event {}
    static class DocEvent extends Event {}

    static class Dispatcher {
        void dispatchEvent(DocEvent e) {
            System.out.println("A");
        }

        void dispatchEvent(MailEvent e) {
            System.out.println("B");
        }
    }

    public static void main(String[] args) {
        Dispatcher d = new Dispatcher();
        Event e = new DocEvent();

        d.dispatchEvent(e);
    }

Answer 5

What's the problem with exploiting the method resolution order?

public void dispatchEvent(DocumentEvent e) {
    documentGenerator.gerenateDocument(event);
}

public void dispatchEvent(MailEvent e) {
    deliveryManager.deliverMail(event);
}

Let Java do the work of matching the correct argument type, then just dispatch the event properly.

Answer 6

This is a typical use case for Sum types, also known as tagged unions. Unfortunately, Java does not support them directly, so they have to be implemented using some variation of the visitor pattern.

interface DocumentEvent {
    // stuff specific to document event
}

interface MailEvent {
    // stuff specific to mail event
}

interface EventVisitor {
    void visitDocumentEvent(DocumentEvent event);
    void visitMailEvent(MailEvent event);
}

class EventDivider implements EventVisitor {
    @Override
    void visitDocumentEvent(DocumentEvent event) {
        documentGenerator.gerenateDocument(event);
    } 

    @Override
    void visitMailEvent(MailEvent event) {
        deliveryManager.deliverMail(event);
    }
}

Here we have defined our EventDivider, now to provide a dispatch mechanism:

interface Event {
    void accept(EventVisitor visitor);
}

class DocumentEventImpl implements Event {
    @Override
    void accept(EventVisitor visitor) {
        visitor.visitDocumentEvent(new DocumentEvent(){
            // concrete document event stuff
        });
    }
}

class MailEventImpl implements Event { ... }

public void divideEvent(Event event) {
    event.accept(new EventDivider());
}

Here I used maximum possible separation of concerns so that responsibility of each class and interface is one and only one. In real life projects DocumentEventImpl, DocumentEvent implementation and DocumentEvent interface declaration are usually merged into a single class DocumentEvent, but that introduces circular dependencies and forces some dependencies between concrete classes (and as we know, one should prefer to depend on interfaces).

Additionally, void should usually be replaced with a type parameter to represent the result type, like this:

interface EventVisitor<R> {
    R visitDocumentEvent(DocumentEvent event);
    ...
}

interface Event {
    <R> R accept(EventVisitor<R> visitor);
}

This allows one to use stateless visitors, which are very nice to deal with.

This technique allows to (almost?) always eliminate instanceof mechanically rather than having to figure out a problem-specific solution.

Answer 7

You could register each of your handler classes against each event type, and perform dispatch when event happens like this.

class EventRegister {

   private Map<Event, List<EventListener>> listerMap;


   public void addListener(Event event, EventListener listener) {
           // ... add it to the map (that is, for that event, get the list and add this listener to it
   }

   public void dispatch(Event event) {
           List<EventListener> listeners = map.get(event);
           if (listeners == null || listeners.size() == 0) return;

           for (EventListener l : listeners) {
                    l.onEvent(event);  // better to put in a try-catch
           }
   }
}

interface EventListener {
    void onEvent(Event e);
}

And then get your specific handlers to implement the interface, and register those handlers with the EventRegister.

Answer 8

You could have a Dispatcher interface, defined like

interface Dispatcher {
    void doDispatch(Event e);
}

with implementations like DocEventDispatcher, MailEventDispatcher, etc.

Then define a Map<Class<? extends Event>, Dispatcher>, with entries like (DocEvent, new DocEventDispatcher()). Then your dispatch method could be reduced to:

public void divideEvent(Event event) {
    dispatcherMap.get(event.getClass()).doDispatch(event);
}

Here's a unit test:

public class EventDispatcher {
    interface Dispatcher<T extends Event> {
        void doDispatch(T e);
    }

    static class DocEventDispatcher implements Dispatcher<DocEvent> {
        @Override
        public void doDispatch(DocEvent e) {

        }
    }

    static class MailEventDispatcher implements Dispatcher<MailEvent> {
        @Override
        public void doDispatch(MailEvent e) {

        }
    }


    interface Event {

    }

    static class DocEvent implements Event {

    }

    static class MailEvent implements Event {

    }

    @Test
    public void testDispatcherMap() {
        Map<Class<? extends Event>, Dispatcher<? extends Event>> map = new HashMap<Class<? extends Event>, Dispatcher<? extends Event>>();
        map.put(DocEvent.class, new DocEventDispatcher());
        map.put(MailEvent.class, new MailEventDispatcher());

        assertNotNull(map.get(new MailEvent().getClass()));
    }
}