junit event driven testing

It's my first time on testing (yes, i've never run a test before, but it's never to late to get started, isn't it?)

I'd a parser that parses a buffer, and dispatch an event).

Calling the parser:

parser.parse((ByteBuffer) readBuffer.duplicate().flip(), key);

parse method:

@Override
public void parse(final ByteBuffer buffer, final SelectionKey key) {
    //Parse some stuff here and then dispatch a new event

    eventDispatcher.dispatch(new ConnectionDataReadyArgs(key, host.toString(), port));
}

event handler:

@Override
public void handle(final ConnectionDataReadyArgs args) {
    //Do stuff here with the recieved args
}

How do I create a junit test that checks if the arguments received on the event handler are right?

Thanks in advance.