Question

I need to implement classes to support a custom event capable of passing varying types/amounts of data based on the event type. For example,

With event Type A I need to pass 2 String names.
With event Type B I need to pass 3 String names and a double.

2 designs I’ve found:

1. An extension of EventObject that contains fields and methods for all of possible data (fields not required for some event types are left blank).

   // event constructors
   public MyEvent(Object source, int type, String a, String b) {}
   
   public MyEvent(Object source, int type, String a, String b, String c, double d) {}
   

2. An extension of EventObject that contains a parameter for an EventInfo object. EventInfo is a base class for change info. Methods firing specific events will pass along subclasses of EventInfo containing the necessary change information (listeners would be required to downcast).

   // event constructors
   public MyEvent(Object source, int type, EventInfo info) {}
   
   // base class for event info
   public class EventInfo {}
   

What other designs exist for implementing this? What are the pros and cons of each?

Thanks.

Answer 1

One common design would be to include a Map in your event class to keep parameters as well as a field to show the event type. Your event interface would therefore look like this:

public interface Event [
    public Object getParameter(String key);
    public void setParameter(String key, Object parameter);
    public String getType();
}

In your event handler class you can use getParameter() and getType() to retrieve the event’s parameters and event type.