Question

In all the Java source code examples I have looked at the listeners have always been declared in inner classes.

Why – what is the reason for coding the classes like this instead of having the listener(s) in their own seperate *.java file \ class?

Would having seperate classes for the listeners be considered a bad design?

If it’s not a bad design \ sackable offence could someone please post a short example demonstrating how to implement this?

Thank for reading.

Edit\Update – 10.8.2010:
Thanks to all who took the time to reply. Lots of insightful points to consider.
Having read all the answers I think that unless there is a very good reason for doing otherwise it is better and easier to declare listeners as inner classes.

Apologies for not coming back to this question sooner, but I don’t always have as much time for coding as I’d like 🙁

Happy coding.

Answer 1

Good reasons for using inner classes:

• Avoids adding extra top-level classes to your package namespace
• Keeps the code locally encapsulated (e.g. within the component that requires the event handling behaviour)
• static inner classes work well when they do not need to access fields of the enclosing class

Possible reasons for using top level classes:

• You have a special type of listener that you expect to be used by external packages or forms part of your API (e.g. in the Java class library itself)
• The listener is used in many places, and is not logically specific to any one of many possible enclosing classes

In short: inner classes are usually preferred, but if you have good reasons then it can be perfectly sensible to create top level classes instead.