What's the difference between Event Listeners & Handlers in Java?

There's no formally defined difference between listeners and handlers. Some people would probably argue that they are interchangeable. To me however, they have slightly different meaning.

A listener is an object that subscribes for events from a source. Cf. the observer pattern. Usually you can have many listeners subscribing for each type of event, and they are added through addXyzListener methods.

Example: The MouseListener in the Java API.

A handler is an object that is responsible for handling certain events. A typical scenario would be to provide a handler for a specific event/task as an argument to a constructor, or set the handler through a setXyzHandler method. In other words, you usually have one handler for each type of event.

Example: The MemoryHandler in the Java API.

The most basic difference is the association

• Listener is associated with Event Source (Ex: key board)
• Handler is associated with an Event (Ex: keydown)

Generally speaking, there will only one central Handler Manager which manages all the events, while in case of Listener each Entity which wants to listen, will have to manage their own Collection of listeners

This is the way I see it:

A listener watches for an event to be fired. For example, a KeyListener waits for KeyEvents, a MessageListener waits for messages to arrive on a queue and so on.

The handler is responsible for dealing with the event. Normally, listeners and handlers go hand-in-hand. For example, the KeyListener tells the ExitHandler that "the letter Q was pressed" and the handler performs logic such as cleaning up resources and exiting the application gracefully. Similary a ButtonClickListener would tell the same ExitHandler that the "Exit button was clicked". So, in this case you have two different events, two different listeners but a single handler.