Скачать или смотреть Java Event and Listeners

When you run Java GUI programs, the program interacts with the user and the events drive its execution. An event can be defined as a signal to the program that something has happened. Events are triggered either by external user actions, such as mouse movements, button clicks, and keystrokes, or by internal program activities, such as a timer. The program can choose to respond to or ignore an event.

Java uses a delegation-based model for event handling: a source object fires an event, and an object interested in the event handles the event. The latter object is called a listener
In NetBeans both events and listeners are automatically generated. For a button event handling just need to double click and both are generated