Ivor Horton's Beginning Java™ 2, JDK™ 5th Edition

18.1. Window-Based Java Programs

Before you get into the programming specifics of window-based programs, you need to understand a little of how such programs are structured, and how they work. There are fundamental differences between the console programs that you have been producing up to now and a window-based Java program. With a console program, you start the program, and the program code determines the sequences of events. Generally everything is predetermined. You enter data when required, and the program will output data when it wants. At any given time, the specific program code that will execute next is generally known.

A window-based application, or an applet for that matter, is quite different. The operation of the program is driven by what you do with the graphical user interface (GUI). Selecting menu items or buttons using the mouse, or through the keyboard, causes particular actions within the program. At any given moment you have a whole range of possible interactions available to you, each of which will result in a different program action. Until you do something, the specific program code that is to be executed next is not known.

18.1.1. Event-Driven Programs

Your actions when you're using the GUI for a window-based program or an applet—clicking a menu item or a button, moving the mouse, and so on—are first recognized by the operating system. For each action, the operating system determines which of the programs currently running on your computer should know ...