Java Reference
In-Depth Information
Chapter 18
Handling Events
WHAT YOU WILL LEARN IN THIS CHAPTER:
• What an event is
• What an event-driven program is and how it is structured
• How events are handled in Java
• How events are categorized in Java
• How components handle events
• What an event listener is and how you create one
• What an adapter class is and how you can use it to make programming the handling of events easier
• What actions are and how you use them
• How to create a toolbar
In this chapter you learn how a window-based Java application is structured and how to respond to user ac-
tions in an application or an applet. This is the fundamental mechanism you use in virtually all of your graph-
ical Java programs. When you understand how user actions are handled in Java, you are equipped to imple-
ment the application-specific code that is necessary to make your program do what you want.
INTERACTIVE JAVA PROGRAMS
Before you get into the programming specifics of interactive window-based applications you need to under-
stand 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 an interactive program with a
graphical user interface (GUI). With a console program, you start the program and the program code determ-
ines the sequences of events. Generally everything is predetermined. You enter data when required, and the
program outputs data when it wants. At any given time, the specific program code that executes next is gen-
erally 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 GUI. Selecting menu items or buttons using the mouse, or through the
keyboard, causes particular actions within the program. At any given moment, the user has a whole range of
possible interactions available, each of which results in a different program action. Until you do something,
the specific program code that is to be executed next is not known.
