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Unformatted text preview: Chapter 6 Introduction to GUI Programming C omputer users today expect to interact with their computers using a graphical user interface (GUI). Java can be used to write GUI programs ranging from simple applets which run on a Web page to sophisticated stand-alone applications. GUI programs differ from traditional “straight-through” programs that you have encoun- tered in the first few chapters of this book. One big difference is that GUI programs are event-driven . That is, user actions such as clicking on a button or pressing a key on the keyboard generate events, and the program must respond to these events as they occur. Event- driven programming builds on all the skills you have learned in the first five chapters of this text. You need to be able to write the subroutines that respond to events. Inside these subrou- tines, you are doing the kind of programming-in-the-small that was covered in Chapter 2 and Chapter 3. And of course, objects are everywhere in GUI programming. Events are objects. Colors and fonts are objects. GUI components such as buttons and menus are objects. Events are handled by instance methods contained in objects. In Java, GUI programming is object-oriented programming. This chapter covers the basics of GUI programming. The discussion will continue in Chap- ter 12 with more details and with more advanced techniques. 6.1 The Basic GUI Application T here are two basic types of GUI program in Java: stand-alone applications and applets . An applet is a program that runs in a rectangular area on a Web page. Applets are generally small programs, meant to do fairly simple things, although there is nothing to stop them from being very complex. Applets were responsible for a lot of the initial excitement about Java when it was introduced, since they could do things that could not otherwise be done on Web pages. However, there are now easier ways to do many of the more basic things that can be done with applets, and they are no longer the main focus of interest in Java. Nevertheless, there are still some things that can be done best with applets, and they are still fairly common on the Web. We will look at applets in the next section. A stand-alone application is a program that runs on its own, without depending on a Web browser. You’ve been writing stand-alone applications all along. Any class that has a main() routine defines a stand-alone application; running the program just means executing this main() routine. However, the programs that you’ve seen up till now have been “command- line” programs, where the user and computer interact by typing things back and forth to each 225 226 CHAPTER 6. INTRODUCTION TO GUI PROGRAMMING other. A GUI program offers a much richer type of user interface, where the user uses a mouse and keyboard to interact with GUI components such as windows, menus, buttons, check boxes, text input boxes, scroll bars, and so on. The main routine of a GUI program creates one or more such components and displays them on the computer screen. Very often, that’s all it does.more such components and displays them on the computer screen....
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