Graphical User Interfaces - Building Java Programs: A Back to Basics Approach - page 820

14.1 GUI Basics
GUIs are potentially very complex entities because they involve a large number of
interacting objects and classes. Each onscreen component and window is represented
by an object, so a programmer starting out with GUIs must learn many new class,
method, and package names. In addition, if the GUI is to perform sophisticated tasks,
the objects must interact with each other and call each other's methods, which raises
tricky communication and scoping issues.
Another factor that makes writing GUIs challenging is that the path of code execu-
tion becomes nondeterministic. When a GUI program is running, the user can click
any of the buttons and interact with any of the other onscreen components in any
order. Because the program's execution is driven by the series of events that occur,
we say that programs with GUIs are event-driven. In this chapter you'll learn how to
handle user events so that your event-driven graphical programs will respond appro-
priately to user interaction.
Graphical Input and Output with Option Panes
The simplest way to create a graphical window in Java is to have an option pane pop
up. An option pane is a simple message box that appears on the screen and presents a
message or a request for input to the user.
The Java class used to show option panes is called JOptionPane . JOptionPane
belongs to the javax.swing package, so you'll need to import this package to use it.
(“Swing” is the name of one of Java's GUI libraries.) Note that the package name
starts with javax this time, not java . The x is because, in Java's early days, Swing
was an extension to Java's feature set.
import javax.swing.*; // for GUI components
JOptionPane can be thought of as a rough graphical equivalent of
System.out.println output and Scanner console input. The following program
creates a “Hello, world!” message on the screen with the use of JOptionPane :
1 // A graphical equivalent of the classic "Hello world" program.
2
3 import javax.swing.*; // for GUI components
4
5 public class HelloWorld {
6 public static void main(String[] args) {
7 JOptionPane.showMessageDialog( null , "Hello, world!");
8 }
9 }
The program produces the following graphical “output” (we'll show screenshots
for the output of the programs in this chapter):