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Nascent Java: GUI
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by James Aldridge, Ph.D.
Fort Worth Country Day School
Fort Worth, Texas
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FWCDS Pre-AP Java Programming
The API Documentation
A First GUI
Learn by Playing Around
There are a lot of ways to achieve "windowed" output with Java, but the easiest uses a Java class called JOptionPane. This class is used to provide simple I/O.
It is instructive to have a look at the Java API documentation. Go to the API specification for the JavaTM 2 Platform, Standard Edition, version 1.4.2 below to find this documentation. Find the entry for the JOptionPane class. Java classes are packaged into related groups. Note that JOptionPane is in the package javax.swing.
API specification for the Java 2 Platform, Standard Edition, version 1.4.2
The next information shows the inheritance hierarchy of the class. In this specific example, JOptionPane inherits certain data and methods from the class JComponent, which in turn inherits from the class Container, and so on. More about inheritance later.
Skip down to the "Method Summary." Here you will find method showMessageDialog(). Notice that it is labeled as a static method. Recall that this means we can use it without creating an instance of class JOptionPane.
Allow me to digress a bit on classes. Think of a class as a blueprint of a container for data and methods that operate on that data. To actually create a usable version of a class (an object of that class), you normally must create a class instance. However, with static methods, no such instance is needed. If this doesn't make a lot of sense just yet, hang in there.
Back to our JOptionPane method. Notice that there are several varieties of method showMessageDialog(). The Java compiler can tell which version is needed by looking at the parameter list. Methods with multiple versions differing only in the nature of their parameter lists are said to be overloaded.
A First GUI
Let's write the code for our simple GUI:
// MyFirstName MyLastName - 10 May 2002
// HelloGUI.java: a Java application to display
// the traditional message as a GUI
import javax.swing.*;
class HelloGUI
{
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public static void main(String args[])
{
JOptionPane.showMessageDialog(null, "Hello World!");
System.exit(0);
}
}
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The import statement makes all classes in the javax.swing package, including JOptionPane, available for our use. Swing is the newest group of Java GUI classes.
Notice how the static method is invoked: the name of the class (JOptionPane) followed by a dot and then the name of the method (showMessageDialog()).
The first parameter to the method is the name of the component (like a window) that is to contain the dialog box. Our box will be an orphan without a parent, so we write the keyword null here. The second parameter is the object to be displayed in the box, in our case an object of type String.
Our code has a second statement. Class System contains a number of useful methods (look at the API docs), including the static method exit(). This method halts the Java Virtual Machine, which is the software that takes the metacode file (*.class) and interprets it into native machine code for execution on the host platform. The method takes a single parameter of type int (integer) indicating the cause of the exit. By convention, a zero indicates a normal end of application.
Drop to a command prompt and compile/run this Java application.
Learn by Playing Around
Experiment with the JOptionPane class using the API docs as a guide. Start by using the static method showConfirmDialog() in lieu of showMessageDialog(). Play around a bit.
Next we'll have a look at a full-fledged GUI that can take user input, process it, and display a result.
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