Java: Dialog Box Output

This is very similar to the first program, but it actually does something. The additional parts are described below.

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// SecondProgram.java
// Michael Maus, 25 August 2004
// This program shows a message in a dialog box.

import javax.swing.*;

public class SecondProgram {
    
    public static void main(String[] args) {
        JOptionPane.showMessageDialog(null, "Hello Earthling");
        
        System.exit(0);  // Stop GUI program
    }
}

Line 5 - import

A strength of Java is that it has many libraries or packages of predefined classes and methods to help you do things. Some of these are automatically known in every Java program, but you have to explicitly import others. This import statement tells the compiler that you will be using part of the Graphical User Interface libraries (everything in javax.swing). Typical programs have a few import statements.

Line 10 - Display a dialog box

This line will display a dialog box. There is a predefined Java class, JOptionPane which is used to display dialog boxes. This statement calls the showMessageDialog method in that class. Every method call must be followed by a parenthesized list of arguments (often called parameters) that tell the method information that it needs to perform its task.

The two arguments here are null, which we'll just ignore for the moment, and "Hello Earthling", which is the message we want to display. Any text messages must be enclosed in quotes. Statements are generally followed by a semicolon.

Line 12 - Stop the program.

When you reach the end of main, a program usually stops automatically. However, if you have any kind of GUI (Graphical User Interface) it is best to stop the program explicitly with a call to the System class's exit method. The 0 argument tells exit that this is a normal termination. Some books don't use this, but it's generally better to use it when you really want your program to stop. Note that you can add a comment to the end of a statement.