Prev: Do Nothing | Next: Dialog Box Input-Output

Java: Dialog Box Output

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

  1 
  2 
  3 
  4 
  5 
  6 
  7 
  8 
  9 
 10 
 11 
 12 
 13 
 14 
 15 
 16 

// Description: This program shows a message in a dialog box.
// File:   dialogOutput/SecondProgram.java
// Author: Michael Maus
// Date:   29 Jan 2005

import javax.swing.*;

public class SecondProgram {

    public static void main(String[] args) {
        JOptionPane.showMessageDialog(null, "Hello Earthling");

        System.exit(0);  // Exit from program.
    }

}

Line 6 - import

One 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 (GUI) library - everything in javax.swing. Typical programs have a few import statements.

Line 11 - Display a dialog box

This line displays this dialog box. The predefined Java class, JOptionPane contains methods to display dialog boxes. A method is a group of Java statements for doing one particular thing. The "." following the class name is followed by the name of the method, showMessageDialog. Every method call must be followed by a parenthesized list of comma-separated arguments (often called parameters) that specify information the method 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. Text messages must be enclosed in quotes. Statements are generally followed by a semicolon.

Line 13 - 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. In some versions of Java, the program continues to sit in memory running, altho nothing is visible. Note that you can add a comment to the end of a statement.