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Java Notes

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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// Description: This program shows a message in a dialog box.
// File:   dialogOutput/SecondProgram.java
// Author: Fred Swartz
// Date:   30 Aug 2005

import javax.swing.*;

public class SecondProgram {

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

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.

Console Output

Beginning textbooks often use console output. In this style text lines appear on the "console", which is often a DOS command window or a separate pane in the development system you're using. Console output doesn't work with normal Graphical User Interface (GUI) programs, so it's preferable to use dialog box output which is compatible.

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// File   : introductory/ConsoleOutput.java
// Purpose: This program shows a message on the console.
// Author : Michael Maus
// Date   : 29 March 2005

public class ConsoleOutput {
    
    public static void main(String[] args) {
        System.out.println("Hello, Earthling");
    }
}

No imports are required

The System class is automatically imported (as are all java.lang classes).

Line 9 - Write the output

You can write one complete output line to the console by calling the System.out.println() method. The argument to this method will be printed. println comes from Pascal and is short for "print line". There is also a similar print method which writes output to the console, but doesn't start a new line after the output.