Introduction to Web Design - 17. Java Applets
17.1 A Simple Java Example
  1. Preface
  2. Markup Languages – A Definition and Some History
  3. Beginning HTML
  4. HTML Lists
  5. HTML Tables
  6. HTML - Color, Fonts and Special Characters
  7. HTML Links
  8. HTML Images
  9. HTML Frames
  10. Cascading Style Sheets
  11. MicroSoft PhotoDraw
  12. JavaScript
  13. HTML Forms and Form Handling
  14. VBScript
  15. MicroSoft FrontPage
  16. Active Server Pages
  17. Java Applets
  18. XML Meaning and More
  19. Macromedia Flash 5.0
  20. References
Although Java and JavaScript have similar names and a couple of similar types of statements, they are very different programming languages. Java is a much more powerful language that was first developed for implementing programs on appliances like Cable TV boxes, microwaves, etc. They wanted the language to be independent of the processor that the programs would be run on, since the processors used in consumer applicances might be vary. They wanted the language to be tight and efficient (not requiring a lot of memory or processing power).

Java programs are somewhat unique in that they are machine independent. The Java compiler translates Java code into byte-code, rather than machine code. Byte-code is an intermediate translation that does not include machine-specific details. To run the byte-code, we can either use a Java interpreter or run the byte-code on a Java-enabled browser. If we use a browser, the program is referred to as an applet, rather than an application. The Java compiler can be obtained free from Sun .

Java is a strongly typed language. That means that you must declare variables to be a certain data type and they cannot hold data of a different type. Java is object-oriented and, in fact, all functions (or methods) must belong to some class in a Java program. There are no stand-alone functions in Java.

17.1 A Simple Java Example

There are three basic steps to create and run a Java applet.

  1. Create the Java Program file using a text editor such as Notepad. For an example, see the file “welcome.java” below.
  2. Create the byte-code for the program using a Java compiler. This should create a file with an extension of .class. The javac command can be accessed on the SVC CIS computers at c:\Program Files\jdk1.3.1\bin. Go to that directory and type in: javac c:\cs105\java\filename.java (or whatever your path and filename are). If there are no errors, a file called filename.class will be created. It is this filename that is used in the applet tag.
  3. Include the class as the value of the code attribute of the <applet > … </applet> tags. See the example file “java.htm” below. 

    //File - welcome.java
import java.awt.*;
import java.awt.event.*;
import java.applet.*;

public class welcome extends Applet implements ActionListener
{
  //create a textbox for user first name
  private Label fnamePrompt = new Label("What is your first name?");
  private TextField fnameField = new TextField(12);
  private String message = "";

  public void init()
  {
    //initializes the webpage
    add(fnamePrompt);
    add(fnameField);
    fnameField.addActionListener(this);
  }


  public void actionPerformed(ActionEvent e)
  {//respond to user entering name
    String fname = fnameField.getText();
    message = "Welcome to my first java applet, " + fname;
    fnameField.setText(" ");
    repaint();
  }


  public void paint(Graphics g)
  {//put message on the screen
    g.drawString(message, 1, 80);
  }
}





    <!--HTML File with Java Applet-->
<html>
<!-- java.htm -->
<head>
<title> Java Applet Example </title>
</head>
<body>
<h2>This page implements a short Java applet. </h2>
<applet  alt = "Java applet should be here."
    align = "CENTER"
    code = "welcome.class"
    width = 200
    height = 300 >
</applet>
</body>
</html>


    References

    1. Dietel, H. M., Dietel, P. J. & Neito, T. R. (2001) Internet & World Wide Web: How to Program. 2nd Edition. Prentice Hall, NJ.

Cynthia J. Martincic
cynthia.martincic@email.stvincent.edu
CIS Department
Saint Vincent College
Latrobe, PA 15650