Learn Java: Passing parameters

Developer's Daily

Java Education

front page | java | perl | unix | DevDirectory


	Front Page Java Education Pure Java Articles

Passing parameters from an HTML file to a Java applet

Introduction

Many times when you're developing Java applets, you want to pass parameters from an HTML page to the applet you're invoking. For instance, you may want to tell the applet what background color it should use, or what font to use, to keep your applet consistent with the rest of your web site.

A great benefit of passing parameters from HTML pages to the applets they call is portability. If you pass parameters into your applets, they can be portable from one web page to another, and from one web site to another. On the other hand, if you don't pass parameters into your Java applets, how will the applet know what font to use? Simple - you hardwire it into the Java source code! As I often say to my new puppy at home, "this is bad".

In this brief article, we'll show you how to pass several parameters from an HTML page to a Java applet. Hopefully you'll be able to use this same technique to make your own applets more flexible and portable.

The sample HTML file

Listing 1 shows the source code for the sample HTML file we created for this article. We named this file ParamTest.html.

<HTML>
<HEAD>
<TITLE>Passing parameters to Java applets</TITLE>
</HEAD>
<BODY>
<APPLET CODE="ParamTest.class" WIDTH="400" HEIGHT="50">
    <PARAM NAME="font"    VALUE="Dialog">
    <PARAM NAME="size"    VALUE="24">
    <PARAM NAME="string" VALUE="Hello, world ... it's me.   :)">
</APPLET>
</BODY>
</HTML>

Listing 1: ParamTest.html - an HTML file that calls a Java applet, and passes several parameters to the applet via the <PARAM> tag.

As you can see, the HTML file identifies the Java applet that we want to load - ParamTest.class. Then, the only thing we need to do to pass parameters to the applet are to include the name and value of the desired parameters in <PARAM> tags. These names and values can be whatever we want - whatever makes sense for your applet. As a final note, notice that the <PARAM> tags are enclosed within the <APPLET> tags.

The ParamTest.java file

So far we've seen to create the HTML code that (1) invokes a Java applet and (2) passes parameters to the applet. Next, let's look at the Java code needed to read the parameters being passed to it. Listing 2 shows the Java code for the ParamTest.java file.

As you can see, this is a fairly simple applet. It simply retrieves the parameters passed to it with the getParameter() method, then uses those parameters to construct it's output. In this case the few parameters we've passed in deal with the font and the string to be printed. In a more elaborate example we might pass in more parameters dealing with the applet colors.

import java.applet.*;
import java.awt.*;
public class ParamTest extends Applet {
   public void paint(Graphics g) {
      String myFont   = getParameter("font");
      String myString = getParameter("string");
      int mySize      = Integer.parseInt(getParameter("size"));
      Font f = new Font(myFont, Font.BOLD, mySize);
      g.setFont(f);
      g.setColor(Color.red);
      g.drawString(myString, 20, 20);
   }
}

Listing 2: The source for ParamTest.java demonstrates how parameters can be passed into Java applets.

Try the on-line demo, and download the source

If you'd like to see the ParamTest applet in action, click here. Of course, once that page is loaded you can also save the HTML source code once it's loaded into your browser.

Click here if you'd like to view/download the ParamTest.java source code.

Listing 1:	`ParamTest.html` - an HTML file that calls a Java applet, and passes several parameters to the applet via the `<PARAM>` tag.

Listing 2:	The source for `ParamTest.java` demonstrates how parameters can be passed into Java applets.

What's Related