Introduction to Java : Building Your First Application part-3

Applet Tag by Example

Here is an example of the code we could use to place the Hello applet we created previously in a web page:

Notice that we gave the applet a specific width and height. For an applet to appear, you must give it a specific width and height. If you don’t want an applet to display (such as when your applet creates a popup frame right away), you can give it a width and height of 0:

However, normally you will want your applet to appear. In our case, we want the applet to be 200 by 100 pixels. Here is a full example of an HTML page we could use to display our Hello applet example.

Using the Applet Viewer

In addition to running the applet using a web browser such as Internet Explorer, you can view it using the “appletviewer” program that comes with the JDK. You can view an applet by executing a command such as:

Notice that we run appletviewer on the HTML file, not the applet itself. The appletviewer program looks for the applet tag in the HTML file you give it to determine what to display. It will not display the HTML around the applet, just the applet itself.

Applet Parameters

There is one more very handy trick that can be performed through HTML. Suppose we wanted to use our Hello applet in a little more generic manner. Instead of simply displaying “Hello Cyberspace”, suppose we wanted it to be able to display any phrase determined by the person who codes the HTML.

To do so, we need only make a simple modification to the Hello applet to change it to display some text we define in the HTML instead of having a hard-coded string defined in the code.

Specifically, you can pass parameters to an applet by specifying them in the applet’s HTML between the start applet tag and the end applet tag. For example the following HTML passes two parameters to the Hello applet:

The first parameter is named “text” and has a value of “Have a nice day”. The second parameter is named “speed” and has a value of “low”.

In your applet, you can access these parameters through the getParameter() method. This method is part of the Applet class.

Here we present a modified init() method for the Hello applet which gets the value to display from HTML parameters. In this case, we look for the parameter named “text” and pass the value to the constructor of the Text control:

Applet Restrictions

Since applets can execute within a web browser session, they are normally restricted in what they can do. Here are some of the basic applet restrictions:

  • An applet can’t read or write files on the local computer
  • An applet can’t communicate with machines other than the one hosting the applet
  • An applet can’t call native system code

These are the restrictions applets operate under by default. There are ways to grant permission to applets so that they can perform the above tasks, however, if you are creating a normal applet, you will probably be restricted in what you can do.