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Introduction to Web Programming
Applet HTML  
  • After creating the Java code for an applet, you will need to compile it to create a Java .class files used to run the program. If you are using the JDK, the following command can be used to compile the above program (as you can see, it is just the same as for applications):

    > javac Hello.java
    

Make sure that you are in the same directory as the .java file you are compiling and make sure that you are careful to type the name of the .java file correctly.

  • When you compile the .java file, you should have a Hello.class file in the same directory as the .java file.

  • Now, you are ready to distribute your applet over the web. To do so, you must create an HTML page that displays the applet using the <APPLET></APPLET> HTML tag to define an applet which you would like the browser to run.

  • When a web browser interprets an HTML page sent to it from a web server, it begins decoding it right away. In other words, each tag that the browser encounters is read and interpreted. When the browser reaches the <APPLET> tag, it either ignores it if it does not understand Java, or it sends a request to the server to retrieve the applet code in the location specified by the tag.

  • For example, the following code gives an example of what an <APPLET> tag might look like in the body of an HTML page:

    
    <HTML>
    <HEAD>
    <TITLE>My Applet!</TITLE>
    </HEAD>
    <BODY>
    <APPLET CODEBASE = "http://www.x.com/MyApplet/"
       CODE = "MyApplet.class"
       ALT = "My Applet"
       NAME = "MyApplet"
       WIDTH = "100"
       HEIGHT = "100"
       ALIGN = "LEFT"
       VSPACE = "5"
       HSPACE = "5">
    <PARAM NAME = "phrase" VALUE = "Hello World!">
    . . .
    Here is some text which can be displayed
    if the browser does not support Java.
    </APPLET>
    </BODY>
    </HTML>
    
    

  • The following table goes over the attributes in the APPLET tag.

Attribute Description
CODE Specifies the name of the class file which starts your applet.
CODEBASE Specifies the base directory on the web server where your class files are stored.
ALT Specifies the text which should be displayed if the browser is capable of running Java applets but is unable to run your applet.
NAME Specifies the name of your aplet within the context of the HTML page and other applets which might be running on the same page.
WIDTH and HEIGHT Specifies the amount of space given for your applet in pixels.
ALIGN Specifies how your applet will be aligned within the browser window. You can pass the following values along with this attribute: TOP, BOTTOM, LEFT, RIGHT, TEXTTOP, MIDDLE, ABSMIDDLE, BASELINE, BOTTOM. These values are standard HTML values such as those for images. If you need an explanation check out an HTMl reference guide.
VSPACE and HSPACE Specifiy how much of a margin to give the applet in pixels.

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:

    
    <APPLET WIDTH = "200" HEIGHT = "100"
               CODE = "Hello.class">
    </APPLET>
    

  • 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:

    <APPLET WIDTH = "0" HEIGHT = "0"
            CODE = "HelloAwtApplet.class">
    </APPLET>
    

  • 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.

    
    <HTML>
    <HEAD>
    <TITLE>Hello</TITLE>
    </HEAD>
    <BODY>
    The Hello applet appears below:
    <P>
    <APPLET WIDTH = "200" HEIGHT = "100"
               CODE = "HelloAwtApplet.class">
    </APPLET>
    </BODY>
    </HTML>
    

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