Create a file named HelloWorld.java with the Java code shown here:
import java.applet.Applet;
import java.awt.Graphics;
public class HelloWorld extends Applet {
public
void paint(Graphics g) {
g.drawString("Hello world!", 50, 25);
}
}
Compile the Source File
Compile the source file using the Java compiler.
Platform-Specific Details: Compiling a Java Source File Using the JDK
UNIX:
javac HelloWorld.java
then Change your permissions to be world readable:
chmod 755 *
DOS shell (Windows 95/NT):
javac HelloWorld.java
MacOS:
Drag the HelloWorld.java file icon onto the Java Compiler icon.
If the compilation succeeds, the compiler creates a file named
HelloWorld.class in the same directory (folder) as the Java source
file
(HelloWorld.java). This class file contains Java bytecodes.
Create an HTML File that Includes the Applet
Using a text editor, create a file named Hello.html in the same directory
that contains HelloWorld.class. This HTML file should contain the
following text:
<HTML>
<HEAD>
<TITLE> A Simple Program </TITLE>
</HEAD>
<BODY>
Here is the output of my program:
<APPLET CODE="HelloWorld.class" WIDTH=150 HEIGHT=25>
</APPLET>
</BODY>
</HTML>
Run the Applet
To run the applet, you need to load the HTML file into an application
that can run Java applets. This application might be a Java-compatible
browser or another Java applet viewing program, such as the Applet Viewer
provided in the JDK. To load the HTML file, you usually need to tell the
application the URL of the HTML file you've created. For example, you might
enter
something like the following into a browser's URL or Location field:
http://www.cs.buffalo.edu/~perini/class/javamain.html
Once you've successfully completed these steps, you should see something
like this in the browser window:
The Anatomy of a Java Applet
Now that you've seen a Java applet, you're probably wondering how it works. Remember that a Java applet is a program that adheres to a set of conventions that allows it to run within a Java-compatible browser.
Here again is the code for the "Hello World" applet.
import java.applet.Applet;
import java.awt.Graphics;
public class HelloWorld extends Applet {
public void paint(Graphics g) {
g.drawString("Hello world!", 50, 25);
}
}
Importing Classes and Packages
The code above starts off with two import statements. By importing
classes or packages, a class can more easily refer to classes in other
packages. In the Java language, packages are used to group classes,
similar to the way libraries are used to group C functions. Importing
Classes and Packages gives you more information about packages and the
import statement.
Besides importing individual classes, you can also import entire packages.
Here's an example:
import java.applet.*;
import java.awt.*;
public class HelloWorld extends Applet {
public void paint(Graphics g) {
g.drawString("Hello world!", 50, 25);
}
}
In the Java language, every class is in a package. If the source code for a class doesn't have a package statement at the top, declaring the package the class is in, then the class is in the default package.
Defining an Applet Subclass
Every applet must define a subclass of the Applet class. In the "Hello
World" applet, this subclass is called HelloWorld. Applets inherit
a great
deal of functionality from the Applet class, ranging from communication
with the browser to the ability to present a graphical user interface
(GUI).
The first bold line of the following listing begins a block that defines
the
HelloWorld class.
import java.applet.Applet;
import java.awt.Graphics;
public class HelloWorld extends Applet {
public void paint(Graphics g) {
g.drawString("Hello world!", 50, 25);
}
}
The extends keyword indicates that HelloWorld is a subclass of the class whose name follows: Applet.
From the Applet class, applets inherit a great deal of functionality. Perhaps most important is the ability to respond to browser requests. For example, when a Java-capable browser loads a page containing an applet, the browser sends a request to the applet, telling the applet to initialize itself and start executing.
An applet isn't restricted to defining just one class. Besides the necessary Applet subclass, an applet can define additional custom classes. When the applet attempts to use a class, the application that's executing the applet first looks on the local host for the class. If the class isn't available locally, it's loaded from the location that the Applet subclass originated from.
Implementing Applet Methods
The HelloWorld applet implements just one method, the paint method.
Every applet must implement at least one of the following methods:
init,
start, or paint. Unlike Java applications, applets do not need to implement
a main method.
The bold lines of the following listing implement the paint method.
import java.applet.Applet;
import java.awt.Graphics;
public class HelloWorld extends Applet {
public void paint(Graphics g) {
g.drawString("Hello world!", 50, 25);
}
}
Every applet must implement one or more of the init, start, and paint
methods.
Besides the init, start, and paint methods, applets can implement two
more
methods that the browser calls when a major event occurs (such as leaving
the applet's page): stop and destroy. Applets can implement any number
of other methods, as well.
Returning to the above code snippet, the Graphics object passed into
the paint method represents the applet's onscreen drawing context. The
first argument to the Graphics drawString method is the string to draw
onscreen. The second and third arguments are the (x,y) position of the
lower left corner of the text onscreen. This applet draws the string "Hello
world!" starting at location (50,25).
The applet's coordinate system starts at (0,0), which is at the upper
left corner of the applet's display area.
Running an Applet
Applets are meant to be included in HTML pages. Using the <APPLET>
tag, you specify (at a minimum) the location of the Applet subclass
and
the dimensions of the applet's onscreen display area. When a Java-capable
browser encounters an <APPLET> tag, it reserves onscreen space for the
applet, loads the Applet subclass onto the computer the browser is executing
on, and creates an instance of the Applet subclass.
The bold lines of the following listing comprise the <APPLET> tag that includes the "Hello World" applet in an HTML page.
<HTML>
<HEAD>
<TITLE> A Simple Program </TITLE>
</HEAD>
<BODY>
Here is the output of my program:
<APPLET CODE="HelloWorld.class" WIDTH=150 HEIGHT=25>
</APPLET>
</BODY>
</HTML>
The above <APPLET> tag specifies that the browser should load the class whose compiled code is in the file named HelloWorld.class. The browser looks for this file in the same directory as the HTML document that contains the tag.
When the browser finds the class file, it loads it over the network, if necessary, onto the computer the browser is running on. The browser then creates an instance of the class. If you include an applet twice in one page, the browser loads the class file once and creates two instances of the class.
The WIDTH and HEIGHT attributes are like the same attributes in an <IMG>
tag:
They specify the size in pixels of the applet's display area. Most
browsers do not let the applet resize itself to be larger or smaller than
this display area. For example, every bit of drawing that the "Hello World"
applet does in its paint method occurs within the 150x25-pixel display
area that the above <APPLET> tag reserves for it.
The <APPLET> Tag:
When you build <APPLET> tags, keep in mind that words such as APPLET
and
CODEBASE can be typed in either as shown or in any mixture of uppercase
and
lowercase. Bold font indicates something you should type in exactly
as shown
(except that letters don't need to be uppercase). Italic font indicates
that you must
substitute a value for the word in italics. Square brackets ([ and
]) indicate that the
contents of the brackets are optional.
< APPLET
[CODEBASE = codebaseURL]
CODE = appletFile
[ALT = alternateText]
[NAME = appletInstanceName]
WIDTH = pixels
HEIGHT = pixels
[ALIGN = alignment]
[VSPACE = pixels]
[HSPACE = pixels]
>
[< PARAM NAME = appletParameter1 VALUE = value
>]
[< PARAM NAME = appletParameter2 VALUE
= value >]
. . .
[alternateHTML]
</APPLET>
CODEBASE = codebaseURL
This optional attribute specifies the base URL of the applet -- the
directory or folder that contains the applet's code. If this attribute
is not specified, then the document's URL is used.
CODE = appletFile
This required attribute gives the name of the file that contains the
applet's
compiled Applet subclass. This file is relative to the base URL of
the applet. It cannot be absolute.
ALT = alternateText
This optional attribute specifies any text that should be displayed
if the browser
understands the APPLET tag but can't run Java applets.
NAME = appletInstanceName
This optional attribute specifies a name for the applet instance, which
makes it possible for applets on the same page to find (and communicate
with) each other.
WIDTH = pixels
HEIGHT = pixels
These required attributes give the initial width and height (in pixels)
of the
applet display area, not counting any windows or dialogs that the applet
brings
up.
ALIGN = alignment
This required attribute specifies the alignment of the applet. The
possible
values of this attribute are the same (and have the same effects) as
those for
the IMG tag: left, right, top, texttop, middle, absmiddle, baseline,
bottom,
absbottom.
VSPACE = pixels
HSPACE = pixels
These optional attributes specify the number of pixels above and below
the applet (VSPACE) and on each side of the applet (HSPACE). They're treated
the same way as the IMG tag's VSPACE and HSPACE attributes.
< PARAM NAME = appletParameter1 VALUE
= value >
<PARAM> tags are the only way to specify applet-specific
parameters. Applets read user-specified values for parameters with the
getParameter() method. See Defining and Using Applet Parameters for information
about the getParameter() method.
alternateHTML If the HTML page containing this <APPLET> tag is viewed
by a browser that doesn't understand the <APPLET> tag, then the browser
will ignore the <APPLET> and <PARAM> tags, instead interpreting any
other HTML code between the <APPLET> and </APPLET> tags. Java-compatible
browsers ignore this extra HTML code. You use the ALT=
tag to give an alternate to the java applet.
In the online version of this tutorial, we use alternate HTML to show a snapshot of the applet running, with text explaining what the applet does. Other possibilities for this area area link to a page that is more useful for the Java-ignorant browser, or text that taunts the user for not having a Java-compatible browser.
For CS 763(Web Seminar) at SUNY Buffalo