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Like applications, applets
are created from classes. However, applets do not have a
main method as an entry point, but instead,
have several methods to control specific aspects of applet execution.
This lesson converts an application from Lesson 2 to
an applet and describes the structure and elements of an
applet.
Application to Applet
The following code is the applet
equivalent to the LessonTwoB application from Lesson 2.
The figure below shows how the running applet looks. The structure and
elements of the applet code are discussed after the section on how to
run the applet just below.
import java.applet.Applet;
import java.awt.Graphics;
import java.awt.Color;
public class SimpleApplet extends Applet{
String text = "I'm a simple applet";
public void init() {
text = "I'm a simple applet";
setBackground(Color.cyan);
}
public void start() {
System.out.println("starting...");
}
public void stop() {
System.out.println("stopping...");
}
public void destroy() {
System.out.println("preparing to unload...");
}
public void paint(Graphics g){
System.out.println("Paint");
g.setColor(Color.blue);
g.drawRect(0, 0,
getSize().width -1,
getSize().height -1);
g.setColor(Color.red);
g.drawString(text, 15, 25);
}
}
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The SimpleApplet class is declared public
so the program that runs the applet (a browser
or appletviewer), which is not local to
the program can access it.
Run the Applet
To see the applet in action, you need an
HTML file with the Applet tag as follows:
<HTML>
<BODY>
<APPLET CODE=SimpleApplet.class WIDTH=200 HEIGHT=100>
</APPLET>
</BODY>
</HTML>
The easiest way to run the applet is with appletviewer shown below where simpleApplet.html is a file that contains the above HTML code:
appletviewer simpleApplet.html
Note:
To run an applet written with Java 2 APIs in a browser, the browser must
be enabled for the Java 2 Platform. If your browser is not enabled
for the Java 2 Platform, you have to use appletviewer to run the applet
or install Java
Plug-in. Java Plug-in lets you run applets on web pages
under the 1.2 version of the Java VM instead
of the web browser's default Java VM.
Applet Structure and Elements
The Java API Applet class provides what you need to
design the appearance and manage the behavior of an applet.
This class provides a graphical user interface (GUI) component
called a Panel and a number of methods.
To create an applet, you extend (or subclass) the Applet
class and implement the appearance and behavior you want.
The applet's appearance is created by drawing onto the Panel or by attaching other GUI components such as push buttons, scrollbars, or text areas to the Panel. The applet's behavior is defined by implementing the methods.
Extending a Class
Most classes of any complexity extend other classes. To extend another
class means to write a new class that can use the fields and methods
defined in the class being extended. The class being extended is the
parent class, and the class doing the extending is the child class.
Another way to say this is the child class inherits the fields and
methods of its parent or chain of parents. Child classes either call or
override inherited methods. This is called single inheritance.
The SimpleApplet class extends Applet class,
which extends the Panel class, which extends the
Container class. The Container class extends
Object, which is the parent of all Java API classes.
The Applet class provides the init,
start, stop, destroy,
and paint methods you saw in the example applet.
The SimpleApplet class overrides these methods
to do what the SimpleApplet class needs them to do.
The Applet class provides no functionality for
these methods.
However, the Applet class does provide functionality
for the setBackground method,which is called in the
init method. The call to setBackground is an
example of calling a method inherited from a parent class in contrast
to overriding a method inherited from a parent class.
You might wonder why the Java language provides methods without
implementations. It is to provide conventions for everyone
to use for consistency across Java APIs. If everyone
wrote their own method to start an applet, for example, but
gave it a different name such as begin or
go, the applet code would not be interoperable with
other programs and browsers, or portable across
multiple platforms. For example, Netscape and Internet Explorer know how
to look for the init and start
methods.
Behavior
An applet is controlled by the software that runs it.
Usually, the underlying software is a browser, but it can also
be appletviewer as you saw in the example. The
underlying software controls the applet by calling the
methods the applet inherits from the Applet class.
The init Method:
The init method is called when the applet is first
created and loaded by the underlying software. This method performs
one-time operations the applet needs for its operation such
as creating the user interface or setting the font.
In the example, the init method initializes the text
string and sets the background color.
The start Method:
The start method is called when the applet is visited such as
when the end user goes to a web page with an applet on it.
The example prints a string to the console to tell you the
applet is starting. In a more complex applet, the start
method would do things required at the start of the applet
such as begin animation or play sounds.
After the start method executes, the event thread
calls the paint method to draw to the applet's
Panel. A thread is a single sequential flow
of control within the applet, and every applet can run in
multiple threads. Applet drawing methods
are always called from a dedicated drawing and event-handling
thread.
The stop and destroy Methods:
The stop method stops the applet when
the applet is no longer on the screen such as when the end user
goes to another web page. The example prints a string to
the console to tell you the applet is stopping. In a more
complex applet, this method should do things like
stop animation or sounds.
The destroy method is called when the browser exits.
Your applet should implement this method to do final cleanup
such as stop live threads.
Appearance
The Panel provided in the Applet class
inherits a paint method from its parent
Container class. To draw something onto the Applet's
Panel, you implement the paint method to do
the drawing.
The Graphics object passed to the paint
method defines a graphics context for drawing on the
Panel. The Graphics object has methods
for graphical operations such as setting drawing colors, and drawing
graphics, images, and text.
The paint method for the SimpleApplet
draws the I'm a simple applet string in red inside a blue
rectangle.
public void paint(Graphics g){
System.out.println("Paint");
//Set drawing color to blue
g.setColor(Color.blue);
//Specify the x, y, width and height for a rectangle
g.drawRect(0, 0,
getSize().width -1,
getSize().height -1);
//Set drawing color to red
g.setColor(Color.red);
//Draw the text string at the (15, 25) x-y location
g.drawString(text, 15, 25);
}
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Packages
The applet code also has three import statements at the top.
Applications of any size and all applets use import
statements to access ready-made Java API classes in packages.
This is true whether the Java API classes come in the Java platform download,
from a third-party, or are classes you write yourself and store in
a directory separate from the program. At compile time, a program
uses import statements to locate and reference
compiled Java API classes stored in packages elsewhere on the local or
networked system. A compiled class in one package can have the same
name as a compiled class in another package. The package name
differentiates the two classes.
The examples in Lessons 1 and 2 did not need a package declaration to
call the System.out.println Java API class
because the System class is in the java.lang
package that is included by default. You never need
an import java.lang.* statement to use
the compiled classes in that package.
More Information
You can find more information on applets in the
Writing
Applets trail in
The Java
Tutorial.
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