Applets
An applet is a piece of Java code that can be run under certain network
browsers (and appletviewer, a Java program). Applets are embedded in html
and invoked by viewing the page (or running appletviewer on the page).
Every applet extends java.applet.Applet, which in turn extends
java.awt.Panel. When an applet is invoked, an instance is created
(i.e., its constructor is called). No arguments are supplied to
the constructor; instead, there is html syntax for providing parameters
to applets. At applet creation time, three methods are called in sequence:
- the applet's constructor
- the applet's public void init() method.
- the applet's public void start() method.
Each of these is provided by java.applet.Applet, but can be
overridden by the subclass. The init method will be called exactly
once. The start method may be called repeatedly, e.g., each time
the applet scrolls off of and then back on to the page. Applets also inherit
stop and destroy methods (both public void,
no parameters) which are called when the applet temporarily disappears
or is permanently removed, respectively. It is conventional to start and
stop any Threads that the applet uses in the applet's start
and stop methods. In this sense, start serves some of
the role of public static void main( String[] ) in standalone
Java applications. (Other parts of that role may be played by init
or even by the constructor.)
The primary differences between applets and standalone applications
are:
- An instance of the applet is always created, and its constructor, init,
and start methods are always run. (These are the only things guaranteed
to run, but both stop and destroy may also be called.)
In addition, because an Applet instance is a Panel instance,
a visible component is created, awt events are (potentially) handled, etc.
- When a standalone application is invoked, only public
static void main( String[] ) (and code called by it) is run.
Other than this information, applets are largely outside the scope of
this course.
