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JAVA APPLET
Applets are built using an application framework.
You inherit from class Applet and override the appropriate methods.
Most of the time you’ll be concerned with only a few important methods
that have to do with how the applet is built and used on a Web page. These
methods are:
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Method
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Operation
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init( )
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Called when the applet is first
created to perform first-time initialization of the applet
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start( )
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Called every time the applet moves
into sight on the Web browser to allow the applet to start up its
normal operations (especially those that are shut off by stop( )).
Also called after init( ).
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paint( )
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Part of the base class Component
(three levels of inheritance up). Called as part of an update( )
to perform special painting on the canvas of an applet.
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stop( )
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Called every time the applet moves
out of sight on the Web browser to allow the applet to shut off
expensive operations. Also called right before destroy( ).
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destroy( )
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Called when the applet is being
unloaded from the page to perform final release of resources when
the applet is no longer used
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Consider the paint( ) method.
This method is called automatically when the Component (in this
case, the applet) decides that it needs to update itself – perhaps
because it’s being moved back onto the screen or placed on the screen
for the first time, or perhaps some other window had been temporarily
placed over your Web browser. The applet calls its update( )
method (defined in the base class Component), which goes
about restoring everything, and as a part of that restoration calls paint( ).
You don’t have to override paint( ), but it turns out to be
an easy way to make a simple applet, so we’ll start out with paint( ).
When update( ) calls paint( )
it hands it a handle to a Graphics object that represents the
surface on which you can paint. This is important because you’re limited
to the surface of that particular component and thus cannot paint outside
that area, which is a good thing or else you’d be painting outside the
lines. In the case of an applet, the surface is the area inside the
applet.
The Graphics object also has a set
of operations you can perform on it. These operations revolve around
painting on the canvas, so most of them have to do with drawing images,
shapes, arcs, etc. (Note that you can look all this up in your online Java
documentation if you’re curious.) There are some methods that allow you
to draw characters, however, and the most commonly used one is drawString( ).
For this, you must specify the String you want to draw and its
starting location on the applet’s drawing surface. This location is
given in pixels, so it will look different on different machines, but at
least it’s portable.
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