Extended Examples
As in the simple pseudo-constructor example given previously, class ShopPanel uses an extension of the Frame class called ShopFrame. The code for ShopFrame is as follows:
Click here to view the first .java code example
Notice that no variables are declared in ShopFrame. It does not contain a constructor. It will be instantiated by means of a pseudo-constructor, contained in ShopPanel.
The core of the Open Source shopping cart program is contained in class ShopPanel. This is probably where most future program modifications would occur.
Click here to view the second .java code example
As mentioned in the pseudo-constructor example, the shopping cart works just as well if a normal, rather than an abstract class, is used. I keep the class abstract because I am then warned, by the fact that the program won't compile, whenever I step outside of the proper rules for generating persistence by means of a pseudo-constructor.
You will notice that some graphics commands are repeated twice. This is because either Netscape or Explorer is not responding correctly to the first command. Probably, there is a timing problem with the peer. Repeating the command at a later point appears to solve the problem. Open Source developers may wish to experiment extensively if they choose to change the position of these repetitions.
Notice that all variables are declared here in ShopPanel, and that many of them are static. Single-stepping through program execution will demonstrate clearly how a simple reference to ShopPanel in the applet "app" causes all static variables in ShopPanel to be instantiated before ShopPanel is ever given control of program execution; everything is always defined, and it is done in the proper order.
A call to "parent.isActive()" is used to determine whether the applet, in its updated form, has been reloaded by the current HTML page. This check could be ommitted, but a security exception would then be thrown by any call to showDocument() that occurred when off-site, and this would need to be handled by the browser. I felt it was better to do things explicitly.
As you notice from the code, the program attempts to load a file called "update.txt" from the directory in which the applet "app" is located. Here's a sample "update.txt":
"1",$14.97
"2",$22.93
"3",$6.22
"4",$5.55
"5",$6.66
"6",$7.77
"update.txt" is a text file written in a format consistent with a two-field Access database, in text format. It allows a merchant to alter prices temporarily, should he want to offer a sale. This file can be multiple megabytes in size. It is cached by the applet. Page to page transfer in HTML is always instantaneous.
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