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Anatomy of a Program with a Graphical UI

This lesson picks apart a simple Java program that has a graphical UI, explaining:

The program converts distance measurements between metric and U.S. units. Here is its source code. Below is the program. If your browser is Java-compatible, you'll see the program running as an applet. Otherwise, you'll see a picture of the program running as an application.

Classes in the Example Program

The example program defines three classes and creates instances of several classes that we provide. It defines an Applet subclass so that it can run as an applet. It creates Components to provide basic controls so that the user can interact with it. Using Containers and LayoutManagers, it groups the Components.

The Component Hierarchy

The Components in the example program are arranged in a hierarchy, with Containers defining the structure of the hierarchy.

Event Handling

User actions result in events, which are passed up the Component hierarchy until an object responds to the event.

Drawing

Components are drawn from the top of the hierarchy -- the program's window -- down to the non-Container Components.


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