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In the first few installments of this column series, I plan to cover some basic object-oriented and structured design techniques as they apply to Java. Many of you undoubtedly are already familiar with these techniques, as they apply equally well to other languages. In my experience in the cubicle, however, I have encountered a lot of code written by programmers who, shall we say, could stand to take a refresher course on the basics. So I think it is important to cover the basics in the early articles of this column.
To maximize the usefulness of these first articles, I will be focusing on how the basic software design principles apply to Java in light of Java's architecture. For example, when I write about designing objects for proper cleanup, I'll discuss Java's finalization and garbage collection mechanisms and show how they affect design. When I write about designing with class variables and class methods, I'll describe Java's mechanisms for class loading and unloading and show how they affect design. For this article, which looks at designing objects for proper initialization, I've written an entire companion article that describes how object initialization works in the Java virtual machine (JVM). In all the articles of this column, I hope to show how the respective architectures of the Java language, virtual machine, and API affect how you should think about designing Java programs.
One way to think of objects is as finite state machines. Thinking of objects in this way as you design classes can help you acquire a mindset that is conducive to good object design.
For example, one finite state machine is a simple traffic light that has three states: red, yellow, and green. A state transition diagram for such a traffic light is shown in Figure 1 (a Java applet).
In Figure 1, states are represented by labeled circles, state changes by arrows, and events by labels next to the arrows. When the finite state machine experiences an event, it responds by performing the state change indicated by the arrow.
The finite state machine shown in Figure 1 could be represented by instances of the following Java class. (As a matter of fact, the following class is used to represent the traffic light finite state machine in the Java applet that is Figure 1. Click here to view the full source code of the traffic light applet.)