Graphics and Animation Examples

This article looks at a short animation example from Section 5.3 of the text. The example is a program called GrowingCircleAnimation.java, and when it runs it displays an animation of circles that grow over time. The circles are semitransparent, which means they are partly see-through.

The text presents this example as an animation first and a programming example second. That order is useful to keep in mind. Start by asking what appears on the screen, and only then ask how the program is organized to produce it. The source describes the example by its purpose, its visual behavior, and one dependency. It does not walk through the internal code, so this article stays with those three things and does not guess at details the source leaves out.

The defining feature of this example is that the circles change while the program runs. A single still picture of a circle cannot show growth. Growth is a change in a circle from one moment to the next, so the learner has to watch the sequence of moments to see it.

Animation of this kind is generally produced by repeating a short cycle: draw the current picture, pause briefly, then draw an updated picture. Each pass through the cycle is one frame. The source does not show the code that carries out this cycle, so treat the diagram below as the general mental model for how a moving picture is built from repeated steps, not as a literal trace of the file.

Each trip around the loop leaves the circle a little larger than before. Because the picture is redrawn many times in quick succession, the eye reads the series of slightly different pictures as one circle that grows. The growth is not a single event in the program. It is many small redraws that look continuous when watched together.

Semitransparency is a visual property of the circles. The source tells us the circles are semitransparent and that they grow over time. It does not say how transparency is set in the program, and it does not say whether the transparency changes during the animation. Those are questions the source leaves open, so this article does not answer them.

The text offers this program as a simple example of programming with objects. Its purpose in the text is practical. It lets a learner see objects being used inside a working graphical program, rather than only in a short exercise with no visible result.

This is why the animation is worth watching before reading any code. The moving circles give the learner a concrete result to connect with the idea of objects. When the learner later meets the program's structure, the question to keep asking is how that structure produces the growing, semitransparent circles seen on screen.

The one structural fact the source states is that GrowingCircleAnimation.java requires CircleInfo.java. The animation program depends on a second file, and the two files together form the example. Neither file is the whole example on its own.

The dependency matters because the example is a pair of files working together. A learner who runs or studies only GrowingCircleAnimation.java has not studied the whole example. Knowing which file is required tells the learner where to look for the rest of the program's support.