Computational Problem Solving with Java and Interactive Object Graphics

The DoodlePad object graphics library includes a collection of classes that encapsulate graphic objects, including a series of shape classes. Rectangle, Oval, RoundRect, Text, Polygon, etc. are among the shape classes in the DoodlePad library.

Think about how you might design these shape classes. The obvious difference is the unique visual representation of each. Class implementations must include a method capable of drawing its unique and recognizable graphic, with adjustments and customizations defined by other object instance variables and their values.

What are the other features of shape classes that must be encapsulated? Every shape needs an (x, y) location and the dimensions (width, height). In DoodlePad we decided that these parameters would describe a shape’s bounding box and that the shape would be drawn within that bounding box. Shape location is the location of the upper left corner of the bounding box and shape dimensions match the dimensions of the bounding box. These four instance variables (x, y, width, height) are common to all shape classes. Does that mean we must repeat these instance variable declarations in all shape classes?

Other shape features include fill color, stroke color, stroke width, whether the shape may be interactively selected or dragged, the text drawn at its center as well as font family, font size, font style, text colors, transformation, etc. If you consider this list, each one of these features is shared by all shape classes. Does that mean we must declare every one of these instance variables in every class definition? That is quite a lot of code repetition, which, by now, should tell you that something is not quite right. All this code repetition multiplies the effort to maintain your code and increases the chances of making a mistake.

Of course, there is a good solution to this code repetition problem. Inheritance gives us a way to share a single instance variable declaration or method implementation with multiple classes.

DoodlePad graphic classes are organized in a hierarchy with a Shape class at its root. For your reference, Figure 9.1.1 illustrates this hierarchy. In addition to the Shape class and all its derived classes, DoodlePad implements several additional classes, including Pad, Timer and Sound. You can find details for all classes in the library’s JavaDocs¹.