First, we will focus on the union design pattern between ABall, WanderBall, CurveBall and StraightBall. In a union design pattern, we see that the superclass represents an abstraction of the union of its subclasses. For instance, a fruit is an abstraction of specific concrete classes such as apple and pear. A fruit embodies the common characteristics of its subclasses even if it doesn't completely describe the exact nature of those characteristics. In a fruit, there is a seed. However, the notion of "fruit" doesn't specify exactly the number, size, color or shape of its seed(s). It only specifies that it does indeed have a seed. Likewise, a fruit has the behavior of ripening. Apples, oranges, and bananas all ripen differently and at different rates. The abstract fruit notion does not specify the specific nature of the ripening behavior, just simply that it does have that behavior. In such, we see that we can never have a fruit that is not a specific class of fruit, such as an orange or grape.
Corresponding to the above notions, abstract classes in Java cannot be instantiated. Abstract classes are denoted by the abstract keyword in the class definition:
public abstract class AFruit {...}
By convention, the classnames of abstract classes always begin with "A".
In Ballworld, ABall is an abstract class representing a circular ball that has a number of properties: a color, a position, a velocity, etc. The abstract ball also has some defining behaviors, such as that all balls should paint a filled, colored circle when requested to display themselves on a graphics context (a panel). Likewise all balls know how to bounce of the walls of the container. These concrete behaviors are called "default behaviors" because all subclasses of ABall, such as StraightBall and CurveBall, automatically get these behaviors by default. One of the most common and useful reasons for using an abstract class is to be able to define the default behaviors for all the subclasses.
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