If class Rectangle is derived from class Quadrilateral (a four-sided shape), then a Rectangle is a more specific version of a Quadrilateral. Any operation (e.g., calculating the perimeter or the area) that can be performed on a Quadrilateral object also can be performed on a Rectangle object. These operations also can be performed on other Quadrilaterals, such as Squares, Parallelograms and Trapezoids. The polymorphism occurs when an app invokes a method through a base-class variable—at execution time, the correct derived-class version of the method is called, based on the type of the referenced object. You’ll see a simple code example that illustrates this process in Section 12.3.

As another example, suppose we design a video game that manipulates objects of many different types, including objects of classes Martian, Venusian, Plutonian, SpaceShip and LaserBeam. Imagine that each class inherits from the common base class SpaceObject, which contains method Draw. Each derived class implements this method. A screen-manager app maintains a collection (e.g., a SpaceObject array) of references to objects of the various classes. To refresh the screen, the screen manager periodically sends each object the same message—namely, Draw. However, each object responds in a unique way. For example, a Martian object might draw itself in red with the appropriate number of antennae. A SpaceShip object might draw itself as a bright silver flying saucer. A LaserBeam object might draw itself as a bright red beam across the screen. Again, the same message (in this case, Draw) sent to a variety of objects has many forms of results.

A screen manager might use polymorphism to facilitate adding new classes to a system with minimal modifications to the system’s code. Suppose we want to add Mercurian objects to our video game. To do so, we must build a Mercurian class that extends Space-Object and provides its own Draw method implementation. When objects of class Mercurian appear in the SpaceObject collection, the screen-manager code invokes method Draw, exactly as it does for every other object in the collection, regardless of its type, so the new Mercurian objects simply “plug right in” without any modification of the screen-manager code by the programmer. Thus, without modifying the system (other than to build new classes and modify the code that creates new objects), you can use polymorphism to include additional types that might not have been envisioned when the system was created.