Polymorphism Examples

We now consider several additional examples of polymorphism. If class Rectangle is derived from class Quadrilateral, then a Rectangle object is a more specific version of a Quadrilateral object. Any operation (e.g., calculating the perimeter or the area) that can be performed on a Quadrilateral object can also be performed on a Rectangle object. These operations can also be performed on other Quadrilaterals, such as Squares, Parallelograms and trapezoids. The polymorphism occurs when a program invokes a method through a superclass variableat execution time, the correct subclass version of the method is called, based on the type of the reference stored in the superclass variable. You will see a simple code example that illustrates this process in Section 10.3.

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 superclass called SpaceObject, which contains method draw. Each subclass implements this method. A screen-manager program 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 messagenamely, 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 polymorphic screen manager might use polymorphism to facilitate adding new classes to a system with minimal modifications to the system's code. Suppose that we want to add Mercurian objects to our video game. To do so, we must build a class Mercurian that extends SpaceObject 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), programmers can use polymorphism to include additional types that were not envisioned when the system was created.

With polymorphism, the same method name and signature can be used to cause different actions to occur, depending on the type of object on which the method is invoked. This gives the programmer tremendous expressive capability.

Software Engineering Observation 10.1

Polymorphism enables programmers to deal in generalities and let the execution-time environment handle the specifics. Programmers can command objects to behave in manners appropriate to those objects, without knowing the types of the objects (as long as the objects belong to the same inheritance hierarchy).

Software Engineering Observation 10.2

Polymorphism promotes extensibility: Software that invokes polymorphic behavior is independent of the object types to which messages are sent. New object types that can respond to existing method calls can be incorporated into a system without requiring modification of the base system. Only client code that instantiates new objects must be modified to accommodate new types.