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Polymorphism is a feature of object-oriented systems that provides great flexibility and consistency. Polymorphism refers to situations where the same message is sent to objects from different classes, and different methods are executed. For example, any 2-dimension shape such as a circle or square have an area, but the way they are calculated differs from shape to shape. Building on the concept of a programmer may wish to use a set of classes designed and implemented as programs by someone else, but they may not wish to have to worry about the complexity of the implementation of each class. If a programmer were writing a 2D drawing program they might wish to use a library of pre-written classes for shapes, each of which provides a "getArea()" method. The programmer can then happily send a "getArea()" message to whichever graphic object he or she is wanting to work with, secure in the knowledge that whichever object is chosen it will return a calculation of the area of the shape. In the following sample of a Java program, both lines look very similar and both objects are being sent the same message. areaOfShape1 = shape1.getArea(); areaOfShape2 = shape2.getArea(); However, the actual methods executed might be different, if shape1 is based on a Circle class and shape2 on a Square class.
Looking at out example for LibraryItem classes, it might be that the charge for late returns is not simply an attribute set and stored for each individual object, but set according to the sub-class of the object. So it might be that objects of subclass TextItem always have a late return charge of 0.05 pounds per day, while objects of subclass Recording have a late return charge of 50% of their weekly loan fee. In such a situation sending an LibraryItem object the message getLateReturnCharge() would result in polymorphism, since depending on which subclass the object was an instance of, different methods would be executed for this same message. Another example: For example, to calculate the monthly pay for a lecturer, one might execute a method to divide the annual salary by 12; however, for a 0.5 part time lecturer one would need to further reduce the 12 of annual salary by half again (or an appropriate amount for a 0.1 or 0.75 part time lecturer). In both cases the classes would have an operation called something like caclulateMonthlyPay(), but the actual methods implemented for the Lecturer class, and the PartTimeLecturer subclass would be different. An important advantage of polymorphism is that it is possible to design systems in such a way that it becomes easy for adding to a system at a later date. For example:
Polymorphism is made possible through the dynamic and late binding of methods to objects when a program is executing (rather than at compile time).
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