This chapter introduced polymorphism—the ability to process objects that share the same base class in a class hierarchy as if they were all objects of the base class. The chapter discussed how polymorphism makes systems extensible and maintainable, then demonstrated how to use overridden methods to effect polymorphic behavior. We introduced the notion of an abstract class, which allows you to provide an appropriate base class from which other classes can inherit. You learned that an abstract class can declare abstract methods that each derived class must implement to become a concrete class. We also discussed that an app can use variables of an abstract class to invoke concrete derived-class implementations of abstract methods polymorphically. You also learned how to determine an object’s type at execution time. We showed how to create sealed methods and classes. Finally, the chapter discussed declaring and implementing an interface as another way to achieve polymorphic behavior, often among objects of different, unrelated classes.

You should now be familiar with classes, objects, encapsulation, inheritance, interfaces and polymorphism—the most essential aspects of object-oriented programming. Next, we take a deeper look at using exception handling to deal with runtime errors.