When you create a class, you subclass from or extend an existing defined class. The class that you subclass or extend must be visible to your application. That is, the existing class must be a public class and it must be defined in one of the following:

Classes that meet at least one of the above criteria are, understandably, called visible classes.

When a class is visible to an application, the application can reuse that class as follows:

A class's definition can be incorrect if the existing class is not visible to your class's containing application. Similarly, a class's behavior can be incorrect if its methods refer to a class that is not visible.