Visual Basic 2005 in a Nutshell, Third Edition

Principles of Object-Oriented Programming | 27

OOP

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Inheritance

Inheritance makes it possible for OOP code to build classes that extend or restrict

features in other existing classes, without the need to fully rewrite the original

class. For instance, a class of Pet may have generic data fields such as Name, Age,

and Color. This single class could be extended into other, more specific classes

through inheritance. A class named Dog that is derived from Pet would automati-

cally include the Name, Age, and Color members, but it may add additional

canine-specific members such as Breed and a ShedsHair flag. In this situation, the

Dog class inherits from the Pet class.

Inheritance used in this manner certainly reduces duplication of code, since the

derived class does not have to rewrite the code for the existing base class’s

members. But inheritance also makes interactions between these objects easier,

since an object of type Dog is also a true object of type Pet. Objects of a derived

class are also objects of the base class, and they can be used in code as if they were

actually members of the base class. (The reverse is not true; objects of type Pet are

not necessarily objects of type Dog).

Some languages allow a class to inherit from multiple base classes at the same

time. Visual Basic does ...