Introduction

• Key components of object-oriented programming are inheritance and polymorphism.
• Inheritance is a method for reusing software where new classes are created from existing ones by inheriting their attributes and behaviors, and then refining them.

  It therefore is especially worthwhile to carefully design, implement, and test classes: They will be well used.

• Polymorphism makes it easy to write programs in a general fashion, so that they can handle a variety of classes.
• Inheritance and polymorphism are powerful tools for creating simple software to perform complex functions.
• When using inheritance, the new class, called a subclass, inherits from an existing class, called a superclass.
• The new class itself has the potential of becoming a superclass for some other new class.
• Although a subclass generally adds instance variables and methods, it represents a more specific class of objects (e.g., rectangle might be the superclass of square).
• Every object of a subclass is also an object of its superclass (e.g., every square is a rectangle).
• In designing classes, we distinguish between "is a" relationships and "has a" relationships.

  The former (e.g., a square "is a" rectangle) suggest the use of inheritance; the latter suggest the use of composition (i.e., making complex objects out of simpler ones). For example, a car "has a" steering wheel.

• Class libraries are emerging that make it feasible to create bullet-proof, complex software rapidly and less expensively.