Classes

A class is the fundamental element in an object oriented system.

All classes of an object oriented system are arranged in a class hierarchy with one root class at its top.

Campione and Walrath define a class in the following way:

What does that mean?

» blueprint: A class can't do anything on its own.

» defines: A class provides something that can be used later.

» objects: A class can only be used, if it had been "brought to life" by instantiating it.

Example: Class "Car"

As can be seen, the class, representing a car, has variables (brand, speed, gear, ...) and methods (accelerate, brake, ...). This class serves as a generic description of any existing car, because each real-world thing that is a car, has, for example, a speed, a number of wheels, ... And when driving a car, you can accelerate it or change the current gear.

But you don't drive a generic car with a number of wheels at a speed.

You usually drive a specific car with, for example, four wheels at 50 mph.

This specific car is an instance of the class "Car" and is then called an object.