Programming in Simple Steps: the Class
Understanding the Concept of a Class in Program Development
Oct 19, 2008
For anyone new to the world of OOP (Object Oriented Programming) it can be little bit of a baptism by fire - not because classes are hard to work with, but because there are so many new terms to deal with. So, rather than jump straight in with data members and functions, inheritance and instantiation, it may be better to start with a simple example - the car.
An Example of a Class: the Car
Mention the phrase 'a xyz is a kind of car' to anyone and they will immediately have a general concept of what an 'xyz' will look like. It will have:
- four wheels on the ground
- a steering wheel
- an engine
- the body of the car will have particular color
The car will also be able to do particular things:
- start
- stop
- accelerate
- decelerate
It's easy to see that the car consists of a number of variables (or data members) and a number of methods (or member functions ), and it's therefore possible to build a stylised definition of the car - in other words the car class:
Now that there is a generic concept of a car then this can be used to produce a rather more specific concept - a Golden Eagle Jeep.
An Example of Inheritance: the Golden Eagle Jeep
At this point a car is still just a concept - an uninstantiated class - but that concept can be built on to produce a new class, the Golden Eagle Jeep:
The Golden Eagle Jeep class is based on a car class and so inherits all of the characteristics of a car (steering wheel, 4 wheels and a manual transmission), but then has some characteristics of its own (no doors and a 2.4 litre engine); and, at this point, it is still just a concept - the next stage is to create an object from the class.
An Example of an Object: My Jeep
About three decades ago Chrysler took the concept of the Golden Eagle Jeep and actually built one - they instantiated the class to produce an object; and that object is my_jeep:
The object my_jeep has all of the characteristics of the class golden_eagle_jeep and therefore also has all of the characteristics of the class car; however, the object itself can be modified once it has been created (for example having a 2.8 litre V6 engine instead of a 2.4 I4).
And so, if programmer were to say "My object is a Golden Eagle Jeep and is a type of car" then any other programmer will know (more or less) what a Golden Eagle Jeep is and how to use it.
Conclusion
A class:
- consists of data members (otherwise known as variables or fields)
- consists of member functions (or methods)
- may be derived from or include another class, in which case it inherits the characteristics of the former class
- cannot be used until it has been instantiated and made into an object
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