Objects vs Classes

Which one to use when

Introduction

I always wondered, “Although definition of a Class states that it's a Blueprint to create different Objects. In reality how to decide that something should be represented as a class or an object”. Object has State and Behaviour. Class defines what kind of states can be stored in the object but the behaviour definition is same.

Use Case - Modelling Vehicles

Let's take an example of three types of Vehicles : Bike, Car and Auto-Rickshaw. All three are specialised kinds of Vehicle. Based on the abstraction we can say that for all these vehicles to move they need wheels. Although the number of wheels differ based on the kind of vehicle. There are two different ways of modelling it :

Single Class Multiple Instances

Model all three using a non-abstract Vehicle class and have noOfWheels value as a state which will enable us to create three different instances which are identified based on the number of wheels

Separate Classes

Separate classes for all three types with a single abstract parent class Vehicle having noOfWheels as a Protected field which will be inherited by the child Classes.

Let's go with Single Class Multiple Instances Approach. Suppose we create on instance of Vehicle class with noOfWheels as two, then it's an instance of Bike. Similarly for Auto-Rickshaw and Car we can have value of noOfWheels instance variable as three and four respectively. The Objects approach suits so far since we have all three instances having different states but same behaviour of movement. Now let's say all three have different turning radius. It's also a state of the object hence we can define that as an instance variable. Hence we can still go ahead with Single Class Multiple Objects approach.

Class Modelling

Summary

Always follow the Generalisation to Specialisation approach ( TOP-DOWN ) for Agile Modelling. Deciding factor for going with classes or objects is whether something can be modelled as a state or a behaviour. Dictionary meaning of State is particular condition something has at that instant. It's a constant thing at that instant. Behaviour means to do something.