Writing an OOP program is essentially writing instructions that the computer will use to,
- create the virtual world of the object network, and
- provide it the inputs to produce the outcome you want.
A class contains instructions for creating a specific kind of objects. It turns out sometimes multiple objects keep the same type of data and have the same behavior because they are of the same kind. Instructions for creating a 'kind' (or ‘class’) of objects can be done once and those same instructions can be used to objects of that kind. You call such instructions a Class.
Classes and objects in an example scenario
Consider the example of writing an OOP program to calculate the average age of Adam, Beth, Charlie, and Daisy.
Instructions for creating objects Adam, Beth, Charlie, and Daisy will be very similar because they are all of the same kind: they all represent ‘persons’ with the same interface, the same kind of data (i.e. name, dateOfBirth, etc.), and the same kind of behavior (i.e. getAge(Date), getName(), etc.). Therefore, you can have a class called Person containing instructions on how to create Person objects and use that class to instantiate objects Adam, Beth, Charlie, and Daisy.
Similarly, you need classes AgeList, Calculator, and Main classes to instantiate one each of AgeList, Calculator, and Main objects.
| Class | Objects |
|---|---|
Person | objects representing Adam, Beth, Charlie, Daisy |
AgeList | an object to represent the age list |
Calculator | an object to do the calculations |
Main | an object to represent you (i.e., the one who manages the whole operation) |