Author: S S B Magesh Puvananthiran

When can we create sub/super classes in an Object Oriented Design?

Answer:

As every Object Pascal developer knows, inheritance is one of the fundamental 
concepts in Object Oriented Design. I’m not going to give you any explanation on 
what Inheritance is since everybody knows the definition already. Instead, I'm 
going to give you some of the tips in designing classes in the early stages of 
Object Oriented Design. 

In any project development, the analysis and design phases will be given importance 
in the initial stage.  In Object Oriented Design/Visual Modeling, once the team 
starts collecting information regarding the project, the team will identify the 
objects involved in each of the activities. 

At one stage, the team will have some sample classes for those objects identified. 
As the design stage matures, there would be more and more classes coming. 
Sometimes, you may need to inherit a new class from an existing one or you may need 
to group two classes into one. At this time, you may use the following 
concepts/techniques to create a sub/super class from existing classes: 

What is a Sub Class and Super Class

It's a class inherited/derived from another class. The new class(sub-class) will 
have all the properties/methods and events of the parent class(from which it 
inherited) and can have additional properties specific to this sub-class. The 
parent class is called Super Class. 

Let me explain this concept with an example. 

Let us suppose we have a class called TCitizen. 

The structure of TCitizen is something like this: 

1   TCitizen = class
2     SocialSecurityNo: string;
3     Name: string;
4     Age: integer;
5     Street: string;
6     City: string;
7     State: string;
8     Zip: integer;
9     {..................
10    ..................etc., }
11  end;


The above attributes are some of the common attributes you can have for a Citizen. 
This citizen could be anybody from a small child to an old man in a country. 

Let us suppose that we have, in our analysis, some Veterans information also. 
Veterans are people who were being in Army and/or some distinguished government 
services and retired now. Those Veterans are also part of normal citizens but they 
would have some special privileges. In this case, we can use the existing TCitizen 
class by adding the special privileges attributes for a Veteran but that would not 
be a better design. In this case, we can call this Veteran as a SPECIALIZED 
Citizen. So we can create a new SUB CLASS derived from the TCitizen, called 
TVeteranCitizen. 

The TVeteranCitizen class may look like something like this: 

12  TVeteranCitizen = class(TCitizen)
13    NoOfYearsOfService: integer;
14    Rewards: string;
15    Ranks: string;
16    DateRetired: TDateTime;
17    {.........................................
18    ......................................etc., }
19  end;


GENERALIZATION: Creating Super Classes 

Let me explain this also thru some sample classes. 

Let us suppose we are designing a library system and we identified two classes 
TStudent, containing student information, and TProfessor, containing professor 
information, among other classes. We take these two classes for our discussion. 

The structure of those two classes would be as follows: 

20  TStudent = class
21    StudentID: string;
22    Name: string;
23    Age: string;
24    Grade: string;
25    {........................
26    .....................etc., }
27  end;
28  
29  TProfessor = class
30    ProfID: string;
31    Name: string;
32    Age: string;
33    {....................
34    ................etc., }
35  end;


The system will allow both the students and professors to login using their student 
and professor ids and do the library related activities. The system will verify the 
student and professor ids at the time of login. 

Here we can GENERALIZE an information pertaining to both the classes as long as 
they both agree in their structure and type. I’m talking about the two attributes 
StudentID in TStudent and ProfID in TProfessor. In this case, they both are of same 
type: String. The second thing is that they should be both of the same 
structure/size string. If StudentID is of seven charactors length and ProfID is of 
four charactors length, then we cannot generalize this info. 

As long as they both agree on those two things, we can generalize. 

They both serve as a way to login to the system after verification. So we can 
GENERALIZE this information and create a SUPER CLASS with a name TUser and inherit 
both TStudent and TProfessor from TUser. 

Although this seems to be a simple issue, I just would like to share this with our Delphi Community.