Extras

Purpose

Additional resources and practice for mastering OOP principles.

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This section demonstrates abstraction, encapsulation, generalisation and inheritance working together in a single, coherent example.

Important

• In exams, students explain the principles.
• In projects, students apply them using code like this.

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1) Abstraction — modelling only what matters

Scenario

A system manages staff members in a school. Only relevant information is included.

C# code

public class StaffMember
{
    public string Name { get; set; } = "";
    public string StaffId { get; set; } = "";
}

Why this is abstraction

• Only essential attributes are included
• Irrelevant real-world details are excluded
• The model fits the purpose of the software

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2) Encapsulation — protect data and control access

Now we improve reliability by restricting direct access.

C# code

public class StaffMember
{
    private string staffId = "";   // private attribute (encapsulated)

    public string Name { get; set; } = "";

    public void SetStaffId(string id)
    {
        if (id != "")
        {
            staffId = id;          // controlled update
        }
    }

    public string GetStaffId()
    {
        return staffId;            // controlled access
    }
}

Why this is encapsulation

• Data and methods are bundled together
• Direct access to staffId is prevented
• Changes only occur through methods

📌 Textbook link

Encapsulation improves data security and reliability.

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3) Generalisation — identify shared features

Teachers and admin staff share common features, so we generalise them.

C# code

public class StaffMember
{
    protected string staffId = "";

    public string Name { get; set; } = "";

    public void SetStaffId(string id)
    {
        staffId = id;
    }

    public string GetStaffId()
    {
        return staffId;
    }
}

Why this is generalisation

• Common attributes and behaviours are placed in one class
• Duplication is reduced
• Maintenance is simplified

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4) Inheritance — specialised classes reuse the general class

Now we create specialised staff types.

C# code

public class Teacher : StaffMember
{
    public string Subject { get; set; } = "";
}

public class AdminStaff : StaffMember
{
    public string OfficeLocation { get; set; } = "";
}

Why this is inheritance

• Teacher is a StaffMember
• AdminStaff is a StaffMember
• Shared features are inherited automatically
• Specialised features are added where needed

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5) FULL mini-program (all principles together)

This example shows:

• abstraction (relevant features only)
• encapsulation (controlled access)
• generalisation (shared class)
• inheritance (specialised classes)

Complete working program

using System;

class Program
{
    static void Main()
    {
        Teacher t1 = new Teacher();
        t1.Name = "Ms Lim";
        t1.SetStaffId("T204");
        t1.Subject = "Mathematics";

        AdminStaff a1 = new AdminStaff();
        a1.Name = "Mr Patel";
        a1.SetStaffId("A118");
        a1.OfficeLocation = "Office B3";

        PrintStaffDetails(t1);
        PrintStaffDetails(a1);
    }

    static void PrintStaffDetails(StaffMember staff)
    {
        Console.WriteLine("Name: " + staff.Name);
        Console.WriteLine("ID: " + staff.GetStaffId());
        Console.WriteLine();
    }
}