Chapter 6 - Object-Oriented Programming (OOP)

6.1 Chapter Overview

Object-Oriented Programming, or OOP, is a programming approach that organizes software around objects. An object represents a real-world entity such as a student, employee, product, bank account, course or vehicle.

OOP helps developers build applications that are easier to maintain, reuse, expand and understand. It is widely used in C++, Java, Python, C#, PHP and many modern application development environments.

Learning Outcome: Apply object-oriented concepts in application development using classes, objects, attributes, methods, constructors, inheritance, encapsulation, polymorphism and abstraction.

6.2 OOP Concepts at a Glance

Concept	Meaning	Example
Class	Blueprint or template for creating objects.	Student class
Object	Actual instance created from a class.	student1, student2
Attribute	Data stored inside an object.	name, marks, age
Method	Function or behavior inside a class.	displayResult()
Constructor	Special method used to initialize an object.	Student("Amin", 85)
Inheritance	One class receives properties from another class.	Employee inherits Person
Encapsulation	Protecting data inside a class.	private balance
Polymorphism	Same method name behaves differently.	calculateSalary()
Abstraction	Showing essential features and hiding details.	Payment interface

6.3 Classes and Objects

A class is a blueprint. An object is a real item created from that blueprint. For example, Student can be a class, while Amin and Siti can be objects.

Working Example: Student Class and Object

C++

#include <iostream>
using namespace std;

class Student {
public:
    string name;
    int marks;

    void display() {
        cout << "Name: " << name << endl;
        cout << "Marks: " << marks;
    }
};

int main() {
    Student s1;
    s1.name = "Amin";
    s1.marks = 85;
    s1.display();
    return 0;
}

Java

class Student {
    String name;
    int marks;

    void display() {
        System.out.println("Name: " + name);
        System.out.println("Marks: " + marks);
    }
}

public class Main {
    public static void main(String[] args) {
        Student s1 = new Student();
        s1.name = "Amin";
        s1.marks = 85;
        s1.display();
    }
}

Python

class Student:
    def display(self):
        print("Name:", self.name)
        print("Marks:", self.marks)

s1 = Student()
s1.name = "Amin"
s1.marks = 85
s1.display()

Expected Output:
Name: Amin
Marks: 85

Note: C is not an object-oriented language in the same way as C++, Java and Python. OOP is usually taught with C++, Java and Python.

Online Working Example

C++ Java Python

Student Name Marks

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6.4 Attributes and Methods

Attributes store data inside an object. Methods define what an object can do. For example, a BankAccount object may have balance as an attribute and deposit() as a method.

Working Example: Bank Account

C++

class BankAccount {
public:
    double balance;

    void deposit(double amount) {
        balance = balance + amount;
    }

    void showBalance() {
        cout << "Balance: RM " << balance;
    }
};

Java

class BankAccount {
    double balance;

    void deposit(double amount) {
        balance = balance + amount;
    }

    void showBalance() {
        System.out.println("Balance: RM " + balance);
    }
}

Python

class BankAccount:
    def deposit(self, amount):
        self.balance = self.balance + amount

    def show_balance(self):
        print("Balance: RM", self.balance)

Online Working Example

C++ Java Python

Opening Balance Deposit Amount

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6.5 Constructors

A constructor is a special method that runs automatically when an object is created. It is commonly used to initialize object attributes.

Working Example: Course Constructor

C++

class Course {
public:
    string title;
    int duration;

    Course(string t, int d) {
        title = t;
        duration = d;
    }

    void display() {
        cout << title << " - " << duration << " days";
    }
};

Java

class Course {
    String title;
    int duration;

    Course(String t, int d) {
        title = t;
        duration = d;
    }

    void display() {
        System.out.println(title + " - " + duration + " days");
    }
}

Python

class Course:
    def __init__(self, title, duration):
        self.title = title
        self.duration = duration

    def display(self):
        print(self.title, "-", self.duration, "days")

Online Working Example

C++ Java Python

Course Title Duration in Days

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6.6 Inheritance

Inheritance allows one class to reuse attributes and methods from another class. The existing class is called the parent/base class, and the new class is called the child/derived class.

Working Example: Person and Trainer

C++

class Person {
public:
    string name;
};

class Trainer : public Person {
public:
    string subject;

    void display() {
        cout << name << " teaches " << subject;
    }
};

Java

class Person {
    String name;
}

class Trainer extends Person {
    String subject;

    void display() {
        System.out.println(name + " teaches " + subject);
    }
}

Python

class Person:
    def __init__(self, name):
        self.name = name

class Trainer(Person):
    def __init__(self, name, subject):
        super().__init__(name)
        self.subject = subject

    def display(self):
        print(self.name, "teaches", self.subject)

Online Working Example

C++ Java Python

Trainer Name Subject

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6.7 Encapsulation

Encapsulation means protecting data by keeping it inside a class and controlling access through methods. This prevents direct unsafe modification of important data.

Working Example: Protected Balance

C++

class Account {
private:
    double balance;

public:
    Account(double b) {
        balance = b;
    }

    double getBalance() {
        return balance;
    }
};

Java

class Account {
    private double balance;

    Account(double b) {
        balance = b;
    }

    public double getBalance() {
        return balance;
    }
}

Python

class Account:
    def __init__(self, balance):
        self.__balance = balance

    def get_balance(self):
        return self.__balance

Online Working Example

C++ Java Python

Account Balance

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6.8 Polymorphism

Polymorphism means “many forms.” It allows the same method name to behave differently depending on the object or class.

Working Example: Different Payment Types

C++

class Payment {
public:
    virtual void pay() {
        cout << "General payment";
    }
};

class CardPayment : public Payment {
public:
    void pay() override {
        cout << "Paid by card";
    }
};

Java

class Payment {
    void pay() {
        System.out.println("General payment");
    }
}

class CardPayment extends Payment {
    void pay() {
        System.out.println("Paid by card");
    }
}

Python

class Payment:
    def pay(self):
        print("General payment")

class CardPayment(Payment):
    def pay(self):
        print("Paid by card")

Online Working Example

C++ Java Python

Select language and click Run.

6.9 Abstraction

Abstraction means showing only essential features and hiding unnecessary details. In application development, abstraction helps developers focus on what an object does, not how it does it internally.

Working Example: Shape Area

C++

class Shape {
public:
    virtual double area() = 0;
};

class Rectangle : public Shape {
public:
    double length, width;
    double area() override {
        return length * width;
    }
};

Java

abstract class Shape {
    abstract double area();
}

class Rectangle extends Shape {
    double length, width;

    double area() {
        return length * width;
    }
}

Python

from abc import ABC, abstractmethod

class Shape(ABC):
    @abstractmethod
    def area(self):
        pass

class Rectangle(Shape):
    def __init__(self, length, width):
        self.length = length
        self.width = width

    def area(self):
        return self.length * self.width

Online Working Example

C++ Java Python

Length Width

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6.10 OOP Application Example

A student result system can be designed using OOP by creating a Student class with attributes and methods.

Online Working Example: Student Result Class

C++ Java Python

Student Name Marks 1 Marks 2 Marks 3

Select language and click Run.

6.11 Practical Activities

Activity 1: Student Class

Create a Student class with name, marks and display method.

Activity 2: Bank Account

Create a BankAccount class with balance, deposit and show balance methods.

Activity 3: Inheritance

Create a Person parent class and Trainer child class.

Activity 4: Encapsulation

Create a class with private balance and public getter method.

Mini Project

Build an OOP Student Result System using class, object, constructor, attributes and methods.

6.12 Interactive Final Assessment Quiz

Each correct answer gives +1 mark. Each wrong answer gives -0.5 mark. Correct answers are mixed.

1. A class is a blueprint for creating objects.

False True

2. Which OOP concept allows one class to reuse another class?

Encapsulation Polymorphism Inheritance Compilation

3. Attributes represent data stored inside an object.

True False

4. A constructor is usually used to initialize an object.

False True

5. Encapsulation means:

Repeating a loop Protecting data inside a class Drawing a chart Converting text to number

6. Polymorphism means the same method name can behave differently.

True False

7. Abstraction hides unnecessary implementation details.

False True

8. Which Python method is commonly used as a constructor?

main() start() __init__() print()

9. An object is an instance of a class.

True False

10. OOP cannot be used in application development.

True False

Your Score: 0

6.13 Chapter Summary

This chapter introduced Object-Oriented Programming concepts including classes, objects, attributes, methods, constructors, inheritance, encapsulation, polymorphism and abstraction.

Remember: OOP helps developers build organized, reusable and scalable applications by modeling real-world entities as objects.