OOP Terms and Java Language Basics for CBSE Class 9

1. Before Java: What are we trying to learn?

Java is a programming language. But in school-level learning, the first target is not to memorize syntax. The first target is to understand how to convert a real-life situation into a program.

So Java is not only about typing commands. Java is about building clear models of real things.

2. What is OOP?

OOP means Object-Oriented Programming. It is a style of programming where we divide the program into objects.

A school itself can be understood through objects:

3. OOP Terms with Realistic Examples

First realistic class: Student record

class Student {
    int rollNo;
    String name;
    int maths;
    int computer;

    int totalMarks() {
        return maths + computer;
    }

    double percentage() {
        return totalMarks() / 2.0;
    }

    void printReport() {
        System.out.println("Roll No: " + rollNo);
        System.out.println("Name: " + name);
        System.out.println("Total: " + totalMarks());
        System.out.println("Percentage: " + percentage());
    }
}

class Main {
    public static void main(String[] args) {
        Student s1 = new Student();
        s1.rollNo = 12;
        s1.name = "Riya Sharma";
        s1.maths = 88;
        s1.computer = 96;

        s1.printReport();
    }
}

Here, Student is the class. s1 is the object reference. rollNo, name, maths and computer are fields. totalMarks(), percentage() and printReport() are methods.

4. Four Pillars of OOP with School Examples

4.1 Encapsulation

Encapsulation means keeping related data and methods together inside one class.

class FeeAccount {
    private int totalFee;
    private int paidFee;

    FeeAccount(int totalFee, int paidFee) {
        this.totalFee = totalFee;
        this.paidFee = paidFee;
    }

    int dueAmount() {
        return totalFee - paidFee;
    }

    void printStatus() {
        System.out.println("Total Fee: " + totalFee);
        System.out.println("Paid Fee: " + paidFee);
        System.out.println("Due Amount: " + dueAmount());
    }
}

The fee data and fee calculations are packed together. That is encapsulation.

4.2 Abstraction

Abstraction means showing only what is needed and hiding unnecessary details.

When the school office prints a fee due report, the operator does not need to know every internal formula. The operator only calls:

account.printStatus();

The internal calculation is hidden inside the class.

4.3 Inheritance

Inheritance means creating a new class from an existing class. Use it when the new thing is a special type of the old thing.

class Person {
    String name;
    String phone;

    void printContact() {
        System.out.println(name + " - " + phone);
    }
}

class Student extends Person {
    int rollNo;

    void printStudentCard() {
        System.out.println("Roll No: " + rollNo);
        printContact();
    }
}

class Teacher extends Person {
    String subject;

    void printTeacherCard() {
        System.out.println("Subject: " + subject);
        printContact();
    }
}

A student and a teacher are both persons, so common information like name and phone can stay in the Person class.

4.4 Polymorphism

Polymorphism means one name can have different forms.

In a school marks system, the word average may be used for two subjects, three subjects, or five subjects.

class MarksCalculator {
    double average(int a, int b) {
        return (a + b) / 2.0;
    }

    double average(int a, int b, int c) {
        return (a + b + c) / 3.0;
    }

    double average(int a, int b, int c, int d, int e) {
        return (a + b + c + d + e) / 5.0;
    }
}

This is called method overloading. The method name is the same, but the number of inputs is different.

5. Other Important Parts of Java Language

5.1 Basic program structure

class Main {
    public static void main(String[] args) {
        System.out.println("Welcome to Java");
    }
}

5.2 Variables and data types

int rollNo = 15;
double percentage = 89.5;
char section = 'A';
boolean passed = true;
String studentName = "Arjun Verma";

int stores whole numbers, double stores decimal numbers, char stores one character, boolean stores true or false, and String stores text.

5.3 Input using Scanner

import java.util.Scanner;

class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        System.out.print("Enter roll number: ");
        int rollNo = sc.nextInt();

        System.out.print("Enter marks: ");
        int marks = sc.nextInt();

        System.out.println("Roll No " + rollNo + " scored " + marks);
    }
}

5.4 Conditions

int attendance = 78;

if (attendance >= 75) {
    System.out.println("Allowed to appear in exam");
} else {
    System.out.println("Attendance shortage");
}

5.5 Loops

for (int rollNo = 1; rollNo <= 5; rollNo++) {
    System.out.println("Printing admit card for roll no " + rollNo);
}

5.6 Arrays

int[] marks = {82, 91, 76, 88, 95};

int total = 0;
for (int i = 0; i < marks.length; i++) {
    total = total + marks[i];
}

System.out.println("Total marks = " + total);

6. Realistic Complete Examples

Example 1: Library book issue system

class LibraryBook {
    String title;
    String accessionNo;
    boolean issued;

    LibraryBook(String title, String accessionNo) {
        this.title = title;
        this.accessionNo = accessionNo;
        this.issued = false;
    }

    void issueBook() {
        if (issued) {
            System.out.println(title + " is already issued.");
        } else {
            issued = true;
            System.out.println(title + " has been issued.");
        }
    }

    void returnBook() {
        issued = false;
        System.out.println(title + " has been returned.");
    }
}

class Main {
    public static void main(String[] args) {
        LibraryBook book = new LibraryBook("Introduction to Java", "LIB-1021");
        book.issueBook();
        book.issueBook();
        book.returnBook();
    }
}

Example 2: Attendance checker

class AttendanceRecord {
    String studentName;
    int totalDays;
    int presentDays;

    AttendanceRecord(String studentName, int totalDays, int presentDays) {
        this.studentName = studentName;
        this.totalDays = totalDays;
        this.presentDays = presentDays;
    }

    double attendancePercentage() {
        return presentDays * 100.0 / totalDays;
    }

    void printEligibility() {
        System.out.println("Student: " + studentName);
        System.out.println("Attendance: " + attendancePercentage() + "%");

        if (attendancePercentage() >= 75) {
            System.out.println("Exam status: Allowed");
        } else {
            System.out.println("Exam status: Shortage");
        }
    }
}

class Main {
    public static void main(String[] args) {
        AttendanceRecord a = new AttendanceRecord("Meera", 200, 160);
        a.printEligibility();
    }
}

Example 3: School transport route

class BusRoute {
    int routeNo;
    String driverName;
    String[] stops;

    BusRoute(int routeNo, String driverName, String[] stops) {
        this.routeNo = routeNo;
        this.driverName = driverName;
        this.stops = stops;
    }

    void printRoute() {
        System.out.println("Route No: " + routeNo);
        System.out.println("Driver: " + driverName);
        System.out.println("Stops:");

        for (int i = 0; i < stops.length; i++) {
            System.out.println((i + 1) + ". " + stops[i]);
        }
    }
}

class Main {
    public static void main(String[] args) {
        String[] stops = {"Gate No. 1", "City Library", "Railway Crossing", "Main Market"};
        BusRoute route = new BusRoute(7, "Mr. Singh", stops);
        route.printRoute();
    }
}

7. Common Mistakes Students Make

int marks = 90;

if (marks == 90) {
    System.out.println("Excellent");
}

String house = "Blue";

if (house.equals("Blue")) {
    System.out.println("Blue house student");
}

String[] subjects = {"English", "Maths", "Computer"};
System.out.println(subjects[0]); // English

8. Practice Projects: From 0 to Infinity

9. Revision and Quick Quiz

Quiz 1

Quiz 2

Quiz 3