OOPs Concepts in Java

Think about a bank. It has accounts, customers, cards and transactions. Each account knows its own balance and can do things: accept a deposit, allow a withdrawal, show a statement. Object-oriented programming (OOP) writes software the same way: as a set of objects, each holding its own data and the actions that work on that data. Java is built around this idea, and Level 2 of this course is all about it.

Class and Object: The Foundation

public class Bank {
    static class BankAccount {
        String holder;
        long balance;
 
        BankAccount(String holder, long balance) {
            this.holder = holder;
            this.balance = balance;
        }
 
        void deposit(long amount) {
            balance += amount;
        }
 
        String summary() {
            return holder + ": Rs. " + balance;
        }
    }
 
    public static void main(String[] args) {
        BankAccount navika = new BankAccount("Navika", 5000);
        BankAccount srihitha = new BankAccount("Srihitha", 12000);
        navika.deposit(1500);
        System.out.println(navika.summary());
        System.out.println(srihitha.summary());
    }
}

Navika: Rs. 6500
Srihitha: Rs. 12000

How it works

1. Lines 2 to 18 define the class BankAccount: two fields, a constructor that fills them in, and two methods.
2. Lines 21 and 22 create two objects with new. Each has its own holder and balance.
3. Line 23 deposits into Navika's account only; Srihitha's balance is untouched.

Try it yourself: add a withdraw(long amount) method and take Rs. 2,000 out of Srihitha's account.

The Four Pillars

This chapter, OOP Fundamentals, builds the foundation: classes, objects, constructors, this, static and final. The next four modules take one pillar each.

Why OOP?

Common Mistakes

Thinking OOP means "put everything in classes". Java requires classes, but OOP is about objects that own their data and behavior, not about wrapping procedural code in a class.

Confusing class and object. "I created a class Navika" is wrong; you create an object of class BankAccount for Navika.

Learning the four pillars as definitions only. Each one solves a real problem; the next modules show which.

Interview Corner

Practice

1. Easy: List three fields and two methods for a Ticket class in a railway app.
2. Harder: For each of the four pillars, write one sentence about how it could apply to a food delivery app.

Key Takeaways