Unit 3 · Classes and Objects · 9 min

Encapsulation

Encapsulation means keeping data private inside a class and giving controlled access through public getter and setter methods.

Why this matters

It protects your object from invalid data and lets you change the inside of a class later without breaking other code.

Real-life analogy

An ATM encapsulates the cash. You cannot reach inside the machine; you request money through buttons, and the machine checks your balance first.

Example

Java

class BankAccount {
    private String holder;
    private double balance;

    public BankAccount(String holder, double balance) {
        this.holder = holder;
        this.balance = balance;
    }

    public String getHolder() {
        return holder;
    }

    public double getBalance() {
        return balance;
    }

    public void deposit(double amount) {
        if (amount <= 0) {
            System.out.println("Invalid deposit amount");
            return;
        }
        balance += amount;
    }

    public void withdraw(double amount) {
        if (amount > balance) {
            System.out.println("Insufficient balance");
            return;
        }
        balance -= amount;
    }
}

public class Main {
    public static void main(String[] args) {
        BankAccount acc = new BankAccount("Ananya Rao", 5000);
        acc.deposit(1500);
        acc.withdraw(10000);
        System.out.println(acc.getHolder() + " balance: " + acc.getBalance());
    }
}

Output

Insufficient balance
Ananya Rao balance: 6500.0

Step by step

1Both fields are private, so `acc.balance = -999;` is impossible from outside.
2getHolder() and getBalance() are getters: read-only access.
3deposit() and withdraw() are the only ways to change balance, and each validates first.
4withdraw(10000) is rejected because the balance is only 6500.

Key points

private fields + public methods = encapsulation
Getters read, setters write, and setters can validate
Also called data hiding
Makes future changes safe

Common mistakes

Writing getters and setters for everything without any validation
Leaving one field public and breaking the whole protection

Try it yourself

Encapsulate a Student class so marks can never be set below 0 or above 100.
Add a toString()-style display method that shows account details safely.

Open the practice notebook