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Encapsulation in Java – Complete Tutorial

Last Updated on: 17th Nov 2025 15:31:43 PM

Welcome to this beginner-friendly tutorial on Encapsulation in Java!
Encapsulation is one of the four fundamental principles of OOP (Object-Oriented Programming). It is the technique of bundling data (variables) and methods (behavior) that operate on the data into a single unit (class), and restricting direct access to some of the object’s components.

 

This tutorial uses a real-world project-based example — a Secure Online Banking System — to show how encapsulation is used in actual production applications.

 

What Is Encapsulation?

Encapsulation means wrapping data (variables) and code (methods) into a single unit (class) and restricting direct access to the data using access modifiers like private.

 

The data can be accessed only through public methods (getters & setters).

 

It means:

  • Keep the internal details (fields) of a class private

  • Provide public methods (getters/setters) to access or modify them safely

  • Hide how things work, expose only what is needed

 

Why Encapsulation Is Important?

✔ Protects sensitive data
✔ Helps achieve security
✔ Controls how data is read/modified
✔ Makes code maintainable
✔ Helps implement real-world objects correctly
✔ Avoids accidental changes in important fields

 

Why Use Encapsulation?

  Benefit

  Explanation

 Data Protection

 Prevent invalid values (e.g., negative balance)

 Flexibility

 Change internal logic without breaking code

 Security

 Hide sensitive data (password, account number)

 Maintainability

 Easy to debug and update

 

Encapsulation With Getters & Setters (Main Concept)

Encapsulation uses:

  • private variables → hidden from outside

  • public getter and setter methods → controlled access

 

Encapsulation Example

class BankAccount {
    private String accountHolder; // private = data hiding
    private double balance;

    // Setter Method
    public void setAccountHolder(String name) {
        accountHolder = name;
    }

    // Getter Method
    public String getAccountHolder() {
        return accountHolder;
    }

    // Setter Method
    public void setBalance(double amount) {
        if (amount > 0) {
            balance = amount;
        } else {
            System.out.println("Invalid Amount");
        }
    }

    // Getter Method
    public double getBalance() {
        return balance;
    }
}

public class Main {
    public static void main(String[] args) {
        BankAccount acc = new BankAccount();

        acc.setAccountHolder("Amit Sharma");
        acc.setBalance(50000);

        System.out.println("Account Holder: " + acc.getAccountHolder());
        System.out.println("Balance: ₹" + acc.getBalance());
    }
}

 

Output:

Account Holder: Amit Sharma
Balance: ₹50000

 

Here:

  • accountHolder and balance are private, so no one can access them directly.

  • Only allowed through getter & setter methods → controlled access.

 

Without Encapsulation (Bad Practice)

class Student {
    public String name;
    public int age;
}

public class Main {
    public static void main(String[] args) {
        Student s = new Student();
        s.age = -10;   // ❌ logically wrong
    }
}

 

Anyone can change age to -10 or name to an empty value → not secure.

 

Encapsulation Fix (Proper Way)

class Student {
    private String name;
    private int age;

    // Setter with validation
    public void setAge(int a) {
        if (a > 0) {
            age = a;
        } else {
            System.out.println("Age must be positive");
        }
    }

    public int getAge() {
        return age;
    }
}

 

Real-Time Project: Secure Online Banking System

Let’s build a BankAccount class with full encapsulation.

 

Step 1: Poor Design (Without Encapsulation)

// BAD EXAMPLE - NO ENCAPSULATION
public class BadBankAccount {
    public String accountNumber = "ACC123";
    public double balance = 10000;
    public String pin = "1234";

    public void withdraw(double amount) {
        balance -= amount;
    }
}

// Usage (Dangerous!)
BadBankAccount acc = new BadBankAccount();
acc.balance = -999999;     // Allowed! Negative balance
acc.pin = "0000";          // Anyone can change PIN
System.out.println(acc.balance); // Direct access

 

→ Anyone can break the system!

 

Step 2: Perfect Encapsulation (Real Project Style)

// BankAccount.java - FULLY ENCAPSULATED
public class BankAccount {
    // 1. Private fields - hidden from outside
    private String accountNumber;
    private String accountHolder;
    private double balance;
    private String pin;
    private boolean isLocked = false;
    private int failedAttempts = 0;

    // 2. Public constructor
    public BankAccount(String accountNumber, String accountHolder, String pin, double initialDeposit) {
        this.accountNumber = accountNumber;
        this.accountHolder = accountHolder;
        this.pin = pin;
        this.balance = initialDeposit;
        System.out.println("Account created for " + accountHolder);
    }

    // 3. Public getter methods (Read-only access)
    public String getAccountNumber() {
        return accountNumber;
    }

    public String getAccountHolder() {
        return accountHolder;
    }

    public double getBalance() {
        return balance;
    }

    public boolean isLocked() {
        return isLocked;
    }

    // 4. Public controlled methods (Safe operations)
    public boolean authenticate(String enteredPin) {
        if (isLocked) {
            System.out.println("Account is locked. Contact bank.");
            return false;
        }
        if (enteredPin.equals(this.pin)) {
            failedAttempts = 0;
            return true;
        } else {
            failedAttempts++;
            if (failedAttempts >= 3) {
                isLocked = true;
                System.out.println("Too many wrong attempts! Account locked.");
            }
            System.out.println("Wrong PIN! Attempts: " + failedAttempts);
            return false;
        }
    }

    public void deposit(double amount) {
        if (amount > 0) {
            balance += amount;
            System.out.println("Deposited: ₹" + amount + " | New Balance: ₹" + balance);
        } else {
            System.out.println("Invalid deposit amount");
        }
    }

    public void withdraw(double amount, String pin) {
        if (!authenticate(pin)) {
            return;
        }
        if (amount > 0 && amount <= balance) {
            balance -= amount;
            System.out.println("Withdrawn: ₹" + amount + " | Remaining: ₹" + balance);
        } else if (amount > balance) {
            System.out.println("Insufficient balance!");
        } else {
            System.out.println("Invalid amount");
        }
    }

    public void changePin(String oldPin, String newPin) {
        if (!authenticate(oldPin)) {
            return;
        }
        if (newPin.length() == 4 && newPin.matches("\\d+")) {
            this.pin = newPin;
            System.out.println("PIN changed successfully!");
        } else {
            System.out.println("New PIN must be 4 digits");
        }
    }

    // Optional: Admin method (not public)
    private void unlockAccount() {
        isLocked = false;
        failedAttempts = 0;
        System.out.println("[ADMIN] Account unlocked");
    }
}

 

Step 3: Main Application – OnlineBankingApp.java

// OnlineBankingApp.java
public class OnlineBankingApp {
    public static void main(String[] args) {
        System.out.println("SECURE ONLINE BANKING SYSTEM\n");

        // Create account
        BankAccount account = new BankAccount("SBI123456", "Rahul Sharma", "4567", 15000);

        // Safe operations
        account.deposit(5000);
        account.withdraw(3000, "4567");     // Correct PIN
        account.withdraw(3000, "9999");     // Wrong PIN
        account.withdraw(3000, "8888");     // Wrong again
        account.withdraw(3000, "7777");     // 3rd wrong → Account locked!

        // Try after lock
        account.withdraw(1000, "4567");     // Blocked

        // Safe read
        System.out.println("Balance: ₹" + account.getBalance());
        System.out.println("Holder: " + account.getAccountHolder());

        // Cannot do this! (Compile error)
        // account.balance = -100000;     // private!
        // account.pin = "1111";          // private!
        // account.isLocked = false;      // private!

        // Change PIN safely
        account.changePin("4567", "8901");

        System.out.println("\nEncapsulation protected the account!");
    }
}

 

Output

SECURE ONLINE BANKING SYSTEM

Account created for Rahul Sharma
Deposited: ₹5000.0 | New Balance: ₹20000.0
Withdrawn: ₹3000.0 | Remaining: ₹17000.0
Wrong PIN! Attempts: 1
Wrong PIN! Attempts: 2
Too many wrong attempts! Account locked.
Account is locked. Contact bank.
Balance: ₹17000.0
Holder: Rahul Sharma
Account is locked. Contact bank.
New PIN must be 4 digits

Encapsulation protected the account!

 

Key Encapsulation Rules Followed

  Rule

  Implemented?

  How

 Private fields

 Yes

 All data is private

 Public getters/setters

 Yes

 Controlled access

 Validation in methods

 Yes

 PIN check, amount check

 Hide internal logic

 Yes

 authenticate() is internal

 No direct field access

 Yes

 Impossible from outside

 

Real Life Example of Encapsulation

  • ATM machine shows balance, but you cannot modify the bank’s balance directly.

  • All sensitive data is hidden behind methods.

  • You interact only through predefined functions like withdraw, deposit, check balance.

 

That is Encapsulation.

 

Benefits of Encapsulation

  Benefit   Description
 Data Hiding  Prevents others from accessing sensitive data
 Security  Protect data from unauthorized access
 Controlled Access  Data can be accessed only via getters & setters
 Improves Flexibility  You can change internal logic without affecting outer code
 Clean Code  Better organization and maintainability

 

When Should You Use Encapsulation?

Use it whenever:

  • Data needs protection (password, balance, age, salary)

  • You want control over how values are set

  • You want flexibility to change implementation later

  • You're building real-world systems (banking, HR, shopping cart, login system, etc.)

 

Conclusion

Encapsulation is a core pillar of OOP in Java.
It hides sensitive data using private and provides controlled access using public getters and setters.

It improves:

  • Security

  • Flexibility

  • Maintainability

  • Data integrity

Using encapsulation makes your code cleaner, safer, and more professional.

 

Project Tip:
In your Banking / E-commerce / Hospital app, always apply encapsulation to:

  • User credentials

  • Account balance

  • Patient records

  • Order details

  • Payment info

 

You now fully understand Encapsulation with a real-world secure banking system!
This is how professional Java developers write safe, maintainable code.

Keep practicing — happy coding!  yes

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