Encapsulation and Abstraction

# CHAPTER 17

Encapsulation and Abstraction #

1. Introduction #

2. Encapsulation — Data Hiding #

public class BankAccount {
    private double balance;

    public double getBalance() { return balance; }

    public void deposit(double amount) {
        if (amount > 0) balance += amount;
    }

    public void withdraw(double amount) {
        if (amount > 0 && amount <= balance) balance -= amount;
        else System.out.println("Insufficient funds!");
    }
}

Why? Direct access (account.balance = -1000) bypasses validation. Getters/setters enforce rules.

3. Abstraction — Hiding Complexity #

abstract class Payment {
    abstract void processPayment(double amount);
    void printReceipt() { System.out.println("Receipt printed."); }
}

class CreditCardPayment extends Payment {
    @Override
    void processPayment(double amount) {
        System.out.println("Processing credit card: $" + amount);
    }
}

class PayPalPayment extends Payment {
    @Override
    void processPayment(double amount) {
        System.out.println("Processing PayPal: $" + amount);
    }
}

4. Abstract Classes #

• Cannot be instantiated directly.

• Can have abstract methods (no body) and concrete methods (with body).

• Child classes MUST implement all abstract methods.

abstract class Shape {
    abstract double area();      // Abstract — no body
    void display() {             // Concrete — has body
        System.out.println("Area: " + area());
    }
}

5. MCQ Quiz with Answers #

6. Interview Questions #

• Q: Difference between Encapsulation and Abstraction?

• Q: Why should fields be private? What problem does it solve?

• Q: Can an abstract class have a constructor even though it can't be instantiated?

7. Summary #

8. Next Chapter Recommendation #