Explain the four pillars of OOP with a real-world example.

Four pillars of OOP

Object-oriented design bundles data + behavior in objects. The four pillars interviewers expect: encapsulation, inheritance, polymorphism, and abstraction.

• Encapsulation: Hide fields; expose behavior via methods (bank account balance).
• Inheritance: Reuse base class behavior (Employee → Manager).
• Polymorphism: Same interface, different runtime behavior (Payment → UPI/card).
• Abstraction: Show essentials, hide complexity (NotificationService.send()).

interface Payment { void pay(Money amount); }
class UpiPayment implements Payment { public void pay(Money m) { /* UPI API */ } }