Open/Closed Principle (OCP) — open for extension, closed for modification

Direct Answer

The Open/Closed Principle says a software element should provide stable extension points for expected kinds of change. New variants can often be added through an interface, strategy, handler, or configuration without rewriting the central algorithm. It is a design goal, not a ban on editing existing code.

Code Example

// Define the abstraction - closed for modification
interface PaymentProcessor {
    void processPayment(double amount);
}

// First implementation
class CreditCardProcessor implements PaymentProcessor {
    public void processPayment(double amount) {
        System.out.println("Processing credit card payment: $" + amount);
        // Credit card-specific logic
    }
}

// Second implementation - extension without modifying existing code
class PayPalProcessor implements PaymentProcessor {
    public void processPayment(double amount) {
        System.out.println("Processing PayPal payment: $" + amount);
        // PayPal-specific logic
    }
}

@Service
class OrderService {
    private final PaymentProcessor processor;
    
    // Depends on abstraction, not concrete type
    public OrderService(PaymentProcessor processor) {
        this.processor = processor;
    }
    
    public void checkout(double total) {
        processor.processPayment(total);  // Works with any implementation
    }
}

Common Mistakes & Pitfalls

• Over-engineering simple cases: Freshers see OCP and immediately create interfaces for everything, even a simple DateFormatter that will never have variants. Start simple, refactor to OCP when variation appears.
• Mixing abstraction levels: Creating an interface but then checking instanceof inside the consuming class defeats the purpose — that's still modification when you add types. Let polymorphism handle the branching.
• Ignoring legitimate modifications: Treating OCP as absolute law and refusing to fix bugs or refactor because it requires touching existing code. OCP applies to feature additions, not all code changes. Bug fixes always modify code, and that's fine.