How can you ensure that a system module is open for extension but closed for modification?

To make a system module open for extension but closed for modification, the Open/Closed Principle from SOLID is applied.

This is achieved by:

• Using abstractions (interfaces, abstract classes) that define the behavior of the module.
• Implementing new functionalities through inheritance or composition without changing the original module code.
• Applying design patterns, such as Strategy or Decorator, which allow adding behavior dynamically.

Example in Dart (Flutter):

abstract class PaymentProcessor {
  void pay(double amount);
}

class CreditCardProcessor implements PaymentProcessor {
  @override
  void pay(double amount) {
    print('Payment of $amount using credit card');
  }
}

class PayPalProcessor implements PaymentProcessor {
  @override
  void pay(double amount) {
    print('Payment of $amount via PayPal');
  }
}

// The module uses the abstraction PaymentProcessor and can be extended with new payment methods without changing existing code.