oop-polymorphism

Interface-based polymorphism lets you define a common contract (interface) with multiple implementations that clients use interchangeably. At runtime, you substitute different implementations—such as CreditCard, PayPal, or BankTransfer in a payment processor—without modifying the code that calls them, enabling flexible, extensible architectures.

Interfaces define a contract that multiple implementations fulfill, decoupling callers from concrete classes and enabling runtime substitution. Concrete classes bind behavior directly, requiring code changes to swap implementations. Polymorphism through interfaces supports extensibility; concrete classes create tight coupling and make testing harder.

Create a PaymentMethod interface with process() and isValid() methods, implement concrete classes for each method (CreditCard, PayPal, BankTransfer), and wire them into a PaymentProcessor that iterates through available methods, calling isValid() until one succeeds, then executes process() on that implementation.

Yes. With interface-based polymorphism, you create new implementations of the PaymentMethod interface—such as ApplePay or cryptocurrency handlers—and register them with the PaymentProcessor. Existing code remains unchanged because it depends on the interface contract, not concrete classes.

Use abstract classes when implementations share common code, state, or initialization logic. Use interfaces when defining a pure contract across unrelated implementations. Both support polymorphic behavior; abstract classes work better for shared implementation details, interfaces for looser coupling across diverse types.

Polymorphism through interfaces lets you inject mock implementations during testing without changing production code, and add new behaviors by implementing the interface rather than modifying existing classes. This isolates changes, reduces regression risk, and makes systems easier to extend and test.