What should you program to: an interface or an implementation?

Choosing to program to an interface aligns with one of the core principles of object-oriented design known as "programming to an interface." This principle emphasizes the importance of using interfaces as a means to define a contract that classes can implement. By focusing on the interface, you enable multiple classes to fulfill the same behavioral contract while remaining decoupled from specific implementations.

When you program to an interface, it allows for greater flexibility and adaptability in your code. If a new implementation is needed, you can create a new class that implements the interface without modifying existing code that relies on the interface. This approach also facilitates easier unit testing, as you can swap in mocks or stubs that adhere to the same interface without changing the dependent code.

In contrast, opting for an abstract class or concrete class locks you into a particular implementation or hierarchy that might not be as easily changed or extended. Abstract classes can provide some shared functionality but still limit the polymorphic capabilities that interfaces allow due to their focus on a specific class structure.