Which statement best describes an Interface in object-oriented design?

• A. A graphical user interface for an application.
• B. A data structure used to hold collections.
• C. A starting point with methods from which many classes can be built using those methods.
• D. A protocol for communicating between services.

Answer: (C)

In object-oriented design, an interface defines a contract of behavior: a set of method signatures without concrete implementations that any class can promise to provide. This creates a common language of capabilities that different classes can share, so you can write code that operates on the interface and work with any class that implements it. The idea is a starting point where the methods are declared and then many classes can be built by implementing those methods in their own ways. This enables polymorphism, letting you substitute one class for another as long as they adhere to the same interface, which makes code more flexible and extensible. The other options describe a graphical user interface for users, a data structure for holding collections, and a communication protocol between services, none of which capture this contract-based, implementable-ahead, shared-behavior concept.