What is an interface in Java or C#?

An interface in Java or C# is best understood as a reference type that defines a contract of methods that implementing classes must provide. This means that an interface specifies a set of method signatures (names, return types, and parameters) but does not provide any implementation for those methods. When a class implements an interface, it agrees to provide concrete implementations for all the methods defined in that interface. This allows for a form of abstraction and promotes flexibility in code design, enabling different classes to be treated uniformly based on the shared interface.

Using interfaces can greatly enhance code modularity and readability, as they support polymorphism. Objects of different classes can be used interchangeably if they implement the same interface, allowing developers to write code that is independent of the specific implementations. Additionally, interfaces enable multiple inheritance of type since a class can implement multiple interfaces, facilitating more complex designs without the constraints of single inheritance that classes face.

In summary, option B captures the essence of what an interface represents in these programming languages, highlighting its role as a foundational mechanism for defining behaviors that multiple classes can share and implement.