If a class implements an interface but does not provide implementation for all of its declared methods, what must happen?

• A. The class must be declared as final
• B. It cannot be instantiated and must be marked as abstract
• C. The methods must be declared static
• D. It will compile without issues

Answer: (B)

When a class implements an interface but fails to provide implementations for all declared methods of that interface, the class must be declared as abstract. This is because an abstract class is allowed to have incomplete implementations and serves as a blueprint that can be extended or further implemented by subclasses.

Declaring the class as abstract indicates that it cannot be instantiated on its own, as it does not provide full functionality, and it enforces that the necessary implementations must be provided by any subclass that derives from it. This ensures that all methods necessary for the interface's contract are eventually implemented in a concrete subclass, maintaining the integrity of the design.

This concept is fundamental in Java, where an interface serves as a contract for behavior that a class must fulfill. If a class chooses to implement an interface, Java's type-checking mechanisms require that all methods be provided unless the class is explicitly marked as abstract. Hence, this requirement is essential for ensuring correct use of interfaces and adherence to polymorphic behavior in object-oriented programming.