What will happen if a class implementing an interface provides implementation for some but not all methods?

• A. The class will compile successfully
• B. The class must be declared abstract to compile
• C. The interface will not compile
• D. The class will throw a runtime error

Answer: (B)

When a class implements an interface, it is required to provide concrete implementations for all the methods declared in that interface unless the class itself is declared as abstract. An abstract class serves as a blueprint for other classes and can contain a mix of fully implemented methods and abstract methods (those without implementations). Therefore, if a class implements only some of the methods from the interface, it is deemed incomplete and cannot be instantiated, which necessitates that it is declared abstract.

In contrast, if the class were to provide implementations for all methods of the interface, it would compile successfully, making it unnecessary to declare the class abstract. The interface itself remains valid and will compile independently of whether the implementing class is abstract or not. No runtime errors occur simply based on the implementation state unless specific logic within the methods imples such issues, which is unrelated to the class's abstract status.

Thus, a class that does not implement all interface methods must indeed be designated as abstract for the code to compile properly.