Which statement about valid implementations in the provided interface and class structure is incorrect?

In this scenario, the statement that Class CL1 uses the interface is regarded as incorrect. For a class to "use" an interface, it would typically need to implement the methods declared in that interface or utilize those methods in its methods. If Class CL1 merely implements the interface, this doesn't inherently mean that it is actively using the interface’s methods in its own implementation.

The other statements support the overall structure of how classes and interfaces interact in object-oriented programming. Class CL2 is confirmed to utilize the interface, while Class CL1 is noted to implement the interface, which suggests a relationship where CL1 is required to have the interface's methods defined (unless it is an abstract class). Additionally, the naming of the interface method as if1~m1 in class CL1 aligns with how methods in interfaces can be referenced in implementing classes.

This reasoning highlights the distinct nuance in terminology between "using" and "implementing" in class-interface relationships. An implementing class must define the method signatures required by the interface, but that does not necessarily imply active usage of those methods within its own methods or functionality.