A functional interface can extends another interface only when it does not have any abstract method.

Can we override functional interface?

The implementation will just print out “A” or “B”. You can't add a new abstract method to the extending interface though, as the resulting type would have two abstract methods and the compiler will error. In both cases, you can override methods from Object without causing problems.

Can a functional interface inherit another interface?

You can't inherit any functional interface to another functional interface. Because it breaks the law of functional interface has exactly one abstract method.