java.util.function.Function as an interface for an mutable object

Question

I have an interface with only a single method that could be replaced by java.util.function.Function from the JDK:

interface DataModel {
  boolean apply(Map<String, Integer> input);
}

The replacement would be

Function<Map<String, Integer>, Boolean>

However, I wonder whether I should keep this interface since at least one implementation (other implementations might follow) contains mutable state and is not thread safe. A factory creates a new instance for each call.

So while technically possible, the functional interface might semantically not the best solution.

The API documentation doesn't say something about mutability, but maybe it would violate the principle of least astonishment or some other kind of contract. Is there any such contract?

Would you go with Function or with the custom DataModel interface and why?

Answer 1

I would got for DataModel interface, to avoid future missunderstandings and problems.
I not a Imperative/OO or Functional purist, but I think keeping things clear is a good thing.
@FunctionalInterfaces has it's own purposes, I think custom interfaces always will fit better for "concrete implementations"; you own them, so you can extend and document them whenever you want or need.
Finally, try to avoid this scenario: "a new change will force us to migrate Funtions implementations to BiFunctions, or anything else"; imagine the refactorings...
Something you can do is extend Function, to have your own copy of the interface and reuse default methods if needed:

public interface DataModel<T, R> extends Function<T, R> {}