On the other hand, testing a method that returns a functional interface can be interpreted as testing the behavior of that functional interface. Let's consider the following method:

public static Function<String, String> reduceStrings(
    Function<String, String> ...functions) {

  Function<String, String> function = Stream.of(functions)
    .reduce(Function.identity(), Function::andThen);

  return function;
}

Now, we can test the behavior of the returned Function<String, String> as follows:

@Test
public void testReduceStrings() throws Exception {

  Function<String, String> f1 = (String s) -> s.toUpperCase();
  Function<String, String> f2 = (String s) -> s.concat(" DONE");

  Function<String, String> f = reduceStrings(f1, f2);

  assertEquals("TEST DONE", f.apply("test"));
}