code: private static <W extends Writable> void verifyWritableClass(Class<W> writableClass,
        boolean isKeyClass, WritableConverter<W> writableConverter) {
      Preconditions.checkNotNull(writableClass, "%s Writable class is undefined;"
          + " WritableConverter of type '%s' does not define default Writable type,"
          + " and no type was specified by user", isKeyClass ? "Key" : "Value", writableConverter
          .getClass().getName());
    }
docs: Tests validity of Writable class, ensures consistent error message for both key and value
  tests.
  
  @param writableClass class being tested.
  @param isKeyClass {@code true} if testing keyClass, {@code false} otherwise.
  @param writableConverter associated WritableConverter instance.