code: public <U extends Comparable<? super U>> Comparator<T> thenComparing(
                  Function<? super T, ? extends U> keyExtractor
          ) {
              return thenComparing(F.comparing(keyExtractor));
          }
docs: See <a href="http://download.java.net/jdk8/docs/api/java/util/Comparator.html#thenComparing(java.util.function.Function)">Java 8 doc</a>
  
  @param keyExtractor
  the function that extract key of type U from instance of type T
  @param <U>
  the key type
  @return a comparator that applied if the result of this comparator is even