public default <U extends Comparable<? super U>> Comparator<T> thenComparing (Function<? super T, ? extends U> keyExtractor)

Returns a lexicographic-order comparator with a function that extracts a Comparable sort key.

<U>    the type of the Comparable sort key
keyExtractor    the function used to extract the Comparable sort key

Returns:  a lexicographic-order comparator composed of this and then the Comparable sort key.

NullPointerException    if the argument is null.

See also:
comparing(Function), thenComparing(Comparator)

Since:  1.8

@implSpec This default implementation behaves as if thenComparing(comparing(keyExtractor)).