Returns a lexicographic-order comparator with a function that extracts a key to be compared with the given Comparator.

Parameters:

<U>		the type of the sort key
keyExtractor		the function used to extract the sort key
keyComparator		the Comparator used to compare the sort key

Returns:  a lexicographic-order comparator composed of this comparator and then comparing on the key extracted by the keyExtractor function

Exceptions:

NullPointerException

if either argument is null.

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

Since:  1.8

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