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

Parameters:

<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.

Exceptions:

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)).