Explanation : In the given questions
If lists are not NULL : Invocation of join will append list m to the end of list n if the lists are not NULL.
Example:Before join operation :
m =1->2->3->4->5->null
n =6->7->8->9->nullAfter join operation :
6->7->8->9->1->2->3->4->5->null
& If lists are NULL : If the list n is empty and itself NULL, then joining and referencing would obviously create NULL pointer issue.