7143230: fix warnings in java.util.jar, sun.tools.jar, zipfs demo, etc

Co-authored-by: Michael Barker <mikeb01@gmail.com>
Co-authored-by: Carl Jokl <carl.jokl@gmail.com>
Co-authored-by: Dinuk Weerasinghe <dinuksw@yahoo.com>
Co-authored-by: Markus Stoy <markus.stoy@timgroup.com>
Co-authored-by: Tom Anderson <tom.anderson@timgroup.com>
Reviewed-by: alanb, chegar, lancea, smarks