The "E" signifies "times 10 to the Nth power", also known as
scientific notation So "6.123233995736766E-17" means 6.123233995736766 * 10^-17, which is something around 0.00000000000000006123233995736766. For your purposes you can probably just round off those numbers to the desired significance -maybe 10 digits- and the result will be zero.
As to why those numbers aren't exactly zero to begin with, have a look at #20 in the
http://faq.javaranch.com/java/JavaBeginnersFaq.
[EJFH: Fix URL]
[ December 21, 2007: Message edited by: Ernest Friedman-Hill ]