If your friend is referring to "hacking" as in "breaking in to a computer", I'd say Java is one of the worst tools for doing that (well, most of the work is done by existing tools such as nmap anyway).
cracking (hacking does NOT mean breaking into computers, no matter what crackers and uninformed journalists want you to believe) doesn't rely on any one language or operating system.
For script kiddies, most of them know nothing at all about the tools they use apart from the buttons on the screen to press... Most of those tools are probably written in QT or VB.
The hardcore crackers most likely use C or Assembly to gain the lowlevel access they crave and need.
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