@ Vijay Kalkundri: I did try that too. Still the same error shows up
@ Campbell Ritchie: Thanks for the greeting
I have uninstalled both JDK7 and JRE7:
and re-installed them, but nothing happened .. the same error shows up
one of my friends suggested to install netbeans and things will turn to be OK. And for my surprise, he was right!!
I still don't know what happened in the underlying OS, and that means I need to read more and more about linux and Ubuntu.
Thanks guys for your concern