/usr/lib/jvm/ has all the JDK's you might have installed. For example, these are the contents of my /usr/lib/jvm/ folder:
lrwxrwxrwx 1 root root 14 2009-11-01 11:19 java-1.6.0-openjdk -> java-6-openjdk drwxr-xr-x 5 root root 4096 2009-08-05 09:59 java-6-openjdk lrwxrwxrwx 1 root root 19 2010-05-03 20:27 java-6-sun -> java-6-sun-184.108.40.206 drwxr-xr-x 8 root root 4096 2010-06-05 11:43 java-6-sun-220.127.116.11
The JDK 'proper' is the one from Sun, in the java-6-sun directory, which actually links to java-6-sun-18.104.22.168, as you might have noticed already. But let's use the symbolic link:
... and so, the full JDK path is /usr/lib/jvm/java-6-sun.
To check it:
ls -l /usr/lib/jvm/java-6-sun/bin/
should get you a list of SuperVeryImportantFiles such as javac (the java compiler), jdb (the java debugger) and etc.
And the reason I wanted to find out the exact JDK path was for using it with Netbeans, which wanted to know where it was (apparently it doesn't want to autodetect it once it's installed). So I just edited the launcher menu item for Netbeans, appended
to its command, reopened Netbeans again and voilà! Netbeans was happy with the JDK.