you do not have to unistall all jdks on the machine. this is how it works:
when you install the linux system you have integrated suport for java. this is ths gij you see when you type "java -version".
when you installed JDK6 it is most possible that the path to it was not exported.
what you have to do:
first go to the path where you installed the JDK6 and change directory there to subfolder bin/
type "./java" to check if it works. if it releases an output it means it is ok. to check it's verson type "./java -version". note that "./" are mandatory for the shell to run the executable from the folder you are in (that is tha subfolder bin of you JDK6 installation) not the one from /usr/bin
second you need to make it be your "default" java command. for this you have to do the following:
1.go to your home directory and open for editing file ".bash_profile". at the end of the file add this:
(<path/to/jdk6/bin> is te path to the bin subfolder of your JDK6 insatallation)
2.save file and exit
3.note that the path is not exported until you open a new terminal (restart the destop manager or the system)
4.you're good to go
troubleshooting: if when you type java you still get the old output do this:
env | grep PATH
the output is the value for environment variable PATH. the first path should be the path to subfolder bin of your JDK6 installlation. If not so, you slipped something from what i said or something went wrong. recheck that you did all the steps from above.