Choosing a default jdk when there are more than 2 jdks installed. (jdk5, jdk6)

If there are more than 2 jdks installed, you need to switch to specific jdk according to the situation.
This can be achieved easily with the following commands (update-java-alternatives)

1. Check the current configuration and possibilities.
# sudo update-java-alternatives -l
2. Set the specific jdk as default.
# sudo update-java-alternatives -s XXXX
3. To check the configuration,
# java -version
or
#javac -version

Source: https://help.ubuntu.com/community/Java