root@kali:/opt# update-alternatives --install /usr/bin/java java /opt/jdk1.6.0_45/bin/java 300
root@kali:/opt# update-alternatives --set java /opt/jdk1.6.0_45/bin/java
root@kali:/opt# update-alternatives --install /usr/bin/javac javac /opt/jdk1.6.0_45/bin/javac 300
root@kali:/opt# update-alternatives --set javac /opt/jdk1.6.0_45/bin/javac