I want to have two JDKs. The first one was installed a long time ago to /usr/lib/jvm/java-7-oracle/. I installed the second version and executed following commands to add it to alternatives:

red-planet@laptop:~$ sudo update-alternatives --install "/usr/bin/java" "java" "/opt/java_1.6.0_35/bin/java" 2

update-alternatives: using /opt/java_1.6.0_35/bin/java to provide /usr/bin/java (java) in auto mode.

red-planet@laptop:~$ sudo update-alternatives --install "/usr/bin/javac" "javac" "/opt/java_1.6.0_35/bin/javac" 2

update-alternatives: using /opt/java_1.6.0_35/bin/javac to provide /usr/bin/javac (javac) in auto mode.

red-planet@laptop:~$ sudo update-alternatives --install "/usr/bin/javaws" "javaws" "/opt/java_1.6.0_35/bin/javaws" 2

update-alternatives: using /opt/java_1.6.0_35/bin/javaws to provide /usr/bin/javaws (javaws) in auto mode.

And configured:

There are 2 choices for the alternative java (providing /usr/bin/java).



  Selection    Path                                 Priority   Status

------------------------------------------------------------

* 0            /opt/java_1.6.0_35/bin/java           2         auto mode

  1            /opt/java_1.6.0_35/bin/java           2         manual mode

  2            /usr/lib/jvm/java-7-oracle/bin/java   1         manual mode



Press enter to keep the current choice[*], or type selection number: 

Why do I have two equal items in the list?