If you are in the same boat as I am, upgrading from Ubuntu 9.10 to 10.04 you will find Sun's JDK not to be present after the upgrade. Trying to apt-get it does not work since the package is not present in the default sources.list. I usually download it from http://www.java.com/, and use

sh jdk-6u20-linux-x64.bin 

to execute it. In my case I received this error:

Do you agree to the above license terms? [yes or no]
yes
Unpacking...
Checksumming...
Extracting...
jdk-6u20-linux-x64.bin: 477: ./install.sfx.1854: not found
Failed to extract the files.  Please refer to the Troubleshooting section of
the Installation Instructions on the download page for more information.

So I tried to create the missing links to /lib64/ld-linux-x86-64.so.2 as I had no /lib64 on my system - my Ubuntu was a pure 64-bit installation. It then failed with:

Do you agree to the above license terms? [yes or no]
yes
Unpacking...
Checksumming...
Extracting...
jdk-6u20-linux-x64.bin: 477: ./install.sfx.11221: Accessing a corrupted shared library
Failed to extract the files.  Please refer to the Troubleshooting section of
the Installation Instructions on the download page for more information.

Obviously I was getting nowhere. I then found this web site which helped me. Basically you need to add the following line to /etc/apt/sources.list:

deb http://archive.canonical.com/ lucid partner

Then run:

apt-get update

as root. Lastly, do:

apt-get install sun-java6-jdk

and you will have a working java:

root@sun:/opt # java -version
java version "1.6.0_20"
Java(TM) SE Runtime Environment (build 1.6.0_20-b02)
Java HotSpot(TM) Client VM (build 16.3-b01, mixed mode, sharing)