This command should tell you what is currently providing the Java virtual machine (java) and the Java compiler (javac):
file /etc/alternatives/java /etc/alternatives/javac
This assumes the "alternatives" system is working properly, which might not be the case, depending on how Java has been "messed up" in the past. To check this, run:
file `which java javac`
If the alternatives system is working correctly and being used by Java, then you should see:
/usr/bin/java: symbolic link to `/etc/alternatives/java'
/usr/bin/javac: symbolic link to `/etc/alternatives/javac'
Otherwise please edit your question to provide details. Then it should be possible to give a more specific answer.
You can remove openjdk-6 with the Software Center. There are multiple packages associated with it, so you may need to remove more than one packages. (All the `openjdk-6 packages are listed here.)
Or you can use the command-line:
sudo apt-get remove openjdk-6-\* icedtea-6-\*
However, whichever method you use, you may want to check first to see what depends on these packages--you might have software installed that specifically needs version 6. (Probably not, but possibly.)
You can check for this by simulating the removal operation on the command-line:
apt-get -s remove openjdk-6-\* icedtea-6-\*
This will show you the effects of removing those packages, including what other packages would be removed as well. (You'll notice that since this is a simulation, you don't need sudo.)
If you want to be able to continue using Java content online in your web browser (this is not the same thing as JavaScript), then before you remove any icedtea-6- or openjdk-6- packages (except perhaps openjdk-6-jdk), you should make sure you have icedtea-7- packages installed corresponding to whatever icedtea-6- packages are installed.
Go to Ubuntu Software Center and search for the package software-properties-common, install that package if it is not installed.
Open your dash by pressing the Super Key (Windows Key) and type software sources. Go into the application that appears and place a check mark on all the packages present there.
After doing it, execute the following commands one by one, in a terminal window ( Ctrl+Alt+T ) :
sudo apt-get update && sudo apt-get upgrade
sudo apt-get dist-upgrade
sudo apt-get autoremove
sudo apt-get autoclean
and then run,
sudo apt-get install default-jre to install the package.
Best Answer
update: see Ubuntu Security Notice USN-1693-1
Probably not for the specific exploit being used in the wild for Oracle's Java 7 plugin. These exploits are usually specifically crafted to run with a specific set of software.
However, OpenJDK can be vulnerable in a similar way, if it's because of a design/architecture error in the way Java works in a browser. I could not find any details on it (at the time of writing) to support that statement with facts, but previous vulnerabilities were specifically for Oracle's JRE/JDK while OpenJDK has its own.
Please note the difference between an exploit and a vulnerability in this context.
Also note that you are probably affected to some extent if you're running Oracle's JRE/JDK on Ubuntu. However, the exploits are probably targeted for Windows hosts, and Oracle's JRE/JDK is no longer distributed by Ubuntu, due to licensing issues (Oracle doesn't allow redistribution anymore).