How can I tell what version of Java I have installed?

The simplest way is:

update-java-alternatives -l shows you all the Java versions you have installed.

java -version shows you the Java version you are using.

java -showversion shows you the Java version you are using and help.

Normally it would be OpenJDK.

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.

java -version
javac -version

These commands display what version of java (the interpreter that runs Java programs) and javac (the compiler that creates them) you have installed.

See the image above for details.