How do I find out my JDK version under Ubuntu Linux operating systems? How do I find out JDK version under any Linux distribution?

Open a command-line terminal (select Applications > Accessories > Terminal), and then type the following command to find out JDK version:
java -version
Sample outputs:

java version "1.6.0_20"
OpenJDK Runtime Environment (IcedTea6 1.9.2) (6b20-1.9.2-0ubuntu1~10.04.1)
OpenJDK Server VM (build 19.0-b09, mixed mode)

To list all installed jdk version under Ubuntu / Debian Linux, enter:
dpkg --list | grep -i jdk
Sample outputs:

ii  default-jdk                                 1.6-34                                          Standard Java or Java compatible Development
ii  icedtea-6-jre-cacao                         6b20-1.9.2-0ubuntu1~10.04.1                     Alternative JVM for OpenJDK, using Cacao
ii  openjdk-6-jdk                               6b20-1.9.2-0ubuntu1~10.04.1                     OpenJDK Development Kit (JDK)
ii  openjdk-6-jre                               6b20-1.9.2-0ubuntu1~10.04.1                     OpenJDK Java runtime, using Hotspot JIT
ii  openjdk-6-jre-headless                      6b20-1.9.2-0ubuntu1~10.04.1                     OpenJDK Java runtime, using Hotspot JIT (hea
ii  openjdk-6-jre-lib                           6b20-1.9.2-0ubuntu1~10.04.1                     OpenJDK Java runtime (architecture independe

A Note About RPM Based Linux Distributions

If you are using RPM based distro, try:
$ java -version
$ javac -version
$ rpm -qa | egrep -i "(jdk|jre)"