Please check if Java is installed or not by typing: java -version in Terminal.
If not installed please follow the instructions in Installing Java section to install Java
Setting up XDM
Installing Java
Debian, Ubuntu, Linux Mint, Elementary OS etc.
On the command line, type:
sudo apt-get install openjdk-7-jre
Fedora, Oracle Linux, Red Hat Enterprise Linux, etc.
On the command line, type:
su -c "yum install java-1.7.0-openjdk"
Installing Java manually
Download Java from tar.gz file from: www.java.com
Extract the archive into some directory, say $HOME/JRE
Now create a symbolic link of that java executable in /bin directory,
To create symbolic link: