If your version of Linux has Java preinstalled, it is probably either the GNU version of Java, or the Open JDK. We have had trouble getting our software to run with these (actually even the installer program doesn't work properly) so we recommend getting the JRE or JDK from Oracle. You can check the default Java version with the java -version command as shown here.
[brucejohnson@localhost ~]$ java -version
java version "1.4.2"
gcj (GCC) 3.4.6 20060404 (Red Hat 3.4.6-8)
Copyright (C) 2006 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
The above result indicates that the installed version is the GNU version (and a pre 1.5 version as well), which will not work. In this case you should download and install the Oracle version. Note that it is entirely possible to have multiple versions of Java installed on one computer. The above command will find the first one in your path. You might need to explicilty type the full path to use other versions (for example, /usr/local/jdk1.6/bin/java -version ).