How do I know the java version I need for Tomcat5.5

[Mary Tata]

Hi,
I'm having problems with the installation of Tomcat 5.5. I'm using Ubuntu,
and it gives me the following error:
Exception in thread "main" java.lang.NoClassDefFoundError: while resolving
class: org.apache.catalina.startup.Bootstrap
  at java.lang.VMClassLoader.transformException(java.lang.Class,
java.lang.Throwable) (/usr/lib/libgcj.so.6.0.0)
  at java.lang.VMClassLoader.resolveClass(java.lang.Class)
(/usr/lib/libgcj.so.6.0.0)
  at java.lang.Class.initializeClass() (/usr/lib/libgcj.so.6.0.0)
  at java.lang.Class.forName(java.lang.String, boolean,
java.lang.ClassLoader) (/usr/lib/libgcj.so.6.0.0)
  at gnu.java.lang.MainThread.run() (/usr/lib/libgcj.so.6.0.0)
Caused by: java.lang.ClassNotFoundException: javax.management.ObjectName not
found in gnu.gcj.runtime.SystemClassLoader{urls=
[file:./,file:/usr/local/tomcat/bin/bootstrap.jar,file:/usr/local/tomcat/bin/commons-
logging-api.jar,file:./], parent=gnu.gcj.runtime.ExtensionClassLoader{urls=[],
parent=null}}
  at java.net.URLClassLoader.findClass(java.lang.String)
(/usr/lib/libgcj.so.6.0.0)
  at java.lang.ClassLoader.loadClass(java.lang.String, boolean)
(/usr/lib/libgcj.so.6.0.0)
  at java.lang.ClassLoader.loadClass(java.lang.String)
(/usr/lib/libgcj.so.6.0.0)
  ...4 more

when I stop the server for the console.
Any idea???
Thank you....