Java Classpath : a classic source of issue
 java Hello
  Exception in thread "main" java.lang.NoClassDefFoundError: Hello
In case of doubt ...
java -version
echo $CLASSPATH
  should contain the '.' directory
java -classpath . Hello
After 'java', the only thing that should appear is the name of a 'public' 
  class with a 'main' method in it ...
How to set your classpath in your .tcshrc file :
setenv CLASSPATH .:/Library/Java/Extensions/JSDT/lib/jsdt.jar
  or
  setenv CLASSPATH "$CLASSPATH":.
TSG Info :
Java 1.4.2 is the default on the Linux lab machines now. /usr/local/java/bin 
  is in the path of anyone using tcsh as their shell, so students should be able 
  to use Java straight away.
  Java 1.3.1 is still the default on the Solaris machines, but 1.4.2 will be the 
  default by this evening (!).