In the event that will trigger the Java code (Like the onclick event of a button) use the following SSJS

importPackage(com.compname.packageName);

var jcode:ClassName = new ClassName();

jcode.callMethod();

If the method returns a value then store the return value to a variable. 

that variable can then be used write the value to an XPage component. 

 Hi paul,

 

 If you are using the default package then omit the package statement and just refer to the class.  Although NOT placing your class in you own package is a REALLY BAD IDEA !!

The code for accessing classes in a jar file is exactly the same.