OAF Error: method add(oracle.jbo.domain.Number) not found in class oracle.jbo.domain.Number

Problem
When compiling the code in Oracle Jdeveloper 9i the following error occured:
method add(oracle.jbo.domain.Number) not found in class oracle.jbo.domain.Number

Solution that worked for me:
Adding the following  folder from server to myclasses folder:
JAVA_TOP/oracle/jbo
Move the entire folder to myclasses/oracle  and compile
This solution worked for me.