poi convertNumToColString

Trying to define and use poi convertNumToColString and getting a method error. when i decompile the java class CellReference, i see convertNumToColString defined.

Code
Dcl-PR convertNumToColString like(jString)
         EXTPROC(*JAVA:'org.apache.poi.ss.util.CellReference':' ');
  col int(10) value;
End-PR;

dcl-s colNum int(10);
dcl-s colStr like(jString);

colNum = 1;
colStr = convertNumToColString(ssworkbook:colNum);

Path to poi.jar
/java/poi/poi.jar

Message . . . . :   Java exception received when calling Java method.
Cause . . . . . :   RPG procedure myprogram in program mylib/myprogram
  received Java exception "java.lang.NoSuchMethodError:
org/apache/poi/ss/util/CellReference.convertNumToColString(I)Ljava/lang"
  when calling method "convertNumToColString" with signature
  "(I)Ljava.lang.String;" in class "org.apache.poi.ss.util.CellReference".
Recovery  . . . :   Contact the person responsible for program maintenance to
  determine the cause of the problem.
Technical description . . . . . . . . :   If the exception indicates that the
  Java class was not found, ensure the class for the method is in the class
  path. If the exception indicates that the Java method was not found, check
  the method name and signature. If the signature is not correct, change the