DevOps, CI/CD and Automation

Hi, I have problems implementing own java classes into the Report Builder. OK, let say I am running a simple report using STUDENT_ID and STUDENT_NAME. What I wanted to do is to concatenate a string "XXXX" to one of the records in STUDENT_NAME (e.g. STUDENT_NAME = 'Mary').

The Java class that I wrote has a static method:
public class MyAppend
{
public static String myMethod(String s)
{
s = s +"XXXX";
return s;
}
}

I also wrote a Formula Column in Report Builder called "TestFormula":
function TestFormula return varChar2 is
StuName varchar2(40);
myStr varchar2(40);
begin
select STUMST.STU_NAME
into StuName
from STUMST
where STUMST.STU_NAME= 'Mary';
->myStr := myappend.myMethod(myPackage.aobj, StuName);
return myStr;
end;

The error message I get when I compile this Formula Column PL/SQL is (with '->' pointing to the line):
Wrong number or types of arguments in call to 'MYMETHOD'

How do I correct this error?

Thank you.

Joey