JavaCC simple example not working - javacc

Question

I am trying javacc for the first time with a simple naive example which is not working. My BNF is as follows:
<exp>:= <num>"+"<num>
<num>:= <digit> | <digit><num>
<digit>:= [0-9]
Based on this BNF, I am writing the SimpleAdd.jj as follows:
options
{
}
PARSER_BEGIN(SimpleAdd)
public class SimpleAdd
{
}
PARSER_END(SimpleAdd)
SKIP :
{
" "
| "\r"
| "\t"
| "\n"
}
TOKEN:
{
< NUMBER: (["0"-"9"])+ >
}
int expr():
{
int leftValue ;
int rightValue ;
}
{
leftValue = num()
"+"
rightValue = num()
{ return leftValue+rightValue; }
}
int num():
{
Token t;
}
{
t = <NUMBER> { return Integer.parseInt(t.toString()); }
}
using the above file, I am generating the java source classes. My main class is as follows:
public class Main {
public static void main(String [] args) throws ParseException {
SimpleAdd parser = new SimpleAdd(System.in);
int x = parser.expr();
System.out.println(x);
}
}
When I am entering the expression via System.in, I am getting the following error:
11+11^D
Exception in thread "main" SimpleAddTest.ParseException: Encountered "<EOF>" at line 0, column 0.
Was expecting:
<NUMBER> ...
at SimpleAddTest.SimpleAdd.generateParseException(SimpleAdd.java:200)
at SimpleAddTest.SimpleAdd.jj_consume_token(SimpleAdd.java:138)
at SimpleAddTest.SimpleAdd.num(SimpleAdd.java:16)
at SimpleAddTest.SimpleAdd.expr(SimpleAdd.java:7)
at SimpleAddTest.Main.main(Main.java:9)
Any hint to solve the problem ?

Answer 1

Edit Note that this answer answers an earlier version of the question.
When a BNF production uses a nonterminal that returns a result, you can record that result in a variable.
First declare the variables in the declaration part of the BNF production
int expr():
{
int leftValue ;
int rightValue ;
}
{
Second, in the main body of the production, record the results in the variables.
leftValue = num()
"+"
rightValue = num()
Finally, use the values of those variables to compute the result of this production.
{ return leftValue+rightValue; }
}