Why the Ex_1_1_16.java print 311361142246?

Hello:
In the Ex_1_1_16 Java code, I don't understand why the result is 311361142246??The question is to examine what?
Can you explain it for me ? :)

[vettukal]

First of all note the return type is String. So everything is appended to each other.
So the expansion takes place as follows:
exR1(6)
exR1(3) +6+ exR1(4)+ 6
exR1(0)+3+exR1(1)+3+ +6+ exR1(1)+4+exR1(2)+4 +6
"" +3+ exR1(-2)+1+exR1(-1)+1 +3+ +6+ exR1(-2)+1+exR1(-1)+1 +4+ exR1(-1)+2+exR1(0)+2 +4+6
3+1+1+3+6+1+1+4+2+2+4+6

Nice, thanks for you