Random variable Logic

I would like to test an app with random variable logic through JMeter. it is voting application.

Flow: Login-voting-ReVoting-Logout for 250 users

Problem: I have to vote 1 person out of 3 members and again I have to re-vote to another person. I have defined vote members id in user defined variables,so first I have used bean shell code for the first vote. here is the code.(Beanshell1)

import java.util.Random;

String[] categories = (vars.get("categories")).split(",");

int idx = new Random().nextInt(categories.length);
String category = (categories[idx]);

vars.put("rnd_cat",category);

for re-voting, I want to exclude the previous vote member id, so I have used below code for that (Beanshell2)

while(category==rnd_cat)
{
     String[] categories = (vars.get("categories")).split(",");

     int idx2 = new Random().nextInt(categories.length);
     category = (categories[idx2]);
}

but I am getting void in the response.