RMI client , server communication

How to put the random values for a variable and how to refresh that random value every 30 seconds ? i have got some idea from the site for getting random value as follows,

Random r = new Random();
String i = r.next()%33;
switch (i) {
  case 0:
    myVariable = "a";
  case 1:
    myVariable = "b";
  case 2:
    myVariable = "c";
}

But i could not find exactly to refresh the value every 30 seconds