i saw that but i want random alphanumeric values
Ah, the dreaded "requirement creep". You will do better here to post your full requirements at the beginning of the thread rather than later. What you may do here is create a random number between say 33 and 126 and then cast the number to a char.
for instance:
int delta = '~' - '!' + 1; // the difference between the top and bottom ascii character
Random random = new Random();
for (int i = 0; i < 500; i++)
{
char randomChar = (char)(random.nextInt(delta) + '!');
System.out.print(randomChar + " ");
if ((i+1) % 20 == 0)
{
System.out.println();
}
}