public class Homework17;{
public static void main (string[] args){
String str = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
Random rand = new Random();
StringBuilder a = new StringBuilder(4);
for(int i=0;i<4;i++){
char c = str.charAt(rand.nextInt(str.length()));
a.append(c);
}
System .out.println(a.toString());
}
}