List<String> list = new ArrayList<String>();
Set<String> set = new HashSet<>();
for (int i = 0; i < 9000; i++) {
String invcode = randomCode(6);
set.add(invcode);
}
list.addAll(set);
System.out.println(set.size());
private static String randomCode(int length) {
String val = "";
Random random = new Random();
for (int i = 0; i < length; i++) {
String charOrNum = random.nextInt(2) % 2 == 0 ? "char" : "num";
if ("char".equalsIgnoreCase(charOrNum)) {
val += (char) (random.nextInt(26) + 65);
} else if ("num".equalsIgnoreCase(charOrNum)) {
val += String.valueOf(random.nextInt(10));
}
}
return val;
}