for (int i= 0; i<= 1084; i++) {

if (String.valueOf(i+1).length() == 1) {
username = "sys_000" + (i+1);
}
if (String.valueOf(i+1).length() == 2) {
username = "sys_00" + (i+1);
}
if (String.valueOf(i+1).length() == 3) {
username = "sys_0" + (i+1);
}
if (String.valueOf(i+1).length() == 4) {
username = "sys_" + (i+1);
}
System.out.println(username);
}

sys_0001
sys_0002
sys_0003
sys_0004
sys_0005
sys_0006
sys_0007
sys_0008
sys_0009
sys_0010
sys_0011
sys_0012
sys_0013
sys_0014
sys_0015
sys_0016
sys_0017
sys_0018
sys_0019
sys_0020
sys_0021
sys_0022
sys_0023
sys_0024
sys_0025
sys_0026
sys_0027
sys_0028
sys_0029
sys_0030
sys_0031
sys_0032
sys_0033
sys_0034
sys_0035
sys_0036