for(Long j=i ; j<i+3 ; j++) {
String c = numb.charAt(j);
as you see, you declare j to be long. String.charAt method takes int as a parameter index, and not long. To prevent this think if you really need j to be long. If you do not, change it to be int and it should work fine (I guess your String numb is not going to be longer than int's size anyway). If you do, read about converting long to integer (for example here: Convert Long into Integer).