Search string in sparse array

Given a sorted array of strings which is interspersed with empty strings, write a method to find the location of a given string.
Example: find “ball” in [“at”, “”, “”, “”, “ball”, “”, “”, “car”, “”, “”, “dad”, “”, “”] will return 4
Example: find “ballcar” in [“at”, “”, “”, “”, “”, “ball”, “car”, “”, “”, “dad”, “”, “”] will return -1 

This follows the ordinary binary search. However, notice that when the mid is an empty string, you should either proceed right or left to find the first nonempty string. 

Besides, use start <= last and start = mid + 1

public static int search(String[] strings, String target) {
  if (strings == null || strings.length == 0 || target == null)
   return -1;
  int start = 0;
  int end = strings.length - 1;
  while (start <= end) {
   while (start < end && strings[start].equals(""))
    start++;
   while (start < end && strings[end].equals(""))
    end--;
   int mid = (start + end) / 2;
   while (mid <= end && strings[mid].equals(""))
    mid++;
   if (strings[mid].equals(target))
    return mid;
   else if (strings[mid].compareTo(target) < 0) {
    start = mid + 1;
   }
   else {
    end = mid - 1;
   }
  }
  if (strings[start].equals(target))
   return start;
  return -1;
 }