Implement binary search. What preconditions must hold?

int binarySearch(int[] a, int target) {
  int lo = 0, hi = a.length - 1;
  while (lo <= hi) {
    int mid = lo + (hi - lo) / 2;
    if (a[mid] == target) return mid;
    if (a[mid] < target) lo = mid + 1;
    else hi = mid - 1;
  }
  return -1;
}