We already know from the previous problems that BufferedReader is very efficient for reading text files. Therefore, we can use it to read a big file as well. While reading, for each line obtained via BufferedReader.readLine(), we need to count the number of occurrences of the searched string via countStringInString():

public static int countOccurrences(Path path, String text, Charset ch)
    throws IOException {

  int count = 0;

  try (BufferedReader br = Files.newBufferedReader(path, ch)) {
    String line;
    while ((line = br.readLine()) != null) {
      count += countStringInString(line, text);
    }
  }

  return count;
}