code: private void fill() throws IOException {
      int n;
      previousBufferSize += nChars;
      do {
        n = in.read(cb, 0, cb.length);
      } while (n == 0);
      if (n > 0) {
        nChars = n;
        nextChar = 0;
      }
    }
docs: Fill.
  
  @throws IOException Signals that an I/O exception has occurred.