public int read(long pos) {
    if (pos >= size) {
      return -1;
    }

    byte[] block = blocks[blockIndex(pos)];
    int off = offsetInBlock(pos);
    return UnsignedBytes.toInt(block[off]);
  }
Reads the byte at position {@code pos} in this file as an unsigned integer in the range 0-255.
If {@code pos} is greater than or equal to the size of this file, returns -1 instead.
jimfs/src/main/java/com/google/common/jimfs/RegularFile.java