code: public int readInt(InputStream in) throws IOException {
          return ((byte) in.read() & 0xff) << 24
              | ((byte) in.read() & 0xff) << 16
              | ((byte) in.read() & 0xff) << 8
              | (byte) in.read() & 0xff;
      }
docs: InputStream.read 仅 read 一个 byte
  
  @param in InputStream
  @return int value
  @throws IOException if an I/O error occurs.