code: public static Sha256Hash of(File file) throws IOException {
          FileInputStream in = new FileInputStream(file);
          try {
              return of(ByteStreams.toByteArray(in));
          } finally {
              in.close();
          }
      }
docs: Creates a new instance containing the calculated (one-time) hash of the given file's contents.
  
  The file contents are read fully into memory, so this method should only be used with small files.
  
  @param file the file on which the hash value is calculated
  @return a new instance containing the calculated (one-time) hash
  @throws IOException if an error occurs while reading the file