public Sha256Hash(String hexString){
  checkArgument(hexString.length() == 64);
  this.bytes=Hex.decode(hexString);
}

public Sha256Hash ( String hexString ) { checkArgument ( hexString . length ( ) == 64 ) ; this . bytes = Hex . decode ( hexString ) ; }
creates a sha256hash by decoding the given hex string . it must be 64 characters long .
