UnmappableCharacterException needs a length value indicating the number of chars in the input sequence that need to be skipped before conversion can resume normally. It is generally not correct to assume that this value is 1, since the converter may have recognized a valid but not mappable sequence consisting of surrogate characters, combining jamo, or other combining marks. If the client just skipped one char, the next attempt to convert might result in an unjustified MalformedInputException.