question: public Mp4TagByteField ( Mp4FieldKey id , String value , int realDataLength ) throws FieldDataInvalidException { super ( id . getFieldName ( ) , value ) ; this . realDataLength = realDataLength ; try { Long . parseLong ( value ) ; } catch ( NumberFormatException nfe ) { throw new FieldDataInvalidException ( "Value of:" + value + " is invalid for field:" + id ) ; } }
target:  Create new field with known length