unitsofmeasurement/unit-ri

@keilw Thanks for the clarifications. In my view the dimensionless unit ONE is no more or less special than any other unit, and should only be returned if the string that is parsed is a valid representation of the dimensional unit, e.g. 1 or m/m. Personally I can see 3 different cases from a functional point of view: 1) an input string is a valid combination of known unit symbols and operators, and therefore there is no error, e.g., 1, mm, 1/m^2, s^-1, kg*m; 2) an input string is valid but contains an unknown unit symbol, e.g., 1/bla; 3) An input string is invalid, e.g. 1//m^--1. In my opinion only the first one should return a Unit, the second one should throw a special ParserException to indicate the unit symbol is unknown, and the third should throw a ParserException to indicate the syntax is invalid. In no case should the parse method return null; this puts an undesired burden on the client to check the result which 1) will be forgotten a lot resulting in a lot of NullPointerExceptions, 2) leads to ugly if-based error checking (e.g., if (format.parse("1/m") == null) { // do error handling } where it is hard to separate the happy-flow logic and error-logic leading to hard to read code, and 3) makes it impossible to know the reason or details of the error (was it an invalid symbol, and at what position did the parsing fail?).