On the Hackernoon site Nicolò Pignatelli wrote a good guide on how to write Value objects in a good way.

This is the list you must always check it against:

  • it is immutable and no setters defined;
  • it reflects the semantics of the domain;
  • it shows how information flows and is transformed during runtime;
  • it hasn’t default or useless getter methods;
  • it can be compared to other Value Objects of the - - same class by reading private properties directly

https://hackernoon.com/value-objects-like-a-pro-f1bfc1548c72