Unit testing for object immutability

Question

I want to make sure that a given group of objects is immutable.

I was thinking about something along the lines of:

1. check if every field is private final

Answer 1

I doubt you can do this with unit tests. The best way would be to be careful during writing the class or looking into the code. Precisely because of the problem that methods on the object can mutate its state which you might not see from the outside. Just because it's discouraged doesn't mean it doesn't happen :-)