Considering object encapsulation, should getters return an immutable property?

Question

When a getter returns a property, such as returning a List of other related objects, should that list and it's objects be immutable to prevent code outside of the class, changing the state of those objects, without the main parent object knowing?

For example if a Contact object, has a getDetails getter, which returns a List of ContactDetails objects, then any code calling that getter:

1. can remove ContactDetail objects from that list without the Contact object knowing of it.
2. can change each ContactDetail object without the Contact object knowing of it.

So what should we do here? Should we just trust the calling code and return easily mutable objects, or go the hard way and make a immutable class for each mutable class?

Answer 1

It's a matter of whether you should be "defensive" in your code. If you're the (sole) user of your class and you trust yourself then by all means no need for immutability. However, if this code needs to work no matter what, or you don't trust your user, then make everything that is externalized immutable.

That said, most properties I create are mutable. An occasional user botches this up, but then again it's his/her fault, since it is clearly documented that mutation should not occur via mutable objects received via getters.