Is guava's ImmutableXXX really immutable?

Question

I have been using guava for some time now and truly trusted it, until I stumbled of an example yesterday, which got me thinking. Long story short, here it is:

Answer 1

What you're getting at here is the difference between immutable and deeply immutable.

An immutable object will never change, but anything that it refers to might change. Deep immutability is much stronger: neither the base object nor any object you can navigate to from it will change.

Each is appropriate in its own situations. When you create your own class that has a field of type Date, that date is owned by your object; it's truly a part of it. Therefore, you should make defensive copies of it (on the way in and the way out!) to provide deep immutability.

But a collection does not really "own" its elements. Their states are not considered part of the collection's state; it is a different type of class -- a container. (Furthermore, as you allude, it has no deep knowledge of what element type is being used, so it wouldn't know how to copy the elements anyway.)

Another answer states that the Guava collections should have used the term unmodifiable. But there is a very well-defined difference between the terms unmodifiable and immutable in the context of collections, and it has nothing to do with shallow vs. deep immutability. "Unmodifiable" says you cannot change this instance, via the reference you have; "immutable" means this instance cannot change, period, whether by you or any other actor.