Making Transaction mutable usually means its internal state is not 100% controlled by its implementation. The consumer of a Transaction object might be able to

Making Transaction mutable usually means its internal state is not 100% controlled by its implementation. The consumer of a Transaction object might be able to manipulate its state in an expected way (e.g. calling mutation methods in an unexpected ordering) and makes it invalid. So the unit test suite you write for Transaction will never cover all cases. You need to test Transaction in the context where it's used as well, e.g. the Account example. That means when you're writing tests for Account, you are actually testing Transaction at the same time. That's probably why it feels more like an integration test than a unit test.