How to handle concurrent access to a Scala collection?

Question

I have an Actor that - in its very essence - maintains a list of objects. It has three basic operations, an add, update and a remove (where sometimes the remove is called fr

Answer 1

What collection type should I use in a concurrent access situation, and how is it used?

See @hbatista's answer.

Is an Actor actually a multithreaded entity, or is that just my wrong conception and does it process messages one at a time in a single thread

The second (though the thread on which messages are processed may change, so don't store anything in thread-local data). That's how the actor can maintain invariants on its state.