What are the characteristics of a Collector?

They are defined in the Collector.Characteristics enum, and the Stream API implementation uses them to know how it's allowed to drive your collector.

• CONCURRENT — this collector supports concurrency for the Stream API: a parallel stream is allowed to use it.
• UNORDERED — you're collecting data into a container that doesn't care about the order in which it receives elements. That's the case for Collectors.toSet(), for instance. This property can be used to relax some constraints on the computation of parallel streams.
• IDENTITY_FINISH — the finisher of your collector is the identity function, so the implementation doesn't need to call it. That's the case for Collectors.toList() or Collectors.toSet(), but it is not the case for Collectors.joining().

Setting the CONCURRENT characteristic to true does not magically make your collector concurrent — providing a thread-safe implementation is your responsibility.