What are the features of different types of keys in RxJava and how do they differ from each other?

In RxJava, "keys" usually refer to different types of Subject or ways of managing subscriptions, although the term "keys" is not official. If we talk about Subject types, the main ones are:

• PublishSubject — transmits only the events that occur after subscription.
• BehaviorSubject — immediately sends the last (or initial) value upon subscription, then new events.
• ReplaySubject — stores all or a specified number of events and replays them upon subscription.
• AsyncSubject — sends only the last event to subscribers after the sequence completes.

If the context is about keys in managing subscriptions (e.g., CompositeDisposable), then keys can be used for identifying and managing the lifecycle of subscriptions.

Thus, different types of "keys" or Subjects differ in how they buffer and transmit events to subscribers, affecting the behavior of the reactive stream.