Difference between Subject and Observable

In the context of RxJava or reactive programming in general, both Observable and Subject are key components, but they serve different purposes:

1. Observable:

   • An Observable is a stream of data or events. It represents a sequence of items that can be observed over time.
   • It can emit items (such as data or events) to its subscribers.
   • Observables can be hot or cold. A cold observable starts emitting items when an observer subscribes, while a hot observable may emit items regardless of whether there are any subscribers.

2. Subject:

   • A Subject is a special type of object in RxJava that acts both as an observer and as an Observable.
   • It can subscribe to one or more Observables and also act as an Observable, emitting items to multiple observers.
   • Subjects come in different types, such as PublishSubject, BehaviorSubject, ReplaySubject, and AsyncSubject, each with different characteristics.
   • Subjects are typically used to multicast (broadcast) events to multiple subscribers. They are particularly useful for scenarios where you want to make a single stream of events available to multiple observers.

In summary, an Observable is a unidirectional data stream that emits items to its subscribers, while a Subject is a bidirectional bridge that acts both as an observer and as an Observable. Subjects are often used to multicast events to multiple subscribers and are a powerful tool in the RxJava toolkit.