Java: Publisher and Subscriber

To maintain the integrity of the stream contract and avoid undefined behavior.

The Reactive Streams specification mandates that a Publisher must call onComplete() or onError() only once to signal the end of data flow, preventing Subscribers from receiving unexpected or inconsistent data. Violating this rule can lead to unpredictable system behavior. For example, in a live data feed, multiple onComplete() signals could indicate false end-of-stream events, confusing subsequent processing.