Stream

A source of asynchronous data events. Each event is either a data event, also called an element of the stream, or an error event, which is a notification that something has failed. When a stream has emitted all its event, a single "done" event will notify the listener that the end has been reached.

Streams

• Similar to future, but handles multiple events.
• 하나의 스트림에는 하나의 리스너만 붙일수 있다.
• 하나의 스트림에 여러개 리스너를 붙이려면 브로드캐스트를 사용하면 된다.