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Learning Reactive Programming with Java 8 by Nickolay Tsvetinov

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Hot and cold Observable instances

Looking at the previous examples implemented using the Observable.create(), Observable.just(), and Observable.from() methods, we can say that until someone subscribes to them, they are inactive and don't emit anything. However, each time someone subscribes, they start emitting their notifications. For example, if we subscribe three times to an Observable.from(Iterable) object, the Iterable instance will be iterated three times. The Observable instances behaving like that are called cold Observable instances.

All of the factory methods we've been using in this chapter return cold Observables. Cold Observables produce notifications on demand, and for every Subscriber, they produce independent notifications.

There ...

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