RxJava: “java.lang.IllegalStateException: Only one subscriber allowed!”

Question

I\'m using RxJava to calculate the normalized auto correlation over some sensor data in Android. Strangely enough, my code throws an exception (\"java.lang.IllegalStateExcep

Answer 1

In RxJava, the operators groupBy and window return an observable which can be subscribed to only once and if subscribed, they replay their accumulated contents to the sole subscriber and switch to 'hot' mode.

This was a tradeoff between returning a fully hot observable and risk missing values or return an unbounded replaying observable that allows any subscribers but retains the accumulated contents indefinitely.

The middle ground, namely a single subscriber, cold-then-hot observable is thought to be the least surprising behavior and gives the developer the option to apply further operators and pick any point between the two extremes:

source.window(1, TimeUnit.SECONDS)
    .map(w -> w.publish())
    .doOnNext(w -> w.connect())
    .subscribe(...)

source.window(1, TimeUnit.SECONDS)
    .map(w -> w.cache())
    .subscribe(...)