src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Single<Boolean> isEmpty() {
        return all(Functions.alwaysFalse());
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Single<Boolean> isEmpty() {
        return all(Functions.alwaysFalse());
    }
Returns a Single that emits {@code true} if the source ObservableSource is empty, otherwise {@code false}.
<p>
In Rx.Net this is negated as the {@code any} Observer but we renamed this in RxJava to better match Java
naming idioms.
<p>
<img width="640" height="320" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/isEmpty.2.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code isEmpty} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@return a Single that emits a Boolean
@see <a href="http://reactivex.io/documentation/operators/contains.html">ReactiveX operators documentation: Contains</a>