src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Single<Boolean> contains(final Object element) {
        ObjectHelper.requireNonNull(element, "element is null");
        return any(Functions.equalsWith(element));
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Single<Boolean> contains(final Object element) {
        ObjectHelper.requireNonNull(element, "element is null");
        return any(Functions.equalsWith(element));
    }
Returns a Single that emits a Boolean that indicates whether the source ObservableSource emitted a
specified item.
<p>
<img width="640" height="320" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/contains.2.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code contains} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@param element
the item to search for in the emissions from the source ObservableSource
@return a Single that emits {@code true} if the specified item is emitted by the source ObservableSource,
or {@code false} if the source ObservableSource completes without emitting that item
@see <a href="http://reactivex.io/documentation/operators/contains.html">ReactiveX operators documentation: Contains</a>