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

@param item
the item to search for in the emissions from the source Maybe, not null
@return a Single that emits {@code true} if the specified item is emitted by the source Maybe,
or {@code false} if the source Maybe completes without emitting that item
@see <a href="http://reactivex.io/documentation/operators/contains.html">ReactiveX operators documentation: Contains</a>