src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Single<T> first(T defaultItem) {
        return elementAt(0L, defaultItem);
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Single<T> first(T defaultItem) {
        return elementAt(0L, defaultItem);
    }
@ CheckReturnValue @ SchedulerSupport ( SchedulerSupport . NONE ) public final Single < T > first ( T defaultItem ) { return elementAt ( 0L , defaultItem ) ; }
Returns a Single that emits only the very first item emitted by the source ObservableSource, or a default item
if the source ObservableSource completes without emitting any items.
<p>
<img width="640" height="286" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/first.s.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code first} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@param defaultItem
the default item to emit if the source ObservableSource doesn't emit anything
@return the new Single instance
@see <a href="http://reactivex.io/documentation/operators/first.html">ReactiveX operators documentation: First</a>
Returns a Single that emits only the very first item emitted by the source ObservableSource or a default item if the source ObservableSource completes without emitting any items . <p > <img width = 640 height = 286 src = https : // raw . github . com / wiki / ReactiveX / RxJava / images / rx - operators / first . s . png alt = > <dl > <dt > <b > Scheduler : < / b > < / dt > <dd > { @code first } does not operate by default on a particular { @link Scheduler } . < / dd > < / dl >