src/main/java/io/reactivex/Completable.java
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public static <T> Completable fromSingle(final SingleSource<T> single) {
        ObjectHelper.requireNonNull(single, "single is null");
        return RxJavaPlugins.onAssembly(new CompletableFromSingle<T>(single));
    }
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public static <T> Completable fromSingle(final SingleSource<T> single) {
        ObjectHelper.requireNonNull(single, "single is null");
        return RxJavaPlugins.onAssembly(new CompletableFromSingle<T>(single));
    }
Returns a Completable instance that when subscribed to, subscribes to the Single instance and
emits a completion event if the single emits onSuccess or forwards any onError events.
<p>
<img width="640" height="356" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Completable.fromSingle.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code fromSingle} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>
@param <T> the value type of the Single
@param single the Single instance to subscribe to, not null
@return the new Completable instance
@throws NullPointerException if single is null