src/main/java/io/reactivex/Maybe.java
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public static <T> Maybe<T> fromCompletable(CompletableSource completableSource) {
        ObjectHelper.requireNonNull(completableSource, "completableSource is null");
        return RxJavaPlugins.onAssembly(new MaybeFromCompletable<T>(completableSource));
    }
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public static <T> Maybe<T> fromCompletable(CompletableSource completableSource) {
        ObjectHelper.requireNonNull(completableSource, "completableSource is null");
        return RxJavaPlugins.onAssembly(new MaybeFromCompletable<T>(completableSource));
    }
Wraps a CompletableSource into a Maybe.

<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code fromCompletable} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>
@param <T> the target type
@param completableSource the CompletableSource to convert from
@return the new Maybe instance
@throws NullPointerException if completable is null