src/main/java/io/reactivex/Single.java
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <R> Single<R> map(Function<? super T, ? extends R> mapper) {
        ObjectHelper.requireNonNull(mapper, "mapper is null");
        return RxJavaPlugins.onAssembly(new SingleMap<T, R>(this, mapper));
    }
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <R> Single<R> map(Function<? super T, ? extends R> mapper) {
        ObjectHelper.requireNonNull(mapper, "mapper is null");
        return RxJavaPlugins.onAssembly(new SingleMap<T, R>(this, mapper));
    }
Returns a Single that applies a specified function to the item emitted by the source Single and
emits the result of this function application.
<p>
<img width="640" height="305" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Single.map.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code map} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@param <R> the result value type
@param mapper
a function to apply to the item emitted by the Single
@return a Single that emits the item from the source Single, transformed by the specified function
@see <a href="http://reactivex.io/documentation/operators/map.html">ReactiveX operators documentation: Map</a>