src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <R> Observable<R> map(Function<? super T, ? extends R> mapper) {
        ObjectHelper.requireNonNull(mapper, "mapper is null");
        return RxJavaPlugins.onAssembly(new ObservableMap<T, R>(this, mapper));
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <R> Observable<R> map(Function<? super T, ? extends R> mapper) {
        ObjectHelper.requireNonNull(mapper, "mapper is null");
        return RxJavaPlugins.onAssembly(new ObservableMap<T, R>(this, mapper));
    }
Returns an Observable that applies a specified function to each item emitted by the source ObservableSource and
emits the results of these function applications.
<p>
<img width="640" height="305" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/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 output type
@param mapper
a function to apply to each item emitted by the ObservableSource
@return an Observable that emits the items from the source ObservableSource, transformed by the specified
function
@see <a href="http://reactivex.io/documentation/operators/map.html">ReactiveX operators documentation: Map</a>