src/main/java/io/reactivex/Flowable.java
@CheckReturnValue
    @NonNull
    @BackpressureSupport(BackpressureKind.UNBOUNDED_IN)
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <K, V> Single<Map<K, Collection<V>>> toMultimap(
            final Function<? super T, ? extends K> keySelector,
            final Function<? super T, ? extends V> valueSelector,
            final Callable<? extends Map<K, Collection<V>>> mapSupplier,
            final Function<? super K, ? extends Collection<? super V>> collectionFactory) {
        ObjectHelper.requireNonNull(keySelector, "keySelector is null");
        ObjectHelper.requireNonNull(valueSelector, "valueSelector is null");
        ObjectHelper.requireNonNull(mapSupplier, "mapSupplier is null");
        ObjectHelper.requireNonNull(collectionFactory, "collectionFactory is null");
        return collect(mapSupplier, Functions.toMultimapKeyValueSelector(keySelector, valueSelector, collectionFactory));
    }
@CheckReturnValue
    @NonNull
    @BackpressureSupport(BackpressureKind.UNBOUNDED_IN)
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <K, V> Single<Map<K, Collection<V>>> toMultimap(
            final Function<? super T, ? extends K> keySelector,
            final Function<? super T, ? extends V> valueSelector,
            final Callable<? extends Map<K, Collection<V>>> mapSupplier,
            final Function<? super K, ? extends Collection<? super V>> collectionFactory) {
        ObjectHelper.requireNonNull(keySelector, "keySelector is null");
        ObjectHelper.requireNonNull(valueSelector, "valueSelector is null");
        ObjectHelper.requireNonNull(mapSupplier, "mapSupplier is null");
        ObjectHelper.requireNonNull(collectionFactory, "collectionFactory is null");
        return collect(mapSupplier, Functions.toMultimapKeyValueSelector(keySelector, valueSelector, collectionFactory));
    }
Returns a Single that emits a single Map, returned by a specified {@code mapFactory} function, that
contains a custom collection of values, extracted by a specified {@code valueSelector} function from
items emitted by the finite source Publisher, and keyed by the {@code keySelector} function.
<p>
<img width="640" height="305" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/toMultiMap.png" alt="">
<p>
Note that this operator requires the upstream to signal {@code onComplete} for the accumulated map to
be emitted. Sources that are infinite and never complete will never emit anything through this
operator and an infinite source may lead to a fatal {@code OutOfMemoryError}.
<dl>
<dt><b>Backpressure:</b></dt>
<dd>The operator honors backpressure from downstream and consumes the source {@code Publisher} in an
unbounded manner (i.e., without applying backpressure to it).</dd>
<dt><b>Scheduler:</b></dt>
<dd>{@code toMultimap} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@param <K> the key type of the Map
@param <V> the value type of the Map
@param keySelector
the function that extracts a key from the source items to be used as the key in the Map
@param valueSelector
the function that extracts a value from the source items to be used as the value in the Map
@param mapSupplier
the function that returns a Map instance to be used
@param collectionFactory
the function that returns a Collection instance for a particular key to be used in the Map
@return a Single that emits a single item: a Map that contains the collection of mapped items from
the source Publisher
@see <a href="http://reactivex.io/documentation/operators/to.html">ReactiveX operators documentation: To</a>