src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <K> Observable<T> distinct(Function<? super T, K> keySelector) {
        return distinct(keySelector, Functions.createHashSet());
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <K> Observable<T> distinct(Function<? super T, K> keySelector) {
        return distinct(keySelector, Functions.createHashSet());
    }
Returns an Observable that emits all items emitted by the source ObservableSource that are distinct according
to a key selector function and based on {@link Object#equals(Object)} comparison of the objects
returned by the key selector function.
<p>
<img width="640" height="310" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/distinct.key.png" alt="">
<p>
It is recommended the keys' class {@code K} overrides the default {@code Object.equals()}
and {@link Object#hashCode()} to provide meaningful comparison between the key objects as the default
Java implementation only considers reference equivalence.
<p>
By default, {@code distinct()} uses an internal {@link java.util.HashSet} per Observer to remember
previously seen keys and uses {@link java.util.Set#add(Object)} returning {@code false} as the
indicator for duplicates.
<p>
Note that this internal {@code HashSet} may grow unbounded as keys won't be removed from it by
the operator. Therefore, using very long or infinite upstream (with very distinct keys) may lead
to {@code OutOfMemoryError}.
<p>
Customizing the retention policy can happen only by providing a custom {@link java.util.Collection} implementation
to the {@link #distinct(Function, Callable)} overload.
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code distinct} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@param <K> the key type
@param keySelector
a function that projects an emitted item to a key value that is used to decide whether an item
is distinct from another one or not
@return an Observable that emits those items emitted by the source ObservableSource that have distinct keys
@see <a href="http://reactivex.io/documentation/operators/distinct.html">ReactiveX operators documentation: Distinct</a>
@see #distinct(Function, Callable)