src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Observable<T> distinctUntilChanged() {
        return distinctUntilChanged(Functions.identity());
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Observable<T> distinctUntilChanged() {
        return distinctUntilChanged(Functions.identity());
    }
Returns an Observable that emits all items emitted by the source ObservableSource that are distinct from their
immediate predecessors based on {@link Object#equals(Object)} comparison.
<p>
<img width="640" height="310" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/distinctUntilChanged.png" alt="">
<p>
It is recommended the elements' class {@code T} in the flow overrides the default {@code Object.equals()} to provide
meaningful comparison between items as the default Java implementation only considers reference equivalence.
Alternatively, use the {@link #distinctUntilChanged(BiPredicate)} overload and provide a comparison function
in case the class {@code T} can't be overridden with custom {@code equals()} or the comparison itself
should happen on different terms or properties of the class {@code T}.
<p>
Note that the operator always retains the latest item from upstream regardless of the comparison result
and uses it in the next comparison with the next upstream item.
<p>
Note that if element type {@code T} in the flow is mutable, the comparison of the previous and current
item may yield unexpected results if the items are mutated externally. Common cases are mutable
{@code CharSequence}s or {@code List}s where the objects will actually have the same
references when they are modified and {@code distinctUntilChanged} will evaluate subsequent items as same.
To avoid such situation, it is recommended that mutable data is converted to an immutable one,
for example using {@code map(CharSequence::toString)} or {@code map(list -> Collections.unmodifiableList(new ArrayList<>(list)))}.
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code distinctUntilChanged} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@return an Observable that emits those items from the source ObservableSource that are distinct from their
immediate predecessors
@see <a href="http://reactivex.io/documentation/operators/distinct.html">ReactiveX operators documentation: Distinct</a>
@see #distinctUntilChanged(BiPredicate)