src/main/java/io/reactivex/Flowable.java
@CheckReturnValue
    @NonNull
    @BackpressureSupport(BackpressureKind.PASS_THROUGH)
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Flowable<T> filter(Predicate<? super T> predicate) {
        ObjectHelper.requireNonNull(predicate, "predicate is null");
        return RxJavaPlugins.onAssembly(new FlowableFilter<T>(this, predicate));
    }
@CheckReturnValue
    @NonNull
    @BackpressureSupport(BackpressureKind.PASS_THROUGH)
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Flowable<T> filter(Predicate<? super T> predicate) {
        ObjectHelper.requireNonNull(predicate, "predicate is null");
        return RxJavaPlugins.onAssembly(new FlowableFilter<T>(this, predicate));
    }
Filters items emitted by a Publisher by only emitting those that satisfy a specified predicate.
<p>
<img width="640" height="310" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/filter.png" alt="">
<dl>
<dt><b>Backpressure:</b></dt>
<dd>The operator doesn't interfere with backpressure which is determined by the source {@code Publisher}'s backpressure
behavior.</dd>
<dt><b>Scheduler:</b></dt>
<dd>{@code filter} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@param predicate
a function that evaluates each item emitted by the source Publisher, returning {@code true}
if it passes the filter
@return a Flowable that emits only those items emitted by the source Publisher that the filter
evaluates as {@code true}
@see <a href="http://reactivex.io/documentation/operators/filter.html">ReactiveX operators documentation: Filter</a>