@CheckReturnValue
    @NonNull
    @BackpressureSupport(BackpressureKind.UNBOUNDED_IN)
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <U> Single<U> collect(Callable<? extends U> initialItemSupplier, BiConsumer<? super U, ? super T> collector) {
        ObjectHelper.requireNonNull(initialItemSupplier, "initialItemSupplier is null");
        ObjectHelper.requireNonNull(collector, "collector is null");
        return RxJavaPlugins.onAssembly(new FlowableCollectSingle<T, U>(this, initialItemSupplier, collector));
    }