src/main/java/io/reactivex/parallel/ParallelFlowable.java
protected final boolean validate(@NonNull Subscriber<?>[] subscribers) {
        int p = parallelism();
        if (subscribers.length != p) {
            Throwable iae = new IllegalArgumentException("parallelism = " + p + ", subscribers = " + subscribers.length);
            for (Subscriber<?> s : subscribers) {
                EmptySubscription.error(iae, s);
            }
            return false;
        }
        return true;
    }
protected final boolean validate(@NonNull Subscriber<?>[] subscribers) {
        int p = parallelism();
        if (subscribers.length != p) {
            Throwable iae = new IllegalArgumentException("parallelism = " + p + ", subscribers = " + subscribers.length);
            for (Subscriber<?> s : subscribers) {
                EmptySubscription.error(iae, s);
            }
            return false;
        }
        return true;
    }
Validates the number of subscribers and returns true if their number
matches the parallelism level of this ParallelFlowable.

@param subscribers the array of Subscribers
@return true if the number of subscribers equals to the parallelism level