retrofit-adapters/rxjava2/src/main/java/retrofit2/adapter/rxjava2/RxJava2CallAdapterFactory.java
@SuppressWarnings("ConstantConditions") // Guarding public API nullability.
  public static RxJava2CallAdapterFactory createWithScheduler(Scheduler scheduler) {
    if (scheduler == null) throw new NullPointerException("scheduler == null");
    return new RxJava2CallAdapterFactory(scheduler, false);
  }
@SuppressWarnings("ConstantConditions") // Guarding public API nullability.
  public static RxJava2CallAdapterFactory createWithScheduler(Scheduler scheduler) {
    if (scheduler == null) throw new NullPointerException("scheduler == null");
    return new RxJava2CallAdapterFactory(scheduler, false);
  }
Returns an instance which creates synchronous observables that
{@linkplain Observable#subscribeOn(Scheduler) subscribe on} {@code scheduler} by default.