SingleTest.java

import io.reactivex.Maybe;
import io.reactivex.Single;
import org.junit.Test;

public class SingleTest {
  @Test public void testSingleConcat() {
    Single<String> localSource = Single.create(emitter -> {
      emitter.onError(new RuntimeException("1個目のError"));
    });

    Single<String> remoteSource = Single.create(emitter -> {
      emitter.onError(new RuntimeException("2個目のError"));
    });

    Maybe.concat(localSource.toMaybe().onErrorComplete(), remoteSource.toMaybe())
        .firstOrError()
        .subscribe(System.out::println, System.err::println);
  }
}