either.java

import android.util.Log;

import rx.Observable;
import rx.Observer;

class Either<T> {
  private final T value;
  private final Throwable throwable;

  public static <T> Either<T> ofValue(T value) {
    return new Either<>(value, null);
  }

  public static <T> Either<T> ofThrowable(Throwable throwable) {
    return new Either<>(null, throwable);
  }

  private Either(T value, Throwable throwable) {
    this.value = value;
    this.throwable = throwable;
  }

  public boolean hasValue() {
    return value != null;
  }

  public T getValue() {
    return value;
  }

  public Throwable getThrowable() {
    return throwable;
  }
}

abstract class EitherSubscriber<T> implements Observer<T> {
  @Override public void onCompleted() {
    // do nothing
  }

  @Override public void onError(Throwable e) {
    onEvent(Either.<T>ofThrowable(e));
  }

  @Override public void onNext(T t) {
    onEvent(Either.ofValue(t));
  }

  public abstract void onEvent(Either<T> either);
}

class Main {
  public static void main(String[] args) {
    Observable.range(0, 100)
        .subscribe(new EitherSubscriber<Integer>() {
          @Override public void onEvent(Either<Integer> either) {
            if (either.hasValue()) {
              Log.d("TAG", "Cool! " + either.getValue());
            } else {
              Log.d("TAG", "Whoops! " + either.getThrowable().getMessage());
            }
          }
        });
  }
}