Observable<String> myObservable = Observable.create( 
  new Observable.OnSubscribe<String>() { 
    @Override 
    public void call(Subscriber<? super String> sub) { 
      sub.onNext("Hello, world!"); 
      sub.onCompleted(); 
    } 
  } 
); 

Subscriber<String> mySubscriber = new Subscriber<String>() { 
  @Override 
  public void onNext(String s) { System.out.println(s); } 
 
  @Override 
  public void onCompleted() { } 
 
  @Override 
  public void onError(Throwable e) { } 
}; 

myObservable.subscribe(mySubscriber); 

Action1<String> onNextAction = new Action1<String>() { 
  @Override 
  public void call(String s) { 
    System.out.println(s); 
  } 
}; 

myObservable.subscribe(onNextAction, onErrorAction, onCompleteAction); 

 
myObservable.subscribe(onNextAction); 
// Outputs "Hello, world!" 

Observable.just("Hello, world!") 
  .subscribe(new Action1<String>() { 
    @Override 
    public void call(String s) { 
       System.out.println(s); 
    } 
  }); 

Observable.just("Hello, world!") 
  .subscribe(s -> System.out.println(s)); 

Observable.just("Hello, world! -Dan") 
  .subscribe(s -> System.out.println(s)); 

Observable.just("Hello, world!") 
  .subscribe(s -> System.out.println(s + " -Dan")); 

public class WeatherData {
 
  public Coordinates coord;
  public Local sys;
  public List<Weather> weathers;
  public String base;
  public Main main;
  public Wind wind;
  public Rain rain;
  public Cloud clouds;
  public long id;
  public long dt;
  public String name;
  public int cod;
 
  public static class Coordinates {
    public double lat;
    public double lon;
  }
 
  public static class Local {
    public String country;
    public long sunrise;
    public long sunset;
  }
 
  public static class Weather {
    public int id;
    public String main;
    public String description;
    public String icon;
  }
 
  public static class Main {
    public double temp;
    public double pressure;
    public double humidity;
    public double temp_min;
    public double temp_max;
    public double sea_level;
    public double grnd_level;
  }
 
  public static class Wind {
    public double speed;
    public double deg;
  }
 
  public static class Rain {
    public int threehourforecast;
  }
 
  public static class Cloud {
    public int all;
  }
 
}

private interface ApiManagerService {
  @GET("/weather")
  WeatherData getWeather(@Query("q") String place, @Query("units") String units);
}

//...
final RestAdapter restAdapter = new RestAdapter.Builder()
  .setServer("http://api.openweathermap.org/data/2.5")
  .build();
 
final ApiManagerService apiManager = restAdapter.create(ApiManagerService.class);
final WeatherData weatherData = apiManager.getWeather("Budapest,hu", "metric");
//...

public class ApiManager {
 
  private interface ApiManagerService {
    @GET("/weather")
    WeatherData getWeather(@Query("q") String place, @Query("units") String units);
  }
 
  private static final RestAdapter restAdapter = new RestAdapter.Builder()
    .setServer("http://api.openweathermap.org/data/2.5")
    .build();
  private static final ApiManagerService apiManager = restAdapter.create(ApiManagerService.class);
 
  public static Observable<WeatherData> getWeatherData(final String city) {
    return Observable.create(new Observable.OnSubscribeFunc<WeatherData>() {
      @Override
      public Subscription onSubscribe(Observer<? super WeatherData> observer) {
        try {
          observer.onNext(apiManager.getWeather(city, "metric"));
          observer.onCompleted();
        } catch (Exception e) {
          observer.onError(e);
        }
 
        return Subscriptions.empty();
      }
    }).subscribeOn(Schedulers.threadPoolForIO());
  }
 
}

Observable.from(cities)
      .mapMany(new Func1<String, Observable<WeatherData>>() {
        @Override
        public Observable<WeatherData> call(String s) {
          return ApiManager.getWeatherData(s);
        }
      })
      .subscribeOn(Schedulers.threadPoolForIO())
      .observeOn(AndroidSchedulers.mainThread())
      .subscribe(new Action1<WeatherData>() {
        @Override
        public void call(WeatherData weatherData) {
          // do your work
        }
      });