MainActivity.java

Observable<Integer> observable = Observable
  .just(1, 2, 3, 4, 5)
  .filter(new Func1<Integer, Boolean>() {
      @Override
      public Boolean call(Integer integer) {
          //check if the number is odd? If the number is odd return true, to emmit that object.
          return integer % 2 != 0;
      }
  });

Observer<Integer> observer = new Observer<Integer>() {
  @Override
  public void onCompleted() {
      System.out.println("All data emitted.");
  }

  @Override
  public void onError(Throwable e) {
      System.out.println("Error received: " + e.getMessage());
  }

  @Override
  public void onNext(Integer integer) {
      System.out.println("New data received: " + integer);
  }
};

Subscription subscription = observable
  .subscribeOn(Schedulers.io())       //observable will run on IO thread.
  .observeOn(AndroidSchedulers.mainThread())      //Observer will run on main thread.
  .subscribe(observer);               //subscribe the observer