ContainsObservable.java

        Observable.just(1, 2, 3, 4, 5, 6)
                .contains(10)
                .subscribe(new SingleObserver<Boolean>() {
                    @Override
                    public void onSubscribe(Disposable d) {

                    }

                    @Override
                    public void onSuccess(Boolean aBoolean) {
                        System.out.println("Does list contain value 10: " + aBoolean);
                    }

                    @Override
                    public void onError(Throwable e) {

                    }
                });