Create Observable using just method in RxJava

Constructs a reactive type by taking a pre-existing object and emitting that specific object to the downstream consumer upon subscription.

Observable<String> observable = Observable.just("1");

observable.subscribe(s -> {
    System.out.println(s);
});

Observable<String> observable = Observable.just("1", "2", "3", "4");

observable.subscribe(s -> {
    System.out.println(s);
});

References
https://github.com/ReactiveX/RxJava/wiki/Creating-Observables#just