Cache concat+first wrong and correct solutions

package ass.excercises.cvi8.snippets;

import io.reactivex.Observable;

public class CommonMistakeExample {

    public static void main(String[] args) {
        System.out.println("===== Wrong =====");
        wrong();

        System.out.println("===== Correct =====");
        correct();

        /*  Outputs:
         *  ===== Wrong =====
            Done work in source1
            Done work in source2
            For each: 'source1'
            ===== Correct =====
            Done work in source1
            For each: 'source1'
         */
    }

    private static void wrong() {
        Observable.concat(
                sourceWrong(1),
                sourceWrong(2)
        )
        .first("N/A")
        .toObservable()
        .forEach(s -> System.out.printf("For each: '%s'\n", s));
    }

    private static Observable<String> sourceWrong(int num) {
        // This is a wrong approach!

        // first be do some work
        doWork();
        System.out.println("Done work in source"+num);
        String resultOfWork = "source"+num;

        // then we create observable that already has it's content ready
        return Observable.just(resultOfWork);
    }

    private static void correct() {
        Observable.concat(
                sourceCorrect(1)
                .doOnNext(s -> System.out.println("Note that this is a correct way to log activity of Observable")),

                sourceCorrect(2)
        )
        .first("N/A")
        .toObservable()
        .forEach(s -> System.out.printf("For each: '%s'\n", s));
    }


    private static Observable<String> sourceCorrect(int num) {
        // this illustrates very simple way of fixing the problem:

        // defer work to later: deferred code will be called only when someone
        // will subscribe on Observable and with concat+first combination,
        // it will occur only when first Observable in concat will return empty Observable;
        return Observable.defer(() -> {
            doWork();
            System.out.println("Done work in source"+num);
            String resultOfWork = "source"+num;
            return Observable.just(resultOfWork);
        });
    }

    private static void doWork() {
        try {
            Thread.sleep(500);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}