How to chain tasks in RxJava

Question

I am working on android app registration in which I need to perform several tasks one by one let say

task1
task2
task3

I want to chain these tasks one after another and if a task is failed whole process should be failed..

I want to solve this problem by Rxjava can anyone tell me how to achieve this with rxJava.

(I have wasted 5 hours but did not find solution also newbie in RxJava)

What I have tried

Observable.merge(task1,task2,task3).subscribe(new Observer<DataError>() {
            @Override
            public void onSubscribe(Disposable d) {

            }

            @Override
            public void onNext(DataError dataError) {

            }

            @Override
            public void onError(Throwable e) {

            }

            @Override
            public void onComplete() {
             //this method is not called by rxJava
            }
        });

with this method all things are working fine but onComplete() method is not called by rxJava .

Please help~

Edit-
Each task is dependent on previous task result.and there should be one task at one time.

Answer 1

Let's say we have 3 tasks. From given array of Integers, Find even numbers, Multiply each even numbers with 10, Divide each number by 2

//A stream of observable to find even numbers
private Observable<Integer> findEven(Integer number) {
    return Observable
        .just(number)
        .filter(data -> data % 2 == 0);
}

//A stream of observable to multiply each number with 10
private Observable<Integer> multiplyBy10(Integer evenNumber) {
    return Observable.just(evenNumber).map(data -> data * 10);
}

//A stream of observable to divide each number with 2
private Observable<Integer> divideBy2(Integer evenNumber) {
    return Observable.just(evenNumber).map(data -> data / 2);
}

So, how do we chain this task?

Observable
    .just(1, 2, 3, 4, 5, 6, 7, 8)
    //find even numbers
    .flatMap(num -> findEven(num))
    //Now multiply each even number by 10
    .flatMap(num -> multiplyBy10(num))
    //Now to each number divide 2 
    .flatMap(num -> divideBy2(num))
    .subscribe(
        // result: 10, 20, 30, 40
        result -> Log.v("", "result: " + result),
        error -> Log.e("", error.getMessage())
    );

Hope this helps.