In RxJava we have a few operators for joining Observables. The most common are:

Take a look at the documentation in these links. It has interactive marble diagrams showing how the operators work on the streams. You can move marbles along the lines and see how the output stream changes. It really helps to understand how it works. Code snippets in this article are based on RxJava 2.1.0 with JUnit 4.12 and Google Truth 0.32 for unit tests. Let’s say, we have the following Observables:

public Observable<String> emitNumbers() {
  return Observable.fromArray("1", "2", "3", "4").delay(1, TimeUnit.SECONDS);
}

public Observable<String> emitLetters() {
  return Observable.fromArray("a", "b", "c", "d");
}

We can merge them in the different ways.

Concat

Concat operator emits the emissions from two or more Observables without interleaving them. We can perform the following operation:

public Observable<String> concatStreams() {
  return Observable.concat(emitNumbers(), emitLetters());
}

The easiest way to verify, how this operator works, is to create exploratory unit test as follows:

@Test
public void shouldConcatStreams() {
  // given
  Observable<String> observable = playground.concatStreams();
  List<String> expectedValues = Arrays.asList("1","2","3","4","a","b","c","d");
  List<String> joinedValues = new ArrayList<>();

  // when
  observable.blockingSubscribe(s -> joinedValues.add(s));

  // then
  assertThat(joinedValues).isEqualTo(expectedValues);
}

This operation can be represented graphically as well.

         1 --- 2 --- 3 --- 4
                  |
         a --- b --- c --- d
                  |
                  |
                concat
                  |
                 \|/
1 -- 2 -- 3 -- 4 --- a -- b -- c -- d

As we can see one stream is appended to another regardless of the execution time of both streams.

Merge

Merge operator combines multiple Observables into one by merging their emissions. Here we have a similar story, but changed operator:

public Observable<String> mergeStreams() {
  return Observable.merge(emitNumbers(), emitLetters());
}

We are writing another unit test:

@Test
public void shouldMergeStreams() {
  // given
  Observable<String> observable = playground.mergeStreams();
  List<String> expectedValues = Arrays.asList("a","b","c","d","1","2","3","4");
  List<String> joinedValues = new ArrayList<>();

  // when
  observable.blockingSubscribe(s -> joinedValues.add(s));

  // then
  assertThat(joinedValues).isEqualTo(expectedValues);
}

Merge operation should look like that:

         1 --- 2 --- 3 --- 4
                  |
         a --- b --- c --- d
                  |
                  |
                merge
                  |
                 \|/
a -- b -- c -- d --- 1 -- 2 -- 3 -- 4

This operator doesn’t synchronize the streams and merges them as values are emitted. Numbers are emitted later than letters, so letters are placed in the beginning of the output stream. Try to manipulate marble on the interactive diagram on the reactivex.io website to see how it should work.

Zip

The last operator, I’d like to discuss in this article is “Zip” operator. Zip combines the emissions of multiple Observables together via a specified function and emit single items for each combination based on the results of this function. In simple words, it waits until many observables are emitted and then combines them into a pair (or triple Observable, etc. in the case or more Observables). Now, we need to create a function, which will transform our streams and return combined stream.

public Observable<String> zipStreams() {
  return Observable.zip(emitNumbers(), emitLetters(),
      (s1, s2) -> String.format("(%s,%s)", s1, s2));
}

Next, we can verify it with test as usual:

@Test
public void shouldZipStreams() {
  // given
  Observable<String> observable = playground.zipStreams();
  List<String> expectedValues = Arrays.asList("(1,a)","(2,b)","(3,c)","(4,d)");
  List<String> joinedValues = new ArrayList<>();

  // when
  observable.blockingSubscribe(s -> joinedValues.add(s));

  // then
  assertThat(joinedValues).isEqualTo(expectedValues);
}

and it can be represented graphically like that:

        1 --- 2 --- 3 --- 4
                 |
        a --- b --- c --- d
                 |
                 |
                zip
                 |
                \|/
 (1,a) -- (2,b) --- (3,c) -- (4,d)

Now, we have pairs of merged streams.

Summary

Of course, RxJava is complicated library and these methods are not covering all possibilities of merging and combining the Observable streams. Neverhteless, examples in this article are quite basic and may help you to understand how mentioned operators work. After that we can apply the best operator to appropriate situation.

Reference thread on StackOverflow: http://stackoverflow.com/questions/28843318/android-rxjava-joining-lists