title: How to collect all ints from List<Single<List<Int>>> and put it into List<Int> in RxJava?
body: <p>My goal is to unwrap a list of singles of list of ints, and get all it's elements, in order to put it on a list.</p>
  
  <pre><code>List&lt;Single&lt;List&lt;Int&gt;&gt;&gt; listOfSinglesOfListofInts = getListSingleListType(); // random method that gives us that.
  List&lt;Int&gt; intList = new ArrayList&lt;&gt;();
  </code></pre>
  
  <p>My goal is to move move all Int contents from listOfSinglesOfListOfInts to listInt. Here is what I tried:</p>
  
  <pre><code>ListOfSinglesOfListOfInt.stream().map(
      singleListOfInts -&gt; singleListOfInts.map(
          listOfInts -&gt; intList.addAll(listOfInts)
      )
  );
  
  
  return listInt;
  </code></pre>
  
  <p>The size of listInt is always 0. </p>
  
  <p>What would be the correct way of accomplishing this?</p>