title: How to get all elements in nested collection with streams
body: <p>I have a class which contains different nested collections, now I want to receive all the elements of the nested collections, concrete I want to collect all the StrokePoints of the collections. I can solve it with "old" java but how to do it with streams?</p>
  
  <pre><code>    int strokesCounter = 0;
      List&lt;StrokePoint&gt; pointList = new ArrayList&lt;&gt;();
      if (!strokesData.getListOfSessions().isEmpty()) {
          for (SessionStrokes session : strokesData.getListOfSessions()) {
              List&lt;Strokes&gt; strokes = session.getListOfStrokes();
              for (Strokes stroke : strokes) {
                  strokesCounter++;
                  List&lt;StrokePoint&gt; points = stroke.getListOfStrokePoints();
                  pointList.addAll(stroke.getListOfStrokePoints());        
              }
          }
      }
  </code></pre>
  
  <p>I am looking for a way to fill the pointList with the stream functionality.</p>