title: Merge two maps with Java 8
body: <p>I have two maps like this:</p>
  <pre><code>map1 = new Map&lt;String, MyObject&gt;();
  map2 = new Map&lt;String, MyObject&gt;();
  
  MyObject {
     Integer mark1;
     Integer mark2;
  }
  </code></pre>
  <p>What I want to do to is to merge the two maps into a <code>map3</code> <code>&lt;String, MyObject&gt;</code> like this:<br></p>
  <ol>
  <li>If <code>map1.place</code> is not in <code>map2.place</code>, then I add the entry to <code>map3</code>.</li>
  <li>same if <code>map2.place</code> is not in <code>map1.place</code>, I add the entry to <code>map3</code>.</li>
  <li>if <code>map1.place</code> is in <code>map2.place</code>, then I add this entry:</li>
  </ol>
  <ul>
  <li><code>map1.place, (map1.mark1, map2.mark2)</code></li>
  </ul>
  <p>I have read about <code>flatMap</code>, but I really have a hard time using it.
  Any clue how to do this?</p>