title: How to merge List of Multimaps returned from executors Future with Java8
body: <p>Suppose I have the following snippet: (The future is result of Callable task)</p>
  
  <pre><code>List&lt;Map&lt;String, List&lt;Offset&gt;&gt;&gt; results = new ArrayList&lt;&gt;();
  
  for (Future&lt;Map&lt;String, List&lt;Offset&gt;&gt;&gt; future : listFutures)   {
      Map&lt;String, List&lt;Offset&gt;&gt; mapResult = future.get();
      results.add(mapResult);
  }
  </code></pre>
  
  <p>I want to make an aggregation of all <code>"Map&lt;String, List&lt;Offset&gt;&gt;"</code> into <code>flatMap</code> from all <code>Futures</code> together , the use case that will be is duplication key between the maps, in that case I need to add the value into the list of  <code>List&lt;Offset&gt;</code></p>
  
  <p>I expected after flatMap , have result of <code>"Map&lt;String, List&lt;Offset&gt;&gt;"</code></p>
  
  <p>P.S</p>
  
  <p>I saw some examples but they are talking not during iteration.</p>
  
  <pre><code>       Stream.concat(map1.entrySet().stream(), map2.entrySet().stream())
                          .collect(Collectors.toMap(
                              entry -&gt; entry.getKey(), 
                              entry -&gt; entry.getValue()
                          )
                      );
  </code></pre>
  
  <p>Any suggestion?</p>