title: How to sum values of a List of Maps by Java Stream API
body: <p>I have a <code>List&lt;Map&lt;String,Object&gt;&gt; resultList</code> (from DB Query)</p>
  
  <p>like this</p>
  
  <pre><code>Map&lt;String,Object&gt;map1 = new LinkedHashMap&lt;String,Object&gt;();        
          map1.put("01", "1");
          map1.put("02", "5");
          map1.put("03", "7");
  
          Map&lt;String,Object&gt;map2 = new LinkedHashMap&lt;String,Object&gt;();        
          map2.put("01", "2");
          map2.put("02", "4");
          map2.put("03", "6");
  
          Map&lt;String,Object&gt;map3 = new LinkedHashMap&lt;String,Object&gt;();    
          map3.put("01", "8");
          map3.put("02", "5");
          map3.put("03", "6");
  
          Map&lt;String,Object&gt;map4 = new LinkedHashMap&lt;String,Object&gt;();
          map4.put("01", "7");
          map4.put("02", "4");
          map4.put("03", "2");
  
          List&lt;Map&lt;String,Object&gt;&gt; mapList = new ArrayList&lt;Map&lt;String,Object&gt;&gt;();
  
          mapList .add(map1);
          mapList .add(map2);
          mapList .add(map3);
          mapList .add(map4);
  </code></pre>
  
  <p>I want to use mapList to sum values groupBy key</p>
  
  <p>Result what I want:</p>
  
  <p><code>Map&lt;String,Integer&gt; resultMap {01=18, 02=18, 03=21}</code></p>
  
  <p>I have tried many different method but cant work...</p>
  
  <p>Q: How to convert Object to int in stream and sum values groupBy key?</p>