title: how to easily sum two hashMap<String,Integer>?
body: <p>I have two <code>HashMap&lt;String,Integer&gt;</code></p>
  
  <p>How can I sum them easily?</p>
  
  <p>Meaning that for String "a" the key will be sum of (value from Map1 + value from Map2)?</p>
  
  <p>I can iterate every item of Map2 and add manually to Map1.</p>
  
  <p>But thought there might be an easier way?</p>
  
  <p>I prefer summing the Integers into one of the maps. Not creating a new one</p>