title: What's the best way to sum two Map<String,String>?
body: <p>I have the following maps.</p>
  
  <pre><code>Map&lt;String,String&gt; map1= new HashMap&lt;String, String&gt;(){{
         put("no1","123"); put("no2","5434"); put("no5","234");}};
  
  Map&lt;String,String&gt; map1 = new HashMap&lt;String, String&gt;(){{
         put("no1","523"); put("no2","234"); put("no3","234");}};
  
  sum(map1, map2);
  </code></pre>
  
  <p>I want to join them to one, summing up similar keyed values together. What;s the best way I could do it using java 7 or guava libraries ?</p>
  
  <p>expected output  </p>
  
  <pre><code>Map&lt;String, String&gt; output = { { "no1" ,"646"}, { "no2", "5668"}, {"no5","234"}, {"no3","234" }  }
  </code></pre>