title: Java stream Convert list of maps to set
body: <p>I have a data structure in the form of list of maps:
  <code>List&lt; Map&lt;String, List&lt;String&gt;&gt; &gt;</code></p>
  
  <p>I want to collect all the elements of lists (values of maps) in a single <code>Set</code> using java 8 features.</p>
  
  <p>Example:</p>
  
  <pre><code>Input:  [ {"a" : ["b", "c", "d"], "b" : ["a", "b"]}, {"c" : ["a", "f"]} ]
  Output: ["a", "b", "c", "d", "f"]
  </code></pre>
  
  <p>Thanks.</p>