title: Elastic search 6.2 with java API, how to have auto generated ID?
body: <p>I want to insert data with an autogenerated ID. I insert an "user" like this : </p>
  
  <pre><code>java.util.Map&lt;String, Object&gt; jsonMap = new HashMap&lt;String, Object&gt;();
          jsonMap.put("username", user.username);
          jsonMap.put("password", user.password);
          jsonMap.put("mail", user.mail);
          jsonMap.put("friends", user.friends);
          jsonMap.put("maps", user.maps);
          IndexRequest indexRequest = new IndexRequest("users", "doc",user.username)
                  .source(jsonMap)
                  .opType(DocWriteRequest.OpType.CREATE);
  </code></pre>
  
  <p>Here, user.username is my ID. And if I remove this argument, I have an error on execution because I don't have any ID for my user. (I tried that because I read that if I don't put any ID, I should have an auto incremented ID).
  I didn't find anything on the documentation to help me (or maybe I didn't understood it).</p>