title: Check JSONArray for specific value
body: <p>In my Android Activity I am getting a JSONArray via HTTP containg usernames. 
  The Array looks like this: </p>
  
  <pre><code>[{"username":"Julia"},{"username":"Anja"},{"username":"Hans"},{"username":"Sophia"},{"username":"Sarah"}]
  </code></pre>
  
  <p>I want to check in the Android Activity if a given username already exists.</p>
  
  <p>What would be the most efficient way to do it? Or do I have to iterate over the whole array?</p>