title: How do I convert column of unix epoch to Date in Apache spark DataFrame using Java?
body: <p>I have a json data file which contain one property [creationDate] which is unix epoc in "long" number type. The Apache Spark DataFrame schema look like below:</p>
  
  <pre>
  root 
   |-- <b>creationDate</b>: long (nullable = true) 
   |-- id: long (nullable = true) 
   |-- postTypeId: long (nullable = true)
   |-- tags: array (nullable = true)
   |    |-- element: string (containsNull = true)
   |-- title: string (nullable = true)
   |-- viewCount: long (nullable = true)
  </pre>
  
  <p>I would like to do some groupBy "creationData_Year" which need to get from "creationDate".</p>
  
  <p>What's the easiest way to do this kind of convert in <em>DataFrame</em> using Java?</p>