title: Methods of max() and sum() undefined in the Java Spark Dataframe API (1.4.1)
body: <p>Putting sample code of <code>DataFrame.groupBy()</code> into my code, but it shown the methods of <code>max()</code> and <code>sum()</code> undefined. </p>
  
  <pre><code>df.groupBy("department").agg(max("age"), sum("expense"));
  </code></pre>
  
  <p>Which Java package should I import if I want to use <code>max()</code> and <code>sum()</code> method? </p>
  
  <p>Is the Syntax correct of this sample code?</p>