title: Apply function on a single column of a Dataset in Apache Spark using Java
body: <p>Say I have a Dataset:</p>
  
  <pre><code>Dataset&lt;Row&gt; sqlDF = this.spark.sql("SELECT first_name, last_name, age from persons";
  </code></pre>
  
  <p>this will return a <code>Dataset</code> with three columns: first_name, last_name, age.</p>
  
  <p>I want to apply a function that adds 5 to the <code>age</code> column and returns a new Dataset with the same columns as the original Dataset but with the age value changed:</p>
  
  <pre><code>public int add_age(int old_age){
       return old_age + 5;
  }
  </code></pre>
  
  <p>How do I go about doing this with Apache Spark on Java?</p>