title: How to replace null values with a specific value in Dataframe using spark in Java?
body: <p>I am trying improve the accuracy of Logistic regression algorithm implemented in Spark using Java. For this I'm trying to replace Null or invalid values present in a column with the most frequent value of that column. For Example:-</p>
  
  <pre><code>Name|Place
  a   |a1
  a   |a2
  a   |a2
      |d1
  b   |a2
  c   |a2
  c   |
      |
  d   |c1
  </code></pre>
  
  <p>In this case I'll replace all the NULL values in column "Name" with 'a' and in column "Place" with 'a2'. Till now I am able to extract only the most frequent columns in a particular column. Can you please help me with the second step on how to replace the null or invalid values with the most frequent values of that column.</p>