title: How do I get list of only duplicate objects from a list
body: <p>How do I get list of only duplicate objects from a list.</p>
  
  <p>I have an ArrayList of objects</p>
  
  <pre><code>Pojo Class
  Class Abc {
  int id ;
  String name;
  Date startDate;
  int rowNum;
  ......
  }
  
  Example Data:
  id    name      startDate    rowNum
  110   Art       01-Jan-2018   1
  120   Art       01-Jan-2019   1
  121   Science   01-Jan-2018   2
  122   Computer  01-Jan-2018   3
  ..
  190   Math      01-May-2020   40
  180   Math      04-Aug-2040   40
  456   Math      01-Dec-2060   40
  </code></pre>
  
  <p>Abc list contains - 4000 records or more sometimes.
  I want to get id,name,startDate of duplicate rowNum in separate list.</p>
  
  <pre><code>Ex: List2 should contains (duplicate records only)
  id    name      startDate    rowNum
  110   Art       01-Jan-2018   1
  120   Art       01-Jan-2019   1
  .....
  190   Math      01-May-2020   40
  180   Math      04-Aug-2040   40
  456   Math      01-Dec-2060   40
  </code></pre>
  
  <p>How do I get list of only duplicate objects from a list.</p>