This can happen when you cast a null to a primitive type. It can be hard to spot these types of errors:

Main.java:

import java.util.*;

public class Main
{
   public static void main(String []args) {
      // allow only integers to be added to the arraylist
      HashMap<String, Integer> map = new HashMap<String, Integer>();
      
      map.put("1", 10);
      map.put("2", null);
      
      int a = map.get("1");   // works fine
      int b = map.get("2");   // throws NullPointerException
   }
}

The reason for the NullPointerException is much clearer if you look
at the equivalent:

   int b = ((Integer) map.get("2")).intValue();