Unknown null pointer exception-java

Unknown null pointer exception-java

I am trying to execute the following code.
Date date=new Date();
// System.out.println("date "+date.getTime());
List<User> d=sessionfactory.getCurrentSession().createQuery("from
User").list();
List<String> onlineUsers=null;
for(User u:d){
System.out.println(((date.getTime()/1000)-(u.getLastUpdated().getTime()/1000)));
long
s=((date.getTime()/1000)-(u.getLastUpdated().getTime()/1000));
long j=120;
System.out.println(u.getUsername());
if(s<=j){
onlineUsers.add(u.getUsername());
}
}
Even though the output is coming out to be less than 120 its not even
entering into the if loop.Why is it so?Kindly help.Thanks in advance.