What's wrong with this code block?

Question

I am trying to filter out some of the data from a List of Entity Objects. Here is my sample code:

List<Student> lstUniversalEvents = getAllStudentEvents(StudentID,startTime,endTime);
List<Student> lstTriggerEvents=null;

if(lstUniversalEvents.size()>0)
{
    for (Student event1 : lstUniversalEvents) 
    {
        if(strHighSchool.equals("true"))
        {
            if(event1.getClass().equals("HIGH_SCHOOL"))
            {
                lstTriggerEvents.add(event1);
            }
        }
    }
}

The line lstTriggerEvents.add(event1); is throwing a java.lang.NullPointerException.

Trouble-shooting, I found the following:

1. The lstUniversalEvents list has data.
2. condition if(lstUniversalEvents.size()>0) was PASSED
3. condition if(strHighSchool.equals("true")) was PASSED
4. condition if(event1.getClass().equals("HIGH_SCHOOL")) was PASSED

So basically, it's failing to add the event to the new list lstTriggerEvents.

How do I fix this?

Answer 1

You never initialized lstTriggerEvents, it's still null. Change the declaration to

List<Student> lstTriggerEvents=new ArrayList<Student>();

When you declare a variable you have to initialize it, otherwise the JVM assigns a default value. For reference types (like your list) that value is null.