Attempt to de-reference a null object - Public Variable Manipulation

Question

I am trying to build an editable attendance sheet which can record new attendance records and edit old records based on date and course. The user can select a course which then queries the DB for enrolled students and creates a table based on the following APEX in visualforce:

{!Roster} is my query result on students enrolled in a particular class

{!attendanceOptions} are the 3 radio options for students attendance, "present," "tardy," and "absent."

<apex:pageBlockTable value="{!Roster}" var="Roster">
  <apex:column >
    <apex:facet name="header">Student Name</apex:facet>
    <apex:outputText value="{!Roster['Name']}"/> 
  </apex:column>
  <apex:column >
    <apex:facet name="header">Attendance</apex:facet>
    <apex:selectRadio value="{!attendanceRecord}" disabled="{!lockRecords}">
      <apex:selectOptions value="{!attendanceOptions}"/>
    </apex:selectRadio>
  </apex:column>
</apex:pageBlockTable>

I am would like to set the radio buttons value equal to the students' attendance records (present, tardy, or absent) if they exist for a particular day or to "present" is no records exist by changing the {!attendanceRecord} variable.

Here is the relevant class code for this visualforce page:

public with sharing class TestDisplayQueryList {
  public List<String> attendanceRecord {get;set;}
  public List<String> getAttendanceRecord(){
    if(attendanceRecord == null)
        List<String> attendanceRecord = new List<String>();
    return null;
  }
  public List<Contact> getRoster() {
    string record;
    List<Contact> roster = new List<Contact>();
    for(Contact Name : [SELECT Contact.Name, contact.id FROM contact WHERE contact.id IN (SELECT hed__Course_Enrollment__c.hed__Contact__c FROM hed__Course_Enrollment__c WHERE hed__Course_Enrollment__c.hed__Course_Offering__c=:className)]){
        roster.add(Name);
        for(Course_Attendance__c attendance : [SELECT Attendance_Record__c FROM Course_Attendance__c WHERE Date__c =:dat AND Contact__c =:name.id]){
           record = attendance.Attendance_Record__c;
           record = record.toLowerCase();
           attendanceRecord.add(record);
           roster.add(new string attendanceRecord);
        }
    }
 }

The problem is that I keep getting the error "Attempt to de-reference a null object" referencing the attendanceRecord.add(record); line of code as if the variable attendanceRecord doesn't exist. What am I doing wrong that is preventing this public variable from being used in this function?

Answer 1

You need to initialize the attendanceRecord Object.

The below method creates a local variable and the public class level variable is never initialized.

public List<String> getAttendanceRecord(){
    if(attendanceRecord == null)
        List<String> attendanceRecord = new List<String>();
    return null;
  }

I would suggest to initialize the attendanceRecord object like this:

attendanceRecord = new List<String>();

i.e. without providing the type: List<String> again locally

Also, I would suggest to initialize this in the constructor and change the variable name to something like: attendanceRecordObj, in order to avoid confusion what is being referred, getAttendanceRecord() or attendanceRecord

So change to the class code like this, should work for you:

public with sharing class TestDisplayQueryList {
  public List<String> attendanceRecord {get;set;}
  public TestDisplayQueryList(){
      attendanceRecord = new List<String>();
  }
  public List<Contact> getRoster() {
    string record;
    List<Contact> roster = new List<Contact>();
    for(Contact Name : [SELECT Contact.Name, contact.id FROM contact WHERE contact.id IN (SELECT hed__Course_Enrollment__c.hed__Contact__c FROM hed__Course_Enrollment__c WHERE hed__Course_Enrollment__c.hed__Course_Offering__c=:className)]){
        roster.add(Name);
        for(Course_Attendance__c attendance : [SELECT Attendance_Record__c FROM Course_Attendance__c WHERE Date__c =:dat AND Contact__c =:name.id]){
           record = attendance.Attendance_Record__c;
           record = record.toLowerCase();
           attendanceRecord.add(record);
           roster.add(new string attendanceRecord);
        }
    }
 }

or initializing in the method itself, should also work:

public List<Contact> getRoster() {
    attendanceRecord = new List<String>(); //Initialized here
    string record;
    List<Contact> roster = new List<Contact>();
    for(Contact Name : [SELECT Contact.Name, contact.id FROM contact WHERE contact.id IN (SELECT hed__Course_Enrollment__c.hed__Contact__c FROM hed__Course_Enrollment__c WHERE hed__Course_Enrollment__c.hed__Course_Offering__c=:className)]){
        roster.add(Name);
        for(Course_Attendance__c attendance : [SELECT Attendance_Record__c FROM Course_Attendance__c WHERE Date__c =:dat AND Contact__c =:name.id]){
           record = attendance.Attendance_Record__c;
           record = record.toLowerCase();
           attendanceRecord.add(record);
           roster.add(new string attendanceRecord);
        }
    }