System.NullPointerException: Attempt to de-reference a null object in Displaying a list

Question

I have two objects - Object1 & Object2

Object1 has a list of Products say Product1, Product2, Product3 Object2 has a accessories say acc1, acc2, acc3

• and One Text Field.

User has to select applicable accessories for all those products

Product Acc1 Acc2 Acc3 Product1 True False True Product2 True False True Product3 False True False

I am storing all products in one object - Object1 And all Accessories in Object2

• On Save

I will be saving Object2 with Text field filled with Product Name from Object1 and the rest of the checkboxes with filled values - checkbox1, checkbox2, checkbox3

So far I have written the following code, But getting an error as

public class PrdAccSelection{
public  List<Object1__c> lstPrd{get;set;}
public Object2__c chk = new Object1__c();
public List<Object2__c> lstchk{get;set;}

    public PrdAccSelection(){
    lstPrd = [select id, Name, Prd_Description__c from Object1__c];

        for(Object1__c l : lstPrd){
            chk.Question_Description__c = l.Prd_Description__c;
            system.debug('chk'+chk);
            lstchk.add(chk);    
        }

    system.debug('lstchk'+lstchk);    
}}

VF Page:

<apex:page controller="PrdAccSelection" >
  <apex:form >
  <apex:pageBlock title="Product Accessory Selection">
  <apex:pageBlockTable value="{!lstchk}" var="c">
  <apex:column value="{!c.Prd_Description__c}"/>
  <apex:column value="{!c.acc1__c}"/>
  <apex:column value="{!c.acc2__c}"/>
  <apex:column value="{!c.acc3__c}"/>
  </apex:pageBlockTable>
  </apex:pageBlock>
   </apex:form>

</apex:page>

Visualforce Error Help for this Page

System.NullPointerException: Attempt to de-reference a null object Class.Feedback.: line 10, column 1

Answer 1

You forgot to initialize your lstchk list which you are referring on your visualforce page.

public class PrdAccSelection{
public  List<Object1__c> lstPrd{get;set;}
public Object2__c chk;
public List<Object3__c> lstchk{get;set;}

public PrdAccSelection(){
lstchk = new List<Object3__c>();
lstPrd = [select id, Name, Prd_Description__c from Object1__c];

    for(Object1__c l : lstPrd){
        chk1 = new Object3__c();
        chk.Question_Description__c = l.Prd_Description__c;
        system.debug('chk'+chk);
        lstchk.add(chk);    
    }

system.debug('lstchk'+lstchk);    
}}

Answer 2

I want to point out below things:

1) Your Schema(object Model) is not clear from the question, is there any relationship between Object1__c and Object2__c ?
2) Is this VF page only for displaying records? or you trying to do some DML also?
3) You need to have returntype of your method as List<Object1__c> and return lstchk
4) Where are the fields in acc1__c, acc2__c, acc3__c?
5) To return list of records in lstPrd add following piece of code:

public List<Object1__c> getlstPrd() {
    return [select Prd_Description__c, acc1__c, acc2__c, acc3__c from Object1__c}
    }
}

Answer 3

Okay. So this code of yours should populate a text field with Product name and boolean values accourding to accessories. So an example text field should look like this;

Product1 True False True

First your class has wrong assingments to variables. For instance;

public Object2__c chk = new Object1__c();

You can't do this. It should be either

public Object2__c chk = new Object2__c(); 

or

public Object1__c chk = new Object1__c();

I think your final code should look like this.

public class PrdAccSelection{
  public  List<Object1__c> lstPrd{get;set;}
  public Object2__c chk = new Object2__c();
  public List<Object2__c> lstchk{get;set;}

    public PrdAccSelection(){
      lstPrd = [select id, Name, Prd_Description__c from Object1__c];
      lstchk = new List<Object2__c>();
        for(Object1__c l : lstPrd){
            chk.Question_Description__c = l.Prd_Description__c;
            system.debug('chk'+chk);
            lstchk.add(chk);    
        }

      system.debug('lstchk'+lstchk);    
    }
}

But if you want this to work on Save you need a save button and a corresponding save method.