Null Value assigned at initial value of For Loop, does this produce Null Pointer Exception Errors?

Question

I'm getting a very weird scenario for a for loop.

I'm running the following code:

List<Id> conId = new List<Id>();
List<Customer__c> customers = new List<Customer__c>(); 
customers = [SELECT Id FROM Customer__c WHERE IsValid = false  AND Contact__c != null];
for(Customer__c cust : customers){
    system.debug(cust.Id); 
    conId.add((cust.Contact__c)); 
}
List<Opportunity> opps = new List<Opportunity>(); 
 opps=[SELECT Id, Customer__c FROM Opportunity WHERE Customer__c IN:conId]; 
if(opps.size() != 0){
for(Opportunity opp : opps){
    System.debug(opp.Customer_Contact__c); 
}
}

This is used in a batch class and I believe it is causing a Null Pointer Exception error that is causing the batch class to fail. When I search for null in the filter of the debug logs on this script, I see VARIABLE_ASSIGNMENT cust | null in the logs. Is this normal, or is my null value in the batch class related to something else?

Answer 1

This is absolutely normal behavior. If I am not mistaken, for-each loops work with an iterator, so with every loop it tries to get the next element until it hits the end. That is why you are seeing the null variable assignment.

Example from my org:
I just executed these 4 lines:

List<Account> accs = [SELECT Id FROM Account];
for (Account a : accs) {
    System.debug(a);
}

The output was(the last few lines):

11:28:31.1 (18100074)|VARIABLE_SCOPE_BEGIN|[2]|a|Account|true|false
11:28:31.1 (18120892)|VARIABLE_ASSIGNMENT|[2]|a|{"Id":"0014E000015yGYfQAM","RecordTypeId":"012w0000000QFbFAAW"}|0x4f3d8651
11:28:31.1 (18128728)|STATEMENT_EXECUTE|[2]
11:28:31.1 (18130813)|STATEMENT_EXECUTE|[3]
11:28:31.1 (18172773)|USER_DEBUG|[3]|DEBUG|Account:{Id=0014E000015yGYfQAM, RecordTypeId=012w0000000QFbFAAW}
11:28:31.1 (18199138)|HEAP_ALLOCATE|[2]|Bytes:5
11:28:31.1 (18218563)|HEAP_ALLOCATE|[2]|Bytes:12
11:28:31.1 (18229871)|VARIABLE_SCOPE_BEGIN|[2]|a|Account|true|false
11:28:31.1 (18250943)|VARIABLE_ASSIGNMENT|[2]|a|{"Id":"0014E000015yGYpQAM","RecordTypeId":"012w0000000QFbFAAW"}|0x41f2dc71
11:28:31.1 (18258889)|STATEMENT_EXECUTE|[2]
11:28:31.1 (18261042)|STATEMENT_EXECUTE|[3]
11:28:31.1 (18302673)|USER_DEBUG|[3]|DEBUG|Account:{Id=0014E000015yGYpQAM, RecordTypeId=012w0000000QFbFAAW}
11:28:31.1 (18329220)|HEAP_ALLOCATE|[2]|Bytes:5
11:28:31.1 (18352372)|VARIABLE_ASSIGNMENT|[2]|a|null|

If you do not want to see these null variable assignments, you can use a simple loop with a counter:

List<Account> accs = [SELECT Id FROM Account];
for (Integer i = 0; i < accs.size(); i++) {
    Account a = accs[i];
    System.debug(a);
}

Answer 2

Its always a best practice to add size() check if you're trying to retrieve bulk records using IN. Your list of contact ids might not have records and hence it might fetch all Opportunities. Refer updated code below:

List<Id> conId = new List<Id>();
List<Customer__c> customers = new List<Customer__c>(); 
customers = [SELECT Id FROM Customer__c WHERE IsValid = false  AND Contact__c != null];
for(Customer__c cust : customers){
    system.debug(cust.Id); 
    conId.add((cust.Contact__c)); 
}

List<Opportunity> opps = new List<Opportunity>(); 
// added a size check
if (conId.size () > 0) {
    opps=[SELECT Id, Customer__c FROM Opportunity WHERE Customer__c IN:conId]; 
    if(opps.size() != 0){
        for(Opportunity opp : opps){
            System.debug(opp.Customer_Contact__c); 
        }
    }
}