SOQL error index out of bound : 0

Question

I get this error if LargestRevision returns null. ie., fields revision and quote link have no values. Why isn't the else part handling null values? Following is the code I tried.

   if(quoteLink.Quote_Link__c!=null)
   LargestRevision=[SELECT Revision_Number__c FROM OPP_QuoteLink__c WHERE Quote_Link__c=:quoteLink.Quote_Link__c ORDER BY Revision_Number__c DESC LIMIT 1];      
      
      if(LargestRevision.size()!=null){
          clonedQuteLink.Revision_Number__c=LargestRevision.get(0).Revision_Number__c+1;
          clonedQuteLink.Quote_Link__c = quoteLink.Quote_Link__c;                
      }
      else
      {
          clonedQuteLink.Revision_Number__c=1;
          clonedQuteLink.Quote_Link__c = quoteLink.Id;                
      }   

Here

Revision_Number__c is a number field.

Quote_Link__c is a lookup field

Answer 1

LargestRevision.size() will never return null.

The size() method will return 0 if the list is empty.

From the documentation:

size()
Returns the number of elements in the list.

Signature
public Integer size()

Return Value
Type: Integer

Example

List<Integer> myList = new List<Integer>();
Integer size = myList.size();
system.assertEquals(size, 0);

List<Integer> myList2 = new Integer[6];
Integer size2 = myList2.size();
system.assertEquals(size2, 6);

Changing to the following should resolve this:

if(LargestRevision.size() > 0) {
    clonedQuteLink.Revision_Number__c=LargestRevision.get(0).Revision_Number__c+1;
    clonedQuteLink.Quote_Link__c = quoteLink.Quote_Link__c;                
}

Answer 2

You should replace the first part of the if with

 if(LargestRevision!=null and LargestRevision.size()!=0){
      clonedQuteLink.Revision_Number__c=LargestRevision.get(0).Revision_Number__c+1;
      clonedQuteLink.Quote_Link__c = quoteLink.Quote_Link__c;                
  }

because if LargestRevision is null (no values found) then you cannot call .size() on a null