exact equality operator

Question

Found this in a great post on salesforce collections (http://peterknolle.com/collections-eggstravaganza/). Can anyone explain what is going on here that this isn't true?

   Account acct = [
        Select 
            Id, Name,
            (Select Name From Contacts)
        From Account
        Limit 1
    ];

    System.assert(acct.Contacts === acct.Contacts); // fails assertion

Answer 1

This is actually explained briefly in the article you linked to, but I'm assuming you want an expanded answer.

When returning a nested collection (as Contacts is in this example), Apex returns a defensive copy of the collection, rather than the actual collection. To simplify that further, each time you call acct.Contacts the Contacts list is constructed fresh and will occupy a different location in memory.

Here is a simple example of how you could achieve this in your own class:

public class DefensiveListExample
{
    private List<Contact> internalContacts;

    public List<Contact> Contacts 
    { 
        get 
        { 
            return new List<Contact>(internalContacts); 
        } 
    }
}

Generally this type of behaviour is provided to prevent you violating invariants of classes and keep control of the collection within the class itself, or in languages like C# can be used to provide a read-only version of a collection.

The === compares the location in memory of the two compared objects, as opposed to the == which compares the object types and fields of the objects.

Therefore, because the returned value of each call to acc.Contacts resides in a different place in memory, acct.Contacts === acct.Contacts is false.