Performing an update on record/s that appears in two lists. How to stop the update appearing in both lists (my own old and new lists)

Question

I'm working on a class thats outside the normal trigger set up and used as part of an integration.

I'm looking to have two list or maps, one that contains the records data pre update. The second containing the records with the updated data. My own version of old and new list/map.

A simple example of the code and issue:

 List<Account> accListOne = [SELECT name, Email__c 
                            FROM Account 
                            WHERE Email__c != null  LIMIT 1]; 
system.debug('*** PRE UPDATE accListOne *** ' + accListOne );
system.debug('*** PRE UPDATE accListOne - email *** ' + accListOne[0].Email__c );

List<Account> accListTwo = new List<Account>(accListOne);
system.debug('*** PRE UPDATE accListTwo *** ' + accListTwo );
system.debug('*** PRE UPDATE accListTwo - email *** ' + accListTwo[0].Email__c );

for(account a : accListOne) {
  a.Email__c = '[email protected]';
}

system.debug('*** POST UPDATE accListOne *** ' + accListOne );
system.debug('*** POST UPDATE accListOne - email *** ' + accListOne[0].Email__c );

system.debug('*** POST UPDATE accListTwo *** ' + accListTwo );
system.debug('*** POST UPDATE accListTwo - email *** ' + accListTwo[0].Email__c );

As you can see above when I perform the update on the record the email is being updated in both lists.

Whats the best way to stop this happening so the update only appears in accListTwo?

Thanks

Answer 1

Objects are passed by reference, which means that two variables can refer to the same place in memory. You can read a much longer explanation here.

As a simple example:

Account a = new Account(Name='Demo');
Account b = a;
b.Name = 'Demo 2';
System.debug(a.Name); // Demo 2

To fix this, you must create all new objects. For Lists, we have the deepClone method:

List<Account> accListOne = [SELECT name, Email__c 
                            FROM Account 
                            WHERE Email__c != null  LIMIT 1]; 
List<Account> accListTwo = accListOne.deepClone(true, true, true);

At this point, changes to records in accListTwo and accListOne will no longer affect each other.