ESlomskiWeek7Homework.cls

public with sharing class AccountTriggerHandler {
    

    public static void handleBeforeInsert(List<Account> newAccounts){
        for (Account a : newAccounts) {
            if (a.Est_Annual_Sales__c >= 5000000) {
                a.Priority__c = 'Highest';
            } else if (a.Est_Annual_Sales__c >= 3000000) {
                a.Priority__c = 'High';
            } else if (a.Est_Annual_Sales__c >= 1000000) {
                a.Priority__c = 'Standard';
            } else {
                a.Priority__c = 'Low';
            }
        }
        //this is a before trigger, so our handler doesn't have to call DML
    }

    public static void handleAfterInsert(Account a){
        
        User u = [SELECT Id, Email FROM User WHERE Id = :UserInfo.getUserId()];
        String runningUserEmail = u.Email;
        //Week 7 Homework - bulkify this code
        List<Case> caseList = new List<Case>();
        List<String> newAccounts = new List<String>;
        for (Case c : Trigger.new) {
            c.Status = 'New';
            c.Origin = 'New Account'; //Make sure you've added this as a picklist value for this field
            c.Subject = 'Send Welcome Package';
            c.AccountId = a.id;
            c.Description = 'Please follow up with this new Account and send them a Welcome Package.';
            caseList.add(c);
        }
        insert caseList;
    }

    public static void handleAfterUpdate(List<Account> acctsForUpdate, Map<Id, Account> oldAcctMap){
        //First thing we do is query for all the opportunities on accounts involved in this trigger
        //The SOQL query below uses a nested query, this let's us pull back each acccount with a list of its opportunities attached.
        //We won't be covering nested queries in this class, but take a look and see if you can figure out how they work
        Map<Id, Account> acctsWithOpps = new Map<Id, Account>(
            [SELECT Id, (SELECT Id FROM Opportunities WHERE IsClosed = false) FROM Account WHERE Id IN :acctsForUpdate]
        );

        //Let's make a list to hold any opportunities we create for later insertion
        List<Opportunity> newOpportunities = new List<Opportunity>();

        //Now we need to loop through the accounts in this trigger and see if their priority has been changed in the way we're looking for
        for (Account updatedAcct : acctsForUpdate) {

            //ok, so now we have the udpated Account record, but we also need to compare it to the old version to see what has changed
            //We can use the oldAccountMap, pass it the Account Id, and we'll get the old version for comparison
            Account oldAcct = oldAcctMap.get(updatedAcct.Id);

            //ok, now we have the new and old versions of the same record and we can make our comparison
            if ((oldAcct.Priority__c != 'Highest' && oldAcct.Priority__c != 'High') && (updatedAcct.Priority__c == 'Highest' || updatedAcct.Priority__c == 'High')) {
                //we have a winner!  now check and see if the account has any Open Opportunities

                System.debug('Number of Opportunities on this Account' + acctsWithOpps.get(updatedAcct.Id).Opportunities.size());

                if (acctsWithOpps.get(updatedAcct.Id).Opportunities.size() == 0) {
                    //Ok, this account has no open opportunities, let's create one
                    Opportunity opp = new Opportunity();
                    opp.Name = updatedAcct.Name + ' Opportunity';
                    opp.StageName = 'Prospecting';
                    opp.CloseDate = Date.today().addMonths(3);
                    opp.AccountId = updatedAcct.Id;
                    newOpportunities.add(opp);
                }
            }
        }

        //Finally, insert any new Opportunities
        if (newOpportunities.size() > 0) {
            insert newOpportunities;
        }

    }

}

Actually I just realized that for loop is wrong on the line 26. Also Line 25 missing () in the end. You need to remove line 25 all together because it is not used anywhere. Let's recap that method again.

On line 19 we have public static void handleAfterInsert(Account a){ but if we look at AccountTriggerGood we call that method like this
AccountTriggerHandler.handleAfterInsert(Trigger.new); which is a list(Trigger.new). So line 19 should be like this public static void handleAfterInsert(List<Account> newAccounts){

After changing this line 25 needs to be deleted. And the loop should look like this

for (Account a : newAccounts) {
  Case c = new Case();   //we initialize a case
  c.Status = 'New';
  c.Origin = 'New Account'; //Make sure you've added this as a picklist value for this field
  c.Subject = 'Send Welcome Package';
  c.AccountId = a.id;
  c.Description = 'Please follow up with this new Account and send them a Welcome Package.';
  caseList.add(c);
 }

Also next time can you tag me like this @tugce so I can get notified :)