trigger ContactTrigger on Contact (after insert, after update, after delete, after undelete) {

// Trigger to Insert contact Name from Contact CSV field of Account    
     if(Trigger.isInsert){
         if(Trigger.isAfter){
             ContactHandler insertContact = new ContactHandler();
             insertContact.onAfterInsert(Trigger.new);
        }
     }

// Trigger to Update contact Name from Contact CSV field of Account    
     if(Trigger.isUpdate){
         if(Trigger.isAfter){
             ContactHandler updateContact = new ContactHandler();
             updateContact.onAfterUpdate(Trigger.new , Trigger.oldMap);
        }
     }
     
// Trigger to Delete contact Name from Contact CSV field of Account 
     if(Trigger.isDelete){
         if(Trigger.isAfter){
             ContactHandler deleteContact = new ContactHandler();
             deleteContact.onAfterDelete(Trigger.old);
        }
     }
    
// Trigger to Undelete contact Name from Contact CSV field of Account    
     if(Trigger.isUndelete){
         if(Trigger.isAfter){
             ContactHandler undeleteContact = new ContactHandler();
             undeleteContact.onAfterUndelete(Trigger.new);
        }
     }
}   // End of Trigger ContactTrigger

public class ContactHandler {
    
    // Insert Trigger Handler Class
    public void onAfterInsert(List<Contact> lstContact){
        updateContactNameOnAccount(lstContact, new Map<Id, Contact>());
    }      // End of onAfterInsert Class

    // Update Trigger Handler Class
    public void onAfterUpdate(List<Contact> lstContact , Map<Id, Contact> mapContactOld){
        updateContactNameOnAccount(lstContact, mapContactOld);
    }      // End of onAfterUpdate Class

    // Delete Trigger Handler Class
    public void onAfterDelete(List<Contact> lstContact){
        updateContactNameOnAccount(lstContact, new Map<Id, Contact>());
    }      // End of onAfterDelete Class

    // Delete Trigger Handler Class
    public void onAfterUndelete (List<Contact> lstContact){
        updateContactNameOnAccount(lstContact, new Map<Id, Contact>());
    }   // End of onAfterUndelete Class
    
    private void updateContactNameOnAccount(List<Contact> lstContactNew, Map<Id, Contact> mapContactOld){
        Set<Id> setAccountId = new Set<Id>();
        for(Contact objContact : lstContactNew){
            if(mapContactOld.containskey(objContact.Id)) setAccountId.add(mapContactOld.get(objContact.Id).AccountId);
            setAccountId.add(objContact.AccountId);
        }
        List<Account> lstAccount = new List<Account>();
        for(Account objAccount : [SELECT Id, (SELECT Id, Name FROM Contacts) FROM Account WHERE Id IN: setAccountId]){
            List<String> lstContactName = new List<String>();
            for(Contact objContact : objAccount.Contacts){ 
                lstContactName.add(objContact.Name);
            }
            objAccount.Contact_CSV__c = String.join(lstContactName, ', ');
            lstAccount.add(objAccount);
        }
        update lstAccount;
    }
}   // End of ContactTriggerHandler Class