Apex Triggers and Class Best Practices

What it is

An Apex trigger is code that runs before or after records of an object are inserted, updated, deleted or undeleted. Triggers fire for saves from almost every source: the UI, the API, Data Loader, flows and other Apex.

Because a trigger can receive up to 200 records at a time, every trigger must be written for bulk. Best practice also keeps the logic out of the trigger body, in a handler class.

How it works

Trigger events and context variables

trigger OpportunityTrigger on Opportunity (before insert, before update, after insert, after update) {
    OpportunityTriggerHandler.run(Trigger.operationType, Trigger.new, Trigger.oldMap);
}

Before versus after

In before triggers you change Trigger.new directly. In after triggers the records in Trigger.new are read-only; changing them throws an exception, so update copies with DML.

Bulkified trigger code

public with sharing class ContactTriggerHandler {
    public static void setAccountName(List<Contact> contacts) {
        Set<Id> accIds = new Set<Id>();
        for (Contact c : contacts) {
            if (c.AccountId != null) accIds.add(c.AccountId);
        }
        Map<Id, Account> accs = new Map<Id, Account>(
            [SELECT Name FROM Account WHERE Id IN :accIds]);
        for (Contact c : contacts) {
            Account a = accs.get(c.AccountId);
            if (a != null) c.Description = a.Name;
        }
    }
}

One query for all records, a map for lookups, and no DML because this runs before insert.

Best practices

Rejecting records

for (Contact c : Trigger.new) {
    if (String.isBlank(c.Email)) {
        c.Email.addError('Email is required for this record type.');
    }
}

addError() on a record or field marks that record as failed. With partial success (Data Loader, Database.insert(recs, false)), the other records still save.

A worked example

A trigger on Account calls a @future method once per account. A load of 200 accounts fails because a transaction can make only 50 future calls. The fix is to collect all Account Ids in a Set<Id> and make one @future call with the set. The same pattern applies to callouts, emails and queueable jobs: one call for the batch, not one per record.

How the exam asks about this

• "Change a field on the record being saved." A before trigger, no DML.
• "Create a child record that needs the new record's Id." An after insert trigger.
• "Block saving and show a message on the field." addError() in a before trigger.