Trigger on Contacts (after insert, after update) - Example 2

//A common pattern for triggered operations in Apex

public static void updateContactPhone (List<Account> priorAccounts, List<Account> updatedAccounts) { // static method, takes 2 List params of Account sObject type

Set<Id> modifiedAccounts_Ids = new Set<Id>();
for (Integer i=0; i<updatedAccounts.size(); i++) { // Array notation - Alternative is... for (Account a : updatedAccounts) { 
       if (updatedAccounts[i].Phone != priorAccounts[i].Phone) { // if the phone number is different... (use dot notation to get to field value)
           modifiedAccounts_Ids.add(updatedAccounts[i].Id); // add the Account Id to the List collection 
           }
     }

if (modifiedAccounts_Ids.size() > 0) { // typical control flow
    List<Contact> contactQuery = [SELECT Id, Phone, AccountId, Account.Phone FROM Contact WHERE AccountId IN :modifiedAccounts_Ids
                                 LIMIT : (limits.getLimitQueryRows()-limits.getQueryRows())]; // Limits method means we never exceed the limit

List<Contact> contactUpdates = new List<Contact>();
for (Contact c : contactQuery) { 
    // limit to just the records that need it
    if (c.Phone != c.Account.Phone && contactUpdates.size() < limits.getLimitDMLRows()) { 
        c.Phone = c.Account.Phone;
        contactUpdates.add(c);
        }
}

if ( contactUpdates.size() > 0 ){ // typical control flow
    update contactUpdates;
    }
  }
}