When users are editing or creating a record via a Visualforce page, they will often make mistakes or enter invalid data. The required fields will present an error message underneath the field itself, but validation rules or exceptions will simply send the user to a new page with a large error message, telling them that the insert or update failed.
In this recipe we will create a Visualforce page to allow a user to create or edit a contact record. The contact standard controller and a controller extension manage the page. The extension controller checks whether the e-mail address or phone number field has been populated. If either of the fields is populated, the record will be saved, but if both are missing, an error message will be added to both fields asking the user to populate at least one of them.
This recipe makes use of a controller extension, so this will need to be created before the Visualforce page.