Visualforce Standard Controller method addFields(fieldNames)

 Hi ,

When a Visualforce page is loaded, the fields accessible to the page are based on the fields referenced in the Visualforce markup. This method adds a reference to each field specified in "fieldNames" so that the controller can explicitly access those fields as well.

Here "fieldNames" data type is List<String> .

The strings in fieldNames can either be the API name of a field, such as AccountId, or they can be explicit relationships to fields, such as something__r.myField__c.

Usage:

This method should be called before a record has been loaded—typically, it's called by the controller's constructor. If this method is called outside of the constructor, you must use the reset() method before calling addFields().

This method is only for controllers used by dynamicVisualforce bindings.

Sample Example:

public AccountController(ApexPages.StandardController stdController){

            this.controller = stdController;

           List<String> fieldNamesList = new List<String>{Type,Industry};

            stdController.addFields(fieldNamesList); 

}

Reference:

https://developer.salesforce.com/docs/atlas.en-us.pages.meta/pages/apex_ApexPages_StandardController_addFields.htm