What is the difference between Standard Controller, Custom Controller, and Extensions

Let us look at an interview question that revolves around Apex Classes.

Standard Controllers

One of the main advantages of Salesforce is when we create a Sobject we get access to something called a Standard Controller.

Irrespective of the object being a standard object and custom object we get access to it.

<apex:page StandardController="Account">

</apex:page>

In a standard controller, we will be getting access to 9 methods and we can use them to perform some standard out-of-the-box functionalities.

Custom Controllers

If I were to override the methods with my own custom functionality or implement a new method that not part of the 9 out of the methods then I need to create a Custom Controller.

<apex:page controller="CustomController">

</apex:page>

Extensions

An Apex Custom Controller and Apex Extension both are one and the same with the only difference that in an Extension we will be having a mandatory one argument constructor which is not the case with a Custom Controller.

public with sharing class CustomExtension{

	public void foobar(ApexPages.StandardController controller){
    
    }
}

If I want to associate a Visualforce page with more than one Apex Class then we need to make use of an Extension.

The advantage of using an Extension is we can have up to 5 extensions by separating each of them with a comma.

If a method is implemented in both the Extensions, the extension that comes first that has the method implementation gets the priority and gets invoked.

<apex:page controller="CustomController" extensions="CustomExtensionOne,CustomExtensionTwo,CustomExtensionThree,CustomExtensionFour,CustomExtensionFive ">

</apex:page>

Hope this helps!