Explain the need or importance of the controller extension

Explain the need or importance of the controller extension

Controller extension is very useful and important concept introduced by the salesforce recently. It gives the power to programmer to extend the functionality of existing custom controller or standard controller.

A Visualforce can have a single Custom controller or standard controller but many controller extensions.

we can say that the custom extension is the supporter of custom or standard controller.

Consider one example : If there is one controller written and used by the multiple visualforce pages and one of them needs some extra logic. Then instead of writing that logic to controller class (Which is used by many visualforce pages) we can create a controller extension and apply to that page only.

standardController	Controller	extensions
contain the same functionality and logic that are used for standardSalesforce pages.	need finer control for how information is accessed for your page, you can write a custom controller or a controller extension using Apex.	extends the functionality of a standard or custom controller.
Can be used with standard objects and custom objects.	Only one Apex class is used.	Multiple Apex classes separated by comma are used.