What is the concept of creating class instance using interface name?

Question

what is the concept of set variable or object or i don\'t know what it\'s called when i create instance of class and putting in left hand the name of interface,,, I Know tha

Answer 1

The concept here is to accomplish the design principle "Program to an interface, not to implementation".

Here the interface can be an abstract class or the actual interface. The concrete class will have the actual implementation which are being exposed to outside world through interfaces. The client code will access the functionality using interface ,preventing them to directly create an object of concrete class, just giving them access through interface object.

This is accomplished by OO design patterns I believe.