Extract Interface.

Interfaces are very apropos when classes play special roles in different situations. Use Extract Interface to explicitly indicate which role.

Another convenient case arises when you need to describe the operations that a class performs on its server. If it’s planned to eventually allow use of servers of multiple types, all servers must implement the interface.

Create an empty interface.

Declare common operations in the interface.

Declare the necessary classes as implementing the interface.

Change type declarations in the client code to use the new interface.