My wrox java server pages text, includes an example where some user data is posted to servlet.
To process the data posted and store this, they created a interface class and another class AdminManager which implements the interface.
In the servlet they created an object from the interface class, and called the AdminManager methods through the interface object.
In the book this was the only example which used a interface. And I wondered if this design approach was often used, or should an interface normally be used if several classes would implement the interface.