Interfaces
Definition 1-:Any service requirement specification (SRS) is considered as an interface.
Example 1-:
JDBC API access requirement specification to develop database driver.
database vendor is responsible to implement this JDBC API.
Example 2-:
servelet API requirement specification to develop web server.The web server vendor is responsible to implement servlet API.
Definition 2-: From client's point of view an interface defines the set of services what he is accepting.
From the service provider's point of view, an interface defines the set of services what he is offering hence any contract between the client and service provider is considered an interface.
Example-:
through the bank ATM GUI screen bank people highlighting the set of services what they are offering at the same time the same GUI screen represent the set of service what the customer is accepting hence this GUI screen access contract between the customer and the bank people.
Definition 3-:Inside interface every method is always abstract whether we are declaring or not the interface consider as 100% pure abstract class.
Summary Definition-: Any service requirement specification or any contract between client and service provider or 100% pure abstract class is nothing but an interface.