Getting ready

An interface defines the signatures of the methods one can expect to see in the class that implements the interface. It is the public face of the functionality that's accessible to a client and is thus often called an Application Program Interface (API). It supports polymorphism and aggregation, and facilitates a more flexible and extensible design.

An interface is implicitly abstract, which means it cannot be instantiated. No object can be created based on an interface only, without implementing it. It is used to contain abstract methods (without body) only. But since Java 8, it is possible to add default and private methods to an interface, which is the capability we are going to discuss in the following recipes.

Each interface can extend multiple other interfaces and, similar to class inheritance, inherit all default and abstract methods of the extended interfaces. Static members cannot be inherited because they belong to a specific interface.