Chapter 9.3 : Java - Interfaces

An interface is a collection of abstract methods. A class implements an interface, thereby inheriting the abstract methods of the interface.
An interface is not a class. Writing an interface is similar to writing a class, but they are two different concepts. A class describes the attributes and behaviors of an object. An interface contains behaviors that a class implements.
Unless the class that implements the interface is abstract, all the methods of the interface need to be defined in the class.
An interface is similar to a class in the following ways:

·         An interface can contain any number of methods.

·         An interface is written in a file with a .java extension, with the name of the interface matching the name of the file.

·         The bytecode of an interface appears in a .class file.

·         Interfaces appear in packages, and their corresponding bytecode file must be in a directory structure that matches the package name.

However, an interface is different from a class in several ways, including:

·         You cannot instantiate an interface.

·         An interface does not contain any constructors.

·         All of the methods in an interface are abstract.

·         An interface cannot contain instance fields. The only fields that can appear in an interface must be declared both static and final.

·         An interface is not extended by a class; it is implemented by a class.

·         An interface can extend multiple interfaces.