Interfaces

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 and name of the interface matching the name of the file


• The bytecode of an interface appears in a .class files.


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

An interface is different to a class in the following ways

• 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 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.

Interfaces properties

• An interface is implicitly abstract you do not need to use the abstract keyword when declaring an interface.


• Each method in an interface is also implicitly abstract so the abstract keyword is not needed.


• Methods in an interface are implicitly public.

Implementing Interfaces

When a class implements an interface you can think of the class as signing a contract agreeing to perform the specific behaviors of the interface the class must declare itself as abstract.

• A class can implement more than one interface at a time


• A class can extend only one class, but implement many interfaces.


• An interface itself can extend another interface.

Extending Interfaces

An interface can extend another interface similarly to the way that a class can extend another class. The extend keyword is used to extend an interface and the child interface inherits the methods of the parent interface

Extending multiple interfaces

A java class can only extend one parent class. Multiple inheritance is not allowed.
Interfaces are not class however and an interface can extend more than one parent interfaces . the extend keyword is used once, and the parent interfaces are declared in a comma separated list.