15 What is Interface in Java

Interface in Java:

- An interface in Java is a blueprint of a class. It has static constants and abstract methods.

- The interface in Java is a mechanism to achieve abstraction.

- There can be only abstract methods in the Java interface, not method body.

- It is used to achieve abstraction and multiple inheritance in Java.

or

- interfaces can have abstract methods and variables. It cannot have a method body.

- It cannot be instantiated just like the abstract class.

- Since Java 8, we can have default and static methods in an interface.

- Since Java 9, we can have private methods in an interface.

Why use interface?

- It is used to achieve abstraction.

- By interface, we can support the functionality of multiple inheritance.

- It can be used to achieve loose coupling.

How to declare an interface?

The relationship between classes and interfaces

- a class extends another class, an interface extends another interface, but a class implements an interface.

ex:
1. class extends class

2. interface extends interface

3. class implements interface

Nested Interface:

An interface i.e. declared within another interface or class is known as nested interface. The nested interfaces are used to group related interfaces so that they can be easy to maintain. The nested interface must be referred by the outer interface or class. It can't be accessed directly.

Points for nested interfaces:

- Nested interface must be public if it is declared inside the interface but it can have any access modifier if declared within the class.

- Nested interfaces are declared static implicitely.

Syntax

interface interface_name{
...
interface nested_interface_name{
...
}
}

Syntax

class class_name{
...
interface nested_interface_name{
...
}
}