An interface in Java serves as a blueprint for classes and is key to achieving abstraction and multiple inheritance, containing only static constants and abstract methods. It cannot be instantiated, similar to an abstract class, and allows for the implementation of multiple inheritance without ambiguity. Since Java 8, interfaces can include default and static methods with method bodies.

1. INTERFACE IN JAVA

2.  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.

3.  Java Interface also represents IS-A relationship.
 It cannot be instantiated just like abstract class.

4. Why use Java interface?
 There are mainly three reasons to use interface.
They are given below.
 It is used to achieve abstraction.
 By interface, we can support the functionality of
multiple inheritance.

5. Internal addition by compiler

6.  The java compiler adds public and abstract
keywords before the interface method. More,
it adds public, static and final keywords
before data members.

8. Understanding relationship between classes and
interfaces

10. Java Interface Example
interface printable
{
void print();
}
class A implements printable
{
public void print(){System.out.println("Hello");}

11. public static void main(String args[])
{
A obj = new A();
obj.print();
}
}

12. Java Interface Example
interface Drawable{
void draw();
}
class Rectangle implements Drawable{
public void draw()
{System.out.println("drawing rectangle");}
}
class Circle implements Drawable{
public void draw()
{System.out.println("drawing circle");}
}

13. class TestInterface
{
public static void main(String args[])
{
Drawable d=new Circle();
d.draw();
}
}

14. Java Interface Example

15. • interface Bank
• {
• float rateOfInterest();
• }
• class SBI implements Bank
• {
• public float rateOfInterest(){return 9.15f;}
• }
• class PNB implements Bank
• {
• public float rateOfInterest(){return 9.7f;}
• }

16. class TestInterface
{
public static void main(String[] args)
{
Bank b=new SBI();
System.out.println("ROI: "+b.rateOfInterest());
}
}

17. Multiple inheritance in Java by
interface

19. interface Printable
{
void print();
}
interface Showable
{
void show();
}

20. class A implements Printable, Showable
{
public void print()
{
System.out.println("Hello");
}
public void show()
{
System.out.println("Welcome");
}

21. public static void main(String args[])
{
A obj = new A();
obj.print();
obj.show();
}
}

22. Multiple inheritance is not supported through class
in java but it is possible by interface, why?

23. multiple inheritance is not supported in case
of class because of ambiguity. But it is
supported in case of interface because there
is no ambiguity as implementation is provided
by the implementation class.

24. interface Printable
{
void print();
}
interface Showable
{
void print();
}

25. • class A implements Printable, Showable
• {
• public void print()
• {
• System.out.println("Hello");
• }
• public static void main(String args[])
• {
• A obj = new A();
• obj.print();
• }
• }

26. Interface inheritance

27. A class implements interface but one interface
extends another interface.

28. interface Printable
{
void print();
}
interface Showable extends Printable
{
void show();
}

29. class A implements Showable
{
public void print()
{
System.out.println("Hello");
}
public void show()
{
System.out.println("Welcome");
}

30. public static void main(String args[])
{
A obj = new A();
obj.print();
obj.show();
}
}

31. Java 8 Default Method in Interface

32.  Since Java 8, we can have method body in
interface. But we need to make it default
method.

33. interface Drawable
{
void draw();
default void msg()
{
System.out.println("default method");}
}
class Rectangle implements Drawable
{
public void draw()
{
System.out.println("drawing rectangle");}
}

34. class A
{
public static void main(String args[])
{
Drawable d=new Rectangle();
d.draw();
d.msg();
}
}

35. Java 8 Static Method in Interface

36. interface Drawable
{
void draw();
static int cube(int x)
{
return x*x*x;
}
}
class Rectangle implements Drawable
{
public void draw()
{
System.out.println("drawing rectangle");}
}

37. class A
{
public static void main(String args[])
{
Drawable d=new Rectangle();
d.draw();
System.out.println(Drawable.cube(3));
}
}