In Java, An interface is a way through which unrelated objects use to interact with one another. Using interface, you can specify what a class must do, but not how it does it. It is not a class but a set of requirements for classes that implement the interface.

1. Interface in JAVAInterface in JAVA
Presented By :Presented By :
Mr. Dheeraj Kumar Singh
Assistant Professor
I.T. Department,
Faculty of Engg., Parul University,
Vadodara, Gujarat

2. OutlineOutline
 Introduction to Interface
 Multiple Inheritance – Example
 Why Interfaces are needed
 Java's Interface Concept
 Syntax
 Semantic Rules for Interfaces
 Example: An Interface for Shape Classes
 Extending Interface
 Abstract class and Interface
 Benefits of Interfaces
 Java's Most used Interfaces
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3. IntroductionIntroduction toto InterfaceInterface
3
In General, An interface is a device or system that
unrelated entities use to interact.
- The English language is an interface between two people.
- A remote control is an interface between you and a television.

4. IntroductionIntroduction toto InterfaceInterface
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In Computing, An interface is a shared boundary across
which two or more components of a computer
system exchange information.
- The exchange can be between software, hardware,
humans and combinations of these.
Interface/Medium

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In Object oriented programming,
- An interface is a common means for
unrelated objects to communicate with each other.
In Java,
- An interface is a way through which unrelated objects
use to interact with one another.
- Using interface, you can specify what a class must do,
but not how it does it.
- It is not a class but a set of requirements for classes that
implement the interface
IntroductionIntroduction toto InterfaceInterface

6. Multiple InheritanceMultiple Inheritance
- Example- Example
department
cgpa()
Student
department
salary()
Employee
Person
Name
displayDetail()
TeachingAssistantTeachingAssistant
 For a teaching assistant, we want the properties
from both Employee and Student.
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7. Problems with Multiple InheritanceProblems with Multiple Inheritance
Consider following declearation:
ta = new TeachingAssistant();
ta.department;
 Name clash problem: Which department does ta
refers to?
Combination problem: Can department from
Employee and Student be combined in Teaching
Assistant?
Selection problem: Can you select between
department from Employee and department from
Student?
Replication problem: Should there be two
departments in TeachingAssistent?
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8. Why Interfaces are neededWhy Interfaces are needed
• Multiple Inheritance in JAVA is not allowed – cannot
extend more than one class at a time.
• An object may need IS-A relationships with many
type.
Politician
Citizen
Father
President
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9. Solution for multiple inheritance inSolution for multiple inheritance in
JAVAJAVA
public class Person extends Citizen implements Father,
Politician, President {}
public interface Politician {
public void joinParty();
}
public class Citizen {
}
public interface Father {
public void care();
}
public interface President
{public void winPoll();
}
public class Person {
}
public class Person {
} 9

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• An interface defines a protocol of behavior as a
collection of method definitions (without
implementation) and constants, that can be
implemented by any class.
• A class that implements the interface agrees to
implement all the methods defined in the interface.
• If a class includes an interface but does not implement
all the methods defined by that interface, then that class
must be declared as abstract.
Java's Interface Concept

11. SyntaxSyntax
The Declaration of Interface consists of a keyword interfaceinterface,
its name, and the members.
interface InterfaceName {
// constant declaration
static final type variableName = value;
// method declaration
returntype methodname (argumentlist);
}
The Class that Implements Interface called as
Implementation Class uses keyword implementsimplements:
class classname implementsimplements InterfaceName {
... } 11

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• Instantiation
Does not make sense on an interface. Interfaces are not
classes. You can never use the new operator to instantiate
an interface.
public interface Comparable {. . . }
Comparable x = new Comparable( );
• Data Type
An interface can be used as a type, like classes. You can
declare interface variables
class Employee implements Comparable {. . . }
Comparable x = new Employee( );
Semantic Rules for InterfacesSemantic Rules for Interfaces

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• Access modifiers
An interface can be public or “friendly” (default).
All methods in an interface are by default abstract and public.
- Static, final, private, and protected cannot be used.
All variables (“constants”) are public static final by default
-Private, protected cannot be used.
Semantic Rules for InterfacesSemantic Rules for Interfaces

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Example: An Interface for ShapeExample: An Interface for Shape
ClassesClasses
• Creating classes to represent rectangles, circles,
and triangles and compute their area and
perimeter
It may seem as there is an inheritance relationship
here, because rectangle, circle, and triangle are all
shapes.
But code sharing is not useful in this case because
each shape computes its area and perimeter in a
different way.

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Define Interface ShapeDefine Interface Shape
• A better solution would be to write an interface called
Shape to represent the common functionality (to
compute an area and a perimeter ) of all shapes:
public interface Shape {
public double getArea();
public double getPerimeter();
}

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Class Rectangle implementsClass Rectangle implements
interface Shapeinterface Shape
public class Rectangle implements Shape {
private double width;
private double height;
// Constructs a new rectangle with the given
dimensions.
public Rectangle(double width, double height) {
this.width = width;
this.height = height;
}
// Returns the area of this rectangle.
public double getArea() {
return width * height;
}
// Returns the perimeter of this rectangle.
public double getPerimeter() {
return 2.0 * (width + height);
}
}

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Class Circle implements interfaceClass Circle implements interface
ShapeShape
public class Circle implements Shape {
private double radius;
// Constructs a new circle with the given radius.
public Circle(double radius) {
this.radius = radius;
}
// Returns the area of this circle.
public double getArea() {
return Math.PI * radius * radius;
}
// Returns the perimeter of this circle.
public double getPerimeter() {
return 2.0 * Math.PI * radius;
}
}

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ClassClass TriangleTriangle implementsimplements
interface Shapeinterface Shape
public class Triangle implements Shape {
private double a;
private double b;
private double c;
// Constructs a new Triangle given side lengths.
public Triangle(double a, double b, double c) {
this.a = a;
this.b = b;
this.c = c;
}
// Returns this triangle's area using Heron's formula.
public double getArea() {
double s = (a + b + c) / 2.0;
return Math.sqrt(s * (s - a) * (s - b) * (s - c));
}
// Returns the perimeter of this triangle.
public double getPerimeter() {
return a + b + c;
}}

19. Class Mensuration with MainClass Mensuration with Main
FunctionFunction
class Mensuration
{
public static void main (String [] arg)
{// Rectangle object
Rectangle r = new Rectangle(10,20); // Rectangle object
System.out.println("Area of Rectangle =" +(r.getArea()));
System.out.println("Perimeter of Rectangle =" +
(r.getPerimeter()));
Circle c = new Circle(10); // Circleobject
System.out.println("Area of Circle =" +(c.getArea()));
System.out.println("Perimeter of Circle =" +
(c.getPerimeter()));
Triangle t = new Triangle(3,4,5); // Triangle object
System.out.println("Area of Triangle =" +(t.getArea()));
System.out.println("Perimeter of Triangle =" +
(t.getPerimeter()));
}
}
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20. Output ScreenOutput Screen
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21. Extending InterfaceExtending Interface
• One interface can inherit another interface using the
extends keyword and not the implements keyword.
• For example,
interface A extends B {}
• Obviously, any class which implements a “sub-
interface” will have to implement each of the methods
contained in it’s “super-interface” also.
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22. Abstract class and InterfaceAbstract class and Interface
Abstract class Interface
A programmer uses an abstract
class when there are some
common features shared by all the
objects.
A programmer writes an interface
when all the features have
different implementations for
different objects.
Multiple inheritance not possible
(Multiple “parent” interfaces)
Multiple inheritance possible (Only
one “parent” class)
An abstract class contain both
abstract and concrete(non
abstract) method
An interface contain only abstract
method
In abstract class, abstract keyword
is compulsory to declare a method
as an abstract
abstract keyword is optional to
declare a method as an abstract in
interface
An abstract class can have
protected, public abstract method
An interface can have only public
abstract method
Abstract class contain any type of
variable
Interface contain only static final
variable (constant)
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Benefits of InterfacesBenefits of Interfaces
• Following concepts of object oriented programming
can be achieved using Interface in JAVA.
1.Abstraction
2.Multiple Inheritance
3.polymorphism

24. Some of Java's Most usedSome of Java's Most used
InterfacesInterfaces
• Iterator
To run through a collection of objects without knowing how
the objects are stored, e.g., array.
• Cloneable
Used to make a copy of an existing object via the clone()
method on the class Object. This interface is empty.
• Serializable
Used to Pack a group of objects such that it can be send
over a network or stored to disk. This interface is empty.
• Comparable
The Comparable interface contains a compareTo method,
and this method must take an Object parameter and
return an integer
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: dheeraj.singh@paruluniversity.ac.in: dheeraj.singh@paruluniversity.ac.in