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Interface in java By Dheeraj Kumar Singh


Published on

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.

Published in: Engineering

Interface in java By Dheeraj Kumar Singh

  1. 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. 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 2
  3. 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. 4. IntroductionIntroduction toto InterfaceInterface 4 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
  5. 5. 5 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. 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. 6
  7. 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? 7
  8. 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 8
  9. 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
  10. 10. 10 • 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. 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
  12. 12. 12 • 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
  13. 13. 13 • 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
  14. 14. 14 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.
  15. 15. 15 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(); }
  16. 16. 16 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); } }
  17. 17. 17 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; } }
  18. 18. 18 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. 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())); } } 19
  20. 20. Output ScreenOutput Screen 20
  21. 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. 21
  22. 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) 22
  23. 23. 23 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. 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 24
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