Abstract Classes, Interfaces - Web Design and Development - Lecture Slides, Slides of Web Design and Development

Download Abstract Classes, Interfaces - Web Design and Development - Lecture Slides and more Slides Web Design and Development in PDF only on Docsity! Web Design & Development Lecture 9 Docsity.com Abstract Classes a and Interfaces Docsity.com Problem AND Requirements • Suppose that in order to exploit polymorphism, we specify that 2-D objects must be able to compute their area. – All 2-D classes must respond to area() message. • How do we ensure that? – Define area method in class Shape – Force the subclasses of Shape to respond area() message • Java’s Solutions – Abstract Classes – Interfaces Docsity.com Abstract Classes • Idea – To define only part of an implementation  Can contain instance variables & methods that are fully implemented – Leaving the subclasses to provide the details • Any class with an abstract method must be declared abstract – However you can declare a class abstract that has no abstract method. – An abstract method has no implementation (known in C++ as a pure virtual function) Docsity.com Abstract Classes • If subclass overrides all abstract methods of the superclass, than it becomes a concrete class otherwise we have to declare it as abstract or we can not compile it • Any subclass can override a concrete method inherited from the superclass and declare them abstract • An abstract class cannot be instantiated • However references to an abstract class can be declared – Can point to the objects of concrete subclasses Docsity.com Circle.java // providing definition of abstract method public void calculateArea () { double area = 3.14 * (radius * radius); System.out.println(“Area: ” + area); } }//end of class Docsity.com Test.java (Driver class) public class Test { public static void main (String args[ ]){ //can only create references of abstract class Shape s = null; // Shape s1 = new Shape(); //cannot instantiate abstract class //can point to the concrete subclass s = new Circle(); s.calculateArea(); } } Docsity.com AO Chae eC :NexamplesSNabstract> javac Shape. java :\examplesS\abstract> javac Circle. java :\examples\abstract> javac Test. java i :\examples‘\abstract> java Test Area: 314.0 D:\examples ‘abstract > «| | Lb oe Bocsity=com Interfaces Definition Example • Syntax (appears like abstract class): • All methods are abstract and public by default • All constants are static and final by default public interface Speaker { public void speak( ); } Docsity.com Implementing (Using) Interfaces • Classes Implement interfaces – Implementing an interface is like signing a contract. – A class that implements an interface will have to provide the definition of all the methods that are present inside an interface“ – If the class does not provide definitions of all methods, the class would not compile. We have to declare it as an abstract class in order to get it compiled. • “Responds to” relationship – Relationship between a class and interface Docsity.com Interface - Example speak() Politician Coach <<Interface>> Speaker speak() speak() Lecturer speak() Docsity.com area ORS CURFA CITC D:\examples\interface) javac Student. java Student. java:l: Student is not abstract and does not override abstract method print() in Printable public class Student implements Printablef 1 error D:\examplesNinterfaced_ 2 ® Bocsity=com Example Code (cont.) public class Student implements Printable{ private String name; private String address; public String toString () { return "name:"+name +" address:"+address; } public void print() { System.out.println("Name:" +name+" address"+address); } } Implementing Interface (Modification) Docsity.com Compile Docsity.com Interface based Polymorphism Docsity.com Review again – Interface Example speak() Politician Coach <<Interface>> Speaker speak() speak() Lecturer speak() Docsity.com Example: Interface based Polymorphism /* Speaker interface is implemented by the Politician, Coach and Lecturer class. */ public class Test{ public static void main (String args[ ]) { Speaker sp = null; System.out.println("sp pointing to Politician"); sp = new Politician(); sp.speak(); System.out.println("sp pointing to Coach"); sp = new Coach(); sp.speak(); System.out.println("sp pointing to Lecturer"); sp = new Lecturer(); sp.speak(); } } Docsity.com