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Web Design & Development

Lecture 9

Abstract Classes a

and

Interfaces

Class Shape Hierarchy

• Consider the following class hierarchy

Shape

Circle Square

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

• 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

Example of abstract class Shape.java

/* This is an example of abstract class. Note that

this class contains an abstract method with no

definition.

public abstract class Shape {

public abstract void calculateArea();

Circle.java

// providing definition of abstract method public void calculateArea () { double area = 3.14 * (radius * radius); System.out.println(“Area: ” + area); }

}//end of class

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(); } }

Interfaces

Interfaces

• A special java type which
  • Defines a set of method prototypes, but does not

provide the implementation for the prototypes

• Essentially all the methods inside an interface are

Abstract Methods or we can say that an interface

is like a pure abstract class (Zero Implementation)

• Can also define static final constants

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

Interface - Example

speak()

Politician Coach

<>

Speaker

speak() speak()

Lecturer

speak()

Example Code

public interface Printable { public void print(); }

public class Student implements Printable{ private String name; private String address;

public String toString () { return "name:"+name +" address:"+address; } // NOT providing implementation of print method }

Defining Interface

Implementing Interface

Compile