Lecture 8 abstract class and interface

Java Programming
Manish Kumar
(manish87it@gmail.com)
Lecture- 8
Abstract class &
Interface

Contents
 Introduction to Abstract Class
 Upcasting/Dynamic Dispatch Method
 Importance of Abstract Class (Using real life based program)
 Abstract Class Example (with constructor, methods and variables)
 Introduction to Interface
 Interface implementations
 Marker and Nested Interface
 Abstract Class Vs. Interface

Abstract class
 A class declared with “abstract” keyword is called abstract class. An abstract class may or may not contain
an “abstract method”.
 A method that cannot have definition (i.e., method body) part is known as abstract method.
 There are some important points of an abstract class as follows:
1. An abstract class must be declared with abstract keyword.
2. It contain both or none i.e., abstract and non-abstract method.
3. We cannot create an object of an abstract class.
Declaration of an abstract class and abstract method-
abstract class Abc {}
Here abstract is a keyword that is used to make a class as an abstract class.
abstract void display();
As you see above method declaration, there is no body of the method and declared with abstract keyword.

Example-1
abstract class Polygon {
abstract void area();
}
class Tringle extends Polygon {
void area() {
int r=10;
double pi=3.14;
double ar = pi *r*r;
System.out.println("Area of tringle = "+ar);
}
public static void main(String args[]) {
Polygon p = new Tringle();
p.area();
}
}
Tringle.java
Output –
Area of tringle = 314.0

Example-2
Note: - When an abstract class is inherited, then
derived class must provide the implementations
for all the abstract method declared in base class
otherwise it gives an error at compile time.
Tringle.java
abstract class Polygon {
abstract void display();
}
void area() {
int r=10;
double pi=3.14;
double ar = pi *r*r;
System.out.println("Area of tringle = "+ar);
}
Polygon p = new Tringle(); // Upcasting
p.area();
}
}
Output-
error: Tringle is not abstract and does not override
abstract method display() in Polygon
^
1 error

upcasting
Upcasting/Dynamic Dispatch Method
When an object of derived class is assigned to base class reference variable, then it is called Upcasting.
In previous slide’s example – p is a reference variable of the base class and it assigned by an object of
derived class.
In this case, calling method first search the existence of method in base class then it will execute the method
in derived class.
Polygon p = new Tringle(); // Upcasting

Importance of abstract class
//Bank.java
abstract class RBI {
abstract int getROI();
}
class SBI extends RBI {
int getROI() {
return 7;
}}
class PNB extends RBI {
int getROI() {
return 8;
}}
class Bank {
RBI rb;
rb = new SBI();
System.out.println("ROI of SBI(%) = "+rb.getROI());
rb = new PNB();
System.out.println("ROI of PNB(%) = "+rb.getROI());
}
}
Output –
ROI of SBI(%) = 7
ROI of PNB(%) = 8

example
AbstractClassExample.java (constructor, variables
and methods):
abstract class Polygon{
Polygon() {
System.out.println("Polygon created."); }
void display() {
System.out.println("This is non-abstract method");
}}
class Rectangle extends Polygon {
void area() {
int l=10,w=7, area;
area = l*w;
System.out.println("Area of Rectangle = "+area);
}
}
class AbstractClassExample {
Polygon p =new Rectangle();
p.area();
p.display();
}
}
Output –
Polygon created.
Area of Rectangle = 70
This is non-abstract method

interface
An interface is declared with interface keyword. We can achieve 100% abstraction using interface. It can be
defined as “the collection of public, static, final data member and abstract methods.”
There are few points that need to remembered:
1. Interface declaration is just like class; only difference is that it uses the keyword interface in place of
class.
2. The method in interface is by default public and abstract but since java 9; we can have private method in
an interface and since java 8; interface can have default method.
3. The data member in interface is by default public, static and final.
4. It also represents the IS-A relationship.
5. We can support multiple inheritance using interface.
6. We cannot create an object of an interface.

declaration
Interface <interface-name> {
//declare data member and methods
}
Declaring an interface:
Relationship between class and interface
•A class can implements an interface or more than one interface.
•An interface can extends another interface.
•An interface can also implements multiple interface.

Example - 1
interface Animal {
void livingPlace();
}
class Lion implements Animal {
public void livingPlace() {
System.out.println("The living place of Lion is Forest.");
}
Animal an = new Lion();
an.livingPlace();
}
}
Output-
The living place of Lion is Forest.
Lion.java
A class can implements an interface.

Example-2
Lion.java
interface Animal {
void livingPlace();
}
interface Birds {
void live();
}
class Lion implements Animal, Birds {
System.out.println("The living place of Lion is Forest.");
}
public void live() {
System.out.println("The living place of birds in Nest.");
}
Animal an= new Lion();
an.livingPlace();
Birds b = new Lion();
b.live();
}
}
Output-
The living place of birds in Nest.
A class can implements an interface
or more than one interface.

Example-3
interface Animal {
void livingPlace();
}
interface Birds extends Animal {
void live();
}
class Lion implements Birds {
System.out.println("The living place
of Lion is Forest.");
}
public void live() {
System.out.println("The living place
of birds in Nest.");
}
Birds b = new Lion();
b.livingPlace();
b.live();
}
}
Lion.java
Output-
The living place of birds in Nest.
An interface can extends another interface.

Example-4
interface Polygon {
int a = 10;
void area(int x);
}
class Tringle implements Polygon {
public void area(int x) {
double pi = 3.14;
double area = pi*x*x;
System.out.println("Area of tringle =
"+area);
}
Polygon p = new Tringle();
p.area(10);
}
}
Output-
Area of tringle = 314.0
When you override the abstract method
declared in interface it should be noted that
overridden method must be public because
the method in interface is by default public.
When you declare a variable in interface then
it is mandatory to initialize because it is by
default; public, static and final.

Marker & nested interface
An empty interface is called marker interface i.e. an interface without data member and methods called
Marker Interface. For example – Cloneable, Serializable, etc. They are used to provide essential information
to the JVM.
public interface Serializable { }
Nested Interface
An interface inside another interface is known as Nested Interface.
Interface Abc {
void show();
interface Xyz {
void display(); } }

Abstract class vs interface
Abstract Class Interface
Abstract class may have abstract and non-abstract method Interface can have only abstract method
Multiple inheritance not supported by abstract class. Interface supports multiple inheritance.
Abstract class can have static, non-static, final, non-final
variables.
Interface can have only static and final variables.
abstract keyword is used to declare a class as an Abstract
class.
interface keyword is used to declare an interface.
An abstract class can be extended using “extends”
keyword.
An interface can be implemented using “implements”
keyword.
We can achieve 0 to 100% abstraction. Achieve 100% abstraction.
Abstract class can have “private, protected, etc.” class
member.
Members of an interface are public and default only.

Lecture 8 abstract class and interface

Lecture 8 abstract class and interface

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