Inheritance and Polymorphism in Java.

1. Inheritance and Polymorphism
M. Raihan
Computer Science and Engineering Discipline , Khulna University

2. What is inheritance ?
 Inheritance is a fundamental Object Oriented
concept
 A class can be defined as a "subclass" of another
class.
 The subclass inherits all data attributes of its
superclass
 The subclass inherits all methods of its
superclass
 The subclass can:
 Add new functionality
 Use inherited functionality
 Override inherited functionality

3. Declaration
 Inheritance is declared using the "extends" keyword
 To access Super Class from Sub Class we use super keyword.
Example :
class SuperClass{
// rest of codes
}
class SubClass extends SuperClass{
// rest of codes
}

4. Inheritance Hierarchy
 Each Java class has one (and only one) superclass.
 Inheritance creates a class hierarchy
Class A
Class B
Class D Class E
Class C
Class F

5. A Program
public class SuperClass {
Scanner sc = new Scanner(System.in); // Here we take input from Keyboard
public double CircleArea(){
double pi=3.1416,num1=0,r=0,CircleAreaResult=0;
num1=sc.nextDouble(); // num1 input taken
r=sc.nextDouble(); // r input taken
CircleAreaResult = pi*num1*(r*r);
return CircleAreaResult;
}
public double TriangleArea(){
double base,hight,TriangleAreaResult = 0;
base=sc.nextDouble(); // Here base input taken keyboard
hight=sc.nextDouble(); // hight input taken from keyborad
TriangleAreaResult = 0.5*base*hight;
return TriangleAreaResult;
}
public void Result() {
double Result1,Result2;
Result1= CircleArea();
Result2= TriangleArea();
System.out.println(“Circle Area : “+Result1+”nTriangle Area : “+Result2);
}
}

6. A Program (continue)
public class Main extends SuperClass{
public static void main(String[] args) {
SuperClass sp= new SuperClass();
sp.Result();
}
}

7. Some Methods
toString(): return a string that contains the name
of the object’s class and a hash value
equals(): determines if two variables point to the
same object (more shortly).

8. Access Modifiers
 Private data fields are not accessible to derived classes
 Protected visibility allows data fields to be accessed either by the
class defining it or any subclass.
 In general, it is better to use private visibility because
subclasses may be written by different programmers and
it is always good practice to restrict and control access to
the superclass data fields
 Members (variables and methods) declared with public
visibility are accessible, and those with private visibility
are not

9. Method Overloading
When 2 or more methods in a class have the same method
names with different arguments, it is said to be method
overloading.
Overloading does not block inheritance from the superclass.
Overloaded methods must have different method
signatures.

10. Example
public class Calculate{
public void circle(){
}
public int circle(int a){
return ;
}
public float circle(float a){
return ;
}
}

11. Method Overriding
When a method in a class has the same method
name with same arguments as that of the
superclass, it is said to be method overriding.
Overriding blocks inheritance from the superclass.
Overridden methods must have same signature.

12. Example
public class SuperClass{
public void calculate(int a, int b){
int c=a+b;
}
}
public class SubClass extends SuperClass{
public void calculate(int a, int b){
int c;
if(a>b)
c=a-b;
else
c=a+b;
}
}

13. Thank You