INHERITANCE AND SUPER KEYWORD

Single level Inheritance

PROGRAM: Program on single level inheritance

class First
{
   void display()
  {
     System.out.println("In Super Class");
   }
}
class Second extends First
{
   void display()
  {
     System.out.println("In Sub Class");
   }
}
class Singledemo
{
   public static void main(String args[])
  {
     Second s=new Second ();
     s.display();
  }
}

File Name: - Singledemo.java

Commands:-
                      javac Singledemo.java
                      java Singledemo
OUTPUT:-
                      In Sub Class
NOTE:-

• Here the super class method has been overridden by the subclass method. It is known as method overriding.
• If we want to execute the method present in superclass also then we can use super keyword.

super keyword

It has two functions. They are:
1) In a subclass first the superclass constructor is executed.
2) It hides super class variables or methods i.e. method overriding.

PROGRAM: How to use super keyword

class First
{
   int a,b;

   First(int a,int b)
  {
     this.a=a;
     this.b=b;
  }

  void display()
  {
    System.out.println("a value is:"+a);
    System.out.println("b value is:"+b);
  }
}
class Second extends First
{
    int c,d;

   Second(int a,int b,int c,int d)
   {
      super(a,b);
      this.c=c;
      this.d=d;
   }

   void display()
   {
      super.display();
      System.out.println("c value is:"+c);
      System.out.println("d value is:"+d);
    }
}
class Superdemo
{
   public static void main(String args[])
  {
     Second s=new Second (10, 20, 30, 40);
     s.display();
  }
}

File Name: - Superdemo.java

Commands:-
                      javac Superdemo.java
                      java Superdemo
OUTPUT:-
                     a value is: 10
                     b value is: 20
                     c value is: 30
                     d value is: 40

HOW CONSTRUCTORS ARE INVOKED?

• In a subclass if we use super keyword or not, in a subclass always first the superclass constructor is executed.

PROGRAM: How constructors are invoked

class First
{
   First()
  {
      System.out.println("First Constructor");
   }
}
class Second extends First
{
   Second()
  {
     System.out.println("Second Constructor");
   }
}
class Third extends Second
{
   Third()
  {
     System.out.println("Third Constructor");
  }
}
class Consdemo
{
   public static void main(String args[])
  { 
      Third obj=new Third();
   }
}

File Name: - Consdemo.java

Commands:-
                      javac Consdemo.java
                      java Consdemo
OUTPUT:-
                      First Constructor
                      Second Constructor
                      Third Constructor

Multi-Level Inheritance

PROGRAM: 

class Student
{
   int rollno;
}
class Marks extends Student
{
   double m1,m2,m3;
}
class Average extends Marks
{
   double total;

   void show()
   {
      total=m1+m2+m3;
      double result=total/3;
      System.out.println("Roll Number:"+rollno+" Average is:"+result);
   }
}
class Studentdemo
{
   public static void main(String args[])
  {
     Average avg=new Average();
     avg.rollno=10;
     avg.m1=60.5;
     avg.m2=70.5;
     avg.m3=80.5;
     avg.show();
   }
}

File Name: - Studentdemo.java

Commands:-
                      javac Studentdemo.java
                      java Studentdemo
OUTPUT:-
                      Roll Number: 10 Average is: 70.5