Calculator program in JAVA using if else statements

import java.util.Scanner;
public class calculator
{


int c;
public void add()
{
Scanner sc =new Scanner(System.in);
System.out.println("enter first no.");
int a=sc.nextInt();
System.out.println("enter second no. ");
int b=sc.nextInt();
c=a+b;
System.out.println(c);
}
public void sub()
{
Scanner sc =new Scanner(System.in);
System.out.println("enter first no.");
int a=sc.nextInt();
System.out.println("enter second no. ");
int b=sc.nextInt();
c=a-b;
System.out.println(c);
}
public void mul()
{
Scanner sc =new Scanner(System.in);
System.out.println("enter first no.");
int a=sc.nextInt();
System.out.println("enter second no. ");
int b=sc.nextInt();
c=a*b;
System.out.println(c);
}
public void div()
{
Scanner sc =new Scanner(System.in);
System.out.println("enter first no.");
int a=sc.nextInt();
System.out.println("enter second no. ");
int b=sc.nextInt();
c=a/b;
System.out.println("your answer is"+c);
}
}

class scientificcalculator extends calculator
 {
final float pi=3.14f;
float k;
public void square()
{
Scanner sc =new Scanner(System.in);
System.out.println("enter first no.");
int a=sc.nextInt();
k=pi*a*a;
System.out.println("your answer is"+k);
}
public void cube()
{
Scanner sc =new Scanner(System.in);
System.out.println("enter first no.");
int a=sc.nextInt();
c=a*a*a;
System.out.println("your answer is"+c);
}
public void power()
{
Scanner sc =new Scanner(System.in);
System.out.println("enter first no.");
int a=sc.nextInt();

System.out.println("enter second no. ");
int b=sc.nextInt();

c=a^b;
System.out.println("your answer is"+c);
}
public void fact()
{
int x=1;
int i;
Scanner sc =new Scanner(System.in);
System.out.println("enter first no.");
int a=sc.nextInt();
for(i=0;i<=a;i++)
{
x=x*i;
System.out.println(x);
}
}

}

class Main extends scientificcalculator
{
public static void main(String[] arg)
{

scientificcalculator ob =new Main();

Scanner sc =new Scanner(System.in);
System.out.println("enter the no. b/w 1 to 4");
int num=sc.nextInt();

if(num==1)
ob.add();
else if(num==2)
ob.sub();
else if(num==3)
ob.mul();
else if(num==4)
ob.fact();
else if(num==5)
ob.div();
else if(num==6)
ob.square();
else if(num==7)
ob.power();
else 
ob.cube();}
}