Arithmetic Mean & Standard Deviation in JAVA.

DESCRIPTION: 
Calculate arithmetic mean in the superclass and standard deviation in the subclass in JAVA.
CODING:

import java.util.Scanner;
class AMSD
{
 public static void main(String[] args)
  {
   SD c1=new SD();
   System.out.print("How many Enter Number:");
   c1.CalAM(c1.num.nextInt());
   System.out.println("Arithmetic Mean:" + c1.am);
    c1.CalSD();
   System.out.println("Standard Deviation :" + c1.sd);
  }
}
class AM
{
 Scanner num=new Scanner(System.in);
 double am=0;
 double[] a;
 int i;
  public void  CalAM(int n)
   {
    a=new double[n];
    for(i=0;i<n;i++)
     {
      System.out.print("Enter number:" );
      a[i]=num.nextDouble();
      am=a[i]+am;
     }
     am=am/n;
   }
}
class SD extends AM
{
 double sd=0;
 public void CalSD()
  {
    for(i=0;i<a.length;i++)
     {
      sd=Math.pow(a[i]-am,2)+sd;
     }
   sd=Math.sqrt(sd/a.length);
 }
}