import java.text.NumberFormat;
import java.util.Scanner;

public class Heatingmain
{
    public static void main(String argv[])
    {
        NumberFormat nf = NumberFormat.getInstance();
        nf.setMaximumFractionDigits(6);
        nf.setMinimumFractionDigits(6);

        Scanner s = new Scanner(System.in);

        double somme=0;
        double n = s.nextLong();

        for (int i = 0; i < n; i++) {
            somme += s.nextLong();
        }

        System.out.print(nf.format(somme/n));
    }


}

Edited by author 25.11.2010 03:26

The solution is to use format method of String class !!!
System.out.print(String.format("%.6f",somme));

But still don't understand why the format method of NumberFormat class doesn't work !!!

Edited by author 25.11.2010 03:30

import java.util.Scanner;

public class AcmTimusRu {
    public static void main(String argv[]) {
        String[] nf = new String[0];

        Scanner s = new Scanner(System.in);

        double somme = 0;
        double n = s.nextLong();

        for (int i = 0; i < n; i++) {
            somme += s.nextLong();
        }

        System.out.print((somme / n));
    }

}