Luhn Algorithm

package luhnalgorithm;
import java.util.Scanner;

public class LuhnAlgorithm
{
    public static void main(String[] args)
    {
        Scanner sc=new Scanner(System.in);
        System.out.print("Enter an ID: ");
        String id=sc.nextLine();
        int sum=0;
        boolean alt=false;
        for(int c=id.length()-1;c>=0;c--)
        {
            int temp=Integer.parseInt(id.substring(c,c+1));
            if(alt==true)
            {
                temp*=2;
                if(temp>9)temp-=9;
                sum+=temp;
            }
            else sum+=temp;
            alt=!alt;
        }
        if(sum%10==0)System.out.println("ID is valid.");
        else System.out.println("ID is invalid.");
    }
}