Error in main method

Error in main method

import java.io.*;
import java.lang.*;
class Aaaa
{
public static void main(String args[])
{
System.out.println("Executing");
/*int a,b,c;
InputStreamReader obj=new InputStreamReader(System.in);
BufferedReader br=new BufferedReader(obj);
System.out.println("enter the a value:");
a=Integer.parseInt(br.readLine());
b=Integer.parseInt(br.readLine());
c=a+b;
System.out.println("addition of a & b" +c);*/
}
}
it gives java.noclassdeffounderror
how can i recolve it???
i can execute other programs but this is not working....y????