why do I get this runtime error

When I run this:

package scratch;

import java.io.*;

public class Main extends ClassLoader
{
        public Main()
        {
                super(Main.class.getClassLoader());
        }

        public static void main(String[] args)
                throws Throwable
        {
                Main m=new Main();

                File f=null;

                f = new File("/usr/home/plos/obj/scratch/Main.class");
                int size = (int)f.length();
                byte buff[] = new byte[size];
                FileInputStream fis = new FileInputStream(f);
                DataInputStream dis = new DataInputStream(fis);
                dis.readFully(buff);
                dis.close();

                Class klass=m.defineClass("scratch.Main",buff,
0,buff.length);

                Main m2=(Main) klass.newInstance();
        }
}

I get:
Exception in thread "main" java.lang.ClassCastException: scratch.Main
cannot be cast to scratch.Main
          at scratch.Main.main(Main.java:29)

Notes:

This is distilled from attempting to write a class loader and every
attempt ends with the same error (even cutting and pasting web and
junit examples of class loaders and calling them instead of mine).