Throwing Exception

I am trying unsuccessfully to throw an exception TooLongEx if a user input fails. Been stuck on this forever :(

import java.util.Scanner;
public class MessageTooLong extends Exception {
    public static void main(String args[])
    throws TooLongEx {
        Scanner keyboard = new Scanner(System.in);
        String line;
        char preference;
        int length;
        boolean go = true;
        while (go) 开发者_开发技巧{
            System.out.println("Enter a line of text.");
            System.out.println("Use no more than 20 characters.");
            line = keyboard.next();
            length = line.length();
            if (length <= 20) {
                    System.out.println("You entered " + length + " characters, which is an acceptable length.");
                    System.out.println("Would you like to enter another line?");
                    System.out.println("Enter 'y' to continue or 'n' to quit.");
                    preference = keyboard.next().charAt(0);
                if ((preference == 'y') || (preference == 'Y')) {
                    go = true;
                } else {
                    go = false;
                }
            } else {
                throw new TooLongEx();
            }
        }
    }
}

Seems to work fine for me.

For

class TooLongEx extends Exception {}

I get (for an input length > 20)

Exception in thread "main" TooLongEx at MessageTooLong.main(MessageTooLong.java:26)