Exception Closes Program!

[Xibe]

I'm trying to create a program for class that throws an exception if the string entered is longer than 20 characters. Once the exception is thrown, it is supposed to continue and say: "Do you want to enter another line?" But, it keeps ending prematurely after the exception is thrown.

Here's my program:

/*
CS182
Excercise 5
Ashley Glasser
4/30/2011
 */
// Excercise 5 MessageTooLong.java
import java.util.Scanner;
public class MessageTooLong extends Exception {
        public static void main(String args[])
        throws MessageTooLongException
	{

		Scanner keyboard = new Scanner(System.in);
 String line;
 char preference;
 int length;
boolean go = true;
    while (go) {
 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;
        }
                }
      if (length > 20)
                {
                     throw new MessageTooLongException();
                }

        }
    }
}

And here's my exception:

public class MessageTooLongException extends Exception
{
	public MessageTooLongException()
	{
		super("Message Too Long!");
	}

	public MessageTooLongException(String message)
	{
		super(message);
		System.out.println("Message Exception invoked with an argument.");               
	}

}

Why is it closing out the program?

[Fou-Lu]

Of course, the main method is throwing an exception so you're JVM has to deal with it.
I don't really see a purpose of tossing an exception at all. Since you would require the code to check if it has entered >= 20 in the first place, the exception becomes pointless since you would throw it and catch it yourself anyway. If you were to use this in something like a JDialog, than that would make a little more sense since it will be handled outside of the controlling program.