Application.java (simple version)

package runners;

import javax.swing.*;

public class Application

    {
    public static void main(String[] args)
        {
        int runners = 0;
        while(runners <= 0)
            {
            try
                {
                int running = Integer.parseInt(JOptionPane.showInputDialog("Insert number of runners:"));
                if(running > 0) runners = running;
                }
            catch(NumberFormatException e)
                {
                JOptionPane.showMessageDialog(null, "Invalid number", "Error", JOptionPane.ERROR_MESSAGE);
                }
            }
        for(int i = 0; i < runners; i++)
            {
            Runner r = new Runner("Runner " + i);
            Thread t = new Thread(r);
            t.start();
            }
        }
    }