Working with Threads. Thinking in Java Exercise

I'm sorry for the code apperance. One more time original Restaurant.java I
hope i won't break
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class Order
{
    public static int index = 0;
    private int count = ++index;

    public Order( )
    {
        if ( index > 10 )
        {
            java.lang.System.out.println( "Out of food. Closing..." );
            java.lang.System.exit( 0 );
        }
    }

    public String toString()
    {
        return "order #" + count;
    }
}

class Waitperson extends Thread
{
    private Restaurant restaurant;

    public Waitperson( Restaurant rest )
    {
        super( );
        restaurant = rest;
        start( );
    }

    public void run( )
    {
        while( true )
        {
            if( restaurant.order == null)
            {
                synchronized( this )
                {
                    try
                    {
                        wait( );
                    }
                    catch( java.lang.InterruptedException ex )
                    {
                        throw new RuntimeException( );
                    }
                }
            }
                System.out.println( "Waitperson got " + restaurant.order );
                restaurant.order = null;
        }
    }
}

class Chef extends Thread
{
    private Restaurant restaurant;
    private Waitperson waitperson;

    public Chef( Restaurant rest , Waitperson waitperson )
    {
        super( );
        restaurant = rest;
        this.waitperson = waitperson;
        start( );
    }

    public void run( )
    {
        while( true )
        {
            if( restaurant.order == null )
            {
                restaurant.order = new Order( );
                System.out.println("Order up!!");
                synchronized( waitperson )
                {
                    waitperson.notify();
                }
            }
            try
            {
                sleep( 100 );
            }
            catch( java.lang.InterruptedException ex )
            {
                throw new RuntimeException( );
            }
        }

    }
}

public class Restaurant
{
    Order order;

    public static void main( java.lang.String [] args )
    {
        Restaurant restaurant = new Restaurant( );
        Waitperson waitperson = new Waitperson( restaurant );
        Chef chef = new Chef( restaurant, waitperson );
    }
}