public class Cashier implements Runnable {
    private CustomerLine line;
    private static boolean allDoneYet;

    public Cashier(CustomerLine line) {
        this.line = line;
        allDoneYet = false;
    }

    public void run() {
        while (!allDoneYet) {
            if (!line.isEmpty()) {
                Customer c = line.dequeue();
                c.process();
                if (allDoneYet = c.isLast())
                    notifyAll();
            }
            else Thread.wait();
        }
    }
}