gistfile1.java

    class Ticketing {
        Entering: array [1..N] of boolean = {false};
        Number: array [1..N] of integer = {0};
        
        void lock(integer i) {
            Entering[i] = true;
            Number[i] = 1 + max(Number[1], ..., Number[N]);
            Entering[i] = false;
            for (j = 1; j <= N; j++) {
                // Wait until thread j receives its number:
                while (Entering[j]) { /* nothing */ }
                // Wait until all threads with smaller numbers or with the same
                // number, but with higher priority, finish their work:
                while ((Number[j] != 0) && ((Number[j], j) < (Number[i], i))) {
                    /* nothing */
                }
            }
        }

        void unlock(integer i) {
            Number[i] = 0;
        }

        int emptySeat() {
            return { index of first nonzero element in number }
        }
    }

    monitor class BarberShop {
        integer availableSeats = N;

        condition customerCondition = condition();
        boolean customerReady = false;

        condition barberCondition = condition();
        boolean barberReady = false;

        condition rendezvousCondition = condition();
        boolean rendezvous = false;

        void waitForCustomer() {
            while (!customerReady) customerCondition.wait();
            customerReady = false;
        }

        void customerReady() {
            customerReady = true;
            customerCondition.signal();
        }

        void waitForBarber() {
            while (!barberReady) barberCondition.wait();
            barberReady = false;
        }

        void barberReady() {
            barberReady = true;
            barberCondition.signal();
        }

        void waitForRendezvous() {
            while (!rendezvous) rendezvousCondition.wait();
            rendezvous = false;
        }

        void signalRendezvous() {
            rendezvous = true; 
            rendezvousCondition.signal();
        }

        boolean enterShop() {
            if (availableSeats > 0) {
                availableSeats--;
                return true;
            }
            return false;
        }

        void leaveShop() {
            availableSeats++;
        }
    }

    class Customer {
        BarberShop monitor;

        Customer(BarberShop monitor, Ticketing ticketing) {
            this.monitor = monitor;
            this.ticketing = ticketing;
        }

        void run() {
            if (monitor.enterShop()) {
                int index = ticketing.emptySeat();
                ticketing.lock(index);
                monitor.customerReady();
                monitor.waitForBarber();
                // getting a haircut
                monitor.waitForRendezvous();
                ticketing.unlock(index);
                monitor.leaveShop();
            }
        }    
    }

    class Barber {
        BarberShop monitor;

        Barber(BarberShop monitor) {
            this.monitor = monitor;
        }

        void run() {
            while(true) {
                monitor.waitForCustomer();
                monitor.barberReady();
                // cutting hair
                monitor.signalRendezvous();
            }
        }
    }