ConcertTickets.java

import java.util.*;

class PurchaseOrder {

    private final boolean reserved;

    private final int clientId;

    public PurchaseOrder(boolean reserved, int clientId)
    {
        this.reserved = reserved;
        this.clientId = clientId;
    }

    public boolean isReserved()
    {
        return reserved;
    }

    public int getClientId()
    {
        return clientId;
    }

}

class ConcertTicketClient extends Thread {

    private int clientId;

    private ConcertTicketService service;

    private int purchased;

    public ConcertTicketClient(int clientId, ConcertTicketService service)
    {
        this.clientId = clientId;
        this.service = service;
    }

    public int getTotalTicketsPurchased()
    {
        return purchased;
    }

    public void run()
    {
        Random rand = new Random();

        while (true) {

            PurchaseOrder order = service.purchaseTicket(clientId);

            if (order.isReserved()) {
                purchased++;
            } else {
                // we are done.
                break;
            }

            try { Thread.sleep(1); } catch (Exception e) { }

        }
    }

}

class ConcertTicketService extends Thread {

    private int tickets;

    private int first, last;

    private Map<Integer, Integer> history;

    public ConcertTicketService(int tickets)
    {
        this.tickets = tickets;
        history = new HashMap<>();
    }

    public synchronized PurchaseOrder purchaseTicket(int clientId)
    {
        PurchaseOrder order;

        if ( ! isSoldOut()) {
            if (first == 0) {
                first = (int) System.currentTimeMillis();
            }

            tickets--;

            if (isSoldOut()) {
                last = (int) System.currentTimeMillis();
            }

            history.put(clientId, history.containsKey(clientId)
                ? history.get(clientId) + 1
                : 1);

            order = new PurchaseOrder(true, clientId);
        } else {
            order = new PurchaseOrder(false, clientId);
        }

        return order;
    }

    public synchronized boolean isSoldOut()
    {
        return tickets == 0;
    }

    public int getSaleDuration()
    {
        return last - first;
    }

    public int getNumberOfTicketsSoldToClient(int clientId)
    {
        return history.get(clientId);
    }

    public void run()
    {
        while ( ! isSoldOut()) {

            try { Thread.sleep(50); } catch (Exception e) { }

        }
    }

}

public class ConcertTickets {

    public static void main(String[] args)
    {
        ConcertTicketService service = new ConcertTicketService(100000);

        service.start();

        ConcertTicketClient[] clients = new ConcertTicketClient[10];

        for (int i = 0; i < clients.length; i++) {
            clients[i] = new ConcertTicketClient(i, service);
            clients[i].start();
        }

        try {
            service.join();

            for (int i = 0; i < clients.length; i++) {
                clients[i].join();
            }

            System.out.println("Duration: " + service.getSaleDuration());

            int received = 0;

            for (int i = 0; i < clients.length; i++) {
                received += clients[i].getTotalTicketsPurchased();
                System.out.printf(
                    "Client %d: S %d R %d\n",
                    i, service.getNumberOfTicketsSoldToClient(i), clients[i].getTotalTicketsPurchased()
                );
            }

            System.out.println("Total R: " + received);

        } catch (Exception e) { }
    }

}