线程同步有关问题，哪位高手帮忙看看

线程同步问题，谁帮忙看看
需求：编写三各类Ticket、SealWindow、TicketSealCenter分别代表票信息、售票窗口、售票中心。售票中心分配一定数量的票，由若干个售票窗口进行出售。

运行的时候有重复的票号，我已经在卖票的方法上添加了synchronized关键字，还是会出现问题，但是我找不到问题出在哪里了，谢谢大家了

public class TicketDemo {

	public static void main(String[] args) {	
		new Thread(new SealWindow("1号窗口")).start();
		new Thread(new SealWindow("2号窗口")).start();
		new Thread(new SealWindow("3号窗口")).start();
		new Thread(new SealWindow("4号窗口")).start();
	}
}
//票信息
class Ticket {
	private static int ticketid;

	public static int getTicketid() {
		return ticketid;
	}
	
	public static void setTicketid(int ticket) {
		ticketid = ticket;
	}
}
//售票窗口
class SealWindow implements Runnable {
	private String ticketname;
	TicketSealCenter tsc = null;;
	
	public SealWindow(String ticketname){
		this.ticketname = ticketname;
		tsc = TicketSealCenter.getInstance();
	}
	public void sellTicket(){
	
		while(!tsc.hasTicket()){
			System.out.println(ticketname+"卖出了第" + tsc.getid()+"号票！");
			if (tsc.hasTicket()) {
				System.out.println(ticketname+"的票已卖完！！");
			}
		}
		
	}
	public void run() {
		sellTicket();
	}
}
//售票中心
class TicketSealCenter {
	private static TicketSealCenter tsc = new TicketSealCenter();
	private static Ticket ticket = new Ticket();
	
	private static int sumticket = 100;
	private static boolean flag = false;
	
	private TicketSealCenter(){}
	
	public static TicketSealCenter getInstance(){
		return tsc;
	}
	//询问是否有票，设置票面ID，并返回该ID
	public static synchronized boolean hasTicket(){
		if(sumticket > 0){
			ticket.setTicketid(sumticket);
			sumticket--;
		}else {
			flag =true;
		}
		return flag;
	}
	public int getid(){
		return ticket.getTicketid();
	}
}

------解决思路----------------------

public class TicketDemo {
 
    public static void main(String[] args) { 
    	SealWindow myWindow=new SealWindow();
        new Thread(myWindow,"1号窗口").start();
        new Thread(myWindow,"2号窗口").start();
        new Thread(myWindow,"3号窗口").start();
        new Thread(myWindow,"4号窗口").start();
    }
}
//票信息
class Ticket {
    private static int ticketid;
 
    public static int getTicketid() {
        return ticketid;
    }
     
    public static void setTicketid(int ticket) {
        ticketid = ticket;
    }
}
//售票窗口
class SealWindow implements Runnable {
    TicketSealCenter tsc = null;
    public SealWindow(){
        tsc = TicketSealCenter.getInstance();
    }
    public void sellTicket(){
        while(!tsc.hasTicket()){
           // System.out.println(Thread.currentThread().getName()+"卖出了第" + tsc.getid()+"号票！");
            if (tsc.hasTicket()) {
                System.out.println(Thread.currentThread().getName()+"的票已卖完！！");
            }
        }
         
    }
    public void run() {
        sellTicket();
    }
}
//售票中心
class TicketSealCenter {
    private static TicketSealCenter tsc = new TicketSealCenter();
    private static Ticket ticket = new Ticket();
     
    private static int sumticket = 100;
    private static boolean flag = false;
     
    private TicketSealCenter(){}
     
    public static TicketSealCenter getInstance(){
        return tsc;
    }
    //询问是否有票，设置票面ID，并返回该ID