简单JAVA问题，在线等！谢谢！

public class TicketsSystem
{
public static void main(String[] args)
{
SellThread st=new SellThread();

// new Thread(st).start();
try
{
Thread.sleep(1);
}
catch(Exception e)
{
e.printStackTrace();
}
st.b=true;
new Thread(st).start();
new Thread(st).start();
new Thread(st).start();
}
}

class SellThread implements Runnable
{
int tickets=10; //10张车票
Object obj=new Object();
boolean b=false;
public void run()
{
if(b==false)
{
while(true)
sell();
}
else
{
while(true)
{
synchronized(obj)
{
try
{
Thread.sleep(10);
}
catch(Exception e)
{
e.printStackTrace();
}
synchronized(this)
{
if(tickets>0)
{

System.out.println("obj:"+Thread.currentThread().getName()+
" sell tickets:"+tickets);
tickets--;
}
}
}
}
}
}
public synchronized void sell()
{
synchronized(obj)
{
if(tickets>0)
{
try
{
Thread.sleep(10);
}
catch(Exception e)
{
e.printStackTrace();
}
System.out.println("sell():"+Thread.currentThread().getName()+
" sell tickets:"+tickets);
tickets--;
}
}
}
}