java 中synchronized方法的参数问题

代码如下:
package thead_test3;

/**
* Program 的摘要说明
*/
public class multithread
{
public static void main(String[] args)
{
mythread mt = new mythread();
new Thread(mt).start();
new Thread(mt).start();
new Thread(mt).start();
new Thread(mt).start();
}

}
class mythread extends Thread
{
int index = 100;

Object obj = new Object();
public void run()
{
while (true)
{
synchronized (obj)
{
if (index > 0)
{
try
{
Thread.sleep(10);
System.out.println(Thread.currentThread().getName() + "卖了第" + index + "张饭票");
index--;

}
catch (InterruptedException e)
{
e.printStackTrace();
}
}
}
if (index == 0)
{
System.exit(0);
}
}
}