通过继承Thread类实现线程与实现Runnable接口实现线程有什么区别

//下面这个程序是：继承线程类通过线程同步方法实现同步会出错，通过线程同步块实现同步却没有错。

//请问这二者有何区别？？

class SellThread extends Thread
{
static Object obj=new Object();
static int tickets=50;
public void run()
{
while(tickets>0)
{
/*synchronized(obj)//线程同步块
{
if(tickets>0)
{
System.out.println(Thread.currentThread

().getName( )+" sell tickets:"+tickets);
tickets=tickets-1;
try
{
Thread.sleep(1);
}
catch(Exception e)
{
e.printStackTrace();
}
}*/
sell();
}
}

public synchronized void sell()//线程同步方法
{
if(tickets>0)
{
System.out.println(Thread.currentThread().getName( )+" sell

tickets:"+tickets);
tickets--;
}
}
}

class TicketsSystem_this_obj
{
public static void main(String args[])
{
SellThread st1=new SellThread();
SellThread st2=new SellThread();
SellThread st3=new SellThread();
SellThread st4=new SellThread();
st1.start();
st2.start();
st3.start();
st4.start();
/* try
{
Thread.sleep(10);
}
catch(Exception e)
{
e.printStackTrace();
}*/
}
}