public class ThreadSafeTest implements Runnable {.....tickets-1 tickets-2怎么也输出了啊？？

public class ThreadSafeTest implements Runnable {
int num = 10;

public void run() {
while (true) {
if (num > 0) {
try {
Thread.sleep(100);
} catch (Exception e) {
e.printStackTrace();
}
System.out.println("tickets" + num--);
}

}

}// /run

public static void main(String[] args) {
// TODO 自动生成的方法存根
ThreadSafeTest t = new ThreadSafeTest();
Thread A = new Thread(t);
Thread B = new Thread(t);
Thread C = new Thread(t);
Thread D = new Thread(t);
A.start();
B.start();
C.start();
D.start();

}

}
//////////////////////////

tickets10
tickets9
tickets8
...........

tickets1
tickets0
tickets-1
tickets-2

if里面的代码块没有锁，多线程时候会有并发问题，比如num=1的时候，刚好线程ABC都已经进入if代码里面了，这时候结果就是负数了

没看懂什么意思？