public class Test3 {
    public static void main(String[] args) {
        computer3 t = new computer3();
        new Thread(t).start();
        new Thread(t).start();
        new Thread(t).start();
    }
}

class computer3 extends Thread {
    int i = 10;
    static Object obj = new Object();

    public void print() {
        System.out.println(Thread.currentThread().getName() + ":" + i);
        i--;
    }

    public void run() {
        while (i > 0) {
            synchronized (obj) {
                print();
            }
            try {
                sleep(100);
            } catch (Exception e) {
            }
        }
    }
}

  while (i > 0) {
            synchronized (obj) {
    if(i>0){ // 加上这个if吧，后面别忘了加上}