Thread Run() 后实例变量为空 - Java 中的扑克骰子程序

Question

I am creating a poker dice game and I am having trouble obtaining the value of a List that is an instance variable.

    public class ThrowDice { 
          List<String> result = new CopyOnWriteArrayList<>();

The value of this List is changed concurrently in a Thread run() method whose function is to set the five dice "rolling" at the same time. I am adding the resulting value (that is, the value of the die when it stops rolling) in the instance List with add(), however when I attempt to retrieve the value of the List later on, (for example, at the end of the rollDice method itself), the array is empty.

The code:

void rollDice(JLabel k, JLabel q, JLabel j, JLabel n, JLabel t, JLabel a) throws InterruptedException {

    new Thread() {

        @Override
        public void run() {
            String res = "";
            try {
                int times = 8;

                for (int i = 0; i <= times; i++) {

                    int random = (int) (Math.random() * 6) + 1;

                    switch (random) {
                        case 1:
                            k.setVisible(true);

                            res = "k";
                            break;
                        case 2:
                            q.setVisible(true);

                            res = "q";
                            break;
                        case 3:
                            j.setVisible(true);

                            res = "j";
                            break;
                        case 4:
                            t.setVisible(true);

                            res = "t";
                            break;
                        case 5:
                            n.setVisible(true);

                            res = "n";
                            break;
                        case 6:
                            a.setVisible(true);

                            res = "a";
                            break;
                    }

                    Thread.sleep(300);
                    if (i == times) {
                        result.add(res);
                    System.out.println("The result is " + result);// **this works, List has values**
                    } else {
                        setToFalse(k, q, j, t, a, n);

                    }

                } // end for

            } catch (InterruptedException e) {
            }

        }  //end run           

    }.start(); //end thread
 System.out.println("The result is " + result);// **--------------- List is empty (why?)**

}//end rolldice

}

It is as if the values of the List get deleted after Run() ends, and I need to be able to retrieve the ArrayList value so that it can be passed to other methods.

Answer 1

The second println() call almost certainly happens while the rollDice() function still is sleeping in its first call to sleep(300) . (Ie, before anything has been added to the list.)

Johannes Kuhn suggested that you "Try to .join the thread." What he meant was:

Thread t = new Thread() {

    @Override
    public void run() {
        ...
    }  //end run           
};

t.start();
t.join();    // This _waits_ until the thread has finished its work.
System.out.println(...);

---

But there's a problem with that suggestion: That is, it never makes any sense to .join() a thread in the very next statement after .start()ing it. The whole point of threads is that they can work concurrently with each other.

This makes sense:

t.start();
DoSomethingElseConcurrentlyWithThread_t(...);
t.join();