Exception in thread "main" java.lang.IllegalThreadStateException错误

Exception in thread "main" java.lang.IllegalThreadStateException

The same Thread cannot call the start method repeatedly.
Once a thread is started, it can never be restarted. Only a new thread can be started, and only once. A running thread or dead thread can be restarted.

Case number one:

public static void main(String[] args) {
        Test test= new Test();
        test.start();
        test.start();

Case 2:

public static void main(String[] args) {
        Test test= new Test();
        test.start();
        try {
            Thread.sleep(100);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        test.start();
    }

The above two cases will be reported incorrectly: the Exception in thread "main" java.lang.IllegalThreadStateException
following is a correct case:

 public static void main(String[] args) {
        //声明一个Thread类型的List集合
        List<Thread> thread = new ArrayList<>();
        Test test = new Test();
        for (int i = 0; i < 5; i++) {
         //每次new一个新的线程Thread，将对象test放进去  此处有5个线程
            thread.add(new Thread(test,"thread"+i));
        }
        for (int i = 0; i < 5; i++) {
            thread.get(i).start();
        }
    }