join（）方法如何在多线程中工作

Question

According to my understanding of join() method, it allows which allows one thread to wait until another thread completes.

Then, according to my code: as soon as thread (t0) ends the thread (t3) should start, which doesn't happen

public class Threaded extends Thread {

     @Override
     public void run() {

      for (int i=0; i<5; i++) {

           System.out.println(Thread.currentThread().getName() + ": " + i);

       }

       }

}

public class Demo {

    public static void main(String[] args) throws InterruptedException {

        Thread main = Thread.currentThread();

        Threaded t0 = new Threaded();
        Threaded t1 = new Threaded();
        Threaded t2 = new Threaded();
        Threaded t3= new Threaded();

        t0.start();
        t1.start();
        t2.start();

        t0.join();

        t3.start();

    }

}

The output is

main 5  
Thread-1: 0  
Thread-2: 0  
Thread-0: 0  
Thread-2: 1  
Thread-2: 2  
Thread-1: 1  
Thread-2: 3  
Thread-2: 4  
Thread-0: 1  
Thread-0: 2  
Thread-0: 3  
Thread-0: 4  
Thread-1: 2  
Thread-1: 3  
Thread-1: 4  
Thread-3: 0  
Thread-3: 1  
Thread-3: 2  
Thread-3: 3  
Thread-3: 4

In this output the Thread-3 starts after Thread-0, Thread-1 and Thread-2 ends. But, according to me Thread-3 starts as soon as the Thread-0 ends.

Answer 1

Thread-3 starts as soon as the Thread-0 ends.

Thread-3 becomes runnable after Thread-0 finished. But this does not mean it is scheduled immediatly. There might be 3 runnable threads at the same time and it's unpredictable which thread print first.

Answer 2

Thread.join() waits for this thread to die. Basically all Threads runs on top of main thread. So let me try to explain the flow on your code.

public static void main(String[] args) throws InterruptedException {

    Thread main = Thread.currentThread();

    // you have created 4 Thread instances here...
    Threaded t0 = new Threaded();
    Threaded t1 = new Threaded();
    Threaded t2 = new Threaded();
    Threaded t3= new Threaded();

    // you have started t0, t1 and t2 to run on top of main thread
    t0.start();
    t1.start();
    t2.start();

    // here you have used Thread.join()
    // so your main thread will wait here, 
    // it will wait for the completion of t0 
    t0.join();

    // so after the completion of t0, t3 will start
    t3.start();

}

So now for your question, your threads t1, t2 and t3 all are in runnable Thread pool. So, it is entirely in the hand of Thread scheduler to pick which thread at the moment and execute.