通过扩展Thread类来创建线程

Question

This is a simple example about creating a thread by extending the Thread class.

class Count extends Thread {

    Count() {
        super("my extending thread");
        System.out.println("my new thread is started " + this);
        start();
    }

    @Override
    public void run() {
        try {
            for (int i = 0; i < 10; i++) {
                System.out.println("count " + i);
                Thread.sleep(1000);
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            System.out.println("my thread run is over");
        }
    }

}

public class Multi2 {

    public static void main(String[] args) {
        Count c = new Count();
        try {
            while (c.isAlive()) {
                System.out.println("main thread is alive untill child thread is alive");
                Thread.sleep(1500);
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            System.out.println("main thread is over");
        }
    }
}

And my output was this.

my new thread is started Thread[my extending thread,5,main]
main thread is alive untill child thread is alive
count 0
count 1
main thread is alive untill child thread is alive
count 2
main thread is alive untill child thread is alive
count 3
count 4
main thread is alive untill child thread is alive
count 5
main thread is alive untill child thread is alive
count 6
count 7
main thread is alive untill child thread is alive
count 8
main thread is alive untill child thread is alive
count 9
my thread run is over
main thread is over

My questions are,

01. How come main thread is alive untill child thread is alive output printed before
count 0 count 1

02.How come main thread is alive untill child thread is alive output keep printing withing the output of run() method?

Please help me to figure this out.
Thank you.

Answer 1

Count has this line:

Thread.sleep(1000);

Your main program has this line:

Thread.sleep(1500);

Clearly, you're going to get 2 main prints for every 3 count prints. That's why 0 and 1 are next to each other.

As for why you see the main print before the count prints, you can see this:

Count c = new Count();
try {
while (c.isAlive()) {
    System.out.println("main thread is alive untill child thread is alive");
    Thread.sleep(1500);

You've fired off your c but until your JVM performs a context switch to actually run that thread, you might not see results. The truth is you may, on some systems, see your counter before that. Often, because it's so close to when it kicks off and hasn't yielded yet, you'll see the main print before the counter.

For your second part, your main thread is... keeps printing because it has a loop that tells it to print until your counter thread is no longer alive. They both use System.out so when you look at your console you see them both there.