Explain the result of concurrent threads

[hqt]

I have this piece of code and I print answer to see. But I don't know why the answer like that:

public class ThreadExample implements Runnable{
    
    public Thread thread;
    static int i = 0;
    ThreadExample(){
        thread = new Thread(this);
        thread.start();
    }

    public static void main(String[] args) {
        ThreadExample example = new ThreadExample();
        for(int n =0; n<1000; n++){
            System.out.println("main thread "+i);
            i++;
        }
    }

    public void run(){
        for(int index=0; index<1000; index++){
            System.out.println("Sub thread "+i);
            i++;
        }
    }
}

for some results is:

main thread 0
Sub thread 0
Sub thread 2
Sub thread 3
Sub thread 4
Sub thread 5
Sub thread 6
Sub thread 7
Sub thread 8
Sub thread 9
Sub thread 10
Sub thread 11
Sub thread 12
main thread 1
...

The thing I don't understand is: when sub thread take i goto 12. why main thread just print 1. (because they share variable i together)

I don't know exactly what thread does together. Please explain for me this point.

thanks for all :)

[Tolls]

What did you expect to see?

[hqt]

I think at line 14 of result, I think this line should be: main thread:12. because i has been changed to 12 at sub-Thread.

[Tolls]

You're thinking of the lines of code as atomic.
They aren't.

The call to println() in the main thread was clearly interrupted at some point inside the method, so that the other thread could run (12 times).
Then the actual print() took place, with the text "main thread 1" already in place.