Question

Please help me understand this program’s execution and what concepts apply here in the larger sense? An illustration which explains thread(s)/stack(s) creations and destruction would be helpful.

class Joining {

    static Thread createThread(final int i, final Thread t1) {
        Thread t2 = new Thread() {
            public void run() {
                System.out.println(i+1);
                try {
                    t1.join(); 
                } catch (InterruptedException ie) {
                }
                System.out.println(i+2);
            }
        };
        System.out.println(i+3);
        t2.start(); //1
        System.out.println(i+4);
        return t2;
    }
    public static void main(String[] args) {
        createThread(10, createThread(20, Thread.currentThread()));
    }
}

Answer 1

1. The inner createThread call is called at the bottom from the main thread [M], with the arguments 20 and the current thread.
2. This call prints 23.
3. A new thread [A] is started (and will be returned) which prints 21, and waits for the main thread [M] to die (and will print 22 after it does).
4. This call prints 24. It’s impossible to know if this will happen before the new thread [A] prints 21.
5. This call returns the new thread [A], which is waiting for the main thread [M] to die.
6. The new thread [A] is passed as the second argument to the createThread call with 10 as the first argument.
7. This call prints 13.
8. Another new thread [B] is started (and will be returned although nobody is catching this return) which prints 11, and waits for the first created thread [A] to die (and will print 12 after it does).
9. This call prints 14. It’s impossible to know if this will happen before this second new thread [B] prints 11.
10. This call returns the second new thread [B], which is waiting for the first created thread [A] to die, but nothing is being done with this return call.
11. The main thread [M] runs out of things to do and dies.
12. When the main thread [M] dies, the join() call on the first created thread [A] returns.
13. The first created thread [A] prints 22.
14. The first created thread [A] dies.
15. When the first created thread [A] dies, the join() call on the second created thread [B] returns.
16. The second created thread [B] prints 12.
17. The second created thread [B] dies.
18. The JVM shuts down, because all threads have died.