Why does placing a lock on an object of the main thread's instance work?

Question

I have a few questions on multithreading and waiting in Java on the following code.

/*
 * Taken from
 * https://www.wiley.com/en-us/Java+Programming%3A+24+Hour+Trainer%2C+2nd+Edition-p-9781118951453
 */
public class TestThreads3LambdaWait {

    public static void main(String args[]) {
        
        // Lambda expression for Market News
        Runnable mn = () -> {
            try {
                for (int i = 0; i < 10; i++) {
                    Thread.sleep (1000);  // sleep for 1 second
                    System.out.println( "The market is improving " + i);
                }
            } catch(InterruptedException e ) {
                System.out.println(Thread.currentThread().getName() 
                                                + e.toString());
            }  
        }; 

        Thread marketNews = new Thread(mn, "Market News");
        marketNews.start();
         
        // Lambda expression for Portfolio
        Runnable port = () -> {
            try {
                for (int i = 0; i < 10; i++) {
                    Thread.sleep (700);    // Sleep for 700 milliseconds 
                    System.out.println( "You have " +  (500 + i) +  
                                              " shares of IBM");
                }
            } catch(InterruptedException e ) {
                System.out.println(Thread.currentThread().getName() 
                                                + e.toString());
            }   
        };
         
        Thread portfolio = new Thread(port,"Portfolio data");
        portfolio.start();
         
        TestThreads3LambdaWait thisInstance = new TestThreads3LambdaWait();
         
        synchronized (thisInstance) {
            try {
                thisInstance.wait(15000);
                System.out.println("finished wait");
            } catch (InterruptedException e) { 
                e.printStackTrace();
            }
        }
        System.out.println( "The main method of TestThreads3Lambda is finished");
    }
}

1. Do mn and port call notify() when they finish executing?
2. Who is the monitor that gets notified? (Since main() is static, the code in main() is not tied to a specific object.)
3. Why does placing a lock on an object of the main thread's instance work (ie, sending the notification to thisInstance , which is not tied to main() since main() is static). Is it because all TestThreads3LambdaWait objects are notified because mn and port are in a static method and thus tied to every instance?

Answer 1

1. No.
2. Irrelevant. Nothing is being notified in this code - nobody calls notify(All) anywhere inside it.
3. Because Thread.sleep(15000) would also do it, which is effectively what your wait call does, given that nobody notifies.