Question

in main if the following code is used

Timer timer = new Timer(200, new ActionListener() {
    @Override
    public void actionPerformed(ActionEvent e) {
        System.out.println("boo");
    }
});

timer.start();
Thread.sleep(3000);

boo will be printed every 200 milliseconds as expected.

While

Timer timer = new Timer(200, new ActionListener() {
    @Override
    public void actionPerformed(ActionEvent e) {
        System.out.println("boo");
    }
});

timer.start();

nothing will be output!

Answer 1

Presumably the code you’re showing is in main(). When main() returns, the program exits before the timer thread has a chance to get going. The sleep gives the JVM enough time to create the other thread, which then allows the JVM to keep running.