Java create background thread which does something periodically

Question

Is it possible to create a separate background thread which would separately do some stuff? I've tried the following program but it doesn't work as I expect.

public class Test {

    private static class UpdaterThread extends Thread {
        private final int TIMEOUT = 3000;

        public void run() {
            while (true) {
                try {
                    Thread.sleep(TIMEOUT);
                    System.out.println("3 seconds passed");
                } catch (InterruptedException ex) {
                }
            }
        }
    }

    /**
     * @param args
     *            the command line arguments
     */
    public static void main(String[] args) {
        try {
            Thread u = new UpdaterThread();
            u.start();
            while (true) {
                System.out.println("--");
            }
        } catch (Exception ex) {
            ex.printStackTrace();
        }
    }
}

I expected that every 3 seconds "3 seconds passed" will be printed in the flow of multiple "--" strings. In fact "3 seconds passed" is never printed. Why? And how can I create a background thread which would do something independantly from the main thread?

Answer 1

There are lot of answers but nobody says why his example was not working. System.out is output stream, so after you have started write to this stream JAVA locks it and all other threads will wait while lock is applied to stream. After the stream will have unlocked another thread will be able to work with this stream.

To make your example working you should add Thread.sleep into while loop in the main thread.