Question

I have the following thread which simply prints a dot every 200ms:

public class Progress {

    private static boolean threadCanRun = true;
    private static Thread progressThread = new Thread(new Runnable() 
    {
        public void run() {
            while (threadCanRun) {
                System.out.print('.');
                System.out.flush();
                try {
                    progressThread.sleep(200);
                } catch (InterruptedException ex) {}
            }
        }
    });

    public static void stop()
    {
        threadCanRun = false;
        progressThread.interrupt();
    }

    public static void start()
    {
        if (!progressThread.isAlive())
        {
            progressThread.start();
        } else
        {
            threadCanRun = true;
        }
    }

}

I start the thread with this code (for now):

 System.out.println("Working.");
 Progress.start();


 try {
        Thread.sleep(10000); //To be replaced with code that does work.
 } catch (InterruptedException ex) {}

 Progress.stop();

What’s really strange is this:

If I use System.out.println('.'); , the code works exactly as expected. (Apart from the fact that I don’t want a new line each time).

With System.out.print('.');, the code waits for ten seconds, and then shows the output.

System.out.println:

     Print dot, wait 200ms, print dot, wait 200ms etc...

System.out.print:

     Wait 5000ms, Print all dots

What is happening, and what can I do to go around this behaviour?

EDIT:

I have also tried this:

private static synchronized void printDot()
{
    System.err.print('.');
}

and printDot() instead of System.out.print('.');
It still doesn’t work.

EDIT2:

Interesting. This code works as expected:

        System.out.print('.');
        System.out.flush();  //Makes no difference with or without
        System.out.println();

This doesn’t:

        System.err.print('.');
        System.err.flush();
        System.out.print('.');
        System.out.flush();

Solution: The issue was netbeans related. It worked fine when I run it as a jar file from java -jar.

This is one of the most frustrating errors I have seen in my life. When I try to run this code with breakpoints in debug mode, everything works correctly.

Answer 1

The stdout is line buffered.
Use stderr, or flush the PrintStream after each print.