Question

I’m trying to make a cross platform console progress indicator in Java. Therefore I use the System.out.printf method to print out a percentage:

System.out.printf("\t%2.2f%%\b\b\b\b\b\b", percentage);

and I place this in a for loop. The problem I encounter is that it’s not printing anything until the whole for loop is done. This is a program example to show the problem:

public class Test {

    public static void main(String[] args) {
        for(int i =0; i<5000; i++){
            System.out.printf("\b\b\b\b\b\b%2.2f%%", ((float) i/5000f)*100f);
            System.out.flush();
        }
    }
}

I think the problem has something to do with compiler optimisation, but I’m not shure. The strange thing is that System.out.println does print when the for loop is running.

Edit: I forgot to add it to the problem. But I had allready tried to flush the buffer. This makes no difference. Adding %n to the end of my printf line works but it starts a newline, I really need it to reuse the current line.

All opposed solutions work. But they only work in real consoles. Not the netbeans or eclipse console.

Answer 1

That’s because the output stream is line buffered. If you add a “%n” at the end of your format string you also generate a line break and the line will be flushed (i.e. printed). Alternatively call System.out.flush() to manually flush the output stream and force buffered contents to be printed.