CODEWITHSUNDEEP
javaprintingconsole
Daan Pape
Daan Pape

Reputation: 1140

Java printf not printing

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.

Upvotes: 5

Views: 5974

Answers (4)

Sean Reilly
Sean Reilly

Reputation: 21836

Try using Console instead:

for(int i =0; i<5000; i++){
    System.console().format("\b\b\b\b\b\b%2.2f%%", ((float) i/5000f)*100f);
    System.console().flush();
}

Upvotes: 0

Boris Strandjev
Boris Strandjev

Reputation: 46963

And once more the problem is with flushing the stream. Add this line after your printf:

System.out.flush();

System.out.println is flushing (much like C++'s << endl). However, printf is not flushing and is using buffer.

Upvotes: 2

NPE
NPE

Reputation: 500893

Add a call to flush(): 

    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();
    }

Without the flush(), the output gets accumulated in a buffer that only gets flushed once in a while (whenever it's full, or whenever a newline is printed).

The strange thing is that System.out.println does print when the for loop is running.

The reason for that is that the stream is line-buffered. This means that every newline triggers an implicit flush. The difference between your code and println() is that the latter prints out a newline every time it's called.

Upvotes: 1

user268396
user268396

Reputation: 12006

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.

Upvotes: 7

Related Questions