Why can't we read one character at a time from System.in?

Question

The program below prints each character written on standard in, but only after a new-line has been written (at least on my system!).

public class Test {
    public static void main(String[] args) throws java.io.IOException {
        int c;
        while ((c = System.in.read()) != -1)
            System.out.print((char) c);
    }
}

This prevents people from writing stuff like "Press any key to continue" and forces something like "Press enter to continue."

What is the underlying reason for this?
Is it a limitation of Java?
Is this behavior system-dependent (I'm on Ubuntu)? How does it work on Mac? Windows?
Is it dependent on the specific terminal I run the application in? (For me it behaves like this in Eclipse and in gnome-terminal)
Is there a workaround?

see my answer in http://stackoverflow.com/questions/1864076/equivalent-function-to-cs-getch-in-java/10118990#10118990 — Nagaraju Badaeni, Apr 12 '12 at 07:16

score 19 · Accepted Answer · answered Oct 24 '10 at 08:39

What is the underlying reason for this?

Most terminals is line buffered by default, Java does not receive input until a newline.

Is it a limitation of Java?

Some ancient terminals might only have line-buffered input; though it should be possible to disable buffering in most modern terminal.

Is this behavior system-dependent (I'm on Ubuntu)? How does it work on Mac? Windows?

Yes.

Is it dependent on the specific terminal I run the application in? (For me it behaves like this in Eclipse and in gnome-terminal)

Yes.

Is there a workaround?

There are platform specific hacks. curse in Linux and Unix-like platforms, and getch() in Windows. I'm not aware of any cross-platform way.

related: Why "Press any key to continue" is bad idea:

alt text

score 6 · Answer 2 · edited Jun 20 '20 at 09:12

6

I'm on Ubuntu

Is there a workaround?

Runtime.getRuntime().exec("stty -icanon min 1").waitFor();

And after that all reads of System.in in the same process will read 1 character not waiting for EOL.

edited Jun 20 '20 at 09:12

Community

1
1

answered Jan 18 '15 at 08:36

hoat4

1,053
12
9

This doesn't seem to work unless you use the following: `exec(new String[]{"/bin/sh","-c","stty -icanon min 1 – Kelvin May 08 '17 at 20:51

score 0 · Answer 3 · edited May 23 '17 at 11:54

see my answer in Equivalent function to C's "_getch()" in Java?


public static void getCh() {

    final JFrame frame = new JFrame();

    synchronized (frame) {

        frame.setUndecorated(true);

        frame.getRootPane().setWindowDecorationStyle(JRootPane.FRAME);

        frame.addKeyListener(new KeyListener() {

            public void keyPressed(KeyEvent e) {

                synchronized (frame) {

                    frame.setVisible(false);

                    frame.dispose();

                    frame.notify();

                }

            }  



        public void keyReleased(KeyEvent e) {  
        }  

        public void keyTyped(KeyEvent e) {  
        }  
    });  
    frame.setVisible(true);  
    try {  
        frame.wait();  
    } catch (InterruptedException e1) {  
    }  
}

}

Why can't we read one character at a time from System.in?

3 Answers3

Linked