How to delete previous character printed to console/terminal?

Question

Is there a way to dynamically change output in Java? For instance, in a terminal window if I have:

System.out.print("H")

and then I have:

System.out.print("I")

The output will be:

HI

Is there a way to assign a position to outputs that allows you to replace characters dynamically? For instance (and I know this would not output what I want, I merely want to demonstrate my thinking) this:

 System.out.print("H")
 Thread.sleep("1")
 System.out.print("I")

And it would first print out

and then after a second, replace the H with an I?

I'm sure this sounds stupid, I am just interested in dynamically changing content without GUIs. Can someone point me in the direction for this technique? Thank you very much in advance.

Well, in console you can't do that. You cannot omit the output that already has been displayed, without re-executing your code. — Rohit Jain, Oct 30 '12 at 19:55
Possible duplicate: http://stackoverflow.com/questions/7522022/how-to-delete-stuff-printed-to-console-by-system-out-println — dan, Oct 30 '12 at 20:13

Oliver Atkinson · Accepted Answer · 2012-10-30T20:30:59.743

2

You might want to take a look at

System.out.printf

Look at the example shown here: http://masterex.github.com/archive/2011/10/23/java-cli-progress-bar.html

edit:

printf displays formatted strings, which means you can adapt that format and change it for your needs.

for example you could do something like:

String[] planets = {"Mars", "Earth", "Jupiter"};        
String format = "\r%s says Hello";

for(String planet : planets) {
    System.out.printf(format, planet);
try { 
        Thread.sleep(1000); 
    }catch(Exception e) { 
        //... oh dear
    }
}

Using the formatted string syntax found here: http://docs.oracle.com/javase/6/docs/api/java/util/Formatter.html#syntax

As the comment says this solution is only limited to a singular line however dependent on your needs this might be enough.

If you require a solution for the whole screen then a possible solution would be (although quite dirty) would be to hook the operating system using JNA and get a handle on the console window, find its height and then loop println() to "clear" the window then redraw your output.

If you would like to read more then I can answer more questions or here is a link: https://github.com/twall/jna

edited Oct 30 '12 at 20:30

answered Oct 30 '12 at 20:02

Oliver Atkinson

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A limitation of that example is that it only show how to redraw the current line. – millimoose Oct 30 '12 at 20:06
I updated the post with some more content regarding printf and an alternate solution – Oliver Atkinson Oct 30 '12 at 20:22
Another thing that comes to mind: I believe `\r` merely moves the cursor to the start of the line. Meaning that it wouldn't delete everything you've written before – you're overwriting the previous characters instead. This means it's somewhat tricky to replace a line with a shorter line, you have to overwrite the rest with spaces. – millimoose Oct 30 '12 at 20:53
(I just went and verified the above, that's indeed what happens.) – millimoose Oct 30 '12 at 21:00
well \r stands for "The carriage-return character" and means it will return to the beginning of the line, if you wanted to clear the line for a shorter line you could use padding System.out.println(String.format("[%-20s]", "Earth")); will give you: [Earth ] (15 spaces after earth) – Oliver Atkinson Oct 30 '12 at 21:12
Oh, I'm not saying it's a problem with your answer, just that it's a nonobvious caveat that wouldn't show up with either the progressbar example or your sample code. (The text written gets progressively longer in both.) – millimoose Oct 30 '12 at 21:16

score 2 · Answer 2 · answered Oct 30 '12 at 20:07

You can use \b to backspace and erase the previous character.

$ cat T.java
import java.lang.Thread;

public class T {
    public static void main(String... args) throws Exception {
        System.out.print("H");
        System.out.flush();
        Thread.sleep(1000);
        System.out.print("\bI\n");
        System.out.flush();
    }
}
$ javac T.java && java T
I

It will output H, then replace it with I after one second.

Sadly, it doesn't work in Eclipse console, but in normal console it does.

score 2 · Answer 3 · answered Oct 30 '12 at 20:11

2

This is what you need (uses carriage return '\r' to overwrite the previous output):

  System.out.print("H");
  Thread.sleep(1000);
  System.out.print("\rI");

answered Oct 30 '12 at 20:11

Andrey Mormysh

809
5
12

score 1 · Answer 4 · edited Jun 02 '17 at 19:56

1

The C library that is usually used to do this sort of thing is called curses. (Also used from scripting languages that rely on bindings to C libraries, like Python.) You can use a Java binding to it, like JCurses. Google also tells me a pure-Java equivalent is available, called lanterna.

edited Jun 02 '17 at 19:56

try-catch-finally

7,436
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answered Oct 30 '12 at 20:02

millimoose

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How to delete previous character printed to console/terminal?

4 Answers4

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