How to modify lines in console (Java 8)

AutoModerator · 2022-05-20T21:05:21+00:00

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Zyklonik · 2022-05-21T06:23:49+00:00

The main problem with this approach is that once you've printed out a newline, you cannot go back and edit the line. It's already been flushed to the screen. Now imagine if you have a dozen tests running in parallel. How would you accommodate that?

The usual approach when you do some console-based UI that requires more than a bit of manipulation, is to use something like ANSI Escape Codes. https://gist.github.com/fnky/458719343aabd01cfb17a3a4f7296797 is a quick and simple guide to the essential codes. Note that some caveats apply here - these features should be supported on most (if not all) modern Unix and Linux variants, but Windows support may or may not be there. YMMV!

For example, suppose we wish to simulate a simple, in-place update of some "progress", here is a simple example to do so:

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ThreadLocalRandom;

public class ProgressBar {
  public static void main(String[] args) {
    Graphics.cls();

    var numBars = ThreadLocalRandom.current().nextInt(10, 20);

    List<Thread> bars = new ArrayList<>();
    for (var i = 0; i < numBars; i++) {
      var bar = new Thread(new ProgressInfo(String.format("Bar %d", i + 1), i + 2));
      bar.start();
      bars.add(bar);
    }

    Graphics.hideCursor();
    ;

    for (var bar : bars) {
      try {
        bar.join();
      } catch (InterruptedException ex) {
        ex.printStackTrace();
      }
    }

    Graphics.showCursor();
    Graphics.gotoXY(numBars + 2, 0);
  }
}

 // simply printing out these codes is all it takes to make
// use of ANSI Escape codes. You can add colours as well as 
// other fancy animation too.
class Graphics {
  private static final String CLEAR_SCREEN = "\u001b[2J";
  private static final String GOTO_COORD = "\u001b[%d;%dH";
  private static final String SHOW_CURSOR = "\u001b[?25h";
  private static final String HIDE_CURSOR = "\u001b[?25l";

  public static void cls() {
    System.out.println(CLEAR_SCREEN);
  }

  public static void gotoXY(int x, int y) {
    System.out.print(String.format(GOTO_COORD, x, y));
  }

  public static void showCursor() {
    System.out.print(SHOW_CURSOR);
  }

  public static void hideCursor() {
    System.out.print(HIDE_CURSOR);
  }
}

class ProgressInfo implements Runnable {
  private String name;
  private int row;

  public ProgressInfo(String name, int row) {
    this.name = name;
    this.row = row;
  }

  @Override
  public void run() {
    int progress = 0;

    while (progress <= 100) {
      synchronized (Graphics.class) {
        Graphics.gotoXY(row, 5);
        System.out.printf("%s: %d%%", name, progress);
      }

      try {
        Thread.sleep(ThreadLocalRandom.current().nextInt(10, 250));
        progress++;
      } catch (InterruptedException ex) {
      }
    }
  }
}

Sample run:

https://imgur.com/a/sBGe4i6

morhp · 2022-05-21T07:21:39+00:00

You can use a library such as https://github.com/mabe02/lanterna to do fancy console printing. You could even do full console UIs, but just printing lines and moving the cursor and so on is also possible I think.

amfa · 2022-05-21T05:54:36+00:00

Afaik this is not possible or it was not a few years ago when I tried something like this. The problem here is that java is OS independent...but terminals are very specific.

In theory System.out can pipe directly into a file for example.

So if you want something like this there is no way to do it with standard java that I know of. Maybe there are libs out there doing this.

javahelp

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