Detecting and acting on keyboard direction keys in Java

Question

G'day all,

I have a console project where it is intended the user presses the keyboard direction keys (non-numeric keypad) to move an avatar. I am having difficulty coding to check for the press of these keys. In Pascal it was easy enough to use "readkey" and code, for example, for #80 for the down keypress. However, I am stumped how to implement the same functionality in Java, though I think I understand the use of System.in and BufferedInputStream.

Could anyone help me out? Your thoughts or hints are much appreciated.

Answer 1

The Console support issue in Java is well known, I am not sure that this is doable.

This was not initially possible with System.in since it used to work line-based.

Sun eventually added a java.io.Console class.

Here are its JavaDocs: http://java.sun.com/javase/6/docs/api/java/io/Console.html

Once you get the console (I think from System.console), you can get a reader and perhaps read characters from it, but I'm not sure if it includes keys.

Generally, you're supposed to use Swing or AWT if you want access to the keyboard, which is silly.

As of 2007, there was a feature request about it: here

Answer 2

Unfortunately this is not possible in a portable way:

http://forums.sun.com/thread.jspa?threadID=5351637&messageID=10526512

On Windows, reading from System.in will block until enter is pressed, even when you do not use a BufferedReader. Arrows will cycle through the command history. Try it yourself:

import java.io.*;
public class KeyTest {
  public static void main(String[] argv) {
    try {
      InputStreamReader unbuffered = new InputStreamReader(System.in);
      for (int i = 0; i < 10; ++i) {
        int x = unbuffered.read();
        System.out.println(String.format("%08x", x));
      }
    } catch (Exception e) {
      System.err.println(e);
    }
  }
}

Same issue using the Console class (input buffered under Windows, arrow keys intepreted by Windows):

import java.io.*;
public class KeyTest2 {
  public static void main(String[] argv) {
    try {
      Console cons = System.console();
      if (cons != null) {
        Reader unbuffered = cons.reader();
        for (int i = 0; i < 10; ++i ) {
          int x = unbuffered.read();
          System.out.println(String.format("%08x", x));
        }
      }
    } catch (Exception e) {
      System.err.println(e);
    }
  }
}

Answer 3

If java.io.console doesn't work for you (I haven't tried that), try JLine. I used it to solve a vaguely similar problem.

Answer 4

I had this same kind of problem. Finally I decided to switch to Swing. Pity.