How do I check if the user is pressing a key?

Question

In java I have a program that needs to check continuously if a user is pressing a key. So In psuedocode, somthing like

if (isPressing("w")) {
   // do somthing
}

Bend

In java, but using a particular GUI framework (swing?)? – Qnp

Answer 1

In java you don't check if a key is pressed, instead you listen to KeyEvents. The right way to achieve your goal is to register a KeyEventDispatcher, and implement it to maintain the state of the desired key:

import java.awt.KeyEventDispatcher;
import java.awt.KeyboardFocusManager;
import java.awt.event.KeyEvent;

public class IsKeyPressed {
    private static volatile boolean wPressed = false;
    public static boolean isWPressed() {
        synchronized (IsKeyPressed.class) {
            return wPressed;
        }
    }

    public static void main(String[] args) {
        KeyboardFocusManager.getCurrentKeyboardFocusManager().addKeyEventDispatcher(new KeyEventDispatcher() {

            @Override
            public boolean dispatchKeyEvent(KeyEvent ke) {
                synchronized (IsKeyPressed.class) {
                    switch (ke.getID()) {
                    case KeyEvent.KEY_PRESSED:
                        if (ke.getKeyCode() == KeyEvent.VK_W) {
                            wPressed = true;
                        }
                        break;

                    case KeyEvent.KEY_RELEASED:
                        if (ke.getKeyCode() == KeyEvent.VK_W) {
                            wPressed = false;
                        }
                        break;
                    }
                    return false;
                }
            }
        });
    }
}

Then you can always use:

if (IsKeyPressed.isWPressed()) {
    // do your thing.
}

You can, of course, use same method to implement isPressing("<some key>") with a map of keys and their state wrapped inside IsKeyPressed.

Celeriac

Sorry, I updated my code without checking it. See my updated answer - it uses the class object instead of the instance. Try it again (just replace both IsKeyPressed.this with IsKeyPressed.class). The synchronized block, by the way, isn't truly necessary in that case. – Celeriac

Im testing this, and it doesn't seem to be working :{. I have a timer loop that tests to see if (IsKeyPressed.isWPressed()) but it never returns true even when i press or hold the w key. – Bend

@Saucymeatman, did you get the problem solved? If you are busy waiting for isWPressed(), you might have caching issues. It is possible that a volatile boolean can solve this. – Celeriac

100% perfect. Hint: If you want to test for a lot of keys, this solution can easily be expanded to use a HashMap<Integer, Boolean>. Integer for the KeyCode, Boolean for obvious reasons. – Kythera

@Kythera I would use a Set<Integer> instead, so when a key is pressed you add it to the set and when they key is released you remove it. The Set<> would be much smaller than a HashMap, because it has to have the capacity of the maximum number of keys that can be pressed at the same time (suppose it's 10-20) instead of all the keys that can be pressed in a keyboard (dozens). – Sizing

Answer 2

Try this:

import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;
import javax.swing.JFrame;
import javax.swing.JTextField;

public class Main {

    public static void main(String[] argv) throws Exception {

    JTextField textField = new JTextField();

    textField.addKeyListener(new Keychecker());

    JFrame jframe = new JFrame();

    jframe.add(textField);

    jframe.setSize(400, 350);

    jframe.setVisible(true);

}

class Keychecker extends KeyAdapter {

    @Override
    public void keyPressed(KeyEvent event) {

        char ch = event.getKeyChar();

        System.out.println(event.getKeyChar());

    }

}

Edict

You forgot a '}'. – Nordrheinwestfalen

Answer 3

Universal method

I've built a convenience utility class based on @Elist's approach, which works with any key.

import java.awt.*;
import java.awt.event.KeyEvent;
import java.util.HashMap;
import java.util.Map;

public class Keyboard {

    private static final Map<Integer, Boolean> pressedKeys = new HashMap<>();

    static {
        KeyboardFocusManager.getCurrentKeyboardFocusManager().addKeyEventDispatcher(event -> {
            synchronized (Keyboard.class) {
                if (event.getID() == KeyEvent.KEY_PRESSED) pressedKeys.put(event.getKeyCode(), true);
                else if (event.getID() == KeyEvent.KEY_RELEASED) pressedKeys.put(event.getKeyCode(), false);
                return false;
            }
        });
    }

    public static boolean isKeyPressed(int keyCode) { // Any key code from the KeyEvent class
        return pressedKeys.getOrDefault(keyCode, false);
    }
}

Example usage:

do {
    if (Keyboard.isKeyPressed(KeyEvent.VK_W)) System.out.println("W is pressed!");
} while (!Keyboard.isKeyPressed(KeyEvent.VK_ESCAPE));

Arrowroot

Answer 4

You have to implement KeyListener,take a look here: http://docs.oracle.com/javase/7/docs/api/java/awt/event/KeyListener.html

More details on how to use it: http://docs.oracle.com/javase/tutorial/uiswing/events/keylistener.html

Peden