KeyListener how to handle multiple keys pressed?

Question

So I have a key listener in Java that moves a rectangle on a JFrame either up, down, left, or right for which I am using the W, A, S, and D keys. Usually, and especially in games, pressing 2 keys should yeild diagonal movement. An example would be you press W and D at the same time, and the expected behavior is to move diagonally to the right upwards. However, the key listener only detects one key at a time. I know I can do something where I detect multiple keys at a time and then write new code as if it were a new key but is there a way to execute two pieces of code under two different keys at the same time? If not, how can I test multiple keys at a time?

Here is a minimal reproducible that sets up a rectangle that moves with the W, A, S, and D keys.

import javax.swing.*;
import java.awt.event.*;
import java.awt.*;

public class Test extends JPanel implements KeyListener {
    
    private int x = 200, y = 200;

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        new Test().create();
    }
    
    public void paintComponent(Graphics tool) {
        super.paintComponent(tool);
        tool.drawRect(x,  y, 50, 50);
    }
    
    public void create() {
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(500, 500);
        frame.setLocationRelativeTo(null);
        frame.setResizable(false);
        
        frame.add(this);
        frame.addKeyListener(this);
        
        frame.setVisible(true);
    }

    @Override
    public void keyTyped(KeyEvent e) {
        // TODO Auto-generated method stub
        
    }

    @Override
    public void keyPressed(KeyEvent e) {
        // TODO Auto-generated method stub
        switch(e.getKeyCode()) {
        case KeyEvent.VK_W:
            y -= 3;
            break;
        case KeyEvent.VK_S:
            y += 3;
            break;
        case KeyEvent.VK_A:
            x -= 3;
            break;
        case KeyEvent.VK_D:
            x += 3;
            break;
        }
        repaint();
    }

    @Override
    public void keyReleased(KeyEvent e) {
        // TODO Auto-generated method stub
        
    }

}

Answer 1

So after some of @Maarten Bodewes 's advice, I came up with this solution:

import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
import java.util.*;

public class Test extends JPanel implements KeyListener {
    
    private int x = 200, y = 200;
    
    Set<Integer> keys = new HashSet<Integer>();

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        new Test().create();
    }
    
    public void paintComponent(Graphics tool) {
        super.paintComponent(tool);
        tool.drawRect(x,  y, 50, 50);
    }
    
    public void create() {
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(500, 500);
        frame.setLocationRelativeTo(null);
        frame.setResizable(false);
        
        frame.add(this);
        frame.addKeyListener(this);
        
        frame.setVisible(true);
    }

    @Override
    public void keyTyped(KeyEvent e) {
        // TODO Auto-generated method stub
        
    }

    @Override
    public void keyPressed(KeyEvent e) {
        // TODO Auto-generated method stub
        keys.add(e.getKeyCode());
        multiKeys();
        repaint();
    }
    
    public void multiKeys() {
        System.out.println(keys.size());
        for(Integer key: keys) {
            System.out.println(key);
            switch(key) {
            case KeyEvent.VK_W:
                y -= 3;
                break;
            case KeyEvent.VK_S:
                y += 3;
                break;
            case KeyEvent.VK_A:
                x -= 3;
                break;
            case KeyEvent.VK_D:
                x += 3;
                break;
            }
        }
    }


    @Override
    public void keyReleased(KeyEvent e) {
        // TODO Auto-generated method stub
        keys.remove(e.getKeyCode());
        
    }

}

Now when I press say W and D, the rectangle moved diagonally as intended. This is due to the fact that a Set called keys stores the keys pressed, and then executes those keys' codes through a separate method called multiKeys() which loops keys. And of course when a key is no longer pressed, remove it from the keys Set via the KeyListener keyReleased() method.