我试图创建一个记录所有按键按下的链表，但`keyPressed()`和`keyReleased()`方法没有被调用。我已经尝试过`frame.setFocusable(true)`。
Window.java
```java
public class Window {
    protected JFrame frame;
    private boolean clicked = false; // TODO: 鼠标输入
    private LinkedList<Character> keysDown = new LinkedList<>();
    public Window(String title, Dimension resolution, Canvas display) {
        frame = new JFrame(title);
        Dimension decoratedResolution = new Dimension(resolution.width + 16, resolution.height + 38); // 窗口的分辨率不匹配，这样可以确保resolution.height实际上是屏幕底部而不是稍微下面，resolution.width同理。
        frame.setPreferredSize(decoratedResolution);
        frame.setMinimumSize(decoratedResolution);
        frame.setMaximumSize(decoratedResolution);
        frame.setResizable(false);
        frame.setLocationRelativeTo(null); // 居中窗口
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setFocusable(true);
        frame.addKeyListener(new KeyListener() {
            @Override
            public void keyTyped(KeyEvent e) {}
            @Override
            public void keyPressed(KeyEvent e) {
                char character = e.getKeyChar();
                if (!keysDown.contains(character))
                    keysDown.add(character);
            }
            @Override
            public void keyReleased(KeyEvent e) {
                char character = e.getKeyChar();
                keysDown.remove((Object) character);
            }
        });
        frame.add(display);
        frame.setVisible(true);
    }
    public boolean isMouseDown() {
        return clicked;
    }
    public Vector2 getMousePosition() {
        Point point = MouseInfo.getPointerInfo().getLocation();
        SwingUtilities.convertPointFromScreen(point, frame);
        return new Vector2(point);
    }
    public LinkedList<Character> getKeysDown() {
        return keysDown;
    }
}

public class Game extends Canvas implements Runnable {
    // 通用设置
    public static final String TITLE = "游戏";
    public static final Dimension RESOLUTION = new Dimension(800, 450);
    public static final int FPS = 60;
    // 请不要修改
    protected static final long NANO_SECOND = 1000000000;
    public float timeScale = 1f;
    public volatile boolean running = false; // 设置为false以停止游戏
    public Thread thread;
    public Window window;
    public Renderer renderer;
    public static void main(String[] args) {
        new Game();
    }
    public Game() {
        start();
    }
    public void start() {
        init();
    }
    public void init() {
        window = new Window(TITLE, RESOLUTION, this);
    }
    public void run() {
        final float RENDER_INTERVAL = 1f / FPS;
        long then = System.nanoTime();
        long now = then;
        float deltaTime = 0f;
        int frames = 0;
        while (running) {
            now = System.nanoTime();
            deltaTime = (float)(now - then) / NANO_SECOND;
            update(deltaTime * timeScale);
            if (!running)
                break;
            then = now;
            Thread.onSpinWait();
        }
    }
    public void update(float deltaTime) {
        if (window.getKeysDown().contains('d'))
            System.out.println("按下了 'd' 键");
    }
}