Java Graphics 2D和KeyListener

Question

I am teaching myself 2D graphics in Java right now. I am trying to create a block-stacking game. So far, I am able to get the first row moving and stopping, but my aim is to generate a new row below the first while the first row stays in place. I feel like the fix for this is a simple concept I haven't learned yet. However, any advice will be appreciated. If you want to point me in the right direction to where I can teach myself 2D Graphics in general, I will appreciate that also.

My code for the JFrame is as follows:

public class  BlockStacker extends JFrame {
public class AL extends KeyAdapter {
    stack2 s2 = new stack2();
    public void keyPressed(KeyEvent e){
        int keyCode = e.getKeyCode();
    }

}
public static void main(String[] args){
    stack2 s = new stack2();
    JFrame f = new JFrame();
    f.add(s);
    f.setVisible(true);
    f.setSize(1920, 1080);
    f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    f.setTitle("Block Stacker");


}

}

The code with the graphics and the keylistener is this:

public class stack2 extends JPanel implements ActionListener, KeyListener{
    Timer t = new Timer(250, this);
    double x, y, velX = 253;


    public stack2() {
        t.start();
        addKeyListener(this);
        setFocusable(true);
        setFocusTraversalKeysEnabled(false);

    }

    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D) g;

        setBackground(Color.BLACK);
        Color blue = new Color(0,0,255);
        g2.setColor(blue);

        Rectangle2D.Double rectangle = new Rectangle.Double(x + 210, y, 200, 100);
        g2.fill(rectangle);

        Rectangle2D.Double rectangle2 = new Rectangle.Double(x, y, 200, 100);
        g2.fill(rectangle2);

        Rectangle2D.Double rectangle3 = new Rectangle.Double(x + 420, y, 200, 100);
        g2.fill(rectangle3);

    }

    public void actionPerformed(ActionEvent e) {
        if (x < 0) {
            velX = -velX;
        }

        x += velX;
        repaint();

        if (x < 10 || x > 1200) {
          velX = -velX;
          repaint();
        }
    }

    public void space() {
        velX = 0;
        repaint();

    }
public void space2() {


    }


public void keyPressed(KeyEvent e) {
  int code = e.getKeyCode();
     if (code == KeyEvent.VK_SPACE) {
       space();
            }
 }     

public void keyTyped(KeyEvent e) {}
public void keyReleased(KeyEvent e) {
int code = e.getKeyCode();
    if (code == KeyEvent.VK_SPACE) {
        space2();
    }
}

}

Answer 1

There are multiple problems...

1. You attach the KeyListener to JFrame, but a JFrame has a JRootPane, content pane and it's content between it and the user, any of those components could have keyboard focus, preventing the frame from receiving key events
2. You create a instance of Stack in both the KeyAdapter and then the frame, so the instance of Stack you are trying to change isn't the version that's on the screen
3. KeyListener only responds to key events when the component that they are registed to are focus able and have focus...

Use key bindings instead? See How to use key bindings