使矩形在面板上移动

Question

I am creating a snake game, and I want to start at the very basic that moving a triangle on the screen. But I am have trouble on how to make a rectangle move across on the screen

Here's the code I have so far:

Screen.java

public class Screen extends JPanel implements ActionListener, KeyListener {
    public static final JLabel statusbar = new JLabel("Default");
    public static final int WIDTH = 800, HEIGHT = 800;
    Timer t = new Timer(50, this);
    int x = 400;
    int y = 400;
    int velx = 0;
    int vely = 0;

    private BodyPart b;

    public Screen(){

        b = new BodyPart(x, y);
        t.start();
        addKeyListener(this);
        setFocusable(true);
        setFocusTraversalKeysEnabled(false);
    }

    public static void main(String[] args) {

    }

    @Override
    public void paintComponent(Graphics g){
        super.paintComponent(g);
        g.setColor(new Color(10, 50, 0));
        g.fillRect(0, 0, WIDTH, HEIGHT);

        g.setColor(Color.BLACK);
        for(int i = 0; i < WIDTH / 10; i++) {
            g.drawLine(i * 10, 0, i * 10, HEIGHT);
        }

        for(int i = 0; i < HEIGHT / 10; i++) {
            g.drawLine(0, i * 10, WIDTH, i * 10);
        }

        b.draw(g);
    }

    public void up(){
        vely = -10;
        velx = 0;
    }
    public void down(){
        vely = 10;
        velx = 0;
    }
    public void left(){
        vely = 0;
        velx = -10;
    }
    public void right(){
        vely = 0;
        velx = 10;
    }


    @Override
    public void actionPerformed(ActionEvent e) {
        repaint();
        x += velx;
        y += vely;
    }

    @Override
    public void keyTyped(KeyEvent e) {}

    @Override
    public void keyPressed(KeyEvent e) {
        int key = e.getKeyCode();
        switch(key){
        case KeyEvent.VK_DOWN: down();
            System.out.println("Down");
            break;
        case KeyEvent.VK_UP: up();
            System.out.println("up");
            break;
        case KeyEvent.VK_LEFT: left();
            System.out.println("left");
            break;
        case KeyEvent.VK_RIGHT: right();
            System.out.println("right");
            break;
        }        
    }

    @Override
    public void keyReleased(KeyEvent e) {}
}

BodyPart.java

public class BodyPart {

    int x;
    int y;

    public BodyPart(int x, int y) {
        this.x = x;
        this.y = y;
    }

    public void draw(Graphics g) {
    g.setColor(Color.red);
        g.fillRect(x, y, 10, 10);
        g.setColor(Color.white);
        g.drawRect(x, y, 10, 10);
    }
}

Frame.java

public class Frame extends JPanel {
    private static JLabel statusbar = new JLabel("Default");

    public Frame(){
        statusbar = Screen.statusbar;
    }

    public static void main(String[] args) {
        JFrame f = new JFrame();
        Screen s = new Screen();
        BodyPart b = new BodyPart(400,400);
        f.add(s);
        f.add(statusbar, BorderLayout.SOUTH);
        f.setSize(800, 800);
        f.setVisible(true);
        f.setLocationRelativeTo(null);
        f.setResizable(false);
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }
}

So when I run the code, the rectangle only displays in the center which I set it to, and it doesn't move. Is there anyway to solve it? In addition, the status bar also doesn't move. (There's something weird, when I put everything in a single file, it does work)

Also, I am planning to use linkedlist<Point> , is it possible to mix this program with the linkedlist<> ? or I have to change my code in order to use the linkedlist<> ?

Thank you

Answer 1

I am not too familiar with Java but it seems like you are never calling the BodyPart.draw() method to update your BodyPart . Whenever you want to update the position of your BodyPart b , call b.draw(g) and pass the Graphics g object.

For the status bar, I think it will help you to know that the main(String[] args) method only runs in the main/starting class. It is intended to provide a starting point for your program. So you should rename the method in your Frame class and call it or you could move your code to the constructor of the Frame class and it will run when Frame is initialized

I don't see why you shouldn't be able to use a LinkedList<> .

Answer 2

your problem is the following:

You create the BodyPart object with x = 400 and y = 400 from that moment you only change the values of the variables x and y in class Screen, which is wrong because the Screen.x is different from BodyPart.x, so when you do something like Screen.x += 10 you also need to update the x variable from BodyPart.

A quick solution for your problem could be something like this:

Add 2 parameters to your draw method and pass the graphics object, the x and y updated values.

@Override
public void paintComponent(Graphics g){
    super.paintComponent(g);
    g.setColor(new Color(10, 50, 0));
    g.fillRect(0, 0, WIDTH, HEIGHT);

    g.setColor(Color.BLACK);
    for(int i = 0; i < WIDTH / 10; i++) {
        g.drawLine(i * 10, 0, i * 10, HEIGHT);
    }

    for(int i = 0; i < HEIGHT / 10; i++) {
        g.drawLine(0, i * 10, WIDTH, i * 10);
    }

    b.draw(g, x, y);
}

Receive the x and y updated values and update the BodyPart x and y variables.

  public void draw(Graphics g, int x, int y) {
    this.x = x;
    this.y = y;

    g.setColor(Color.red);
    g.fillRect(x, y, 10, 10);
    g.setColor(Color.white);
    g.drawRect(x, y, 10, 10);
}