在Java中重绘无法正常工作

Question

I am trying to draw a rectangle which its position is updated every second, for that I have a class which extends JPanel and in it I have overriden the paint ( or paintComponent) function_ I have tried both _ but apparanetly this function is called only once and as it is shown in the code below when I try to call it in an infinite loop with repaint function it doesnt get called, any ideas what I can do?

public class Board extends JPanel implements KeyListener{
 public  void setUpBoard(){
     JFrame frame = new JFrame();
     Board board = new Board();
     frame.setVisible(true);
     frame.setResizable(false);
     frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
     frame.setSize(600, 600);
     frame.setLocation(350, 80);
     frame.add(board);
     }
 public void paint(Graphics g){
    g.setColor(Color.RED);
    g.fillRect(food.getX(),200,20,20);
       }
}

the code above is the graphic part, below is the main function, which is placed in another class :

 public static void main(String[] args) throws InterruptedException {
    Board board = new Board();
    FoodGenerator food = new FoodGenerator();
    board.setUpBoard();
    while(true){
        board.repaint();
        food.adder();
        try {
            sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }


}

in the code above food.adder is where the position of the rectangle is updated, which I have checked and doesnt have any issues.

Answer 1

The problem is that you're creating a new Board object in setUpBoard and adding that to your JFrame:

Board board = new Board();
// ...
frame.add(board);

So when you use repaint() , you're repainting the instance of Board that you created in the main method, and not the instance you created in setUpBoard , which is the one you add to the frame.

This can be easily fixed by using Board board = this; in setUpBoard , or, even simpler in my opinion, just using frame.add(this) . Subsequent calls to repaint will then schedule a call to paint for the same Board object that you created in the main method.

Also, since you're working with Swing, don't use paint , and instead use paintComponent , making sure that super.paintComponent(g) is the first statement in the method body.

Answer 2

另一个问题是，重新绘制调用是在主线程上完成的，而不是在事件线程上进行的。