Java调试有时只起作用

我有一只奇怪的虫子。 我写了一些非常小的代码，只显示一个矩形，我可以通过按箭头键来移动它

public void paintComponent(Graphics g)
{   
    super.paintComponent(g);

    g.setColor(Color.blue);
    g.fillRect(position_x, position_y, 20, size);
}
public void keyPressed(KeyEvent e)
{
    int code = e.getKeyCode();

    if(code == KeyEvent.VK_UP)
        goUp();
    else if(code == KeyEvent.VK_DOWN)
        goDown();
    else if(code == KeyEvent.VK_RIGHT)
        goRight();
    else if(code == KeyEvent.VK_LEFT)
        goLeft();
}
public void actionPerformed(ActionEvent e)
{
    repaint();
}
public void goDown()
{
    position_y += 20;
    position_x += 0;
}

public void goUp()
{
    position_y += -20;
    position_x += 0;
}
public void goLeft()
{
    position_y += 0;
    position_x += -20;
}
public void goRight()
{
    position_y += 0;
    position_x += 20;
}

每当我调试时，应该直接显示矩形（它有时做什么）。但大多数时候，只有一个空白的JFrame。我已经在另一个系统上试过了。而且它在那里也不起作用

这是我的全部密码

Main.java

import java.awt.Color;
import java.util.ArrayList;

import javax.swing.JFrame;


public class Main 
{
    private static JFrame frame = new JFrame("Snake");
    private static ArrayList<Snake> mySnake = new ArrayList<Snake>();


    public static void main(String[] args) 
    {
        frame.setSize(500,500);
        frame.setBackground(Color.black);
        frame.setVisible(true);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        mySnake.add(new Snake(50, 50, 60, true));
        frame.add(mySnake.get(0));
    }

}

import java.awt.Color;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import javax.swing.JPanel;
import javax.swing.Timer;
public class Snake extends JPanel implements KeyListener, ActionListener
{
    private static final long serialVersionUID = 1L;
    int position_x;
    int position_y;

    int size;

    Timer t = new Timer(500, this);

    public Snake(int position_x, int position_y, int size, boolean isAlive)
    {       
        t.start();
        this.position_x = position_x;
        this.position_y = position_y;
        this.size = size;

        addKeyListener(this);
        setFocusable(true);
        setFocusTraversalKeysEnabled(true);

    }


    public void paintComponent(Graphics g)
    {   
        super.paintComponent(g);
        g.setColor(Color.blue);
        g.fillRect(position_x, position_y, 20, size);
    }


    public void checkSelfEating()
    {

    }


    public void keyPressed(KeyEvent e)
    {
        int code = e.getKeyCode();
        if(code == KeyEvent.VK_UP)
            goUp();
        else if(code == KeyEvent.VK_DOWN)
            goDown();
        else if(code == KeyEvent.VK_RIGHT)
            goRight();
        else if(code == KeyEvent.VK_LEFT)
            goLeft();
    }
    public void actionPerformed(ActionEvent e)
    {
        repaint();
    }

    public void goDown()
    {
        position_y += 20;
        position_x += 0;
    }
    public void goUp()
    {
        position_y += -20;
        position_x += 0;
    }
    public void goLeft()
    {
        position_y += 0;
        position_x += -20;
    }
    public void goRight()
    {
        position_y += 0;
        position_x += 20;
    }

    public void keyReleased(KeyEvent e)
    {
    }
    public void keyTyped(KeyEvent e)
    {
    }
}

---

是否有任何问题的建议？

我建议您最后一次调用是

frame.setVisible（true）。为什么要在之后添加元素？@Eypros就这么简单！工作得很好。非常感谢。