当我运行我的java程序时，我的矩形有时会移动

Question

我应该可以使用箭头键移动矩形但是在运行程序几次之后矩形不会移动。为了使矩形移动，我关闭Netbeans并重新打开它。然后矩形能够移动，但在几次尝试后它会停止移动。我想解决这个问题所以我可以做出改变。

package project;

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.JFrame;
import javax.swing.JPanel;
import javax.swing.Timer;

//KeyListener is use with keyboard
public class main extends JPanel implements ActionListener, KeyListener
{
    Timer tm = new Timer(5, this); //for animation
    int x = 50, y = 50, velX =0, velY = 0; 

    public main()
    {
        tm.start(); //starts timer
        addKeyListener(this); //this refearing to KeyListener
        setFocusable(true); //enable KeyListener
        setFocusTraversalKeysEnabled(false); //shift or tab is not use so F
    }
    public void paintComponent(Graphics g)
    {
        super.paintComponent(g);

        g.setColor(Color.red);
        g.fillRect(x,y,50,50);
    }        

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

        if (x>500)
        {
            velX = 0;
            x = 500;
        }    

        if (y<0)
        {
            velY = 0;
            y=0;
        }  

        if (y>300)
        {
            velY = 0;
            y = 300;
        }    

        x = x + velX;
        y = y + velY;

        repaint(); // repaint rectangle 
    }

    public void keyPressed(KeyEvent e)
    {
        int c = e.getKeyCode(); //get key

        if (c == KeyEvent.VK_LEFT) // VK_Left is left arrow 
        {
            velX = -3;
            velY = 0;
        }
        if (c == KeyEvent.VK_UP) // VK_UP is up arrow 
        {
            velX = 0;
            velY = -3; // means up 
        }
        if (c == KeyEvent.VK_RIGHT)  
        {
            velX = 3;
            velY = 0; 
        }
        if (c == KeyEvent.VK_DOWN)  
        {
            velX = 0;
            velY = 3; 
        }
    }

    public void keyTyped(KeyEvent e){}
    public void keyReleased(KeyEvent e) //when you stop pressing 
    {
        velX = 0;
        velY = 0; 
    }

    public static void main(String[] args)
    { 
        main m = new main();
        JFrame jf = new JFrame();
        jf.setTitle("Tutorial");
        jf.setSize(600,400);
        jf.setVisible(true);
        jf.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        jf.add(m);
    }
}