Moving a rectangle in JPanel with arrow keys using KeyListener

问题

I have been trying to make a JPanel move with the arrow keys. It has not been working. I believe it is my inner class that extends the KeyAdapter. I'm also not sure about the ActionListener implemented were it is. The other class I have made does not matter since it is just the frame.

package jerryWorlds;

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class Jerry extends JPanel implements ActionListener{

int SizeX, SizeY, PosX, PosY, VelX, VelY;
Image img;
Timer time = new Timer(1, this);

public Jerry(){
    ImageIcon i = new ImageIcon();
    addKeyListener(new AL());
    time.start();
    img = i.getImage();
    PosX = 375;
    PosY = 250;
}

public void paint(Graphics g){
    Graphics2D g2d = (Graphics2D)g;
    g2d.fillRect(PosX, PosY, 50, 100);
}
public void actionPerformed(ActionEvent e) {
    PosX = PosX + VelX;
    repaint();
}

private class AL extends KeyAdapter{
    public void keyPressed(KeyEvent e) {
        int key = e.getKeyCode();
        System.out.println("YAY!");
        if(key == KeyEvent.VK_LEFT)
            VelX = -1;
        else if(key == KeyEvent.VK_RIGHT)
            VelX = 1;
    }

    public void keyReleased(KeyEvent e) {
        int key = e.getKeyCode();
        if(key == KeyEvent.VK_LEFT)
            VelX = 0;
        else if(key == KeyEvent.VK_RIGHT)
            VelX = 0;
    }
}

}

回答1:

You will want to search this site for similar questions, as they usually have the same issue and same answer.
They will tell you that focus is one problem since a component's KeyListener won't work unless it has focus.
They will tell you that regardless you shouldn't use KeyListener at all but rather Key Bindings.
They will tell you not to override paint(...) but rather paintComponent(...) unless you are sure that you want to override painting of a component's borders and children (you don't).
They will tell you to be sure to call the super method inside of paintComponent(...).

Also please have a look at this animation and key bindings example.

来源：https://stackoverflow.com/questions/17246443/moving-a-rectangle-in-jpanel-with-arrow-keys-using-keylistener

标签

java

swing

jpanel

awt

keylistener