尝试使用JFrame在Core Java中借助箭头键移动矩形

Question

I have made a program in which I am trying to move a rectangle down with the help of arrow keys. But it is not moving. Here is my code. Can anyone please help me out and tell why is it not moving ? I have also added a System.out.print( ) in the key down event but its not showing any output on the console. It seems the keyevent part is not working.

  `import java.awt.event.KeyAdapter;
   import java.awt.event.KeyEvent;
   import java.awt.event.KeyListener;

   import javax.swing.ImageIcon;
   import javax.swing.JFrame;
   import javax.swing.JLabel;

public class MovingBoxWithArrowKeys extends JFrame
{
    JLabel l ;

public MovingBoxWithArrowKeys(String title)
{
    super(title);


    l = new JLabel(new ImageIcon("download-box-icon.png"));
    //l.setLocation(10 , 10 );
    l.setBounds(10 , 10 , 400 , 400 );
    l.setVisible(true);
    l.isOptimizedDrawingEnabled();
    l.requestFocusInWindow();
    //l.requestFocus();
    l.addKeyListener(new KeyAdapter() 
    {
        public void keyPressed(KeyEvent k)
        {
            if(k.getKeyCode() == KeyEvent.VK_DOWN)
            {
                l.setLocation(l.getX(), l.getY()+1);
                repaint();
                System.out.print("Down Pressed");
            }
        }
    });



    setLayout(null);
    setSize(this.getMaximumSize());
    setDefaultCloseOperation(EXIT_ON_CLOSE);
    setResizable(false);
    setVisible(true);
    //requestFocus();

    getContentPane().add(l);
    //add(l);
}
public static void main(String args[])
{
    MovingBoxWithArrowKeys m = new MovingBoxWithArrowKeys("Moving Box With Keys");

}
}`

Answer 1

Your KeyListener needs to be add to the JFrame because it only works with the component that has the actual focus. And thats not your JLabel its your JFrame .
Even if you request the Focus with l.requestFocusInWindow(); in your initialization code you will lose it to the JFrame because it is one of the components that is not capable of holding a focus

So simply change this:

l.addKeyListener(new KeyAdapter() {...}

to this

addKeyListener(new KeyAdapter() {...}