如何使用箭头键在JPanel中移动对象

Question

I am trying to create a small program with Windowbuilder that simply paints a red rectangle (called car1) in a JPanel and move it around by pressing the arrow keys; to do that I associated to the arrows a method to change the x position call the repaint method but the rectangle doesn't move at all - therefore I am probably messing up something with the KeyEvent and/or with repaint.

What should I do to make the rectangle each time I press the proper arrow key move and refresh the Panel ?

public class Car extends JPanel {
    int x;
    int y;

    public Car(int x,int y){
        this.x=x;
        this.y=y;
    }


    public void paint(Graphics g) {
        g.setColor(Color.RED);
        g.fillRect(x, y, 20, 20);
    }

    public void move_right(){
        x=x+20;
    }

    public void move_left(){
        x=x-20;
    }

}



public class Form extends JFrame {

    //private JPanel contentPane;
    Car car1;

    /**
     * Launch the application.
     */
    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                    Form frame = new Form();
                    frame.setVisible(true);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }

    /**
     * Create the frame.
     */
    public Form() {
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setBounds(100, 100, 727, 550);
        getContentPane().setLayout(null);
        car1 = new Car(350, 480);
        car1.addKeyListener(new KeyAdapter() {
            public void keyTyped(KeyEvent e) {
                int key = e.getKeyCode();
                if (key == KeyEvent.VK_KP_LEFT) {
                    car1.move_left();
                    car1.repaint();
                }
                if (key == KeyEvent.VK_KP_RIGHT) {
                    car1.move_right();
                    car1.repaint();
                }
            }
        });
        car1.setBounds(0, 0, 700, 500);
        car1.setBackground(new Color(255, 255, 255));
        getContentPane().add(car1);
    }

}

Answer 1

There are at least 3 problems:

1. Using KeyListener . KeyListener is well known for only responding to key events which occur on components which are focusable AND have keyboard focus. A JPanel by default is not focusable, therefore it can't recieve keyboard focus. A better solution is to use the key bindings API, which allows to define the level of focus a component must have before the bindings are triggered and allows you to re-use a Action for multiple keys, reducing code duplication
2. Overriding paint . It's highly recommended to override paintComponent instead of paint when performing custom painting. You've also failed to maintain the paint chain, which is going to cause no end of weird and wonderful paint artifacts. Painting in AWT and Swing and Performing Custom Painting for more details
3. Using null layouts. Avoid using null layouts, pixel perfect layouts are an illusion within modern ui design. There are too many factors which affect the individual size of components, none of which you can control. Swing was designed to work with layout managers at the core, discarding these will lead to no end of issues and problems that you will spend more and more time trying to rectify

For example...

import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.ActionEvent;
import java.awt.event.KeyEvent;
import javax.swing.AbstractAction;
import javax.swing.Action;
import javax.swing.ActionMap;
import javax.swing.InputMap;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.KeyStroke;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class Test {

    public static void main(String[] args) {
        new Test();
    }

    public Test() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (Exception ex) {
                    ex.printStackTrace();
                }

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.add(new TestPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class TestPane extends JPanel {

        private int xPos;

        public TestPane() {
            Action leftAction = new AbstractAction() {
                @Override
                public void actionPerformed(ActionEvent e) {
                    xPos -= 2;
                    if (xPos < 0) {
                        xPos = 0;
                    }
                    repaint();
                }
            };
            Action rightAction = new AbstractAction() {
                @Override
                public void actionPerformed(ActionEvent e) {
                    xPos += 2;
                    if (xPos + 10 > getWidth()) {
                        xPos = getWidth() - 10;
                    }
                    repaint();
                }
            };

            bindKeyStroke(WHEN_IN_FOCUSED_WINDOW, "move.left", KeyStroke.getKeyStroke(KeyEvent.VK_LEFT, 0), leftAction);
            bindKeyStroke(WHEN_IN_FOCUSED_WINDOW, "move.left", KeyStroke.getKeyStroke(KeyEvent.VK_KP_LEFT, 0), leftAction);
            bindKeyStroke(WHEN_IN_FOCUSED_WINDOW, "move.left", KeyStroke.getKeyStroke(KeyEvent.VK_4, 0), leftAction);
            bindKeyStroke(WHEN_IN_FOCUSED_WINDOW, "move.left", KeyStroke.getKeyStroke(KeyEvent.VK_A, 0), leftAction);

            bindKeyStroke(WHEN_IN_FOCUSED_WINDOW, "move.right", KeyStroke.getKeyStroke(KeyEvent.VK_RIGHT, 0), rightAction);
            bindKeyStroke(WHEN_IN_FOCUSED_WINDOW, "move.right", KeyStroke.getKeyStroke(KeyEvent.VK_KP_RIGHT, 0), rightAction);
            bindKeyStroke(WHEN_IN_FOCUSED_WINDOW, "move.right", KeyStroke.getKeyStroke(KeyEvent.VK_6, 0), rightAction);
            bindKeyStroke(WHEN_IN_FOCUSED_WINDOW, "move.right", KeyStroke.getKeyStroke(KeyEvent.VK_D, 0), rightAction);
        }

        protected void bindKeyStroke(int condition, String name, KeyStroke keyStroke, Action action) {
            InputMap im = getInputMap(condition);
            ActionMap am = getActionMap();

            im.put(keyStroke, name);
            am.put(name, action);
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(200, 200);
        }

        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            Graphics2D g2d = (Graphics2D) g.create();
            int yPos = (getHeight() - 10) / 2;
            g2d.drawRect(xPos, yPos, 10, 10);
            g2d.dispose();
        }

    }

}

Answer 2

This is a solution:

addKeyListener(new KeyAdapter() {
    public void keyPressed(KeyEvent e) {
        int key = e.getKeyCode();
        if (key == KeyEvent.VK_LEFT) {
            car1.move_left();
            car1.repaint();
        }
        if (key == KeyEvent.VK_RIGHT) {
            car1.move_right();
            car1.repaint();
        }
    }
});

Where are the mistakes:

1. addKeyListener : add key listener to frame, not to panel
2. VK_KP_ : use VK_ prefix instead
3. keyTyped : use keyPressed instead

Next step you will have to solve is removing the previous rectangles