Java moving a circle in a gui with arrow keys

问题

I am trying to move a circle left with a keyEvent. So far, the circle is drawn on the window but it does not move left! I feel like the problem is where I add the Window() constructor to the container. The is no output on the console to tell me that it is working. So I dont think it even reaches the KeyEvent class. Here is my code:

import java.awt.*;
import java.awt.event.*;
import java.awt.geom.Ellipse2D;
import javax.swing.*;


public class Window extends JPanel {

    private static Ellipse2D.Double circle;

    public Window() {
        super();
        int width = 400;
        int height = 400;
        circle = new Ellipse2D.Double(0.5 * width, 0.9 * height,
                0.1 * width, 0.05 * height);
        addKeyListener(new MoveCircle());
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponents(g);
        Graphics2D brush = (Graphics2D) g;
        int width = getWidth();
        int height = getHeight();
        g.clearRect(0, 0, width, height);
        brush.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                RenderingHints.VALUE_ANTIALIAS_ON);
        brush.draw(circle);
    }

    public class MoveCircle implements KeyListener {

        @Override
        public void keyPressed(KeyEvent e) {
            System.out.println("Working on top!");
            if (e.getKeyCode() == KeyEvent.VK_LEFT) {
                System.out.println("Working on bottom!");
                circle.x++;
                repaint();
            }
        }

        @Override
        public void keyReleased(KeyEvent e) {
            // TODO Auto-generated method stub
        }

        @Override
        public void keyTyped(KeyEvent e) {
            // TODO Auto-generated method stub
        }
    }

    public static void main(String[] args) {
        Window window = new Window();
        JFrame frame = new JFrame();
        Container container = frame.getContentPane();
        container.add(new Window());
        frame.addKeyEvent(window.new MoveCircle());
        frame.setSize(800, 700);
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        frame.setVisible(true);
    }
}

回答1:

Actually what is happening is this, you are adding Window to the JFrame, but the focus lies with the JFrame, so when you type on your Keyboard that thing goes to the JFrame not the KeyListener attached to the Window class. So in order to get over it, you simply have to call requestFocusInWindow() on the Window class's Object. Try this code, I had done some modification regarding EDT and stuff.

import java.awt.*;
import java.awt.event.*;
import java.awt.geom.Ellipse2D;
import javax.swing.*;


public class Window extends JPanel {

    private static Ellipse2D.Double circle;
    private JFrame frame;

    public Window() {
        super();
        int width = 400;
        int height = 400;
        circle = new Ellipse2D.Double(0.5 * width, 0.9 * height,
                0.1 * width, 0.05 * height);        
    }

    public Dimension getPreferredSize()
    {
        return (new Dimension(frame.getWidth(), frame.getHeight()));
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponents(g);
        Graphics2D brush = (Graphics2D) g;
        int width = getWidth();
        int height = getHeight();
        g.clearRect(0, 0, width, height);
        brush.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                RenderingHints.VALUE_ANTIALIAS_ON);
        brush.draw(circle);
    }

    public class MoveCircle implements KeyListener {

        @Override
        public void keyPressed(KeyEvent e) {
            System.out.println("Working on top!");
            if (e.getKeyCode() == Event.ENTER) {
                System.out.println("Working on bottom!");
                double newX = circle.x - 1;
                circle.x = newX;
                repaint();
            }
        }

        @Override
        public void keyReleased(KeyEvent e) {
            // TODO Auto-generated method stub
        }

        @Override
        public void keyTyped(KeyEvent e) {
            // TODO Auto-generated method stub
        }
    }

    private void createAndDisplayGUI(Window window)
    {
        frame = new JFrame();
        Container container = frame.getContentPane();       
        container.add(window);
        window.addKeyListener(new MoveCircle());        
        frame.setSize(800, 700);
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        frame.setVisible(true);
        window.requestFocusInWindow();
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable()
        {
            public void run()
            {
                Window window = new Window();
                window.createAndDisplayGUI(window);
            }
        });
    }
}

回答2:

Only the focussed component will get key events. You need to call requestFocus() at some point.

回答3:

A solution would be, to add the KeyListener to the JFrame. This way all the key strokes will throw an event, when the JFrame has the focus.

来源：https://stackoverflow.com/questions/9727192/java-moving-a-circle-in-a-gui-with-arrow-keys