java中的簡單十字線

Question

我必須畫一個簡單的十字線。我所看到的只是一個空白麪板。

class ChartPanel extends JPanel implements MouseMotionListener{ 
    Graphics2D g; 
    Dimension dimFrame; 
    ChartPanel() { 
     addMouseMotionListener(this); 
    } 
    public void mouseMoved(MouseEvent e) { 
     drawCrosshair(e.getX(),e.getY()); 
    } 
    public void mouseDragged(MouseEvent e) {} 
    protected void paintComponent(Graphics g2) { 
     g = (Graphics2D)g2; 
     g.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON); 
     dimFrame = getSize(); 
     setBackground(Color.WHITE); 
    } 
    public Dimension getPreferredSize() { 
     return new Dimension(700, 500); 
    } 
    void drawCrosshair(double x, double y) { 
     double maxx = dimFrame.getWidth(); 
     double maxy = dimFrame.getHeight(); 
     g.setPaint(Color.BLACK); 
     g.draw(new Line2D.Double(0, y, maxx, y)); 
     g.draw(new Line2D.Double(x, 0, x, maxy)); 
    } 
} 
public class pra { 
    public static void main(String[] args) { 
     JFrame jFrame = new JFrame(); 
     ChartPanel chartPanel = new ChartPanel(); 
     jFrame.add(chartPanel); 
     jFrame.pack(); 
     jFrame.setVisible(true); 
     jFrame.setExtendedState(Frame.MAXIMIZED_BOTH); 
     jFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
    } 
} 

它正在進入drawCrosshair（）方法的正確值。我不知道我做錯了什麼。

Answer 1

你可以只處置drawCrosshair()，並繪製在paint方法的十字線，這將取代paintComponent方法（其實我覺得你不應該永遠覆蓋paintComponent）：

Graphics2D g; 
    Dimension dimFrame; 
    int x, y; 
    ChartPanel() { 
     addMouseMotionListener(this); 
     setPreferredSize(new Dimension(700, 500)); 
    } 

    public void mouseMoved(MouseEvent e) { 
     x = e.getX(); 
     y = e.getY(); 
     repaint(); 
    } 

    public void mouseDragged(MouseEvent e) { 
    } 

    public void paint(Graphics g2) { 
     super.paint(g2); 
     g = (Graphics2D) g2; 
     g.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON); 
     dimFrame = getSize(); 
     g.clearRect(0, 0, dimFrame.width, dimFrame.height);//clears previous drawings 
     g.setColor(Color.BLACK); 
     g.drawLine(x - 10, y, x + 10, y); 
     g.drawLine(x, y - 10, x, y + 10); 
    } 

，這應該這樣做（實際上它的確如我測試過的那樣））