Как нарисовать линию на JLabel в обработчике событий в Java?

Question

Я очень стараюсь, и я снова и снова сталкиваюсь с такой же ошибкой.

В основном я пытаюсь загрузить .png, используя, а затем нарисую строку после двух щелчков.

public class GroundTruthMarker extends JPanel { 

private JFrame frame; 
boolean leftSideDone = false; 

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

/** 
* Create the application. 
*/ 
public GroundTruthMarker() { 
    initialize(); 
} 

/** 
* Initialize the contents of the frame. 
*/ 
private void initialize() { 
    frame = new JFrame(); 
    frame.setBounds(100, 100, 1178, 844); 
    frame.setResizable(true); 
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
    frame.getContentPane().setLayout(null); 

    // points to hold the clicked coordinates 
    Point leftSide = new Point(); 
    Point rightSide = new Point(); 

    // status label 
    JLabel lblStatus = new JLabel(""); 
    lblStatus.setBounds(373, 807, 134, 20); 
    frame.getContentPane().add(lblStatus); 

    JButton btnSave = new JButton("Save"); 
    btnSave.addActionListener(new ActionListener() { 
     public void actionPerformed(ActionEvent e) { 
      lblStatus.setText("Saved!"); 
     } 
    }); 
    btnSave.setBounds(215, 807, 117, 25); 
    frame.getContentPane().add(btnSave); 

    JLabel lblpicture = new JLabel(""); 
    lblpicture.addMouseListener(new MouseAdapter() { 
     @Override 
     public void mouseClicked(MouseEvent e) { 

      if (leftSideDone == false) { 
       leftSide.x = e.getX(); 
       leftSide.y = e.getY(); 
       lblStatus.setText("Left Side Done!"); 
       leftSideDone = true; 
      } else { 
       rightSide.x = e.getX(); 
       rightSide.y = e.getY(); 
       lblStatus.setText("Right Side Done!"); 
       leftSideDone = false; 

       Graphics g = getGraphics(); 
       g.setColor(Color.cyan); 
       g.drawLine(leftSide.x, leftSide.y, rightSide.x, rightSide.y); 
      } 

     } 
    }); 

    // picture label 
    lblpicture.setBackground(Color.WHITE); 
    lblpicture.setBounds(12, 12, 1154, 765); 
    // create a line border with the specified color and width 
    Border border = BorderFactory.createLineBorder(Color.BLUE, 1); 
    // set the border of this component 
    lblpicture.setBorder(border); 
    frame.getContentPane().add(lblpicture); 

    JButton btnLoadImages = new JButton("Load Images"); 
    btnLoadImages.addActionListener(new ActionListener() { 
     public void actionPerformed(ActionEvent e) { 
      // set up the file chooser 
      JFileChooser fileChooser = new JFileChooser(); 
      // only directories 
      fileChooser.setFileSelectionMode(JFileChooser.DIRECTORIES_ONLY); 
      if (fileChooser.showOpenDialog(null) == JFileChooser.APPROVE_OPTION) { 
       File dir = fileChooser.getSelectedFile(); 
       // load from the directory 
       System.out.println(dir.getName()); 
       // loop over the image directory 
       if (dir.isDirectory()) { // make sure it's a directory 
        for (final File f : dir.listFiles()) { 
         BufferedImage img = null; 
         // read every image and display it in the gui 
         try { 
          lblStatus.setText("New Image!"); 
          img = ImageIO.read(f); 
          lblpicture.setIcon(new ImageIcon(img)); 

         } catch (final IOException e1) { 
          // handle errors here 
          System.out.println("Error:" + e1.getMessage()); 
         } 
        } 
       } 
      } 
     } 
    }); 
    btnLoadImages.setBounds(23, 807, 144, 25); 
    frame.getContentPane().add(btnLoadImages); 
} 

}

И это ошибка я получаю во время выполнения, когда я ожидаю линии:

java.lang.NullPointerException 
    at GroundTruthMarker$3.mouseClicked(GroundTruthMarker.java:101) 
    at java.awt.Component.processMouseEvent(Component.java:6538) 
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3324) 
    at java.awt.Component.processEvent(Component.java:6300) 
    at java.awt.Container.processEvent(Container.java:2236) 
    at java.awt.Component.dispatchEventImpl(Component.java:4891) 
    at java.awt.Container.dispatchEventImpl(Container.java:2294) 
    at java.awt.Component.dispatchEvent(Component.java:4713) 
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4888) 
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4534) 
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4466) 
    at java.awt.Container.dispatchEventImpl(Container.java:2280) 
    at java.awt.Window.dispatchEventImpl(Window.java:2750) 
    at java.awt.Component.dispatchEvent(Component.java:4713) 
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758) 
    at java.awt.EventQueue.access$500(EventQueue.java:97) 
    at java.awt.EventQueue$3.run(EventQueue.java:709) 
    at java.awt.EventQueue$3.run(EventQueue.java:703) 
    at java.security.AccessController.doPrivileged(Native Method) 
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76) 
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:86) 
    at java.awt.EventQueue$4.run(EventQueue.java:731) 
    at java.awt.EventQueue$4.run(EventQueue.java:729) 
    at java.security.AccessController.doPrivileged(Native Method) 
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76) 
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:728) 
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201) 
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116) 
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105) 
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101) 
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93) 
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:82) 

Имейте в виду, что рисунок должен быть проведен в обработчике события , Я пробовал некоторые методы, но сталкивался с ошибками «Невозможно сделать это с помощью мыши-события-обработчика», поэтому они не работали.

Я ценю любую помощь.

Спасибо.

Answer 1

Измените код, как этот

public class GroundTruthMarker extends JPanel { 

    private BufferedImage img = null; // ADD THIS LINE 
    [...] 
    private void initialize() { 
     [...] 
     // JLabel lblpicture = new JLabel(""); // REMOVE THIS LINE 
     // ADD THE FOLLOWING INSTEAD 
     JPanel lblpicture = new JPanel(){ 
      @Override 
      protected void paintComponent(Graphics g) { 
       super.paintComponent(g); 
       g.drawImage(img,0,0,null); 
       g.setColor(Color.cyan); 
       g.drawLine(leftSide.x, leftSide.y, rightSide.x, rightSide.y); 
      } 
     }; 
     [...] 
     public void mouseClicked(MouseEvent e) { 
      [...] 
      // REMOVE THIS! 
      //Graphics g = getGraphics(); 
      //g.setColor(Color.cyan); 
      //g.drawLine(leftSide.x, leftSide.y, rightSide.x, rightSide.y); 
      // ADD THIS INSTEAD 
      lblpicture.repaint(); 
     } 
     [...] 
     public void actionPerformed(ActionEvent e) { 
      [...] 
      // REMOVE THIS LINE 
      //BufferedImage img = null; 
      [...] 
      // REMOVE THIS LINE 
      //lblpicture.setIcon(new ImageIcon(img)); 
      // ADD THIS LINE INSTEAD 
      lblpicture.repaint(); 
      [...] 
     } 
    } 
}