从ActionListener方法访问类变量

Question

I am working with GUIs in Java using JFrame and JPanel, as well as ActionListener to edit an image when I click a button. Currently, I am having a lot of trouble getting my JPanel class called ButtonPanel to interact with my BufferedImage img. I am trying to display the height of the image but nothing is working and I have tried various solutions. My code for the ButtonPanel class is:

class ButtonPanel extends JPanel
   {
        //JButton makeBlue;
      BufferedImage img;
       ButtonPanel(BufferedImage x)
      {
            final JButton makeBlue = new JButton ("Make Blue");
            img = x;
            //int width, height;
         setBackground(Color.RED);
         int height = 0;
         add(makeBlue);
         ActionListener action = 
             new ActionListener()
            {
                public void actionPerformed(ActionEvent e)
               {


                  if (e.getSource()== makeBlue)
                  {
                            //img = x;
                         height = img.getHeight();
//                       System.out.println(rgbvalue);

                             //System.out.println(width + " " + height);
                      setBackground(Color.GREEN);
                     repaint();

                  }
               }

            };

         makeBlue.addActionListener(action);
      }
   }

Whenever I try to use a method from the BufferedImage API in order to edit the image, such as in the above code, I get this error:

ImageEditorDeluxe.java:184: error: local variable height is accessed from within inner class; needs to be declared final
                         height = img.getHeight();

I have played around with where I initialize height but nothing has worked. Any help would be appreciated.

I have another class called ProgramWindow in which I add all the different JPanels of the image editor to one main JFrame and I think this is where my problem might be, as the BufferedImage is null. Here is the code for ProgramWindow:

class ProgramWindow extends JFrame  
   {
       ProgramWindow()
      {
         ImagePanel ip = new ImagePanel();
         ChooseFile cf = new ChooseFile();
         ButtonPanel bp = new ButtonPanel(ip.getImg());

         add(ip, BorderLayout.CENTER);
         add(cf, BorderLayout.SOUTH);
         add(bp, BorderLayout.WEST);
      }
   }

I have concluded that the ButtonPanel in ProgramWindow is being passed a null parameter but I do not know why this is. I have a method called getImg in the ImagePanel class which I am calling as the parameter for ButtonPanel. Here is the code for ImagePanel:

class ImagePanel extends JPanel
   {
      BufferedImage img;

       ImagePanel()
      {
         setBackground(Color.BLUE);  //to test
         final JButton button = new JButton ("Display picture");
         add(button);       

         ActionListener action = 
             new ActionListener()
            {
                public void actionPerformed(ActionEvent e)
               {
                  if (e.getSource()==button)
                  {
                     try
                     {
                        img = ImageIO.read(ChooseFile.getFile());
                     }
                         catch(IOException f)
                        {
                           f.printStackTrace();
                        }

                     repaint();

                  }
               }

            };

         button.addActionListener(action);
      }

       public void paintComponent(Graphics g)
      {
         super.paintComponent(g);
         if (img != null)
            g.drawImage(img, 0, 0, this);
      }

       public void setImage(BufferedImage i)
      {
         img = i;
         repaint();
      }
        public BufferedImage getImg()
        {
            return img;
        }
   }

Answer 1

You're declaring height in the constructor and so it is local to the constructor. Perhaps it would be better to make it an instance field of the class instead.

public class ButtonPanel extends JPanel {
    private BufferedImage img;
    private int height;

Having said that, why even have a height variable field, since you can obtain it any time you'd like by calling img.getHeight() ?

---

Edit
Your second problem, one we can't help you on yet, is that your img variable holds a null reference. Given your new code, this suggests that ip.getImg() is returning null , but don't take my word on this, test it. Put this line in your code:

System.out.println("is ip.getImg() returning null? " + (ip.getImg()));

---

Edit 2
Of course you're getting null. You're extracting img from ImagePanel before the user has had a chance to interact with it. Since it only gets an image when the user presses the button, and since you're extracting the image on class creation, before the user has had a chance to do squat, the only possibility is that when you extract the image on class creation, you're going to get null. Solution: don't do that.