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我想在Java中開發一個圖像編輯器，目前使用BufferedImage，JFrame和JPanel，但我無法讓我的按鈕與我從文件中讀取的BufferedImage進行交互。這裏是我的代碼：如何解決空指針異常
import java.awt.*;
import java.awt.Graphics;
import java.awt.event.*;
import javax.swing.*;
import java.io.*;
import javax.imageio.ImageIO;
import java.awt.image.BufferedImage;
import java.io.IOException;
/**
This class just holds the main
*/
public class ImageEditorDeluxe
{
public static void main(String[] args)
{
ProgramWindow p = new ProgramWindow();
p.setBounds(100, 100, 500, 500);
p.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
p.setVisible(true);
// p.pack();
}
}
/**
This is where all of the JPanels will come together
*/
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);
}
}
/**
This is where the image will be displayed
*/
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;
}
}
/**
This is where the JFileChooser will exist
*/
class ChooseFile extends JPanel
{
static File file;
JButton bOpen, bDisplay;
BufferedImage img;
ChooseFile()
{
setBackground(Color.GREEN); //to test
bOpen = new JButton("Open File");
add(bOpen);
// bDisplay = new JButton("Display File");
// add(bDisplay);
final JFileChooser chooser = new JFileChooser();
ActionListener action =
new ActionListener()
{
public void actionPerformed(ActionEvent e)
{
//open file
if (e.getSource()==bOpen)
{
chooser.showOpenDialog(null);
file = chooser.getSelectedFile();
}
//display file
/* if (e.getSource()== bDisplay)
{
try
{
img = ImageIO.read(file);
}
catch(IOException f)
{
f.printStackTrace();
}
}*/
}
};
bOpen.addActionListener(action);
// bDisplay.addActionListener(action);
}
//returns String name of file
static File getFile() throws IOException
{
return file;
}
}
class ButtonPanel extends JPanel
{
JButton makeBlue;
public static BufferedImage img;
public static int width, height;
//BufferedImage img;
ButtonPanel(BufferedImage x)
{
img = x;
int width, height;
setBackground(Color.RED);
makeBlue = new JButton("Make Blue");
add(makeBlue);
ActionListener action =
new ActionListener()
{
public void actionPerformed(ActionEvent e)
{
int width, height;
if (e.getSource()== makeBlue)
{
//img = x;
width = img.getWidth();
height = img.getHeight();
System.out.println(width + " " + height);
repaint();
}
}
};
makeBlue.addActionListener(action);
}
}
的錯誤是在ButtonPanel類的地方，但我不知道它是什麼。我目前得到的錯誤，當我按下makeBlue按鈕是這樣的：
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at ButtonPanel$1.actionPerformed(ImageEditorDeluxe.java:185)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
at java.awt.Component.processMouseEvent(Component.java:6505)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3320)
at java.awt.Component.processEvent(Component.java:6270)
at java.awt.Container.processEvent(Container.java:2229)
at java.awt.Component.dispatchEventImpl(Component.java:4861)
at java.awt.Container.dispatchEventImpl(Container.java:2287)
at java.awt.Component.dispatchEvent(Component.java:4687)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)
at java.awt.Container.dispatchEventImpl(Container.java:2273)
at java.awt.Window.dispatchEventImpl(Window.java:2719)
at java.awt.Component.dispatchEvent(Component.java:4687)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:735)
at java.awt.EventQueue.access$200(EventQueue.java:103)
at java.awt.EventQueue$3.run(EventQueue.java:694)
at java.awt.EventQueue$3.run(EventQueue.java:692)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
at java.awt.EventQueue$4.run(EventQueue.java:708)
at java.awt.EventQueue$4.run(EventQueue.java:706)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:705)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)
任何幫助，將不勝感激。謝謝。
ImageEditorDeluxe.java:185，該行有什麼？ – perencia
您應該知道如何查看異常堆棧跟蹤並識別失敗的行。然後從那裏追溯並找出指針爲空的原因是一件相對簡單的事情。 –
我對Java很新，這是一個雄心勃勃的項目。我不知道如何查看異常堆棧跟蹤或者是什麼。 – masonc15