我有一个图像编辑程序。它有几种方法，如灰度，缩放，合并图像等。每种方法都可以完美地完成。但是，在调用mergeImg方法后调用grayScale方法时出现错误。如果我先应用灰度，就不会发生这种情况。
这是错误：
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at Picture.width(Picture.java:51)
at Picture$4.actionPerformed(Picture.java:222)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
at javax.swing.AbstractButton.doClick(AbstractButton.java:376)
at javax.swing.plaf.basic.BasicMenuItemUI.doClick(BasicMenuItemUI.java:833)
at javax.swing.plaf.basic.BasicMenuItemUI$Handler.mouseReleased(BasicMenuItemUI.java:877)
at java.awt.Component.processMouseEvent(Component.java:6504)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3321)
at java.awt.Component.processEvent(Component.java:6269)
at java.awt.Container.processEvent(Container.java:2229)
at java.awt.Component.dispatchEventImpl(Component.java:4860)
at java.awt.Container.dispatchEventImpl(Container.java:2287)
at java.awt.Component.dispatchEvent(Component.java:4686)
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:2713)
at java.awt.Component.dispatchEvent(Component.java:4686)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:707)
at java.awt.EventQueue.access$000(EventQueue.java:101)
at java.awt.EventQueue$3.run(EventQueue.java:666)
at java.awt.EventQueue$3.run(EventQueue.java:664)
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:680)
at java.awt.EventQueue$4.run(EventQueue.java:678)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:677)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:211)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:128)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:117)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:113)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:105)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:90)
这是我的代码：
import java.awt.*;
import java.awt.event.*;
import java.util.*;
import java.io.*;
import javax.swing.*;
import java.awt.image.*;
import javax.imageio.ImageIO;
import java.lang.Math;
public class Picture{
JFileChooser fileChooser = new JFileChooser(); //file chooser
final JFrame frame = new JFrame("ImageEdit"); //creates JFrame
Container content; //creates container to place GUI objects in
static BufferedImage image; //original image
BufferedImage image2; //image after changes are made
BufferedImage mergeImage; //used for mergeImg method
JLabel imageLabel; //used to display image
//constructor; welcomes user, asks for image input
public Picture() {
//pops up prior to JFileChooser, intstructing user on what to do
Object[] options = {"Browse...", "Exit"};
ImageIcon welcomeIcon = new ImageIcon("GUI-images/welcome-icon.png");
int getFile = JOptionPane.showOptionDialog(frame, "Welcome to ImageEdit. To begin, please select an image file to edit.",
"Welcome!", JOptionPane.YES_NO_OPTION, JOptionPane.INFORMATION_MESSAGE, welcomeIcon, options, options[0]);
//if user selects browse option, do this:
if (getFile == JOptionPane.YES_OPTION) {
//asks for image file as input
browse();
}
//otherwise, exit program
else {
//exit program
System.exit(0);
}
}
//method returns width of image
public int width() {
int width = image.getWidth();
return width;
}
//method returns height of image
public int height() {
int height = image.getHeight();
return height;
}
//method sets updated image as "original" image
public void setImage() {
this.image = image2;
}
//method writes image in destination
public void saveImage() {
//gets file name & destination from user through JFileChooser
fileChooser.setDialogTitle("Save As...");
fileChooser.showSaveDialog(frame);
//writes image to new file with given name & location
try {
ImageIO.write(this.image, "JPG", fileChooser.getSelectedFile());
}
catch (IOException f) {
System.out.println("Saving failed! Could not save image.");
}
}
//method browses for new file
public void browse() {
//asks for new image file
fileChooser.setDialogTitle("Choose an image file:");
fileChooser.showOpenDialog(frame);
File selectedFile = fileChooser.getSelectedFile();
//if user has selected image file, continue
if (fileChooser.getSelectedFile() != null) {
try {
//reads selectedFile as image
image = ImageIO.read(selectedFile);
}
catch (IOException e) {
System.out.println("Invalid image file: " + selectedFile);
System.exit(0);
}
}
//else print error message
else {
System.out.println("Error! No File Selected.");
}
}
//method creates frame, adds menubar with options, provides parameters for other methods
public void show() {
//set frame title, set it visible, etc
content = frame.getContentPane();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setResizable(false);
//add the image to the frame
ImageIcon icon = new ImageIcon(image);
imageLabel = new JLabel(icon);
frame.setContentPane(imageLabel);
//adds a menubar on the frame with program name, File & Edit menus
JMenuBar menuBar = new JMenuBar();
frame.setJMenuBar(menuBar);
JMenu progName = new JMenu("ImageEdit");
progName.setBackground(Color.RED);
menuBar.add(progName);
JMenu fileMenu = new JMenu("File");
menuBar.add(fileMenu);
JMenu editMenu = new JMenu("Edit");
menuBar.add(editMenu);
//adds options to JMenus
//option to exit application
ImageIcon exitIcon = new ImageIcon("GUI-images/app-exit.png");
JMenuItem exitAction = new JMenuItem("Exit", exitIcon);
progName.add(exitAction);
//if exit option is selected, do this:
exitAction.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
//prompts to save file before exiting
ImageIcon saveIcon = new ImageIcon("GUI-images/save-icon.png");
int askSave = JOptionPane.showConfirmDialog(null,"Save image before exit?", "Save...",
JOptionPane.YES_NO_OPTION, JOptionPane.QUESTION_MESSAGE, saveIcon);
if (askSave == JOptionPane.YES_OPTION) {
//opens save image method, then exits
saveImage();
System.exit(0);
}
else {
//exits without saving
System.exit(0);
}
}
});
//option to open a new image
ImageIcon newIcon = new ImageIcon("GUI-images/new-image.png");
JMenuItem newAction = new JMenuItem("Open Image", newIcon);
fileMenu.add(newAction);
//if new option is selected, do this:
newAction.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
//prompts to save image before opening new image
ImageIcon saveIcon = new ImageIcon("GUI-images/save-icon.png");
int askSave = JOptionPane.showConfirmDialog(null,"Save current image?", "Save...",
JOptionPane.YES_NO_OPTION, JOptionPane.QUESTION_MESSAGE, saveIcon);
//if they do want to save first, do this:
if (askSave == JOptionPane.YES_OPTION) {
//opens save image method, then asks asks for new image file
saveImage();
//clears old image
imageLabel.setIcon(null);
//browses for new image
browse();
//displays new image
imageLabel.setIcon(new ImageIcon(image));
//resizes canvas to fit new image
frame.setSize(width(), height());
}
//if they don't want to save, do this:
else {
//erases old image
imageLabel.setIcon(null);
//browses for new image
browse();
//displays new image
imageLabel.setIcon(new ImageIcon(image));
//resizes canvas to fit new image
frame.setSize(width(), height());
}
}
});
//option to save current image
ImageIcon saveIcon = new ImageIcon("GUI-images/save-image.png");
JMenuItem saveAction = new JMenuItem("Save Image As...", saveIcon);
fileMenu.add(saveAction);
//if save option is selected, do this:
saveAction.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
//opens save image method
saveImage();
}
});
//option to make current image grayscale
ImageIcon gsIcon = new ImageIcon("GUI-images/grayscale-image.png");
JMenuItem grayScale = new JMenuItem("Grayscale", gsIcon);
editMenu.add(grayScale);
//if grayscale option is selected, do this:
grayScale.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
//grabs height and width of image,
//then calls grayscale method
grayscale(width(), height());
}
});
//option to scale current window to new dimensions
ImageIcon scaleIcon = new ImageIcon("GUI-images/scale-image.png");
JMenuItem scaleImg = new JMenuItem("Scale Image", scaleIcon);
editMenu.add(scaleImg);
//if scale option is selected, do this:
scaleImg.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
//asks for height and width to create new image
ImageIcon widthIcon = new ImageIcon("GUI-images/LR-arrows.png");
String scaleWidth = (String)JOptionPane.showInputDialog(null,"What should the new width be?",
"Scale Image", JOptionPane.QUESTION_MESSAGE, widthIcon, null, null);
ImageIcon heightIcon = new ImageIcon("GUI-images/UD-arrows.png");
String scaleHeight = (String)JOptionPane.showInputDialog(null,"What should the new height be?",
"Scale Image", JOptionPane.QUESTION_MESSAGE, widthIcon, null, null);
//turns user input strings into doubles
double x = Double.parseDouble(scaleWidth);
double y = Double.parseDouble(scaleHeight);
//casts doubles as ints
int newWidth = (int)x;
int newHeight = (int)y;
//resizes frame to fit new image dimensions
frame.setSize(newWidth, newHeight);
//calls scale method to resize image using given dimensions
scale(newWidth, newHeight);
}
});
//option to merge two images together
ImageIcon mergeIcon = new ImageIcon("GUI-images/merge-image.png");
JMenuItem mergeImg = new JMenuItem("Merge Image", mergeIcon);
editMenu.add(mergeImg);
//if merge option is selected, do this:
mergeImg.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
//asks for image file as input
fileChooser.setDialogTitle("Choose an image file to merge current image with.");
fileChooser.showOpenDialog(frame);
File mergeFile = fileChooser.getSelectedFile();
//if user has selected image file, continue
if (fileChooser.getSelectedFile() != null) {
try {
//reads selectedFile as image
mergeImage = ImageIO.read(mergeFile);
}
catch (IOException f) {
System.out.println("Invalid image file: " + mergeFile);
System.exit(0);
}
}
//else print error message
else {
System.out.println("Error! No File Selected.");
}
//if two images are same size, merge them
if (width() == mergeImage.getWidth() && height() == mergeImage.getHeight()) {
mergeImg(width(), height(), mergeImage);
}
//else, resize the second image to size of original, then merge
else {
scale(width(), height());
mergeImg(width(), height(), mergeImage);
}
}
});
//option to rotate image by x degrees
ImageIcon rotateIcon = new ImageIcon("GUI-images/rotate-image.png");
JMenuItem rotateImage = new JMenuItem("Rotate Image", rotateIcon);
editMenu.add(rotateImage);
//if rotate option is selected, do this:
rotateImage.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
String rotateAngle = (String)JOptionPane.showInputDialog(null,"By what angle would you like to rotate?",
"Input Degrees", JOptionPane.QUESTION_MESSAGE, null/*icon goes here*/, null, null);
//turns user input strings into doubles
double angleDegs = Double.parseDouble(rotateAngle);
//converts degrees to rads
double angle = Math.toRadians(angleDegs);
//applies sine and cosine functions
int sin = (int)Math.sin(angle);
int cos = (int)Math.cos(angle);
//gets new width of rotated image
int newWidth = width()*sin + height()*cos;
int newHeight = height()*sin + width()*cos;
//sets frame to new image size
frame.setSize(newWidth, newHeight);
//calls rotate method to rotate image
rotate(newWidth, newHeight, angle);
}
});
//paint the frame
frame.pack();
frame.repaint();
frame.setVisible(true);
}
//method converts image to grayscale; 6 lines of code
public void grayscale(int width, int height) {
// create a grayscale image with original dimensions
image2 = new BufferedImage(width, height, BufferedImage.TYPE_BYTE_GRAY);
// convert colored image to grayscale
ColorConvertOp grayScale = new ColorConvertOp(image.getColorModel().getColorSpace(),
image2.getColorModel().getColorSpace(),null);
grayScale.filter(image,image2);
imageLabel.setIcon(new ImageIcon(image2));
//sets new image as "original"
setImage();
}
//method scales image to user-input dimensions; 5 lines of code
public void scale(int width, int height){
//uses user-input dimensions to create new image
image2 = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
Graphics g = image2.createGraphics();
//gets new dimensions and resizes image
g.drawImage(image, 0, 0, image2.getWidth(), image2.getHeight(), 0, 0, width(), height(), null);
imageLabel.setIcon(new ImageIcon(image2));
//sets new image as "original"
setImage();
}
//method merges two images together; 14 lines of code
public void mergeImg(int width, int height, BufferedImage mergeImage) {
//creates new image from two images of same size
BufferedImage image2 = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
for (int i = 0; i < width; i++) {
for (int j = 0; j < height; j++) {
//get color from original image
Color c = new Color(image.getRGB(i, j));
//get colors from merge image
Color c2 = new Color(mergeImage.getRGB(i, j));
//average the colors
int r = (c.getRed()+c2.getRed())/2;
int g = (c.getGreen()+c2.getGreen())/2;
int b = (c.getBlue()+c2.getBlue())/2;
Color avgColor = new Color(r, g, b);
//set colors of new image to average of the two images
image2.setRGB(i, j, avgColor.getRGB());
imageLabel.setIcon(new ImageIcon(image2));
}
}
mergeImage = null;
//sets new image as "original"
setImage();
}
//method rotates image by user-input angle; 18 lines of code
public void rotate(int width, int height, double angle) {
//rotates image around center point
BufferedImage image2 = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
//defines sin and cos functions
double sin = Math.sin(angle);
double cos = Math.cos(angle);
//gets coordinates of image center
double Xc = width/2;
double Yc = height/2;
//rotate
for (int i = 0; i < width; i++) {
for (int j = 0; j < height; j++) {
//new i,j at center of image
double iPrime = i - Xc;
double jPrime = j - Yc;
//i,j at new points after rotation
int xPrime = (int) (iPrime * cos - jPrime * sin + Xc);
int yPrime = (int) (iPrime * sin + jPrime * cos + Yc);
// plot pixel (i, j) the same color as (xPrime, yPrime) if it's in bounds
if (xPrime >= 0 && xPrime < width && yPrime >= 0 && yPrime < height) {
image2.setRGB(xPrime, yPrime, image.getRGB(i, j));
imageLabel.setIcon(new ImageIcon(image2));
}
}
}
//sets new image as "original"
setImage();
}
//main method; starts program
public static void main(String[] args) {
//creates new picture from image file
Picture p = new Picture();
//shows picture on JFrame
p.show();
}
}
任何想法可能是什么？感谢帮助！
答案 0 :(得分：5)
从堆栈跟踪和代码判断，看起来像这个方法
//method returns width of image
public int width() {
int width = image.getWidth();
return width;
}
抛出NullPointerException。唯一可能导致此问题的是image是null。
我建议您逐步解决有问题的方法，并检查null变量的image的位置和原因。
一个可能的问题是您在方法image2和rotate中重新声明mergeImg。
BufferedImage image2 = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
这有效地“隐藏”this.image2。稍后再拨打setImage并执行
this.image = image2;
image2未引用您准备的局部变量。
答案 1 :(得分：2)
从堆栈跟踪中，您的实例变量图像似乎为空。
public int width() {
int width = image.getWidth();
return width;
}
尝试添加：
public int width() {
int width = 0;
if (null != image) {
width = image.getWidth();
}
return width;
}
喜欢
public void setImage(BufferedImage workingImg)
依赖于许多类方法修改的不同实例变量的状态是获得不可预测结果的最佳方法。
祝你好运