我在网上查找并发现了如何向
Netbeans自动生成了以下内容（与jpanel相关的代码片段）：
请注意，在调试模式下，我无法进入
JPanel添加图像的方法（how to add an image to a JPanel）。这个方法很好用（当图像在ImagePanel构造函数中加载时），但是我想能够动态更改图像（例如：按下按钮键）。我尝试实现这一点，但每次单击按钮时都会出现Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException异常。有人可以帮忙吗？我是Java新手。谢谢！这是我扩展的JPanel类：import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;
import javax.imageio.ImageIO;
import javax.swing.JPanel;
public class ImagePanel extends JPanel{
private BufferedImage image;
public ImagePanel() {
this.loadImage("/home/raph/Images/shortcurt_unity.png");
}
public void loadImage(String filename) {
try {
this.image = ImageIO.read(new File(filename));
this.repaint();
} catch (IOException ex) {
// handle exception...
}
}
@Override
public void paintComponent(Graphics g) {
super.paintComponent(g);
if (image != null) {
// Scale it by width
int scaledWidth = (int)((image.getWidth() * getHeight()/image.getHeight()));
// If the image is not off the screen horizontally...
if (scaledWidth < getWidth()) {
// Center the left and right destination x coordinates.
int leftOffset = getWidth() / 2 - scaledWidth / 2;
int rightOffset = getWidth() / 2 + scaledWidth / 2;
g.drawImage(image, leftOffset, 0, rightOffset, getHeight(), 0, 0, image.getWidth(), image.getHeight(), null);
}
// Otherwise, the image width is too much, even scaled
// So we need to center it the other direction
else {
int scaledHeight = (image.getHeight() * getWidth()) / image.getWidth();
int topOffset = getHeight() / 2 - scaledHeight / 2;
int bottomOffset = getHeight() / 2 + scaledHeight / 2;
g.drawImage(image, 0, topOffset, getWidth(), bottomOffset, 0, 0, image.getWidth(), image.getHeight(), null);
}
}
}
}
我在我的JFrame中进行了初始化：
private static ImagePanel jPanel1;
Netbeans自动生成了以下内容（与jpanel相关的代码片段）：
javax.swing.JPanel jPanel1 = new ImagePanel();
javax.swing.GroupLayout jPanel1Layout = new javax.swing.GroupLayout(jPanel1);
jPanel1.setLayout(jPanel1Layout);
jPanel1Layout.setHorizontalGroup(
jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGap(0, 1260, Short.MAX_VALUE)
);
jPanel1Layout.setVerticalGroup(
jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGap(0, 124, Short.MAX_VALUE)
);
我正在从按钮发出的调用：
private void jButton10ActionPerformed(java.awt.event.ActionEvent evt) {
jPanel1.loadImage("/home/raph/Images/shortcurt_unity.png");
}
请注意，在调试模式下，我无法进入
loadImage，我在到达它之前就会收到异常：Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at musicchecker.MusicCheckerGUI.jButton10ActionPerformed(MusicCheckerGUI.java:276)
at musicchecker.MusicCheckerGUI.access$400(MusicCheckerGUI.java:26)
at musicchecker.MusicCheckerGUI$5.actionPerformed(MusicCheckerGUI.java:124)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2318)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:236)
at java.awt.Component.processMouseEvent(Component.java:6267)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3267)
at java.awt.Component.processEvent(Component.java:6032)
at java.awt.Container.processEvent(Container.java:2041)
at java.awt.Component.dispatchEventImpl(Component.java:4630)
at java.awt.Container.dispatchEventImpl(Container.java:2099)
at java.awt.Component.dispatchEvent(Component.java:4460)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4577)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4238)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4168)
at java.awt.Container.dispatchEventImpl(Container.java:2085)
at java.awt.Window.dispatchEventImpl(Window.java:2478)
at java.awt.Component.dispatchEvent(Component.java:4460)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)