无论何时编译此代码，它都可以在文件选择选项中正常工作，但在行ImageIcon ic = new ImageIcon(sname)处显示NullPointerException。我用print语句检查了它。变量sname接收正确的路径但无法显示图像。
以下是代码：
package Interface;
import java.awt.Component;
import java.io.File;
import javax.swing.ImageIcon;
import javax.swing.JFileChooser;
import javax.swing.JLabel;
/**
*
* @author Rajdeep
*/
public class InputPage2 extends javax.swing.JFrame {
/**
* Creates new form InputPage2
*/
public InputPage2() {
initComponents();
}
/**
* This method is called from within the constructor to initialize the form.
* WARNING: Do NOT modify this code. The content of this method is always
* regenerated by the Form Editor.
*/
@SuppressWarnings("unchecked")
// <editor-fold defaultstate="collapsed" desc="Generated Code">
private void initComponents() {
fileChooser = new javax.swing.JFileChooser();
panel = new javax.swing.JPanel();
gray = new javax.swing.JButton();
filter = new javax.swing.JButton();
jMenuBar1 = new javax.swing.JMenuBar();
jMenu1 = new javax.swing.JMenu();
Open = new javax.swing.JMenuItem();
Exit = new javax.swing.JMenuItem();
setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
javax.swing.GroupLayout panelLayout = new javax.swing.GroupLayout(panel);
panel.setLayout(panelLayout);
panelLayout.setHorizontalGroup(
panelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGap(0, 576, Short.MAX_VALUE)
);
panelLayout.setVerticalGroup(
panelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGap(0, 652, Short.MAX_VALUE)
);
gray.setText("Gray Scale");
gray.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
grayActionPerformed(evt);
}
});
filter.setText("Filter");
filter.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
filterActionPerformed(evt);
}
});
jMenu1.setText("File");
Open.setText("Open");
Open.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
OpenActionPerformed(evt);
}
});
jMenu1.add(Open);
Exit.setText("Exit");
Exit.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
ExitActionPerformed(evt);
}
});
jMenu1.add(Exit);
jMenuBar1.add(jMenu1);
setJMenuBar(jMenuBar1);
javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
getContentPane().setLayout(layout);
layout.setHorizontalGroup(
layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(layout.createSequentialGroup()
.addContainerGap()
.addComponent(panel, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
.addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
.addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addComponent(gray, javax.swing.GroupLayout.Alignment.TRAILING, javax.swing.GroupLayout.PREFERRED_SIZE, 94, javax.swing.GroupLayout.PREFERRED_SIZE)
.addComponent(filter, javax.swing.GroupLayout.Alignment.TRAILING, javax.swing.GroupLayout.PREFERRED_SIZE, 94, javax.swing.GroupLayout.PREFERRED_SIZE))
.addContainerGap())
);
layout.setVerticalGroup(
layout.createParallelGroup(javax.swing.GroupLayout.Alignment.TRAILING)
.addGroup(layout.createSequentialGroup()
.addGap(165, 165, 165)
.addComponent(gray, javax.swing.GroupLayout.PREFERRED_SIZE, 49, javax.swing.GroupLayout.PREFERRED_SIZE)
.addGap(156, 156, 156)
.addComponent(filter, javax.swing.GroupLayout.PREFERRED_SIZE, 50, javax.swing.GroupLayout.PREFERRED_SIZE)
.addContainerGap(javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
.addGroup(layout.createSequentialGroup()
.addContainerGap(16, Short.MAX_VALUE)
.addComponent(panel, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
.addContainerGap())
);
pack();
}// </editor-fold>
private void OpenActionPerformed(java.awt.event.ActionEvent evt)throws java.lang.NullPointerException {
// TODO add your handling code here:
fileChooser = new JFileChooser();
int result = fileChooser.showOpenDialog(null);
if(result == JFileChooser.APPROVE_OPTION){
File file=fileChooser.getSelectedFile();
String sname= file.getAbsolutePath();
sname= "/" +sname;
System.out.println(sname);
ImageIcon ic = new ImageIcon(sname);
JLabel j= new JLabel("",ic,JLabel.CENTER);
panel.add(j);
panel.revalidate();
panel.repaint();
}
}
private void ExitActionPerformed(java.awt.event.ActionEvent evt) {
// TODO add your handling code here:
System.exit(0);
}
private void grayActionPerformed(java.awt.event.ActionEvent evt) {
// TODO add your handling code here:
System.out.println("In Gray");
}
private void filterActionPerformed(java.awt.event.ActionEvent evt) {
// TODO add your handling code here:
System.out.println("In Filter");
}
/**
* @param args the command line arguments
*/
public static void main(String args[]) {
/* Set the Nimbus look and feel */
/* //<editor-fold defaultstate="collapsed" desc=" Look and feel setting code (optional) ">*/ /* If Nimbus (introduced in Java SE 6) is not available, stay with the default look and feel.
* For details see http://download.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html
*/
try {
for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) {
if ("Nimbus".equals(info.getName())) {
javax.swing.UIManager.setLookAndFeel(info.getClassName());
break;
}
}
} catch (ClassNotFoundException ex) {
java.util.logging.Logger.getLogger(InputPage2.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
} catch (InstantiationException ex) {
java.util.logging.Logger.getLogger(InputPage2.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
} catch (IllegalAccessException ex) {
java.util.logging.Logger.getLogger(InputPage2.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
} catch (javax.swing.UnsupportedLookAndFeelException ex) {
java.util.logging.Logger.getLogger(InputPage2.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
}
//</editor-fold>
/* Create and display the form */
java.awt.EventQueue.invokeLater(new Runnable() {
@Override
public void run() {
new InputPage2().setVisible(true);
}
});
}
// Variables declaration - do not modify
private javax.swing.JMenuItem Exit;
private javax.swing.JMenuItem Open;
private javax.swing.JFileChooser fileChooser;
private javax.swing.JButton filter;
private javax.swing.JButton gray;
private javax.swing.JMenu jMenu1;
private javax.swing.JMenuBar jMenuBar1;
private javax.swing.JPanel panel;
// End of variables declaration
}
答案 0 :(得分：2)
构建GUI时应添加JLabel。然后代码很简单。
private void OpenActionPerformed(java.awt.event.ActionEvent evt)throws java.lang.NullPointerException {
fileChooser = new JFileChooser();
int result = fileChooser.showOpenDialog(null);
if(result == JFileChooser.APPROVE_OPTION){
File file=fileChooser.getSelectedFile();
ImageIcon ic = new ImageIcon(file.toURI().toURL());
j.setIcon(ic);
}
// ..
注意：请详细了解课程，方法和广告Java naming conventions（特别是用于名称的情况）属性名称＆amp;一贯地使用它们。 OpenActionPerformed应为openActionPerformed。