我不明白我的代码出了什么问题。该程序运行良好，直到我使用"randPath（）"方法执行它。我总是收到此错误：
线程"AWT-EventQueue-0"中的异常 java.lang.NullPointerException
我的代码如下：
package org.me.mycardapp;
import java.io.File;
import java.util.Random;
public class CardDisplay extends javax.swing.JFrame {
public CardDisplay() {
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() {
jPanel1 = new javax.swing.JPanel();
jPanel2 = new javax.swing.JPanel();
jLabel1 = new javax.swing.JLabel();
jButton1 = new javax.swing.JButton();
jButton2 = new javax.swing.JButton();
setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
jPanel1.setBorder(javax.swing.BorderFactory.createTitledBorder(""));
jPanel2.setBorder(javax.swing.BorderFactory.createTitledBorder(""));
jPanel2.addMouseListener(new java.awt.event.MouseAdapter() {
public void mousePressed(java.awt.event.MouseEvent evt) {
jPanel2MousePressed(evt);
}
});
jLabel1.setIcon(new javax.swing.ImageIcon(getClass().getResource("/org/me/mycardapp/newpackage/10_of_diamonds.png"))); // NOI18N
javax.swing.GroupLayout jPanel2Layout = new javax.swing.GroupLayout(jPanel2);
jPanel2.setLayout(jPanel2Layout);
jPanel2Layout.setHorizontalGroup(
jPanel2Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(jPanel2Layout.createSequentialGroup()
.addGap(267, 267, 267)
.addComponent(jLabel1)
.addContainerGap(343, Short.MAX_VALUE))
);
jPanel2Layout.setVerticalGroup(
jPanel2Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(jPanel2Layout.createSequentialGroup()
.addGap(24, 24, 24)
.addComponent(jLabel1)
.addContainerGap(47, Short.MAX_VALUE))
);
jButton1.setText("Change");
jButton1.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
jButton1ActionPerformed(evt);
}
});
javax.swing.GroupLayout jPanel1Layout = new javax.swing.GroupLayout(jPanel1);
jPanel1.setLayout(jPanel1Layout);
jPanel1Layout.setHorizontalGroup(
jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(javax.swing.GroupLayout.Alignment.TRAILING, jPanel1Layout.createSequentialGroup()
.addContainerGap()
.addComponent(jPanel2, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
.addContainerGap())
.addGroup(javax.swing.GroupLayout.Alignment.TRAILING, jPanel1Layout.createSequentialGroup()
.addContainerGap(javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
.addComponent(jButton1)
.addGap(343, 343, 343))
);
jPanel1Layout.setVerticalGroup(
jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(jPanel1Layout.createSequentialGroup()
.addContainerGap()
.addComponent(jPanel2, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
.addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
.addComponent(jButton1, javax.swing.GroupLayout.PREFERRED_SIZE, 23, javax.swing.GroupLayout.PREFERRED_SIZE))
);
jButton2.setText("Exit");
jButton2.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
jButton2ActionPerformed(evt);
}
});
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()
.addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addComponent(jPanel1, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
.addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
.addGap(0, 0, Short.MAX_VALUE)
.addComponent(jButton2)))
.addContainerGap())
);
layout.setVerticalGroup(
layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(layout.createSequentialGroup()
.addContainerGap(javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
.addComponent(jPanel1, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
.addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
.addComponent(jButton2)
.addGap(5, 5, 5))
);
pack();
}// </editor-fold>
private void jButton2ActionPerformed(java.awt.event.ActionEvent evt) {
System.exit(0);
}
private void jPanel2MousePressed(java.awt.event.MouseEvent evt) {
jLabel1.setLocation(evt.getX()-63, evt.getY()-91);
}
private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {
jLabel1.setIcon(new javax.swing.ImageIcon(getClass().getResource(randPath())));
}
public String randPath () {
Random rand = new Random();
String path;
File file = new File("/org/me/mycardapp/newpackage/");
File[] files = file.listFiles();
int r = rand.nextInt(files.length);
path = files[r].getPath();
return path;
}
/**
* @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(CardDisplay.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
} catch (InstantiationException ex) {
java.util.logging.Logger.getLogger(CardDisplay.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
} catch (IllegalAccessException ex) {
java.util.logging.Logger.getLogger(CardDisplay.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
} catch (javax.swing.UnsupportedLookAndFeelException ex) {
java.util.logging.Logger.getLogger(CardDisplay.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
}
//</editor-fold>
/* Create and display the form */
java.awt.EventQueue.invokeLater(new Runnable() {
public void run() {
new CardDisplay().setVisible(true);
}
});
}
// Variables declaration - do not modify
private javax.swing.JButton jButton1;
private javax.swing.JButton jButton2;
private javax.swing.JLabel jLabel1;
private javax.swing.JPanel jPanel1;
private javax.swing.JPanel jPanel2;
// End of variables declaration
}
尝试编写确切的目录。与其这样"/org/me/mycardapp/newpackage/"不如试试这个"c:/.../org/me/mycardapp/newpackage/"。填充点。
有两种方法可以做到这一点：
File file = new File("c:/.../org/.."); // enter the exact path
String[] files = file.list();
int r = rand.nextInt(files.length);
path = file.getPath()+File.separatorChar+files[r];
File file = new File("c:/.../org/.."); // enter the exact path
File[] files = file.listFiles();
int r = rand.nextInt(files.length);
path = files[r].getPath();
您的问题出在randPath字符串中的这一行中。
File file = new File("/org/me/mycardapp/newpackage/");
将其更改为以下行，它应该可以解决问题
File file = new File("org/me/mycardapp/newpackage/");
编辑
在运行您提供的代码时，我发现了异常。问题在于下面显示的代码。
getClass().getResource(...))
您有两种选择。 If you are using an IDE such as Eclipse and don't want to change code go for Option 2
选项 1
改变
jLabel1.setIcon(new javax.swing.ImageIcon(getClass().getResource("/org/me/mycardapp/newpackage/10_of_diamonds.png"))); // NOI18N
和
private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {
jLabel1.setIcon(new javax.swing.ImageIcon(getClass().getResource(randPath())));
}
自
jLabel1.setIcon(new javax.swing.ImageIcon("org/me/mycardapp/newpackage/10_of_diamonds.png")); // NOI18N
和
private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {
jLabel1.setIcon(new javax.swing.ImageIcon(randPath()));
}
选项 2
观看此视频，它将说明如何在 IDE （如 Eclipse）中修复该问题。
通过这种方式，您可以保持代码相同，只需将图像放入包中即可，如视频所示
注意：很抱歉，一个很小的孩子提出了它，但调高了音量，因为他解释得很好