Question

JAVA运行时出现 Exception in thread "main" java.lang.NullPointerException

import java.awt.BorderLayout;
import java.awt.CardLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.net.URL;
import javax.swing.Icon;
import javax.swing.ImageIcon;
import javax.swing.JFrame;
import javax.swing.JButton;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.WindowConstants;

public class UserCardLayoutFrame extends JFrame{
private JTextField txtld;
private URL url = null;
private Icon icon = null;
private JPanel imgPanel;
private CardLayout cardlayout;
public UserCardLayoutFrame(){
setTitle("使用卡片布局浏览照片");
setBounds(100,100,365,252);
setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE );
final JPanel northPanel = new JPanel();
getContentPane().add(northPanel,BorderLayout.NORTH );
final JLabel label = new JLabel();
label.setText("请输入照片编号：");
northPanel.add(label);
txtld = new JTextField();
northPanel.add(txtld);
imgPanel = new JPanel();
cardlayout = new CardLayout();
imgPanel.setLayout(cardlayout);
getContentPane().add(imgPanel,BorderLayout.CENTER );
for(int i=1;i<=6;i++){
url = UserCardLayoutFrame.class.getResource("/image/"+i+".jif");
icon = new ImageIcon(url);
imgPanel.add(String.valueOf(i),new JLabel(icon));
}
final JButton button = new JButton();
button.addActionListener(new ActionListener(){
public void actionPerformed(final ActionEvent arg0){
String id = txtld.getText();
cardlayout.show(imgPanel, id);
}
});
button.setText("显示照片");
northPanel.add(button);
final JPanel southPanel = new JPanel();
getContentPane().add(southPanel, BorderLayout.SOUTH );
final JButton dButton = new JButton();
dButton.addActionListener(new ActionListener(){
public void actionPerformed(final ActionEvent arg0){
cardlayout.first(imgPanel);
}
});
dButton.setText("第一张");
southPanel.add(dButton);
final JButton gButton = new JButton();
gButton.addActionListener(new ActionListener(){
public void actionPerformed(final ActionEvent arg0){
cardlayout.next(imgPanel);
}
});
gButton.setText("下一张");
southPanel.add(gButton);
final JButton button_1 = new JButton();
button_1.addActionListener(new ActionListener(){
public void actionPerformed(final ActionEvent arg0){
cardlayout.last(imgPanel);
}
});
button_1.setText("最后一张");
southPanel.add(button_1);
}

public static void main(String[] args) {
UserCardLayoutFrame frame = new UserCardLayoutFrame();
frame.setVisible(true);
}

}
问题：
Exception in thread "main" java.lang.NullPointerException
at javax.swing.ImageIcon.<init>(ImageIcon.java:167)
at UserCardLayoutFrame.<init>(UserCardLayoutFrame.java:39)
at UserCardLayoutFrame.main(UserCardLayoutFrame.java:90)
我建了一个LayoutProject项目，然后就在里面的src文件夹里新建一个image文件夹，把图片放到里面。

Answer 1

获取图片的url地址写错了。程序我跑出来了，一张一张查看6张图片的小程序。
请注意看以下的两个地址不同处。图片的扩展名错了。 有.jif的图片？？图片路径，当然要正确。你说的图片位置是对的。
url = UserCardLayoutFrame.class.getResource("/image/"+i+".jif");//源程序错误的
url = UserCardLayoutFrame.class.getResource("/image/"+i+".gif");//修改成这个

追问

我还是运行不了。
我把“.jif”改成“.gif”后，还是出现和原来一样的错误提示。我把鼠标放在("/image/"+i+".jif"）上面时显示“Note:The Javadoc for this element could neither be found in the attached source nor the attached Javadoc”。

追答

那个只是提示信息，找不到相关说明注解而已。不是错误信息。要不我远程帮你看下？这样小程序应该是没问题的

Answer 2

Exception in thread "main"
有个地方出现了异常，你一直把他抛到MAIN 函数了
java.lang.NullPointerException
空指针
at javax.swing.ImageIcon.<init>(ImageIcon.java:167)
应该错误在167行，很有可能是你的图片出了问题

Answer 3

空指针异常是经常发生的；
这句icon = new ImageIcon(url); 的时候就发生了空指针异常了，因为你上面刚定义了url和icon，而且都是null。当然要发生空指针异常了。

Answer 4

楼上说法都正确！

Answer 5

你这个类使用的有问题ImageIcon