这是怎么回事啊

Question

E:\JavaWork>javac EventTest.javaE:\JavaWork>java EventTest
Exception in thread "main" java.lang.NullPointerException
        at java.awt.Container.addImpl(Container.java:1010)
        at java.awt.Container.add(Container.java:348)
        at EventTest.<init>(EventTest.java:29)
        at EventTest.main(EventTest.java:44)源代码import java.awt.*;
import javax.swing.*;
import java.awt.event.*;public class EventTest
{
         JFrame frame;
         JPanel panel;
         JButton btnAdd;
         JButton btnDelete;
         JButton btnModify;
         JLabel lblStatus;
         FlowLayout fl;
       
         EventTest()
         {
              frame=new JFrame("Event Test!!");
              panel=(JPanel)frame.getContentPane();
              fl=new FlowLayout();
              panel.setLayout(fl);              btnAdd=new JButton("Add User");
              btnDelete=new JButton("Delete User");
              btnAdd=new JButton("Modify User");
              lblStatus=new JLabel();
          
              panel.add(btnAdd);
              panel.add(btnDelete);
              panel.add(btnModify);
              panel.add(lblStatus);              ButtonHander bh=new ButtonHander();
              btnAdd.addActionListener(bh);
              btnDelete.addActionListener(bh);
              btnModify.addActionListener(bh);
     
              frame.pack();
              frame.setVisible(true);               
         }
         public static void main(String args[])
         {
              new EventTest();
         }         class ButtonHander implements ActionListener
         {
                public void actionPerformed(ActionEvent e)
                {
                       Object obj=e.getSource();                       if(obj==btnAdd)
                       {
                              lblStatus.setText("Add Button Clicked!!");
                       }
                       if(obj==btnDelete)
                       {
                              lblStatus.setText("Delete Button Clicked!!");
                       }
                       if(obj==btnModify)
                       {
                              lblStatus.setText("Modify Button Clicked!!");
                       }
                }
         }
}哪里空指针异常了 实在是找不到

Answer 1

你的btnModify只是声明，没有new 出来

Answer 2

btnAdd=new JButton("Add User");
  btnDelete=new JButton("Delete User");
  btnAdd=new JButton("Modify User");//这一行
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Answer 3

  btnAdd=new JButton("Add User");
  btnDelete=new JButton("Delete User");
  btnAdd=new JButton("Modify User");//此处应该是btnModify  btnModify.addActionListener(bh);//由于上面未对btnModify实例化，导致空指针异常