Java 中的 ActionListener 对我不起作用

Question

I was watching a tutorial on how to crate a login system in Java and when I recreated it, it just wouldn't work. When I start the program I can see all my JLabels, JButtons, and my JPasswordField but when I press the reset button I get hit with this error message in the console

in thread "AWT-EventQueue-0" java.lang.NullPointerException: Cannot invoke java.util.EventObject.getSource()" because "this.e" is null
   at LoginPage.actionPerformed(LoginPage.java:67)
   at java.desktop/javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1967)
   at java.desktop/javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2308)
   at java.desktop/javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:405)
   at java.desktop/javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:262)
   at java.desktop/javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:279)
   at java.desktop/java.awt.Component.processMouseEvent(Component.java:6636)
   at java.desktop/javax.swing.JComponent.processMouseEvent(JComponent.java:3342)
   at java.desktop/java.awt.Component.processEvent(Component.java:6401)
   at java.desktop/java.awt.Container.processEvent(Container.java:2263)
   at java.desktop/java.awt.Component.dispatchEventImpl(Component.java:5012)
   at java.desktop/java.awt.Container.dispatchEventImpl(Container.java:2321)
   at java.desktop/java.awt.Component.dispatchEvent(Component.java:4844)
   at java.desktop/java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4919)
   at java.desktop/java.awt.LightweightDispatcher.processMouseEvent(Container.java:4548)
   at java.desktop/java.awt.LightweightDispatcher.dispatchEvent(Container.java:4489)
   at java.desktop/java.awt.Container.dispatchEventImpl(Container.java:2307)
   at java.desktop/java.awt.Window.dispatchEventImpl(Window.java:2764)
   at java.desktop/java.awt.Component.dispatchEvent(Component.java:4844)
   at java.desktop/java.awt.EventQueue.dispatchEventImpl(EventQueue.java:772)
   at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:721)
   at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:715)
   at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
   at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:85)
   at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:95)
   at java.desktop/java.awt.EventQueue$5.run(EventQueue.java:745)
   at java.desktop/java.awt.EventQueue$5.run(EventQueue.java:743)
   at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
   at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:85)
   at java.desktop/java.awt.EventQueue.dispatchEvent(EventQueue.java:742)
   at java.desktop/java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:203)
   at java.desktop/java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:124)
   at java.desktop/java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:113)
   at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:109)
   at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
   at java.desktop/java.awt.EventDispatchThread.run(EventDispatchThread.java:90)

So at this point I thought I had misspelled something and so I copied the code from the guy that was making the tutorial and when I pasted it I got the same message. At this point I am pretty desperate and I would take any help I can get. TIA to anyone who helps.

import java.awt.Font;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.EventObject;
import java.util.HashMap;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPasswordField;
import javax.swing.JTextField;
public class LoginPage  implements ActionListener {
    JFrame frame = new JFrame();
    JButton loginButton = new JButton("Login");
    JButton resetButton = new JButton("Reset");
    JTextField userIDField = new JTextField();
    JPasswordField userPasswordField = new JPasswordField();
    JLabel userIDLabel = new JLabel("userID:");
    JLabel userPasswordLabel = new JLabel("Password:");
    JLabel messageLabel = new JLabel("");
    HashMap<String,String> logininfo = new HashMap<String,String>();
    private EventObject e; 
    
        LoginPage(HashMap<String,String> loginInfoOriginal){
            logininfo = loginInfoOriginal;
            
            //Labels
            userIDLabel.setBounds(50, 100, 75, 25);
            userPasswordLabel.setBounds(50, 150, 75, 25);
            messageLabel.setBounds(125, 250, 250, 35);
            messageLabel.setFont(new Font(null,Font.ITALIC,25));
            
            //Fields
            userIDField.setBounds(125,100,200,25);
            userPasswordField.setBounds(125,150,200,25);
            
            //Buttons
            loginButton.setBounds(125, 200, 100, 25);
            loginButton.addActionListener(this);
            loginButton.setFocusable(false);
            resetButton.setBounds(225, 200, 100, 25);
            resetButton.setFocusable(false);
            resetButton.addActionListener(this);
            
            //Window
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            frame.setSize(420,420);
            frame.setLayout(null);
            frame.setVisible(true);
            frame.add(userIDLabel);
            frame.add(userPasswordLabel);
            frame.add(messageLabel);
            frame.add(userIDField);
            frame.add(userPasswordField);
            frame.add(loginButton);
            frame.add(resetButton);
        }

        @Override
        public void actionPerformed(ActionEvent arg0) {
            if(e.getSource()==resetButton); 
                userIDField.setText("");
                userPasswordField.setText("");
        }
}

Answer 1

Instead of calling e.getSource() , use arg0.getSource() .