尝试从输入框中获取密码时出错

问题描述

正在做一些学校课程，目前正在创建一个基本的登录系统，将用户名和密码存储在文本文件中。我是 Java 新手，发现了一个关于制作类似系统的旧教程。到目前为止一切正常，包括程序能够从输入中获取用户名，但是任何时候我尝试输入密码时都会出现大量错误。我已经搜索了半个小时的大部分时间，但似乎找不到相关信息，所以只是想知道是否有人可以帮助我或将我链接到涵盖我需要的内容的帖子或网站（对于上下文，我目前正在尝试获取它将用户名和密码输入终端只是为了测试它是否可以正确获取它们）

代码：

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.jpasswordfield;
import javax.swing.JTextField;

public class GUI implements ActionListener {
    
    private static JLabel userLabel;
    private static JTextField userText;
    private static JLabel passwordLabel;
    private static jpasswordfield passwordText;
    private static JButton button;
    private static JLabel success;
    
    public static void main(String[] args) {
        JFrame frame = new JFrame();
        JPanel panel = new JPanel();
        frame.setSize(400,200);
        frame.setDefaultCloSEOperation(JFrame.EXIT_ON_CLOSE);
        
        frame.add(panel);
     
        panel.setLayout(null);
        
        userLabel = new JLabel("User");
        userLabel.setBounds(10,20,80,25);
        panel.add(userLabel);
        userText = new JTextField(20);
        userText.setBounds(100,165,25);
        panel.add(userText);
        
        passwordLabel = new JLabel("Password");
        passwordLabel.setBounds(10,50,25);
        panel.add(passwordLabel);
        jpasswordfield passwordText = new jpasswordfield();
        passwordText.setBounds(100,25);
        panel.add(passwordText);
        
        button = new JButton("Login");
        button.setBounds(10,25);
        button.addActionListener(new GUI());
        panel.add(button);
        
        success = new JLabel("");
        success.setBounds(10,110,300,25);
        panel.add(success);

        frame.setVisible(true);
    }
    @Override
    public void actionPerformed(ActionEvent e) {
        String user = userText.getText();
        String passwordString = String.valueOf(passwordText.getpassword());
        System.out.println(user + "," + passwordString);
        System.out.println(user);
    } 

}

错误（点击按钮后）：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException: Cannot invoke "javax.swing.jpasswordfield.getpassword()" because "GUI.passwordText" is null
    at GUI.actionPerformed(GUI.java:57)
    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:6614)
    at java.desktop/javax.swing.JComponent.processMouseEvent(JComponent.java:3342)
    at java.desktop/java.awt.Component.processEvent(Component.java:6379)
    at java.desktop/java.awt.Container.processEvent(Container.java:2263)
    at java.desktop/java.awt.Component.dispatchEventImpl(Component.java:4990)
    at java.desktop/java.awt.Container.dispatchEventImpl(Container.java:2321)
    at java.desktop/java.awt.Component.dispatchEvent(Component.java:4822)
    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:2769)
    at java.desktop/java.awt.Component.dispatchEvent(Component.java:4822)
    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)

解决方法

JPasswordField passwordText = new JPasswordField();
passwordText.setBounds(100,50,165,25);
panel.add(passwordText);

您正在重新声明 passwordText。这样，您就会模糊 passwordText 类变量，并且永远不会初始化它，因此当您在 actionPerformed 方法中引用它时，会抛出 NullPointerException。

去掉重新声明即可，即

passwordText = new JPasswordField();
passwordText.setBounds(100,25);
panel.add(passwordText);