尝试从输入框中获取密码时出错

Question

正在做一些学校课程，目前正在创建一个基本的登录系统，它将用户名和密码存储在一个文本文件中。我是 java 的新手，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，但任何时候我尝试输入密码时都会出现一大堆错误。我已经搜索了半小时的大部分时间，但似乎找不到相关信息，所以我只是想知道是否有人可以帮助我或 link 我到 post 或涵盖我需要的内容的网站（用于上下文我目前正在尝试让它将用户名和密码发送到终端，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, 20, 165 ,25);
        panel.add(userText);
        
        passwordLabel = new JLabel("Password");
        passwordLabel.setBounds(10, 50, 80, 25);
        panel.add(passwordLabel);
        JPasswordField passwordText = new JPasswordField();
        passwordText.setBounds(100, 50, 165, 25);
        panel.add(passwordText);
        
        button = new JButton("Login");
        button.setBounds(10, 80, 80, 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$Handler.actionPerformed(AbstractButton.java:2308)
    at java.desktop/javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:405)
    at java.desktop/javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:262)
    at java.desktop/javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:279)
    at java.desktop/java.awt.Component.processMouseEvent(Component.java:6614)
    at java.desktop/javax.swing.JComponent.processMouseEvent(JComponent.java:3342)
    at java.desktop/java.awt.Component.processEvent(Component.java:6379)
    at java.desktop/java.awt.Container.processEvent(Container.java:2263)
    at java.desktop/java.awt.Component.dispatchEventImpl(Component.java:4990)
    at java.desktop/java.awt.Container.dispatchEventImpl(Container.java:2321)
    at java.desktop/java.awt.Component.dispatchEvent(Component.java:4822)
    at java.desktop/java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4919)
    at java.desktop/java.awt.LightweightDispatcher.processMouseEvent(Container.java:4548)
    at java.desktop/java.awt.LightweightDispatcher.dispatchEvent(Container.java:4489)
    at java.desktop/java.awt.Container.dispatchEventImpl(Container.java:2307)
    at java.desktop/java.awt.Window.dispatchEventImpl(Window.java:2769)
    at java.desktop/java.awt.Component.dispatchEvent(Component.java:4822)
    at java.desktop/java.awt.EventQueue.dispatchEventImpl(EventQueue.java:772)
    at java.desktop/java.awt.EventQueue.run(EventQueue.java:721)
    at java.desktop/java.awt.EventQueue.run(EventQueue.java:715)
    at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
    at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:85)
    at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:95)
    at java.desktop/java.awt.EventQueue.run(EventQueue.java:745)
    at java.desktop/java.awt.EventQueue.run(EventQueue.java:743)
    at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
    at java.base/java.security.ProtectionDomain$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)

Answer 1

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

您要重新声明 passwordText。这样你就模糊了 passwordText class 变量，并且从不初始化它，所以当你在 actionPerformed 方法中引用它时，会抛出 NullPointerException。

只需删除重新声明，即

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

尝试从输入框中获取密码时出错

Getting error when trying to fetch password from input box

java

jpasswordfield