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一个文本文件中。我是java新手，发现了一本关于制作类似系统的老教程。到目前为止，一切都正常，包括程序能够从输入中获取用户名，但每当我尝试密码，我也会看到一大堆错误。我已经搜索了半个小时的大部分时间，似乎找不到相关的信息，所以我只是想知道是否有人可以帮助或链接到一个职位或网站，涵盖了我所需要的（为上下文，我目前正试图让它Spring的用户名和密码到终端，只是为了测试它可以得到他们正确的）
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（单击按钮后）：

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)