java - 尝试从输入框获取密码时出错
问题描述
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,但是每当我尝试密码时,我都会遇到一大堆错误。我已经搜索了半小时的最佳部分,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或将我链接到涵盖我需要的帖子或网站(对于我目前正在尝试获取的上下文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$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)
解决方案
JPasswordField passwordText = new JPasswordField();
passwordText.setBounds(100, 50, 165, 25);
panel.add(passwordText);
你在重新声明passwordText
. 这样你就隐藏了类变量,并且从不初始化它,所以当你在你的方法passwordText
中引用它时会抛出 a 。actionPerformed
NullPointerException
只需删除重新声明,即
passwordText = new JPasswordField();
passwordText.setBounds(100, 50, 165, 25);
panel.add(passwordText);
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