java - ClassCastException : javax. Swing .JButton

Question

我只想做一个简单的用户密码登录。

我在那里放了一个 ActionListener，当我按下登录时，只需弹出密码并检查它是否正确。

import java.awt.BorderLayout;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.GridLayout;
import java.awt.Insets;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.JPasswordField;
import javax.swing.JTextField;

public class PasswordForm 
{
    private static String password = "mypass";
    public static void main(String[] args)
    {
        // Basic form create
        JFrame frame = new JFrame("Form 1");
        frame.setSize(300,300);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        // Creating the grid
        JPanel panel = new JPanel(new GridBagLayout());
        frame.getContentPane().add(panel, BorderLayout.NORTH);
        GridBagConstraints c = new GridBagConstraints();

        // Create some elements
        JTextField usernameInput = new JTextField(10);
        c.gridx = 0;
        c.gridy = 1;
        panel.add(usernameInput,c);

        JPasswordField passwordInput = new JPasswordField(10);
        c.gridx = 0;
        c.gridy = 2;
        panel.add(passwordInput,c);

        JButton loginInput = new JButton("Login");
        c.gridx = 0;
        c.gridy = 3;
        loginInput.addActionListener(new LoginButton());
        panel.add(loginInput,c);


        frame.setVisible(true);
    }

    static class LoginButton implements ActionListener
    {
        public void actionPerformed(ActionEvent e)
        {
            JTextField usernameInput = (JTextField)e.getSource();
            JOptionPane.showMessageDialog(null,"Text is:");
        }
    }
}

有人帮忙吗？

错误

    Exception in thread "AWT-EventQueue-0" java.lang.ClassCastException: javax.swing.JButton
at PasswordForm$LoginButton.actionPerformed(PasswordForm.java:56)
at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
at java.awt.Component.processMouseEvent(Unknown Source)
at javax.swing.JComponent.processMouseEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)

Answer 1

当您忽略代码存在编译错误这一事实并尝试运行它时，会发生以下异常。 (我假设您使用的是 Eclipse。在相关源文件中查找红色错误标记，并检查 Problems View 。)

    Exception in thread "AWT-EventQueue-0" java.lang.Error: 
        Unresolved compilation problem:
        The method getText() is undefined for the type ActionEvent

奇怪的是，嵌入式编译错误消息似乎与您发布的源代码不对应。要么您更改了代码，要么构建过程中的某些缺陷导致您运行陈旧的类文件。

---

其他几点:

• 您在嵌套类中违反了 Java 的命名约定。类名应该总是以大写字母开头。将“登录按钮”更改为“登录按钮”。

• 您的 PasswordForm 类过多地使用了 static。 static 内部类没问题，但是将 password 声明为静态，并将所有逻辑放入静态 main 方法会导致问题从长远来看。 (好吧，这段代码显然是实验性的……就目前的形式而言。)