2

这是我的代码，其中包含我在控制台中遇到的错误的复制和粘贴。

package com.bob;

import java.awt.FlowLayout;
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 BobLogin extends JFrame implements ActionListener {

    // Declare all of the component variables.
    private JButton login;
    private JTextField usernameField;
    private JPasswordField passwordField;
    private JLabel username;
    private JLabel password;
    private JFrame loginFrame;

    private String usernameText = "Username:";
    private int usernameColumns = 20;
    private String passwordText = "Password:";
    private int passwordColumns = 20;

    //Declare all of the constants.
    private static final int WIDTH = 300;
    private static final int HEIGHT = 300;

    //Login password and username.
    private static final String correctUsername = "Admin";
    private static final String correctPassword = "Password";

    //Constructor that calls initliaziation method when an object is created.
    public BobLogin() {
        initializeGUI();
    }

    //Sets up the login GUI.
    private void initializeGUI() {
        JButton login = new JButton("Login");
        JTextField usernameField = new JTextField(usernameColumns);
        JPasswordField passwordField = new JPasswordField(passwordColumns);
        JLabel username = new JLabel(usernameText);
        JLabel password = new JLabel(passwordText);
        JFrame loginFrame = new JFrame();
        JPanel loginPanel = new JPanel(new FlowLayout());
        loginPanel.add(username);
        loginPanel.add(usernameField);
        loginPanel.add(password);
        loginPanel.add(passwordField);
        loginPanel.add(login);
        login.addActionListener(this);
        loginFrame.add(loginPanel);
        // loginFrame.setSize(WIDTH, HEIGHT);
        loginFrame.pack();
        loginFrame.setLocationRelativeTo(null);
        loginFrame.setResizable(false);
        loginFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        loginFrame.setVisible(true);
        //username.req

    }

    public static void main(String[] args) {
        BobLogin bobLogin = new BobLogin();
    }

    //What happens when the "Login" button is clicked.
    @Override
    public void actionPerformed(ActionEvent e) {
        String usernameResponse = usernameField.getText();
        //String passwordResponse = passwordField.getText();
        if(usernameResponse.equalsIgnoreCase("Admin")){
            System.out.println("Correct");
        }else{
            System.exit(0);
        }
    }

}

这是我点击“登录”时遇到的错误。一切正常，但每当我单击登录按钮时，我都会在控制台中收到此错误：

 at com.bob.BobLogin.actionPerformed(BobLogin.java:75)
        at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
        at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)
        at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
        at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
        at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
        at java.awt.Component.processMouseEvent(Component.java:6505)
        at javax.swing.JComponent.processMouseEvent(JComponent.java:3321)
        at java.awt.Component.processEvent(Component.java:6270)
        at java.awt.Container.processEvent(Container.java:2229)
        at java.awt.Component.dispatchEventImpl(Component.java:4861)
        at java.awt.Container.dispatchEventImpl(Container.java:2287)
        at java.awt.Component.dispatchEvent(Component.java:4687)
        at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
        at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492)
        at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)
        at java.awt.Container.dispatchEventImpl(Container.java:2273)
        at java.awt.Window.dispatchEventImpl(Window.java:2719)
        at java.awt.Component.dispatchEvent(Component.java:4687)
        at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:723)
        at java.awt.EventQueue.access$200(EventQueue.java:103)
        at java.awt.EventQueue$3.run(EventQueue.java:682)
        at java.awt.EventQueue$3.run(EventQueue.java:680)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
        at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
        at java.awt.EventQueue$4.run(EventQueue.java:696)
        at java.awt.EventQueue$4.run(EventQueue.java:694)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:693)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)
4

2 回答 2

3

您正在使用实例变量在触发usernameField时获取文本。ActionEvent问题是，您正在方法内部创建新的局部usernameField变量，initializeGUI而它已经创建为实例变量但尚未初始化。要解决您的问题，请在方法usernameField之外初始化您的变量initializeGUI。这是工作代码：

import java.awt.FlowLayout;
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 BobLogin extends JFrame implements ActionListener {

    // Declare all of the component variables.
    private JButton login;
    //Here is an issue fix!
    private JTextField usernameField = new JTextField(20);
    private JPasswordField passwordField = new JPasswordField(20);;
    private JLabel username;
    private JLabel password;
    private JFrame loginFrame;

    private String usernameText = "Username:";
    private int usernameColumns = 20;
    private String passwordText = "Password:";
    private int passwordColumns = 20;

    //Declare all of the constants.
    private static final int WIDTH = 300;
    private static final int HEIGHT = 300;

    //Login password and username.
    private static final String correctUsername = "Admin";
    private static final String correctPassword = "Password";

    //Constructor that calls initliaziation method when an object is created.
    public BobLogin() {
        initializeGUI();
    }

    //Sets up the login GUI.
    private void initializeGUI() {
        JButton login = new JButton("Login");
        JLabel username = new JLabel(usernameText);
        JLabel password = new JLabel(passwordText);
        JFrame loginFrame = new JFrame();
        JPanel loginPanel = new JPanel(new FlowLayout());
        loginPanel.add(username);
        loginPanel.add(usernameField);
        loginPanel.add(password);
        loginPanel.add(passwordField);
        loginPanel.add(login);
        login.addActionListener(this);
        loginFrame.add(loginPanel);
        // loginFrame.setSize(WIDTH, HEIGHT);
        loginFrame.pack();
        loginFrame.setLocationRelativeTo(null);
        loginFrame.setResizable(false);
        loginFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        loginFrame.setVisible(true);
        //username.req

    }

    public static void main(String[] args) {
        BobLogin bobLogin = new BobLogin();
    }

    //What happens when the "Login" button is clicked.
    @Override
    public void actionPerformed(ActionEvent e) {
        String usernameResponse = usernameField.getText();
        //String passwordResponse = passwordField.getText();
        if(usernameResponse.equalsIgnoreCase("Admin")){
            System.out.println("Correct");
        }else{
            System.exit(0);
        }
    }

}
于 2013-07-10T23:11:40.843 回答
2

You have this line in your initialize() method

JTextField usernameField = new JTextField(usernameColumns);

Which is actually creating a local instance of usernameField.

Try

usernameField = new JTextField(usernameColumns);

to initialize your class variable.

This concept also applies to a few other class variables you declared.

于 2013-07-10T23:13:49.377 回答