java - Java内部类空指针异常

Question

我正在处理的聊天客户端的代码会生成以下错误：

"Exception in thread "Thread-0" java.lang.NullPointerException at ChatClient$FieldLengthChecker.run(ChatClient.java:26)"

此错误仅发生大约 50% 的时间。
另外 50% 的时间，程序按预期运行。

该线程用于“侦听” userAuthField 中的字符，以确保用户在我设置 authLoginButton.setEnabled(true) 之前至少输入了 4 个字符。

如果 authUserField 内容的长度低于 4 个字符，则应禁用该按钮。

如果我运行程序并在字段中输入 4 个字符，它要么给我上面列出的异常，要么运行良好，直到我关闭应用程序（如，我可以添加和删除字符，按钮将按预期激活/停用，直到我关闭程序）。

非常感谢任何和所有的见解。

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.io.*;
import java.net.*;

public class ChatClient {
    JFrame authFrame;
    JPanel authPanel;
    JLabel authUserLabel;
    JLabel authPassLabel;
    JLabel authMessageLabel;
    JTextField authUserField;
    JTextField authPassField;
    JButton authLoginButton;
    JButton authRegisterButton;

    String authUserData;
    String authPassData;

    //This inner-class is used in a thread to listen
    //for characters entering and exiting the userAuthField
    //in order to change the status of authLoginButton
    public class FieldLengthChecker extends Thread {

        public void run() {
            while(true) {
               String username = authUserField.getText();
               if (username.length() > 3) {
                   authLoginButton.setEnabled(true);
                }
                else {
                     authLoginButton.setEnabled(
                }
            }
        }
    }

    public static void main(String[] args) {
        ChatClient chat = new ChatClient();
        chat.start();
    }

    public void start() {
        FieldLengthChecker FieldLengthChecker = new FieldLengthChecker();
        authFrame = new JFrame("Authentication");
        authFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        authPanel = new JPanel();
        authUserLabel = new JLabel("Username");
        authPassLabel = new JLabel("Password");
        authMessageLabel = new JLabel();
        authUserField = new JTextField(20);
        authPassField = new JTextField(20);
        authLoginButton = new JButton("Login");
        authLoginButton.addActionListener(new AuthLoginButtonListener());
        authLoginButton.setEnabled(false);
        authRegisterButton = new JButton("Register");
        authFrame.getContentPane().add(authPanel);
        authPanel.add(authUserLabel);
        authPanel.add(authUserField);
        authPanel.add(authPassLabel);
        authPanel.add(authPassField);
        authPanel.add(authLoginButton);
        authPanel.add(authMessageLabel);
        authFrame.setSize(250, 300);
        authFrame.setVisible(true);
        FieldLengthChecker.start();
    }

    //This inner-class takes the user and pass information from
    //authUserField and authPassField and sends it to the 
    //Authentication server to query the associated database
    class AuthLoginButtonListener implements ActionListener {
        Socket authSock;
        BufferedReader authReader;
        PrintWriter authWriter;
        public void actionPerformed(ActionEvent event) {
            try {
                authSock = new Socket("127.0.0.1", 5001);
                InputStreamReader streamReader = new InputStreamReader(authSock.getInputStream());
                authReader = new BufferedReader(streamReader);
                authWriter = new PrintWriter(authSock.getOutputStream());
                System.out.println("Connection established with Authentication server");

                authUserData = authUserField.getText();
                authPassData = authPassField.getText();

                authWriter.println(authUserData + " " + authPassData);
                authWriter.close();
            }
            catch(Exception ex) {
                ex.printStackTrace();
                System.out.println();                                                   
                System.out.println("Failed to establish a connection with the authentication server.");
                System.out.println("See above stack trace for further information.");
                authFrame.dispose();
            }
        }
    }
}

Answer 1

试试这样的东西？

public class FieldLengthChecker extends Thread {

    public void run() {
        while(true) {
           try {
                String username = authUserField.getText();
                if (username.length() > 3) {
                    authLoginButton.setEnabled(true);
                } else {
                    authLoginButton.setEnabled(false);
                }
            } catch (NullPointerException e) {
                // handle exception
            }
        }
    }
}