我想创建一个程序来玩抽认卡。程序随机给出抽认卡一侧的内容，并要求用户输入另一侧。我希望该程序能够实时检查，而不是需要一个按钮来提交答案。据我了解，我需要一个 DocumentListener 来做到这一点。但是，在编码方面非常新，我在实现它时遇到了麻烦。 该程序运行，但每当我按下一个键时，它都会在控制台中出现以下错误：

线程“AWT-EventQueue-0”中的异常java.lang.NullPointerException：无法调用“javax.swing.JTextField.getText()”，因为“Play.Answer”为空

关注

在 Play$MyDocumentListener.answer(Play.java:75)

在 Play$MyDocumentListener.insertUpdate(Play.java:60)

[..]

在 java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)

在 java.desktop/java.awt.EventDispatchThread.run(EventDispatchThread.java:90)

这是代码：

import java.awt.Color;
import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;

import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.event.DocumentEvent;
import javax.swing.event.DocumentListener;


public class Play {
    
    public static JFrame frame;
    public static JPanel panel;
    public static JLabel givenSide; //side of the flashcard the player gets
    public static JTextField Answer; //player inputs answer here
    static String lineToPrint;      //this string contains the info the player gets
    //static String lineGiven;  //input given by player
    static String correctAnswer; //this string contains the correct answer

public static void PlayGUI() throws IOException  {
        
        frame = new JFrame(); 
        frame.setSize(500,300);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
        
        panel = new JPanel();
        panel.setSize(100,100);
        panel.setBackground(Color.WHITE);
        panel.setVisible(true);
        frame.add(panel);
        
        panel.setLayout(null);
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
                        
        printGivenSide();
        
        JTextField Answer = new JTextField(20);
        Answer.setbounds(10,50,100,25);
        panel.add(Answer);
                
        Answer.getDocument().addDocumentListener(new MyDocumentListener());
        Answer.setVisible(true);
            
            }
        
    static class MyDocumentListener implements DocumentListener {
        
        @Override
        public void insertUpdate(DocumentEvent arg0) {
            answer();               
        }

        @Override
        public void removeUpdate(DocumentEvent arg0) {
            answer();               
        }

        @Override
        public void changedUpdate(DocumentEvent arg0) {
            answer();
        }
        
        public void answer() {
            //lineGiven = Answer.getText();
            if (Answer.getText().equals(correctAnswer)) {correct();}
        }
        
    }

public static void printGivenSide() throws IOException {
    
    File file = new File("C:\\Users\\39340\\Desktop\\storageapp\\flashcard0.txt");
    
    //counts lines in the txt document
    int lines = 0;
    BufferedReader br = new BufferedReader(new FileReader(file));
    while (br.readLine() != null) {
        lines++;
        
    }
    
    //randomly selects a line to read 
    int lineToRead = 0 + (int)(Math.random() * ((lines - 1) + 1));  
    
    //reads lineToRead
    lineToPrint = Files.readAllLines(Paths.get("C:\\Users\\39340\\Desktop\\storageapp\\flashcard0.txt")).get(lineToRead);
    
    if (lineToRead == 1) {
        
        correctAnswer = Files.readAllLines(Paths.get("C:\\Users\\39340\\Desktop\\storageapp\\flashcard0.txt")).get(0);
    } else correctAnswer = Files.readAllLines(Paths.get("C:\\Users\\39340\\Desktop\\storageapp\\flashcard0.txt")).get(1);
    
    
    System.out.println("linea mostrata: " + lineToPrint);
    System.out.println("risposta corretta: " + correctAnswer);
    
    //System.out.println(lineToPrint);
    //System.out.println("ho generato "+lineToPrint +" e le linee sono " + lines);
    
    //creates a label to display one side of the flashcard
    JLabel givenSide = new JLabel(lineToPrint);
    givenSide.setbounds(10,20,25);
    panel.add(givenSide);
    givenSide.setVisible(true);
    
    }


public static void correct() {
    
    JLabel correct = new JLabel ("correct");
    correct.setbounds(10,75,25);
    panel.add(correct);
    correct.setVisible(true);
    
    }

    public static void main(String[] args) throws IOException {

        PlayGUI();
    
    }

}