线程“AWT-EventQueue-0”Java错误中的异常

我的GUI出了问题 . 代码没有完成，但它仍然可以工作 . 按下阅读按钮后出现此错误：

线程“AWT-EventQueue-0”中的异常java.lang.NumberFormatException：在sun.misc.FloatingDecimal.parseDouble（FloatingDecimal.java:110）处的sun.misc.FloatingDecimal.readJavaFormatString（FloatingDecimal.java:1842）处的空字符串java.lang.Double.parseDouble（Double.java:538）在coursework.Coursework $ 2.actionPerformed（Coursework.java:135）at javax.swing.AbstractButton.fireActionPerformed（AbstractButton.java:2022）at javax.swing.AbstractButton $ Handler.actionPerformed（AbstractButton.java:2346）位于javax.swing.plaf.basic.BasicButtonListener的javax.swing.DefaultButtonModel.setPressed（DefaultButtonModel.java:259）的javax.swing.DefaultButtonModel.fireActionPerformed（DefaultButtonModel.java:402） .mouseReleased（BasicButtonListener.java:252）at java.awt.Component.processMouseEvent（Component.java：6525）at java.awt.Component.processEvent（Component . ）的javax.swing.JComponent.processMouseEvent（JComponent.java:3324） . java：6290）java.awt.Comp的java.awt.Container.processEvent（Container.java:2234） java.awt.Light上的java.awt.Container.dispatchEventImpl（Container.java:2292）中的onent.dispatchEventImpl（Component.java:4881）java.awt.LightweightDispatcher.retargetMouseEvent上的java.awt.Component.dispatchEvent（Component.java:4703） .java：4898）java.awt.LightweightDispatcher.processMouseEvent（Container.java：4533）at java.awt.LightweightDispatcher.dispatchEvent（Container.java:4462）at java.awt.Container.dispatchEventImpl（Container.java:2278）在Java.awt.Window.dispatchEventImpl（Window.java:2750）的java.awt.Component.dispatchEvent（Component.java:4703），java.awt的java.awt.EventQueue.dispatchEventImpl（EventQueue.java:751） . 在java.security.AccessController的java.awt.EventQueue $ 3.run（EventQueue.java:696）的java.awt.EventQueue $ 3.run（EventQueue.java:702）上的EventQueue.access $ 500（EventQueue.java:97） . doPrivileged（本机方法）在java.security.ProtectionDomain $ 1.doIntersectionPrivilege（ProtectionDomain.java:75）at java.security.ProtectionDomain $ 1.doIntersectionPrivilege（ProtectionDomain.java:86 ）java.awt.EventQueue $ 4.run（EventQueue.java:724）at java.awt.EventQueue $ 4.run（EventQueue.java:722）at java.security.AccessController.doPrivileged（Native Method）at java.security . 保护域$ 1.doIntersectionPrivilege（ProtectionDomain.java:75）at java.awt.EventQueue.dispatchEvent（EventQueue.java:721）at java.awt.EventDispatchThread.pumpOneEventForFilters（EventDispatchThread.java:201）at java.awt.EventDispatchThread.pumpEventsForFilter（ EventDispatchThread.java:116）java.awt.EventDispatchThread.pumpEventsForHierarchy（EventDispatchThread.java:105）at java.awt.EventDispatchThread.pumpEvents（EventDispatchThread.java:101）at java.awt.EventDispatchThread.pumpEvents（EventDispatchThread.java:93） ）在java.awt.EventDispatchThread.run（EventDispatchThread.java:82）

和代码

package coursework;

import java.awt.Color;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.io.Writer;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;


public class Coursework extends JFrame  {    
    public static void main(String[] args) {
     JFrame frame=new JFrame("myframe");
     JPanel panel = new JPanel();
     panel.setLayout(new GridLayout(18,1));
    JLabel StudentNameLabel = new JLabel("Student Name: ");
     final JTextField TypeNameArea = new JTextField(" "); 
     JLabel StudentSurnameLabel = new JLabel("Student Surname: ");
      final JTextField TypeSurnameArea = new JTextField(" ");
     JLabel StudentIDLabel = new JLabel("Student ID: ");
     final JTextField StudentIDArea = new JTextField("M00");
     JLabel LabTestLabel = new JLabel("Lab Test: ");
     final JTextField LabTestArea = new JTextField(" ");
     JLabel QuizLabel = new JLabel("Quizes: ");
     final  JTextField QuizArea = new JTextField(" ");
     JLabel ICourseworkLabel = new JLabel("Indiwidual CW: ");
     final JTextField ICourseworkArea = new JTextField(" ");
     JLabel GCourseworkLabel = new JLabel("Group CW: ");
     final JTextField GCourseworkArea = new JTextField(" ");
     panel.setBackground(Color.BLACK);
     StudentSurnameLabel.setForeground(Color.YELLOW);
     StudentNameLabel.setForeground(Color.YELLOW);
     LabTestLabel.setForeground(Color.YELLOW);
     StudentIDLabel.setForeground(Color.YELLOW);
     QuizLabel.setForeground(Color.YELLOW);
     ICourseworkLabel.setForeground(Color.YELLOW);
     GCourseworkLabel.setForeground(Color.YELLOW);
     JButton buttonSubmit = new JButton("Submit");
     buttonSubmit.setBackground(Color.BLACK);
     buttonSubmit.setForeground(Color.WHITE);
     JButton buttonRead = new JButton("Read");
     buttonRead.setBackground(Color.BLACK);
     buttonRead.setForeground(Color.WHITE);
     JButton buttonCancel = new JButton("Cancel");
     buttonCancel.setBackground(Color.BLACK);
     buttonCancel.setForeground(Color.WHITE);
     JButton buttonNext = new JButton("Next");
     buttonNext.setBackground(Color.BLACK);
     buttonNext.setForeground(Color.WHITE);

     panel.add(StudentNameLabel);
     panel.add(TypeNameArea);
     panel.add(StudentSurnameLabel);
     panel.add(TypeSurnameArea);
     panel.add(StudentIDLabel);
     panel.add(StudentIDArea);
     panel.add(LabTestLabel);
     panel.add(LabTestArea);
     panel.add(QuizLabel);
     panel.add(QuizArea);
     panel.add(ICourseworkLabel);
     panel.add(ICourseworkArea);
     panel.add(GCourseworkLabel);
     panel.add(GCourseworkArea);
     panel.add(buttonSubmit);
     panel.add(buttonRead);
     panel.add(buttonCancel);
     panel.add(buttonNext);
     frame.setSize(300, 800);
     frame.add(panel);
     frame.setVisible(true);

     frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);



    buttonNext.addActionListener(new ActionListener() {

            @Override
            public void actionPerformed(ActionEvent e)
            {
                String name;
                String surname;
                String number;
                String gradeLabTest;
                String gradeQuiz;
                String gradeICourswork;
                String gradeGCourswork;
                name = TypeNameArea.getText();
                surname = TypeSurnameArea.getText();
                number = StudentIDArea.getText();
                gradeLabTest = LabTestArea.getText();
                gradeQuiz = QuizArea.getText();
                gradeICourswork = ICourseworkArea.getText();
                gradeGCourswork = GCourseworkArea.getText();
                String line = name + " " + surname + " " + number + " " + gradeLabTest + " " + gradeQuiz + gradeICourswork + gradeGCourswork + "\r\n";
                Writer output;
                try {
                    output = new BufferedWriter(new FileWriter("C:/Hola/test.txt",true));  //clears file every time
                    output.append(line);
                    output.close();
                } catch (IOException ex) {
                    Logger.getLogger(Coursework.class.getName()).log(Level.SEVERE, null, ex);
                }
            }
        }); 

     buttonRead.addActionListener(new ActionListener() {

            @Override
            public void actionPerformed(ActionEvent e)
            {
                BufferedReader br ;
                try {
                    br = new BufferedReader(new FileReader("C:/Hola/test.txt"));
                    for(String line; (line = br.readLine()) != null; ) {
                    String[] token = line.split(" ");
                    String name = token[0];
                    String surname = token[1];
                    String number = token[2];
                    double g1 = Double.parseDouble(token[3]);
                    double g2 = Double.parseDouble(token[4]);
                    double g3 = Double.parseDouble(token[5]);
                    double g4 = Double.parseDouble(token[6]);

                    double av = 0.4*g1 + 0.2*g2 + 0.2*g3 + 0.2*g4;
                    int grade = 0;
                    String gradestring;




                    String line_input = name + " " + surname + " " + number + " " + av + " " + grade + "\r\n";
                    Writer output;
                    try {
                        output = new BufferedWriter(new FileWriter("C:/Hola/test2.txt",true));  //clears file every time
                        output.append(line_input);
                        output.close();
                    } catch (IOException ex) {
                        Logger.getLogger(Coursework.class.getName()).log(Level.SEVERE, null, ex);
                    }


                // line is not visible here.
            }
                } catch (FileNotFoundException ex) {
                    Logger.getLogger(Coursework.class.getName()).log(Level.SEVERE, null, ex);
                } catch (IOException ex) {
                    Logger.getLogger(Coursework.class.getName()).log(Level.SEVERE, null, ex);
                }

            }
        }); 
    }

}