在将学生对象添加到“链接列表”之后，我尝试显示该学生的详细信息，但始终在输出端出现NullPointerException。
StudentRecords（GUI）类中的addStudent()方法旨在调用方法： Manager 类中的addStudent(Student aNewStudentObj)一个由用户输入的详细信息的学生的新对象，并将该对象存储到链接列表中。然后，Manager类中的displayStudent(Student studentId)方法应该能够使用给定的studentId显示学生及其详细信息，但就我而言，我只得到了一个NullPointerException。
我已经尝试了所有可以想到的一切，现在由于压力我开始脱发，因此我非常感谢我所能提供的所有帮助。拜托。
下面的所有代码：
下面的StudentRecords（GUI）类：
import java.awt.*;
import java.awt.event.*;
import java.util.LinkedList;
import java.util.Queue;
import javax.swing.*;
public class StudentRecords implements ActionListener {
private JTextField firstNameTextField;
private JTextField surnameTextField;
private JTextField yearOfStudyTextField;
private JTextField emailTextField;
private JTextField studentIdTextField;
private JTextField moduleCodeTextField;
private JTextField moduleMarkTextField;
private JButton addStudentButton;
private JButton addModuleButton;
private JButton displayStudentButton;
private JButton displayAllButton;
private JButton displayMarksButton;
private JButton deleteStudentButton;
private JFrame frame;
private Manager studentList = new Manager();
private static Student student;
public StudentRecords(Student student) {
this.student = student;
frame = new JFrame("Student Records");
Container contentPane = frame.getContentPane();
contentPane.setLayout(new GridLayout(9, 3));
JLabel firstNameLabel = new JLabel("First Name:");
contentPane.add(firstNameLabel);
JLabel surnameLabel = new JLabel("Surname:");
contentPane.add(surnameLabel);
JLabel nothing = new JLabel(" ");
contentPane.add(nothing);
firstNameTextField = new JTextField(15);
contentPane.add(firstNameTextField);
surnameTextField = new JTextField(15);
contentPane.add(surnameTextField);
addStudentButton = new JButton("Add Student");
contentPane.add(addStudentButton);
addStudentButton.addActionListener(this);
JLabel yearOfStudyLabel = new JLabel("Year of Study:");
contentPane.add(yearOfStudyLabel);
JLabel emailLabel = new JLabel("Email:");
contentPane.add(emailLabel);
JLabel nothing1 = new JLabel(" ");
contentPane.add(nothing1);
yearOfStudyTextField = new JTextField(15);
contentPane.add(yearOfStudyTextField);
emailTextField = new JTextField(15);
contentPane.add(emailTextField);
JLabel nothing2 = new JLabel(" ");
contentPane.add(nothing2);
JLabel studentIdLabel = new JLabel("Student ID:");
contentPane.add(studentIdLabel);
JLabel nothing3 = new JLabel(" ");
contentPane.add(nothing3);
JLabel nothing4 = new JLabel(" ");
contentPane.add(nothing4);
studentIdTextField = new JTextField(15);
contentPane.add(studentIdTextField);
displayStudentButton = new JButton("Display Student");
contentPane.add(displayStudentButton);
displayStudentButton.addActionListener(this);
displayMarksButton = new JButton("Display Marks");
contentPane.add(displayMarksButton);
displayMarksButton.addActionListener(this);
JLabel moduleCodeLabel = new JLabel("Module Code:");
contentPane.add(moduleCodeLabel);
JLabel moduleMarkLabel = new JLabel("Module Mark:");
contentPane.add(moduleMarkLabel);
JLabel nothing5 = new JLabel(" ");
contentPane.add(nothing5);
moduleCodeTextField = new JTextField(15);
contentPane.add(moduleCodeTextField);
moduleMarkTextField = new JTextField(15);
contentPane.add(moduleMarkTextField);
addModuleButton = new JButton("Add Module");
contentPane.add(addModuleButton);
addModuleButton.addActionListener(this);
displayAllButton = new JButton("Display All");
contentPane.add(displayAllButton);
displayAllButton.addActionListener(this);
JLabel nothing6 = new JLabel(" ");
contentPane.add(nothing6);
deleteStudentButton = new JButton("Delete Student");
contentPane.add(deleteStudentButton);
deleteStudentButton.addActionListener(this);
frame.pack();
frame.setVisible(true);
}
public static void main(String[] args) {
StudentRecords sRecord = new StudentRecords(student);
}
public void actionPerformed(ActionEvent event) {
String command = event.getActionCommand();
if (command.equals("Add Student")) {
addStudent();
}
if (command.equals("Add Module")) {
addModule();
}
if (command.equals("Display All")) {
displayAll();
}
if (command.equals("Display Student")) {
displayStudent();
}
if (command.equals("Display Marks")) {
displayMarks();
}
if (command.equals("Delete Student")) {
deleteStudent();
}
}
public String getFirstName() {
String firstName
= firstNameTextField.getText();
return firstName;
}
public String getSurname() {
String surname
= surnameTextField.getText();
return surname;
}
public int getYearOfStudy() {
int yearOfStudy
= Integer.parseInt(yearOfStudyTextField.getText());
return yearOfStudy;
}
public String getEmail() {
String email
= emailTextField.getText();
return email;
}
public int getStudentId() {
int studentId
= Integer.parseInt(studentIdTextField.getText());
return studentId;
}
public String getModuleCode() {
String moduleCode
= moduleCodeTextField.getText();
return moduleCode;
}
public int getModuleMark() {
int moduleMark
= Integer.parseInt(moduleMarkTextField.getText());
return moduleMark;
}
public void addStudent() {
studentList.addStudent(student);
}
public void addModule() {
//search for the student withh the ID
Student module = new Student(getFirstName(), getSurname(), getEmail(), getYearOfStudy(), getStudentId());
module.addModule(getStudentId(), getModuleCode(), getModuleMark());
}
public void displayAll() {
studentList.displayAll();
}
public void displayStudent() {
studentList.displayStudent(student);
}
public void displayMarks() {
studentList.displayMarks(student);
}
public void deleteStudent() {
studentList.deleteStudent(student);
}
}
学生课程如下：
import java.util.Arrays;
import java.util.Collections;
import java.util.LinkedList;
import java.util.Queue;
/**
*
* @author azim1
*/
public class Student {
private String firstName;
private String surname;
private String email;
private int yearOfStudy;
private int studentId;
private Queue<Module> mods = new LinkedList<>();
public Student(String firstName, String surname, String email, int yearOfStudy, int studentId) {
this.firstName = firstName;
this.surname = surname;;
this.email = email;
this.yearOfStudy = yearOfStudy;;
this.studentId = studentId;
}
public String getFirstName() {
return firstName;
}
public String getSurname() {
return surname;
}
public String getEmail() {
return email;
}
public int getYearOfStudy() {
return yearOfStudy;
}
public int getStudentId() {
return studentId;
}
public String print() {
return "Student ID: " + studentId + "\n"
+ "First Name: " + firstName + "\n"
+ "Surname: " + surname + "\n"
+ "Email: " + email + "\n"
+ "Year of Study: " + yearOfStudy;
}
public void addModule(int id, String mCode, int mMark) {
if (mods.size() == 4) {
mods.remove();
}
Module module = new Module();
mods.add(module);
}
//this method returns the module list of this student sorted by marks
public String getModulesSortedByMarks(Student id) {
Object[] sortedMods;
sortedMods = mods.toArray();
Arrays.sort(sortedMods);
String sortedModulesList = "";
for (int i = 0; i < sortedMods.length; i++) {
sortedModulesList = "\n" + ((Module) sortedMods[i]).print();
}
return sortedModulesList;
}
}
模块类如下：
package studentrecords;
/**
*
* @author azim1
*/
public class Module implements Comparable {
private String moduleCode;
private int moduleMark;
public String getModuleCode() {
return moduleCode;
}
public int getModuleMark() {
return moduleMark;
}
public String print() {
return "Module Code: " + moduleCode + "\n"
+ "Mark: " + moduleMark;
}
@Override
public int compareTo(Object aModule) {
if (this.moduleMark == ((Module) aModule).getModuleMark()) {
return 0;
} else if (this.moduleMark < ((Module) aModule).getModuleMark()) {
return -1;
} else {
return 1;
}
}
}
Manager 类别如下：
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
/**
*
* @author azim1
*/
public class Manager {
List<Student> studs = new LinkedList<>();
Student student;
public void addStudent(Student aNewStudentObj) {
studs.add(aNewStudentObj);
}
public void displayStudent(Student studentId) {
Iterator<Student> it = studs.iterator();
while (it.hasNext()) {
Student s = it.next();
if (s.equals(studentId)) {
System.out.println(s);
}
}
}
public void displayMarks(Student studentId) {
student.getModulesSortedByMarks(studentId);
}
public void deleteStudent(Student studentId) {
Iterator<Student> it = studs.iterator();
while (it.hasNext()) {
if (it.next().equals(studentId)) {
it.remove();
}
}
}
public void displayAll() {
Iterator<Student> it = studs.iterator();
while (it.hasNext()) {
System.out.println(it.next());
}
}
}
错误跟踪：
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at studentrecords.Manager.displayStudent(Manager.java:31)
at studentrecords.StudentRecords.displayStudent(StudentRecords.java:225)
at studentrecords.StudentRecords.actionPerformed(StudentRecords.java:155)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2348)
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:6539)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3324)
at java.awt.Component.processEvent(Component.java:6304)
at java.awt.Container.processEvent(Container.java:2239)
at java.awt.Component.dispatchEventImpl(Component.java:4889)
at java.awt.Container.dispatchEventImpl(Container.java:2297)
at java.awt.Component.dispatchEvent(Component.java:4711)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4904)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4535)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4476)
at java.awt.Container.dispatchEventImpl(Container.java:2283)
at java.awt.Window.dispatchEventImpl(Window.java:2746)
at java.awt.Component.dispatchEvent(Component.java:4711)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:760)
at java.awt.EventQueue.access$500(EventQueue.java:97)
at java.awt.EventQueue$3.run(EventQueue.java:709)
at java.awt.EventQueue$3.run(EventQueue.java:703)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:74)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:84)
at java.awt.EventQueue$4.run(EventQueue.java:733)
at java.awt.EventQueue$4.run(EventQueue.java:731)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:74)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:730)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:205)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)