我是java的新手，所以请原谅我的无知。我正在开发一个将记录写入MySQL数据库的项目。我收到一个空指针异常错误。我无法弄清楚我做错了什么。以下是代码：
package studentrecord;
import java.io.PrintStream;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;
import java.util.logging.Level;
import java.util.logging.Logger;
public class StudentRecord {
private Scanner in = new Scanner(System.in);
private PrintStream out = System.out;
private Map<Integer, Student> students = new HashMap<Integer, Student>();
private Map<String, Mentor> mentors = new HashMap<String, Mentor>();
private static Connection connection = null;
/**
* @return the connection
*/
public static Connection getConnection() {
return connection;
}
static {
// Create a connection to the registrar database
String url = "jdbc:mysql://localhost:3306/registrar";
String userName = "root";
String password = "Stef1234";
try {
// Register the JDBC driver for MySQL.
Class.forName("com.mysql.jdbc.Driver");
connection = DriverManager.getConnection(url, userName, password);
System.out.println("** CONNECTED TO DATABASE **");
} catch (Exception ex) {
Logger.getLogger(StudentRecord.class.getName()).log(Level.SEVERE, null, ex);
}
}
class InvalidStudentException extends Exception {
@Override
public String toString() {
return "** INVALID STUDENT **";
}
}
class NoStudentsFoundException extends Exception {
@Override
public String toString() {
return "** NO STUDENTS FOUND **";
}
}
class InvalidMentorException extends Exception {
@Override
public String toString() {
return "** INVALID MENTOR **";
}
}
class NoMentorsFoundException extends Exception {
@Override
public String toString() {
return "** NO MENTORS FOUND **";
}
}
/**
* Load in students from the database.
*/
public StudentRecord() {
try {
Statement statement = connection.createStatement();
ResultSet results = statement.executeQuery("SELECT * FROM student");
// Read in each student from the results
int numStudents = 0;
while (results.next()) {
int studentID = results.getInt("studentID");
String firstName = results.getString("firstName");
String lastName = results.getString("lastName");
// Attempt to determine what type of student this is
if (!results.getString("level").isEmpty()) { // undergraduate
students.put(studentID, new Undergraduate(firstName, lastName, studentID));
} else if (!results.getString("thesisTitle").isEmpty() ||
!results.getString("thesisAdvisor").isEmpty()) { // graduate
students.put(studentID, new Graduate(firstName, lastName, studentID));
} else if (!results.getString("company").isEmpty()) { // part-time
students.put(studentID, new PartTime(firstName, lastName, studentID));
} else {
out.println("** UNKNOWN STUDENT TYPE: (" + studentID + ") "
+ firstName + ", " + lastName + " **");
continue;
}
++numStudents;
}
statement.close();
System.out.println("** LOADED " + numStudents + " STUDENTS **");
} catch (SQLException ex) {
Logger.getLogger(PartTime.class.getName()).log(Level.SEVERE, null, ex);
}
}
public void run() {
// Display menu options
while (true) {
System.out.print("\nStudent Record Managing System\n"
+ "==============================\n"
+ "1) Add Student\n"
+ "2) Delete Student\n"
+ "3) Query Student\n"
+ "4) Update Student\n"
+ "5) Add Mentor\n"
+ "6) Query Mentor\n"
+ "7) Exit\n\n"
+ "Enter Option: ");
// Do the correct action
try {
switch (Integer.parseInt(in.nextLine())) {
case 1:
addStudent();
break;
case 2:
deleteStudent();
break;
case 3:
queryStudent();
break;
case 4:
updateStudent();
break;
case 5:
addMentor();
break;
case 6:
queryMentor();
break;
case 7:
cleanup();
return;
}
} catch (Exception e) {
out.println(e.toString());
}
}
}
/**
* Adds a student.
*/
private void addStudent() {
out.println();
out.print("Enter First Name: ");
String firstName = in.nextLine();
out.print("Enter Last Name: ");
String lastName = in.nextLine();
out.print("Enter ID: ");
int ID = Integer.parseInt(in.nextLine());
int option;
do {
out.print("Select Status (1 = Resident, 2 = Non-Resident): ");
option = Integer.parseInt(in.nextLine());
} while (!(option == 1 || option == 2));
Student.Status status = option == 1 ? Student.Status.Resident : Student.Status.NonResident;
int type;
do {
out.print("Select Type (1 = Undergraduate, 2 = Graduate, 3 = Part-Time): ");
type = Integer.parseInt(in.nextLine());
} while (type < 1 || type > 3);
// Request additional attributes depending on student type
Student student = null;
switch (type) {
case 1: // undergraduate
int levelOption;
do {
out.print("Select Level: (1 = Freshman, 2 = Sophomore, 3 = Junior, 4 = Senior): ");
levelOption = Integer.parseInt(in.nextLine());
} while (levelOption < 1 || levelOption > 4);
Undergraduate.Level level = levelOption == 1 ? Undergraduate.Level.Freshman
: levelOption == 2 ? Undergraduate.Level.Sophomore
: levelOption == 3 ? Undergraduate.Level.Junior
: Undergraduate.Level.Senior;
student = new Undergraduate(firstName, lastName, ID, status, level);
break;
case 2: // graduate
out.print("Enter Thesis Title: ");
String thesisTitle = in.nextLine();
out.print("Enter Thesis Advisor: ");
String thesisAdvisor = in.nextLine();
student = new Graduate(firstName, lastName, ID, status, thesisTitle, thesisAdvisor);
break;
case 3: // part-time
in.nextLine();
out.print("Enter Company: ");
String company = in.nextLine();
student = new PartTime(firstName, lastName, ID, status, company);
break;
}
// Add the student to both the internal map and the database
students.put(ID, student);
student.add();
out.println("** STUDENT ADDED **");
}
/**
* Selects a student from the current list of students.
* @return
* @throws StudentRecord.NoStudentsFoundException
* @throws StudentRecord.InvalidStudentException
*/
private int selectStudent() throws NoStudentsFoundException, InvalidStudentException {
// If there are no students, throw an exception
if (students.size() == 0) {
throw new NoStudentsFoundException();
}
// Display all the available students
out.println("\nList of students\n"
+ "================");
for (Student s : students.values()) {
out.printf("(%d) %s %s\n", s.getStudentID(), s.getFirstName(), s.getLastName());
}
out.print("\nEnter ID: ");
int ID = Integer.parseInt(in.nextLine());
if (!students.containsKey(ID)) {
throw new InvalidStudentException();
}
return ID;
}
/**
* Selects a mentor from the current list of mentors.
* @return
* @throws StudentRecord.NoMentorsFoundException
* @throws StudentRecord.InvalidMentorException
*/
private String selectMentor() throws NoMentorsFoundException, InvalidMentorException {
// If there are no mentors, throw an exception
if (mentors.size() == 0) {
throw new NoMentorsFoundException();
}
// Display all the available mentors
out.println("\nList of mentors\n"
+ "===============");
for (Mentor m : mentors.values()) {
out.printf("%s %s\n", m.getFirstName(), m.getLastName());
}
out.print("\nEnter Full Name: ");
String fullName = in.nextLine();
if (!mentors.containsKey(fullName)) {
throw new InvalidMentorException();
}
return fullName;
}
/**
* Deletes a student.
* @throws StudentRecord.NoStudentsFoundException
* @throws StudentRecord.InvalidStudentException
*/
private void deleteStudent() throws NoStudentsFoundException, InvalidStudentException {
int index = selectStudent();
students.get(index).delete();
students.remove(index);
out.println("** STUDENT DELETED **");
}
/**
* Display a student's information.
* @throws StudentRecord.NoStudentsFoundException
* @throws StudentRecord.InvalidStudentException
*/
private void queryStudent() throws NoStudentsFoundException, InvalidStudentException {
students.get(selectStudent()).query();
}
/**
* Updates a student's information.
* @throws StudentRecord.NoStudentsFoundException
* @throws StudentRecord.InvalidStudentException
*/
private void updateStudent() throws NoStudentsFoundException, InvalidStudentException {
Student student = students.get(selectStudent());
// Generate base menu
String menu = "\nUpdate\n"
+ "======\n"
+ "1) GPA\n"
+ "2) Status\n";
// Add additional attributes to the menu depending on type of student
if (student instanceof Undergraduate) {
menu += "3) Level\n";
} else if (student instanceof Graduate) {
menu += "3) Thesis Title\n"
+ "4) Thesis Advisor\n";
} else if (student instanceof PartTime) {
menu += "3) Company\n";
}
menu += "\nEnter Choice: ";
out.print(menu);
switch (Integer.parseInt(in.nextLine())) {
case 1:
out.print("Enter GPA: ");
student.setGPA(Float.parseFloat(in.nextLine()));
break;
case 2:
int option;
do {
out.print("Select Status (1 = Resident, 2 = Non-Resident): ");
option = Integer.parseInt(in.nextLine());
} while (!(option == 1 || option == 2));
Student.Status status = option == 1 ? Student.Status.Resident : Student.Status.NonResident;
student.setStatus(status);
break;
case 3:
if (student instanceof Undergraduate) {
int levelOption;
do {
out.print("Select Level: (1 = Freshman, 2 = Sophomore, 3 = Junior, 4 = Senior): ");
levelOption = Integer.parseInt(in.nextLine());
} while (levelOption < 1 || levelOption > 4);
((Undergraduate) student).setLevel(levelOption == 1 ? Undergraduate.Level.Freshman
: levelOption == 2 ? Undergraduate.Level.Sophomore
: levelOption == 3 ? Undergraduate.Level.Junior
: Undergraduate.Level.Senior);
} else if (student instanceof Graduate) {
out.print("Enter Thesis Title: ");
((Graduate) student).setThesisTitle(in.nextLine());
} else if (student instanceof PartTime) {
out.print("Enter Company: ");
((PartTime) student).setCompany(in.nextLine());
}
break;
case 4:
if (!(student instanceof Graduate)) {
out.println("** INVALID CHOICE **");
}
out.print("Enter Thesis Advisor: ");
((Graduate) student).setThesisAdvisor(in.nextLine());
break;
default:
out.println("** INVALID CHOICE **");
return;
}
student.update();
out.println("** STUDENT UPDATED **");
}
/**
* Adds a mentor.
*/
private void addMentor() {
out.println();
out.print("Enter First Name: ");
String firstName = in.nextLine();
out.print("Enter Last Name: ");
String lastName = in.nextLine();
Mentor mentor = new Mentor(firstName, lastName);
out.print("\nAdd Students\n"
+ "============\n");
for (Student s : students.values()) {
out.printf("(%d) %s %s\n", s.getStudentID(), s.getFirstName(), s.getLastName());
}
out.print("Enter a list of IDs (separated by commas): ");
String[] ids = in.nextLine().split(",");
for (String ID : ids) {
try {
mentor.addStudent(students.get(Integer.parseInt(ID)));
} catch (Exception e) {
}
}
mentors.put(firstName + ' ' + lastName, mentor);
out.println("** MENTOR ADDED **");
}
/**
* Displays the mentor's information and his list of students.
* @throws StudentRecord.NoMentorsFoundException
* @throws StudentRecord.InvalidMentorException
*/
private void queryMentor() throws NoMentorsFoundException, InvalidMentorException {
out.println("\n\nMentor Info\n"
+ "============\n"
+ mentors.get(selectMentor()).toString());
}
/**
* Cleans up resources.
*/
private void cleanup() {
if (getConnection() != null) {
try {
getConnection().close();
} catch (SQLException ex) {
Logger.getLogger(StudentRecord.class.getName()).log(Level.SEVERE, null, ex);
}
}
}
public static void main(String[] args) {
StudentRecord manager = new StudentRecord();
manager.run();
}
}
Student.java：
package studentrecord;
/**
*
* @author Gwen
*/
import java.sql.Connection;
public abstract class Student {
private String firstName;
private String lastName;
private String mentor;
private int studentID;
private float GPA;
protected Connection connection = StudentRecord.getConnection();
/**
* @return the firstName
*/
public String getFirstName() {
return firstName;
}
/**
* @return the lastName
*/
public String getLastName() {
return lastName;
}
/**
* @return the mentor
*/
public String getMentor() {
return mentor;
}
/**
* @param mentor the mentor to set
*/
public void setMentor(String mentor) {
this.mentor = mentor;
}
/**
* @return the studentID
*/
public int getStudentID() {
return studentID;
}
/**
* @param GPA the GPA to set
*/
public void setGPA(float GPA) {
this.GPA = GPA;
}
/**
* @return the GPA
*/
public double getGPA() {
return GPA;
}
/**
* @return the status
*/
public Status getStatus() {
return status;
}
/**
* @param status the status to set
*/
public void setStatus(Status status) {
this.status = status;
}
public enum Status { Resident, NonResident };
private Status status;
/**
* Creates a student.
* @param firstName
* @param lastName
* @param studentID
* @param mentor
* @param status
* @param GPA
*/
public Student(String firstName, String lastName, int studentID) {
this(firstName, lastName, studentID, Status.NonResident);
}
/**
* Creates a student.
* @param firstName
* @param lastName
* @param studentID
* @param mentor
* @param status
* @param GPA
*/
public Student(String firstName, String lastName, int studentID, Status status) {
this.firstName = firstName;
this.lastName = lastName;
this.studentID = studentID;
this.mentor = "";
this.GPA = 4.f;
}
/**
* Calculates the tuition for the student.
* @param creditHours the number of credit hours the student has
* @return
*/
public abstract double calculateTuition(int creditHours);
/**
* Updates the database entry for the student
*/
public abstract void update();
/**
* Creates the database entry for the student
*/
public abstract void add();
/**
* Deletes the database entry for the student
*/
public abstract void delete();
/**
* Queries the database for the student's information
* and prints it to the screen
*/
public abstract void query();
@Override
public String toString() {
return "First Name: " + firstName + '\n' +
"Last Name: " + lastName + '\n' +
"Student ID: " + studentID + '\n' +
"Status: " + status.toString() + '\n' +
"Mentor: " + mentor + '\n' +
"GPA: " + GPA;
}
}
任何帮助都会非常感激!!谢谢！
答案 0 :(得分：0)
检查ResultSet.getString的文档：
http://docs.oracle.com/javase/7/docs/api/java/sql/ResultSet.html#getString%28int%29
String getString（int columnIndex）抛出SQLException
的返回： 列值;如果值为SQL NULL，则返回的值为空
根据这一点，您的代码在以下位置易受null pointer exception攻击：
if (!results.getString("level").isEmpty()) {
.......
} else if (!results.getString("thesisTitle").isEmpty() ||
!results.getString("thesisAdvisor").isEmpty()) {
..........
} else if (!results.getString("company").isEmpty()) {
............
如果这些列中的任何一个包含空值，则getString( .. )会返回null，因此getString( ... ).isEmpty()会引发null pointer exception。