我正在开发一个java测验游戏，它涉及从数据库中获取一系列问题。游戏编译并运行，但是当我单击“测试”按钮时，我得到上述错误。 这是我的代码：
import java.awt.*;
import java.awt.event.*;
import java.sql.*;
import java.util.*;
public class PhysicsQuiz extends Frame implements WindowListener, ActionListener{
public Connection getConnection() throws SQLException{
String user = "root";
String pass = "usbw";
Connection c = null;
c = DriverManager.getConnection("jdbc:mysql://localhost:3307/pastpapers", user, pass);
System.out.println("Connected to database");
return c;
}
private Random rand = new Random();
private Label menuheader;
private Choice topiclist;
private Button test;
private Button scores;
private Button quit;
private Label questionheader;
private Choice multiquestion;
private TextField calculation;
private TextField equation;
public PhysicsQuiz(){
setLayout(new FlowLayout());
menuheader = new Label("Choose the topic you wish to revise.");
add(menuheader);
topiclist = new Choice();
topiclist.add("Mechanics");
topiclist.add("Materials");
topiclist.add("Waves");
topiclist.add("DC Electricity");
topiclist.add("Nature of Light");
add(topiclist);
test = new Button("Test!");
add(test);
test.addActionListener(this);
scores = new Button("View previous scores on this topic");
add(scores);
scores.addActionListener(this);
quit = new Button("Quit");
add(quit);
quit.addActionListener(this);
addWindowListener(this);
setTitle("Physics Quiz");
setSize(250, 300);
setVisible(true);
}
public void Test() throws SQLException{
String topic = topiclist.getSelectedItem();
remove(topiclist);
remove(test);
remove(scores);
remove(quit);
question[] questions = new question[10];
for(int j=0;j<questions.length;j++){
try{
Connection c = getConnection();
Statement st = c.createStatement();
int random = rand.nextInt(33);
System.out.println(random);
String sqlcontent = "SELECT QuestionContent FROM question WHERE QuestionTopic = '" + topic + "' AND QuestionID = " + random;
ResultSet rs1 = st.executeQuery(sqlcontent);
try{
while (rs1.next()){
questions[j].content = rs1.getString(1);
System.out.println(questions[j].content);
}
} finally {
rs1.close();
}
String sqltype = "SELECT QuestionType FROM question WHERE QuestionTopic = '" + topic + "' AND QuestionID = " + random;
ResultSet rs2 = st.executeQuery(sqltype);
try{
while (rs2.next()){
questions[j].type = rs2.getString(1);
System.out.println(questions[j].type);
}
} finally {
rs2.close();
}
st.close();
c.close();
}catch(SQLException ex){
System.out.println(ex);
}
}
for(int i = 0;i<questions.length;i++){
if(questions[i].type.equals("Multiple Choice")){
questionheader = new Label("Question " + i + " : " + questions[i].content);
multiquestion = new Choice();
}
if(questions[i].type.equals("Calculation")){
questionheader = new Label("Question " + i + " : " + questions[i].content);
calculation = new TextField();
equation = new TextField();
}
else{
System.out.println("Error. Cannot get results from query");
}
}
}
public static void main(String[] args){
PhysicsQuiz app = new PhysicsQuiz();
}
public void actionPerformed(ActionEvent e){
String action = e.getActionCommand();
if(action.equals("Test!")){
try{
Test();
}
catch(SQLException ex){
System.out.println(ex);
}
}
if(action.equals("View previous scores on this topic")){
}
if(action.equals("Quit")){
System.exit(0);
}
}
public void windowClosing(WindowEvent e) {
System.exit(0);
}
public void windowOpened(WindowEvent e) { }
public void windowClosed(WindowEvent e) { }
public void windowIconified(WindowEvent e) { }
public void windowDeiconified(WindowEvent e) { }
public void windowActivated(WindowEvent e) { }
public void windowDeactivated(WindowEvent e) { }
}
这是我的完整错误消息：
E:\Documents\GitHub\COMP4-Project>java PhysicsQuiz
Connected to database
2
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at PhysicsQuiz.Test(PhysicsQuiz.java:70)
at PhysicsQuiz.actionPerformed(PhysicsQuiz.java:116)
at java.awt.Button.processActionEvent(Button.java:409)
at java.awt.Button.processEvent(Button.java:377)
at java.awt.Component.dispatchEventImpl(Component.java:4881)
at java.awt.Component.dispatchEvent(Component.java:4703)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:751)
at java.awt.EventQueue.access$500(EventQueue.java:97)
at java.awt.EventQueue$3.run(EventQueue.java:702)
at java.awt.EventQueue$3.run(EventQueue.java:696)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDo
main.java:75)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDo
main.java:86)
at 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.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDo
main.java:75)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:721)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThre
ad.java:201)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.
java:116)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThre
ad.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)
我将非常感谢所有帮助，因为这是一个截止日期的项目。谢谢！