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3
Another Problem.
STUDENTS:
/** A class that models a Student of a driving school
*/
public class Students extends Lessons{
private String name;
private String address;
private int number;
private int noLessons;
public Students() {
name = "No Name Specified";
address = "No Address Specified";
number = 0;
noLessons = 0;
}
public Students (String name, String addr, int num, int noLes) {
setName(name);
setAddress(addr);
setNumber(num);
setNoLessons(noLes);
}
public String getName() {
return name;
}
public String getAddress() {
return address;
}
public int getNumber() {
return number;
}
public int getNoLessons() {
return noLessons;
}
public void setName(String name) {
name = name;
}
public void setAddress(String addr) {
name = addr;
}
public void setNumber(int num) {
number = num;
}
public void setNoLessons(int noLes) {
noLessons = noLes;
}
public String toString(){
return "Name: " + name + "\nAddress: " + address + "\nPhone Number: " + number + "\nNumber of Lessons: " + noLessons;
}
}
LESSONS:
/**A class that models the Lessons in a driving school
*/
public class Lessons{
private String location;
private String time;
private double price;
// 'accessor' methods to return a copy of an attribute
public String getLocation() { return location;}
public String getTime() { return time;}
public double getPrice() { return price;}
// 'mutator' methods to change the value of an attribute
public void setLocation (String location) {
this.location = location;
}
public void setTime( String time) {
this.time = time;
}
public void setPrice(double price) {
this.price = price;
}
// 'constructor' methods to give lessons a sensible initial state
/** full-args constructor, to create a lesson, which everything is known
*/
public Lessons(String location, String time, double price) {
setLocation(location);
setTime(time);
setPrice(price);
}
/** no-args constructor
*/
public Lessons() {
this("Not Given","00:00",00.00);
}
/* String summary of the values of all the object's attributes
*/
public String toString() {
return "Location: " + getLocation() + "\nPrice: €" + getPrice() + "\nTime: " + getTime();
}
}
DRIVER PROGRAM:
/**Driving School Tester*/
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.io.*;
public class DrivingSchoolTest extends JFrame implements ActionListener{
JMenu fileMenu,lessonMenu,studentMenu;
Students [] student;
Lessons [] lesson;
int count;
int count1;
public static void main( String[] args ) {
DrivingSchoolTest frame = new DrivingSchoolTest();
frame.setVisible(true);
}
public DrivingSchoolTest( ) {
setTitle ( "Driving School" );
setSize ( 400,200 );
setLocation ( 350,300 );
Container pane = getContentPane();
pane.setBackground(Color.WHITE);
setDefaultCloseOperation( EXIT_ON_CLOSE );
createFileMenu();
createStudentMenu();
createLessonMenu();
JMenuBar menuBar = new JMenuBar();
setJMenuBar(menuBar);
menuBar.add(fileMenu);
menuBar.add(studentMenu);
menuBar.add(lessonMenu);
}
public void newSystemStudent() {
student = new Students[10];
count = 0;
}
/** writes the array of students to the file "student.dat"
*/ // NEW
public void saveStudent() throws IOException {
ObjectOutputStream os;
os = new ObjectOutputStream(new FileOutputStream ("student.dat"));
os.writeObject(student);
os.close();
}
/** loads an array of students from the file "student.dat"
*/ // NEW
public void openStudent() {
count = 0;
try{
ObjectInputStream is;
is = new ObjectInputStream(new FileInputStream ("student.dat"));
student = (Students []) is.readObject();
is.close();
}
catch(Exception e){
JOptionPane.showMessageDialog(null,"open didn't work");
e.printStackTrace();
}
// how many valid student records?
while (student[count] !=null)
count++;
} // end open()
public void addStudent(){
Students s1 = new Students();
s1.setName(JOptionPane.showInputDialog("Enter Student Name:"));
s1.setAddress(JOptionPane.showInputDialog("Enter address for Student 2: "));
s1.setNumber(JOptionPane.showInputDialog("Enter phone number for Student 2: "));
s1.setNoLessons(Integer.parseInt(JOptionPane.showInputDialog("Enter number of lessons for Student 2: ")));
student[count] = s1; // 'default student
// set the attributes: ask the user for name etc
count++; // now there is one more student in the system
}
public void displayStudent(){
JTextArea area = new JTextArea();
if (count>0) {
area.setText("Student List:");
for (int i = 0; i<count; i++) // loop over existing bikes, rather than array size
area.append("Students: " + i + " " +student[i].toString()+"\n");
showMessage(area);
}
else
showMessage("No students in the system");
} // end display
public void newSystemLesson() {
lesson = new Lessons[10];
count1 = 0;
}
/** writes the array of lessons to the file "lesson.dat"
*/ // NEW
public void saveLesson() throws IOException {
ObjectOutputStream os;
os = new ObjectOutputStream(new FileOutputStream ("lesson.dat"));
os.writeObject(lesson);
os.close();
}
/** loads an array of lessons from the file "lesson.dat"
*/ // NEW
public void openLesson() {
count1 = 0;
try{
ObjectInputStream is;
is = new ObjectInputStream(new FileInputStream ("lesson.dat"));
lesson = (Lessons []) is.readObject();
is.close();
}
catch(Exception e){
JOptionPane.showMessageDialog(null,"open didn't work");
e.printStackTrace();
}
// how many valid lesson records?
while (lesson[count1] !=null)
count1++;
} // end open()
public void addLesson(){
Lessons l2 = new Lessons();
l2.setLocation(JOptionPane.showInputDialog("Enter location for Lesson 2: "));
l2.setTime(JOptionPane.showInputDialog("Enter time for Lesson 2: "));
l2.setPrice(Double.parseDouble(JOptionPane.showInputDialog("Please enter a price for lesson 2: ")));
lesson[count1] = l2; // 'default lesson
// set the attributes: ask the user for location etc
count++; // now there is one more lesson in the system
}
public void displayLesson(){
JTextArea area = new JTextArea();
if (count1>0) {
area.setText("Lesson List:");
for (int i = 0; i<count1; i++) // loop over existing bikes, rather than array size
area.append("Lessons: " + i + " " +lesson[i].toString()+"\n");
showMessage(area);
}
else
showMessage("No lessons in the system");
} // end display
public void actionPerformed (ActionEvent e) {
if (e.getActionCommand() .equals ("Quit")){
showMessage("Shutting down the system");
System.exit(0);
}
else if (e.getActionCommand() .equals ("Add")){
addStudent(); // branch to a separate method
}
else if (e.getActionCommand() .equals ("Display")){
displayStudent();
}
else if (e.getActionCommand() .equals ("Add")){
addLesson(); // branch to a separate method
}
else if (e.getActionCommand() .equals ("Display")){
displayLesson();
}
else if (e.getActionCommand() .equals ("New File")){
newSystemStudent();
newSystemStudent();
}
else if (e.getActionCommand() .equals ("Save")){
// Error Handling
try{
saveStudent();
saveLesson();
showMessage("Data saved successfully");
} // try
catch (IOException f){
showMessage("Not able to save the file:");
f.printStackTrace();
}// catch
}// else if
else if (e.getActionCommand() .equals ("Open")){
openStudent();
openLesson();
displayStudent();
displayLesson();
}
else
showMessage("Error");
} // actionPerformed
private void createFileMenu(){
// create the menu
fileMenu = new JMenu("File");
// declare a menu item (re-usable)
JMenuItem item;
item = new JMenuItem("Save");
item.addActionListener(this);
fileMenu.add(item);
item = new JMenuItem("Open");
item.addActionListener(this);
fileMenu.add(item);
item = new JMenuItem("New File");
item.addActionListener(this);
fileMenu.add(item);
// create the 'quit' option
fileMenu.addSeparator();
item = new JMenuItem("Quit");
item.addActionListener(this);
fileMenu.add(item);
}
private void createStudentMenu(){
// create the menu
studentMenu = new JMenu("Student");
// declare a menu item (re-usable)
JMenuItem item;
item = new JMenuItem("Add");
item.addActionListener(this);
studentMenu.add(item);
item = new JMenuItem("Display");
item.addActionListener(this);
studentMenu.add(item);
}
private void createLessonMenu(){
// create the menu
lessonMenu = new JMenu("Lesson");
// declare a menu item (re-usable)
JMenuItem item;
item = new JMenuItem("Add");
item.addActionListener(this);
lessonMenu.add(item);
item = new JMenuItem("Display");
item.addActionListener(this);
lessonMenu.add(item);
}
/** utility methods to make the code simpler */
public void showMessage (String s){
JOptionPane.showMessageDialog(null,s);
}
public void showMessage (JTextArea s){
JOptionPane.showMessageDialog(null,s);
}
}
This program compiles Perfectly no errors. My problem is that when i try and add new student the program crashes and i get this error:
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException at DrivingSchoolTest.addStudent(DrivingSchoolTest.java:83) at DrivingSchoolTest.actionPerformed(DrivingSchoolTest.java:170) at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995) at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2318) at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387) at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242) at javax.swing.AbstractButton.doClick(AbstractButton.java:357) at javax.swing.plaf.basic.BasicMenuItemUI.doClick(BasicMenuItemUI.java:1170) at javax.swing.plaf.basic.BasicMenuItemUI$Handler.mouseReleased(BasicMenuItemUI.java:1211) at java.awt.Component.processMouseEvent(Component.java:6038) at javax.swing.JComponent.processMouseEvent(JComponent.java:3260) at java.awt.Component.processEvent(Component.java:5803) at java.awt.Container.processEvent(Container.java:2058) at java.awt.Component.dispatchEventImpl(Component.java:4410) at java.awt.Container.dispatchEventImpl(Container.java:2116) at java.awt.Component.dispatchEvent(Component.java:4240) at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4322) at java.awt.LightweightDispatcher.processMouseEvent(Container.java:3986) at java.awt.LightweightDispatcher.dispatchEvent(Container.java:3916) at java.awt.Container.dispatchEventImpl(Container.java:2102) at java.awt.Window.dispatchEventImpl(Window.java:2429) at java.awt.Component.dispatchEvent(Component.java:4240) at java.awt.EventQueue.dispatchEvent(EventQueue.java:599) at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:273) at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:183) at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:173) at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:168) at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:160) at java.awt.EventDispatchThread.run(EventDispatchThread.java:121)
Can somebody help with a solution to this and possibly straighten out my code to make it run better.
Thanks for any help.
Aaron.
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