Thread: Multiple nullpointerexceptions in my code.

I thought I had this working but I'm getting nullpointerexceptions now on lines 21,31, & 80. I think I've been looking at it for too long.
Can someone help me out?

 
1 package student;
2
3 import java.sql.Connection;
4 import java.sql.ResultSet;
5 import java.sql.Statement;
6 import java.util.HashMap;
7 import java.util.Map.Entry;
8 import java.util.Scanner;
9
10 public class StudentApp {
11 private HashMap<String, Mentor> mapMentors = new HashMap<String, Mentor>();
12 private HashMap<Integer, Student> mapStudents = new HashMap<Integer, Student>();
13
14 private int maxID = 0;
15 private Scanner scan = new Scanner(System.in);
16
17 /**
18 * @param args the command line arguments
19 */
20 public static void main(String[] args) {
21 new StudentApp().run();
22 }
23
24 /**
25 * Run the app
26 */
27 private void run() {
28 System.out.println("\tStudent Database System");
29 int cmd;
30
31 init();
32
33 while ((cmd = getMenu()) != 9)
34 {
35 switch (cmd)
36 {
37 case 1:
38 addStudent();
39 break;
40 case 2:
41 updateStudent();
42 break;
43 case 3:
44 deleteStudent();
45 break;
46 case 4:
47 queryStudent();
48 break;
49 case 5:
50 calculateTuition();
51 break;
52 case 6:
53 createMentor();
54 break;
55 case 7:
56 assignMentor();
57 break;
58 case 8:
59 listAll();
60 break;
61 default:
62 System.out.println("Unknown command.");
63 break;
64 }
65 System.out.print("Press enter to continue...");
66 scan.nextLine();
C:/Users/jwhite/Desktop/Student/src/student/StudentApp.java
1.1 of 7 2011.10.23 20:27:45
66 scan.nextLine();
67
68 }
69 System.out.println("\nThanks for using the system.");
70 }
71
72 /**
73 * Init the database
74 */
75 private void init() {
76 try
77 {
78 Student.connect();
79 Connection con = Student.getConnection();
80 Statement stmt = con.createStatement();
81 ResultSet rs = stmt.executeQuery("select * from student");
82 do {
83 if (!rs.next())
84 break;
85 int id = rs.getInt("studentID");
86 String fname = rs.getString("firstName").trim();
87 String lname = rs.getString("lastName").trim();
88 double gpa = rs.getDouble("gpa");
89 String status = rs.getString("status").trim();
90 String mentor = rs.getString("mentor").trim();
91 String level = rs.getString("level").trim();
92 String thesisTitle = rs.getString("thesisTitle").trim();
93 String thesisAdvisor = rs.getString("thesisAdvisor").trim();
94 String company = rs.getString("company").trim();
95
96 if (id > maxID)
97 maxID = id;
98
99 Student st;
100 if (!level.isEmpty())
101 st = new Undergraduate(fname, lname, id, gpa, status, mentor, level);
102 else if (!company.isEmpty())
103 st = new PartTime(fname, lname, id, gpa, status, mentor, company);
104 else
105 st = new Graduate(fname, lname, id, gpa, status, mentor, thesisTitle, thesisAdvisor);
106 mapStudents.put(id, st);
107
108 if (!mentor.isEmpty()) {
109 String[] names = mentor.split(" ");
110 String mfname = "";
111 String mlname = "";
112 if (names != null && names.length > 0)
113 mfname = names[0];
114 if (names != null && names.length > 1)
115 mlname = names[1];
116 Mentor mt;
117 if (!mapMentors.containsKey(mentor)) {
118 mt = new Mentor(mfname, mlname);
119 mapMentors.put(mentor, mt);
120 }
121 else
122 mt = mapMentors.get(mentor);
123 mt.addStudent(st);
124 }
125 } while (!rs.isLast());
126 } catch (Exception e)
127 {
128 e.printStackTrace();
129 }
130 }
131
132 /**
C:/Users/jwhite/Desktop/Student/src/student/StudentApp.java
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132 /**
133 * Display the user menu and return the choice.
134 *
135 */
136 private int getMenu() {
137 System.out.println();
138 System.out.println("1. Add a student.");
139 System.out.println("2. Update a student.");
140 System.out.println("3. Delete a student.");
141 System.out.println("4. Query a student");
142 System.out.println("5. Calculate tuition");
143 System.out.println("6. Create a mentor ");
144 System.out.println("7. Add students to a mentor");
145 System.out.println("8. List all");
146 System.out.println("9 . Exit");
147 System.out.print("Enter your choice: ");
148 // skip non integer
149 while (!scan.hasNextInt())
150 scan.next();
151 int cmd = scan.nextInt();
152 scan.nextLine(); // skip others.
153 return cmd;
154 }
155
156 /**
157 * Add the new student
158 */
159 private void addStudent() {
160 System.out.print("Enter the stduentID: ");
161 while (!scan.hasNextInt())
162 scan.next();
163 int id = scan.nextInt();
164 scan.nextLine();
165 if (mapStudents.containsKey(id)) {
166 System.out.println("Student already exists");
167 return;
168 }
169 System.out.print("Choose the type:\n1=Undergraduate 2=Grauate 3=PartTime: ");
170 int type = 0;
171 Student st;
172 do {
173 while (!scan.hasNextInt())
174 scan.next();
175 type = scan.nextInt();
176 scan.nextLine();
177 } while (type < 0 || type > 3);
178 if (type == 1)
179 st = new Undergraduate();
180 else if (type == 2)
181 st = new Graduate();
182 else
183 st = new PartTime();
184 st.setStudentID(id);
185 readStudent(st);
186 st.add();
187 mapStudents.put(id, st);
188 }
189
190 /**
191 * Update the student
192 */
193 private void updateStudent() {
194 Student st = getStudent();
195 if (st == null) {
196 System.out.println("Cannot find the student.");
197 }
198 else {
C:/Users/jwhite/Desktop/Student/src/student/StudentApp.java
3.1 of 7 2011.10.23 20:27:45
198 else {
199 readStudent(st);
200 st.update();
201 }
202 }
203
204 /**
205 * Read student information from the user
206 */
207 private void readStudent(Student st) {
208 System.out.print("Enter the first name: ");
209 st.setFirstName(scan.next());
210 scan.nextLine(); // skip
211 System.out.print("Enter the last name: ");
212 st.setLastName(scan.next());
213 scan.nextLine();
214 System.out.print("Enter the GPA: ");
215 while (!scan.hasNextDouble())
216 scan.next();
217 st.setGPA(scan.nextDouble());
218 scan.nextLine();
219 System.out.print("Choose the status(1=resident, other=nonresident): ");
220 while (!scan.hasNextInt())
221 scan.next();
222 if (scan.nextInt() == 1)
223 st.setStatus("resident");
224 else
225 st.setStatus("nonresident");
226 scan.nextLine();
227
228 if (st instanceof Graduate) {
229 Graduate gst = (Graduate)st;
230 System.out.print("Enter the thesisTitle: ");
231 gst.setThesisTitle(scan.nextLine());
232 System.out.print("Enter the thesisAdvisor: ");
233 gst.setThesisAdvisor(scan.nextLine());
234 }
235 else if (st instanceof Undergraduate) {
236 Undergraduate ust = (Undergraduate)st;
237 System.out.print("Choose the leve:\n 1=freshman, 2=sophomore, 3=junior, 4=senior: ");
238 int id = 0;
239 do {
240 while (!scan.hasNextInt())
241 scan.next();
242 id = scan.nextInt();
243 scan.nextLine();
244 } while (id < 0 || id > 4);
245 if (id == 1)
246 ust.setLevel("freshman");
247 else if (id == 2)
248 ust.setLevel("sophomore");
249 else if (id == 3)
250 ust.setLevel("junior");
251 else if (id == 4)
252 ust.setLevel("senior");
253 }
254 else if (st instanceof PartTime) {
255 PartTime pst = (PartTime)st;
256 System.out.print("Enter the company: ");
257 pst.setCompany(scan.nextLine());
258 }
259 }
260 /**
261 * Delete the student
262 */
263 private void deleteStudent() {
C:/Users/jwhite/Desktop/Student/src/student/StudentApp.java
4.1 of 7 2011.10.23 20:27:45
264 Student st = getStudent();
265 if (st == null) {
266 System.out.println("Cannot find the student.");
267 }
268 else {
269 String mname = st.getMentor();
270 Mentor mt = mapMentors.get(mname);
271 if (mt != null)
272 mt.removeStudent(st);
273 mapStudents.remove(st.getStudentID());
274 st.delete();
275 }
276 }
277
278 /**
279 * Query a student
280 */
281 private void queryStudent() {
282 Student st = getStudent();
283 if (st == null) {
284 System.out.println("Cannot find the student.");
285 }
286 else {
287 st.query();
288 }
289 }
290
291 /**
292 * Calculate the tuition of a student
293 */
294 private void calculateTuition() {
295 Student st = getStudent();
296 if (st == null) {
297 System.out.println("Cannot find the student.");
298 return;
299 }
300 System.out.print("Enter the hours: ");
301 while (!scan.hasNextInt())
302 scan.next();
303 int hours = scan.nextInt();
304 scan.nextLine();
305 double value = st.calculateTuition(hours);
306 System.out.printf("The tuition is %.2f\n", value);
307 }
308
309 /**
310 * Create a new Mentor
311 */
312 private void createMentor() {
313 Mentor mt = getMentor(true);
314 if (mt == null) {
315 System.out.println("Failed: The Mentor exists.");
316 return;
317 }
318
319 System.out.println(mt);
320 System.out.println("New Mentor added!");
321 String name = (mt.getFirstName() + " " + mt.getLastName()).trim();
322 mapMentors.put(name, mt);
323 }
324
325 /**
326 * Assign a student to a mentor
327 */
328 private void assignMentor() {
329 Mentor mt = getMentor(false);
C:/Users/jwhite/Desktop/Student/src/student/StudentApp.java
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329 Mentor mt = getMentor(false);
330 if (mt == null) {
331 System.out.println("Cannot find The Mentor.");
332 return;
333 }
334 Student st = getStudent();
335 if (st == null) {
336 System.out.println("Cannot find the student.");
337 return;
338 }
339 String oldmname = st.getMentor();
340 Mentor oldmt = mapMentors.get(oldmname);
341 if (oldmt != null)
342 oldmt.removeStudent(st);
343
344 mt.addStudent(st);
345 st.update();
346 }
347
348 /**
349 * Query and return a Mentor
350 * @param newMentor true : create a new Mentor
351 * false : query from the list
352 * @return Mentor object
353 */
354 private Mentor getMentor(boolean newMentor) {
355 System.out.println("Enter the name of the mentor:");
356 System.out.print("Enter the first name: ");
357 String fname = scan.nextLine();
358 System.out.print("Enter the last name: ");
359 String lname = scan.nextLine();
360 String name = (fname + " " + lname).trim();
361 for (Entry<String, Mentor> m : mapMentors.entrySet()) {
362 Mentor mt = m.getValue();
363 String mname = (mt.getFirstName() + " " + mt.getLastName()).trim();
364
365 if (mname.equals(name)) {
366 if (newMentor)
367 return null;
368 return mt;
369 }
370 }
371 if (newMentor)
372 return new Mentor(fname, lname);
373 return null;
374 }
375
376 /**
377 * Query for a student
378 * @return student object
379 */
380 private Student getStudent() {
381 System.out.print("Enter the stduentID: ");
382 while (!scan.hasNextInt())
383 scan.next();
384 int id = scan.nextInt();
385 scan.nextLine();
386 return mapStudents.get(id);
387 }
388
389 /**
390 * List all the mentors and the students
391 */
392 private void listAll() {
393 for (Entry<String, Mentor> m : mapMentors.entrySet()) {
394 System.out.println(m.getValue().toString());
395 System.out.println();
C:/Users/jwhite/Desktop/Student/src/student/StudentApp.java
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395 System.out.println();
396 }
397 System.out.println("Students Without Mentor");
398 for (Entry<Integer, Student> s : mapStudents.entrySet()) {
399 if (s.getValue().getMentor().isEmpty()) {
400 System.out.println(s.getValue().toString());
401 System.out.println();
402 }
403 }
404 System.out.println("Total " + mapMentors.size() + " Mentor(s)");
405 System.out.println("Total " + mapStudents.size() + " Student(s)");
406
407 }
408
409 }

Student.connect();

What is processed in connect()?

Post your Student class code here too, so we could hav a look over it.

 
 
1 package student;
2
3 import java.sql.Connection;
4 import java.sql.DriverManager;
5
6 public abstract class Student {
7 protected String firstName;
8 protected String lastName;
9 protected int studentID;
10 protected double GPA;
11 protected String status; // resident or nonresident
12 protected String mentor;
13
14 protected static Connection dbCon;
15
16 /**
17 * Default constructor
18 */
19 Student() {
20 firstName = "";
21 lastName = "";
22 studentID = 0;
23 GPA = 0;
24 status = "resident";
25 mentor = "";
26 }
27
28 /**
29 * Paramter Constructor
30 * @param firstName
31 * @param lastName
32 * @param studentID
33 */
34 Student(String firstName, String lastName, int studentID) {
35 this.firstName = firstName;
36 this.lastName = lastName;
37 this.studentID = studentID;
38 this.GPA = 0;
39 this.status = "resident";
40 this.mentor = "";
41 }
42
43 /**
44 * Parameter Constructor 2
45 * @param firstName
46 * @param lastName
47 * @param studentID
48 * @param gpa
49 * @param status
50 * @param mentor
51 */
52 Student(String firstName, String lastName, int studentID, double gpa, String status, String mentor) {
53 this.firstName = firstName;
54 this.lastName = lastName;
55 this.studentID = studentID;
56 this.GPA = gpa;
57 this.status = status;
58 this.mentor = mentor;
59 }
60
61 /**
62 * Return the database connection.
63 */
64 public static Connection getConnection() {
65 return dbCon;
66 }
67
68 /**
69 * Connect the database
70 */
71 public static void connect() {
72 try {
73 Class.forName("com.mysql.jdbc.Driver");
74 if (dbCon != null)
75 dbCon.close();
76 dbCon = DriverManager.getConnection("jdbc:mysql://localhost:3306/registrar",
77 "root", "555680");
78 } catch (Exception e) {
79 e.printStackTrace();
80 }
81 }
82
83 /**
84 * Calculate the tuition
85 * @param hours Number of credit hours
86 * @return the tuition
87 */
88 public abstract double calculateTuition(int hours);
89
90 /**
91 * Update the student record in the database
92 */
93 public abstract boolean update();
94
95 /**
96 * Add the student record to the database
97 */
98 public abstract boolean add();
99
100 /**
101 * Delete the record in the database
102 */
103 public abstract boolean delete();
104
105 /**
106 * Retrieve the record from the database
107 */
108 public abstract boolean query();
109
110 @Override
111 /**
112 * Return the String representation of the Student
113 */
114 public String toString() {
115 String s = "Student Name: " + firstName + " " + lastName + "\n";
116 s += "ID: " + studentID + "\n";
117 s += "GPA: " + GPA + "\n";
118 s += "Status: " + status + "\n";
119 s += "Mentor: " + mentor + "\n";
120 return s;
121 }
122
123 // Getters and Setters
124 public String getFirstName() {
125 return firstName;
126 }
127
128 public void setFirstName(String firstName) {
129 this.firstName = firstName;
130 }
131
132 public String getLastName() {
133 return lastName;
134 }
135
136 public void setLastName(String lastName) {
137 this.lastName = lastName;
138 }
139
140 public int getStudentID() {
141 return studentID;
142 }
143
144 public void setStudentID(int studentID) {
145 this.studentID = studentID;
146 }
147
148 public double getGPA() {
149 return GPA;
150 }
151
152 public void setGPA(double gpa) {
153 GPA = gpa;
154 }
155
156 public String getStatus() {
157 return status;
158 }
159
160 public void setStatus(String status) {
161 this.status = status;
162 }
163
164 public String getMentor() {
165 return mentor;
166 }
167
168 public void setMentor(String mentor) {
169 this.mentor = mentor;
170 }
171
172 }

Thanks for the insight!
Let me go back and reevaluate with a fresh point of view and see what I can do.

Originally Posted by copeg

I'm not sure how an explicit import - something that affects compilation - would rectify a NullPointerException at runtime.

I know that it will not make any difference, yet i guess, may be, 1 out of 100