Multiple nullpointerexceptions in my code.

[javastudnt]

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
2.1 of 7 2011.10.23 20:27:45
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
5.1 of 7 2011.10.23 20:27:45
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
6.1 of 7 2011.10.23 20:27:45
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 }

[Junky]

79 Connection con = Student.getConnection();
80 Statement stmt = con.createStatement();

The only thing on line 80 that could be null is con. Therefore on line 79 Student.getConnection must be returning null. Add a print statement in between to print out con. If it is null then you need to find out why getConnection is returning null.

[Mr.777]

Student.connect();

What is processed in connect()?

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

[javastudnt]

 
 
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 }

[Mr.777]

Why don't you try

java.sql.Statement stmt=con.createStatement();

instead of

Statement stmt = con.createStatement();

EDIT: A suggestion, i will recommend you, not to use static connection, as it can take your system resources and can cause performance issues. If you used it coz of not to open connection, everytime, you must see connection pools that just keep opening multiple connections and use them only when it is needed. So, creating a static connection is highly discouraged.

[copeg]

Why don't you try

java.sql.Statement stmt=con.createStatement();

instead of

Statement stmt = con.createStatement();

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

javastudnt, does the connection actually get established? The client has no checks to verify the connection is actually created, and if not the return connection will be null. You should redesign how the connection is established and how exceptions are thrown and caught. For example, make use of the try/catch/finally - have the connect method throw the exception, and force the client to catch the exception - in which case the client knows the error occurred and can deal with it accordingly, as well as close the connection in the finally clause if all is well. Moving onto things like connection pools as mr777 pointed out is something to keep in mind

[javastudnt]

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

[Mr.777]

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