Having issues with Serialization

[Janeice DelVecchio]

I'm sure I'm missing something simple here... the problem is when I try to access either student or mentor objects after they are deserialized, I get null pointer exceptions. The application behaves appropriately if I access the objects before the application is closed and reopened.

This is the deserialization:

public static void main(String[] args) {
		File f = new File("registrar.ser");
		// check if file exists. If it doesn't, create it.
		try {
			if (!f.exists()) {
				try {
					System.out.println("Creating new save file!");
					FileOutputStream fs = new FileOutputStream(f);
					ObjectOutputStream os = new ObjectOutputStream(fs);
					os.writeInt(0);
					os.writeInt(0);
					os.close();
				}
				catch (Exception ex) {
					ex.printStackTrace();
				}
			}
		}
		catch (Exception ex) {
			ex.printStackTrace();
		}
		// unzip objects
		try {
			FileInputStream fs = new FileInputStream(f);
			ObjectInputStream is = new ObjectInputStream(fs);
			int total = is.readInt();
			Student.setNumOfStudents(total);
			SortedMap<Integer, Student> students = Student.getStudentMap();
			for (int x = 0; x < total; x++) {
				Student s = (Student)is.readObject();
				Student.StudentType t = s.getStudentType();
				switch (t) {
					case UNDERGRADUATE:
						Undergraduate w = (Undergraduate) s;
						students.put(w.getStudentID(), w);
						break;
					case GRADUATE:
						Graduate v = (Graduate) s;
						students.put(v.getStudentID(), v);
						break;
					case PARTTIME:
						PartTime u = (PartTime) s;
						students.put(u.getStudentID(), u);
						break;	
				}
			}
			
			total = is.readInt();
			Mentor.setNumOfMentors(total);
			SortedMap<Integer, Mentor> mentors = Mentor.getMentorMap();
			for (int x = 0; x < total; x++) {
				Mentor m = (Mentor)is.readObject();
				System.out.println(m);  // I put this in here to see if anything was coming out... I get a null pointer
				mentors.put(m.getMentorID(), m);
			}
			is.close();
		} 
		catch (Exception ex) {
			ex.printStackTrace();
		}
		    

And here's where I serialize the objects:

		try {
			FileOutputStream fs = new FileOutputStream("registrar.ser");
			ObjectOutputStream os = new ObjectOutputStream(fs);
		
			SortedMap<Integer, Student> students = Student.getStudentMap();
			os.writeInt(students.size());
			for (int x = 0; x < students.size() ; x++) {
				os.writeObject(students.get(x));
			}
			SortedMap<Integer, Mentor> mentors = Mentor.getMentorMap();
			os.writeInt(mentors.size());
			for (int x = 0; x < mentors.size(); x++) {
				os.writeObject(mentors.get(x));
			}
			os.close();
		} catch (Exception ex) {
			ex.printStackTrace();
		}     

The behavior is similarly incorrect whether I am trying to view Students or Mentors.

I feel like this should be something simple and I'm not seeing it.

Thanks a bunch!
Janeice

[Christophe Verré]

System.out.println(m);

How can you get a NullPointerException here ? What does Mentor#toString look like ? What is the exact stack trace ?

[Janeice DelVecchio]

I don't get a null pointer there (although my comment implied that and I'm sorry).... it just says "null".... this has given me enough information to determine the cause of my null pointer exception when I try to use/manipulate the objects down the road is the object apparently not deserializing correctly.

Here's the Mentor toString()

	public String toString() {
		String m = this.getLName() + ", " + this.getFName();
		return m;
	}    

Here's the stack trace:

C:\Users\Janeice\Documents\ABT1 Task4\Application>java -cp mysql.jar;. StudentRe
gisterApp
null
java.lang.NullPointerException
        at StudentRegisterApp.main(StudentRegisterApp.java:58)
**************************************************
* Welcome to the Student Register Application.   *
*   What would you like to do?                   *
*    1. Register new student                     *
*    2. Edit existing student                    *
*    3. View student records                     *
*    4. Create / Edit a mentor                   *
*    5. View mentor information                  *
*    6. Save/Quit                                *
**************************************************    

and this is line 58.... it's in context above.... but I get a similar error whenever I try to manipulate the objects.

 				mentors.put(m.getMentorID(), m);   

Thanks for the help!

[Christophe Verré]

In your serialization process, you are using a loop starting from index 0. Then you get a Student/Mentor, using that index. Are you sure that the index exists in the Map ? If not, you'll serialize a null object ! You should be looping the Map using its keys, or using its values. Like :

for ( Student s : students.values() ) {
...
}
for ( Mentor s : mentors.values() ) {
...
}

[Janeice DelVecchio]

Christophe Verré wrote:In your serialization process, you are using a loop starting from index 0. Then you get a Student/Mentor, using that index. Are you sure that the index exists in the Map ? If not, you'll serialize a null object ! You should be looping the Map using its keys, or using its values. Like :

for ( Student s : students.values() ) {
...
}
for ( Mentor s : mentors.values() ) {
...
}

I knew it would be something easy!

Lemme try that today. Thanks!

Janeice

[Janeice DelVecchio]

This is what I ended up with.... it works!

void quit() {
		// serialize objects
		try {
			FileOutputStream fs = new FileOutputStream("registrar.ser");
			ObjectOutputStream os = new ObjectOutputStream(fs);
		
			SortedMap<Integer, Student> students = Student.getStudentMap();
			os.writeInt(students.size());
			for (Student s : students.values()) {
				os.writeObject(s);
			}
			SortedMap<Integer, Mentor> mentors = Mentor.getMentorMap();
			os.writeInt(mentors.size());
			for (Mentor m : mentors.values()) {
				os.writeObject(m);
			}
			os.close();
		} catch (Exception ex) {
			ex.printStackTrace();
		} 
		
		
		System.exit(0);
	}    

Thanks!!