readObject() method not printing object

Question

In an assignment that I have to do, I have to write a Student object which comprises of a name, social security number, number of courses passed, and the grades for each of those courses. I made a gpa method to calculate the gpa based on and also made overrides of the writeObject and readObject methods with my own methods. Here is the complete source code- http://pastebin.com/dNMrc1zK

This is what it looks like without the getter and setter methods for each instance:

import java.io.*;

public class Student implements Serializable{
        private String name;
        private int ssNum;
        private int coursesCompleted;
        private char[] grades;

        public Student(String n, int num, int c){
                n = name;
                num = ssNum;
                c = coursesCompleted;
        }

        public String getName() {
                return name;
        }

        public void setName(String name) {
                this.name = name;
        }

        public int getSsNum() {
                return ssNum;
        }

        public void setSsNum(int ssNum) {
                this.ssNum = ssNum;
        }

        public int getCoursesCompleted() {
                return coursesCompleted;
        }

        public void setCoursesCompleted(int coursesCompleted) {
                this.coursesCompleted = coursesCompleted;
        }

        public char[] getGrades() {
                return grades;
        }

        public void setGrades(char[] grades) {
                this.grades = grades;
        }
        public double gpa(char[] grades){

                double gpa = 0;

                for (int i = 0; i < grades.length; i++){
                        if (grades[i] == 'A'){
                                gpa += 4.0;
                        }
                        else if (grades[i] == 'B'){
                                gpa += 3.0;
                        }
                        else if (grades[i] == 'C'){
                                gpa += 2.0;
                        }
                        else if (grades[i] == 'D'){
                                gpa += 1.0;
                        }
                        else if (grades[i] == 'F'){
                                gpa += 0.0;
                        }
                }

                return gpa / grades.length;

        }

        public void ReadObject(ObjectInputStream in) throws IOException{
                String readTitle = "";

                for(int i = 0; i < name.length(); i++){
                        readTitle += in.readChar();
                }
                ssNum = in.readInt();
                coursesCompleted = in.readInt();

                for (int i = 0; i < grades.length; i++){
                        grades[i] = in.readChar();
                }
        }
        public void WriteObjectOverride(ObjectOutputStream out) throws IOException, ClassNotFoundException{

                out.writeChars(name);
                out.writeInt(ssNum);
                out.writeInt(coursesCompleted);

                for (int i = 0; i < grades.length; i++){
                        out.writeChar(grades[i]);
                }
                gpa(grades);
        }
        public String toString(){
                   String student = name + " " +  "\nGPA: " + gpa(grades);
                   return student;
                }
}


public void ReadObject(ObjectInputStream in) throws IOException{
    String readTitle = "";

    for(int i = 0; i < name.length(); i++){
        readTitle += in.readChar();
    }
    ssNum = in.readInt();
    coursesCompleted = in.readInt();

    for (int i = 0; i < grades.length; i++){
        grades[i] = in.readChar();
    }
}
public void WriteObjectOverride(ObjectOutputStream out) throws IOException, ClassNotFoundException{

    out.writeChars(name);
    out.writeInt(ssNum);
    out.writeInt(coursesCompleted);

    for (int i = 0; i < grades.length; i++){
        out.writeChar(grades[i]);
    }
    gpa(grades);
}
public String toString(){
       String student = name + " " +  "\nGPA: " + gpa(grades);
       return student;
    }
}

I then have to write a separate program which writes the object to a file. I made a constructor so that it gives the name, ss number, and courses completed a value and then I used a setter method to set the grades array value. I then used the values I had to use writeObject and wrote the object like this:

            Student john = new Student("John Doe", 123456, 4);
    Student jane = new Student("Jane Doe", 987654, 3);
    Student jack = new Student("Jack Doe", 528491, 4);

    ObjectOutputStream out;

    john.setGrades(student1);
    jane.setGrades(student2);
    jack.setGrades(student3);

    try {
        out = new ObjectOutputStream(new BufferedOutputStream(new FileOutputStream("students.bin")));

        out.writeObject(john);
        out.writeObject(jane);
        out.writeObject(jack);

        out.close();
    } catch (IOException e) {
        e.printStackTrace();
    }
}

}

The problem I'm having is that when I use another program to execute the readObject() method, like this:

    public static void main (String[] args)throws FileNotFoundException, IOException, ClassNotFoundException{
    Student student = null;
    try(ObjectInputStream in = new ObjectInputStream(new FileInputStream("students.bin"));){

        student = (Student)in.readObject();
        System.out.println(student);
    }

It prints a line which not something I expected:

UnitSeven.Student@70dea4e

Does anyone know how to fix this problem so that it shows the actual object? Any help would be appreciated.

Answer 1

In addition to your overrides having the wrong names as Kumar Abhinav pointed out, you have several issues.

Your readObject is using names.length and grades.length but neither has been initialized. You will probably need to modifiy writeObject to output the length of the name and then read that length before reading the name or just write and read the name as a String object. You can use coursesCompleted to determine how many grades to read. You also don't do anything with the readTitle variable.

Your writeObject method calls the gpa method but does not use the return value.

Answer 2

The code you are using is completely wrong. You are using ReadObject with capital R, use readObject and you have not implemented writeObject. These two private methods must have an exact signature as defined as they are call-back methods. The way your code currently works, it's calling the default way of serializing/deserializing Objects.

1. The method signature of your ReadObject method is not matching the one required.

   public void ReadObject(ObjectInputStream in) throws IOException { ... }
   

   should be

   private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException { ... }
   

2. The same applies for your attempt at the writeObject method.

   public void WriteObjectOverride(ObjectOutputStream out) throws IOException, ClassNotFoundException {
   

   should be

   private void writeObject(ObjectOutputStream out) throws IOException { ... }
   

Also note: All String Objects and primitives are themselves serializble, so you may as well change the logic of writing data to your stream

To prevent your method from printing out a hashcode next to the class name, implement your own toString() method in the Student class. Please consult this question for more details:

To print something different when you call System.out.println(myObject), you must override the toString() method in your own class. Here's a simple example:

public class Person {

    private String name;

    // constructors and other methods omitted

    @Override   public String toString() {
     return name;
    }
}

Now if we print a Person, we see their name rather than com.foo.Person@12345678.

Answer 3

When you execute:

System.out.println(student);

what actually happens is that student object's toString() method is called. Since you did not override toString() method for student's class what you run is Object class's, which is the parent of all classes in java, toString() method. This Object class's toString() returns the student's class name and a hashcode. Hashcode is the mumbo jumbo number that you are seeing. This hashcode is the standard identifier in java.

What you should do is override Object class's toString() method in your Student class in order to print your desired output.