ClassCastException when casting array to readObject

Question

I have to run this array of object Student (where there's 3 students):

        try
    {
        out = new ObjectOutputStream (new BufferedOutputStream (new FileOutputStream ("Students.dat")));
        out.writeObject(s[0]);
        out.writeObject(s[1]);
        out.writeObject(s[2]);
        out.close();
    }
    catch (IOException e)
    {
        System.out.println("Error writing student data to file.");
    }
}

And each Student object must be set like this:

for (int i = 0; i<3; i++)
    {
        System.out.println("The following information applies to student " + (i+1));
        System.out.println("What is the student's name?");
        String name = input.nextLine();
        if (i == 0)
        {
            System.out.println("Please enter the name again.");
        }
        name = input.nextLine();
        System.out.println("What is the Social Security Number of this student?");
        String ssn = input.next();
        System.out.println("How many courses has the student completed?");
        int numCourses = input.nextInt();
        int [] grades = new int [numCourses];
        int credits = (5*numCourses);

        double points = 0;
        for(int k = 0; k<numCourses; k++)
        {
            System.out.println("Type a number to represent the letter grade of course " + (k+1) + ". Type 1 for an A. Type 2 for a B. Type 3 for a C. Type 4 for a D. Type 5 for an F.");
            grades[k] = input.nextInt();
            switch (grades[k])
            {
                case 1:
                    points += 4;
                    break;
                case 2:
                    points += 3;
                    break;
                case 3:
                    points += 2;
                    break;
                case 4:
                    points += 1;
                    break;
                case 5:
                    break;
            }

            s[i] = new Student (name, ssn, numCourses, grades, credits);
        }
    }

I keep getting the ClassCastException when running these lines:

Object obj = new Object();
obj = in.readObject();
Student[] ns = (Student[])obj;

The exception looks like this:

java.lang.ClassCastException: UnitSeven.Student cannot be cast to [LUnitSeven.Student;
at UnitSeven.StudentGPA.main(StudentGPA.java:21)

And line 21 is the last line in the code mentioned above. Does anyone know how to fix this so that I can cast it properly? Thanks in advance for any help.

Can you post some more code on how you serialize the object? These three lines are not enough to identify the issue — Ravindra babu
– Ravindra babu, Commented Aug 16, 2015 at 21:39

Luiggi Mendoza · Accepted Answer · 2015-08-16 21:52:01Z

1

You're reading a single Student but trying to cast it as an Student[]. Just remove the array reference:

Student s = (Student)obj;

This is because you're storing each element of your array in your ObjectOutputStream by itself, noted here:

out = new ObjectOutputStream (new BufferedOutputStream (new FileOutputStream ("Students.dat")));
//you write each reference of Student
out.writeObject(s[0]);
out.writeObject(s[1]);
out.writeObject(s[2]);
out.close();

If you want/need to read it as an array, then store it as an array as well:

out = new ObjectOutputStream (new BufferedOutputStream (new FileOutputStream ("Students.dat")));
out.writeObject(s);
out.close();

So you can read it properly:

Object obj = new Object();
obj = in.readObject();
Student[] ns = (Student[])obj;

Or in a single line:

Student[] ns = (Student[])in.readObject();

edited Aug 16, 2015 at 21:52

answered Aug 16, 2015 at 21:39

Luiggi Mendoza

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2 Comments

Foxwood211 Over a year ago

Thanks, your advice helped. But I'm now getting an ArrayOutOfBoundsException. I don't know how to fix it. Could you help me with that? Here's the code for displaying the objects: pastebin.com/pUi5gKcP The line that's giving the runtime error is: Object cred = ns[5];

Luiggi Mendoza Over a year ago

That's a different issue that must be covered in a new question/answer

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