Reading ArrayList as object from file?

Question

Alright, so I have done the following:

I've added objects to an ArrayList and written the whole list as an object to a file.
The problem is when trying to read them back as a whole. I get the following error:

Exception in thread "main" java.lang.ClassCastException: java.util.Arrays$ArrayList cannot be cast to java.util.ArrayList at persoana.Persoana.main(Student.java:64)

Here's my code: (Everything is in a try catch so nothing to worry about that)

Writing

Student st1 = new Student("gigi","prenume","baiat","cti");
        Student st2= new Student("borcan","numegfhfh","baiat cu ceva","22c21");

        List <Student> studenti = new ArrayList<Student>();
        studenti = Arrays.asList(st1,st2);

FileOutputStream  fos = new FileOutputStream("t.ser");
            ObjectOutputStream oos = new ObjectOutputStream(fos);

            oos.writeObject(studenti);
            oos.close();

Reading

FileInputStream fis = new FileInputStream("t.ser");
             ObjectInputStream ois = new ObjectInputStream(fis);

             ArrayList <Student> ds;

             ds = (ArrayList <Student>)ois.readObject(); 

             ois.close();

The problem occurs at this line:

ds = (ArrayList <Student>)ois.readObject();

A note, here: List <Student> studenti = new ArrayList<Student>(); studenti = Arrays.asList(st1,st2); you are wasting an ArrayList — Jack
– Jack, Commented Jan 5, 2013 at 15:20
Arrays.asList() doesn't return an ArrayList. Check its javadoc. — JB Nizet
– JB Nizet, Commented Jan 5, 2013 at 15:21

Jack · Accepted Answer · 2013-01-05 15:23:47Z

9

I guess that the problem is that you are creating the List of Student through Arrays.asList. This method doesn't return an ArrayList but an Arrays.ArrayList which is a different class, meant to backen an Array and to be able to use it as a List. Both ArrayList and Arrays.ArrayList implement List interface but they are not the same class.

You should cast it to appropriate object:

List<Student> ds = (List<Student>)ois.readObject();

answered Jan 5, 2013 at 15:23

Jack

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Comments

munyengm · Accepted Answer · 2013-01-05 15:20:12Z

4

Change the following lines:

ArrayList <Student> ds;
ds = (ArrayList<Student>)ois.readObject();

to

List<Student> ds = (List<Student>)ois.readObject();

answered Jan 5, 2013 at 15:20

munyengm

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1 Comment

munyengm Over a year ago

No problem. See Jack's answer for an explanation of why your original code didn't work.

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