ObjectInputStream being initialised causes an EOFException of the type IOException

Question

I have been writing a to-do list app in java and each todo item is stored as an object of the class ToDo (which I created).
The ToDo class is serializable and I am using an ObjectOutputStream to write the objects to a text file. I close the ObjectOutputStream after doing this.
I should probably mention that currently my text file is empty as there are no todos in it and GUI.items is a static ArrayList in my GUI class.

When I run the method that reads the files, an IO exception is thrown on the line:

ObjectInputStream objectInputStream = new ObjectInputStream(fileInputStream);

Here is the method that reads the files:

public void read() {
    try (FileInputStream fileInputStream = new FileInputStream("todo.txt")) {

        ObjectInputStream objectInputStream = new ObjectInputStream(fileInputStream);

        GUI.items.clear();

        while (objectInputStream.readObject() != null) {
            GUI.items.add((ToDo) objectInputStream.readObject());
        }

        GUI.updateInterface();

        objectInputStream.close();

    } catch (FileNotFoundException e) {
        e.printStackTrace();
        //JOptionPane.showMessageDialog(null, "Error: To-Do List not found.\nPlease contact the developer.", "Error", JOptionPane.ERROR_MESSAGE);
    } catch (IOException e) {
        JOptionPane.showMessageDialog(null, "Error: To-Do List could not be opened.\nPlease contact the developer.", "Error", JOptionPane.ERROR_MESSAGE);
    } catch (ClassNotFoundException e) {
        JOptionPane.showMessageDialog(null, "Error: To-Do List object type could not be found.\nPlease contact the developer.", "Error", JOptionPane.ERROR_MESSAGE);
    }
}

Why is this exception being thrown and how can I fix it? Thanks.

Answer 1

Yes, this is behaving as documented:

Creates an ObjectInputStream that reads from the specified InputStream. A serialization stream header is read from the stream and verified.

...

Throws:
IOException - if an I/O error occurs while reading stream header

If your file is empty, it doesn't contain the stream header. A file which has been created using an ObjectOutputStream which has been closed after writing 0 objects is not the same thing as an empty file.

Answer 2

I discovered that this is the line using the Eclipse line by line debugger.

The stack trace would also show you the line.

ObjectInputStream being initialised causes an EOFException

OIS has a strange habit of throwing EOF when actually the file format is invalid (or you are not reading it as expected) Given the file name is todo.txt I suspect it is text file which cannot be read as an Object stream.

Another possibility is that you are getting further than this and it is your loop which is breaking as you are checking every second object is null. This is a bit like reading a text file line by line but wrong for BufferedReader and completely wrong for ObjectInputStream.

If you want to serialize a List into an object stream, I suggest you write and read a List, not read one element at a time with no idea when to finish.