Java reading/writing several objects to file

Question

I'm trying to learn how to use steams and I can't seem to figure out. The thing I'm trying to do is add several separate objects to a file then retrieve them all at once. When I try to retrieve them I only get the "last" inserted object. For now I'm trying to figure out how to print the objects but later I would like to import them into an ArrayList for example. Here is my code:

public class ExpenseDB {
private final static File DB = new File("C:\\Expense2s3.dat");

public static void addExpense(Expense ex) throws AddException {
        try {
            ObjectOutputStream out;
            out = new ObjectOutputStream(new FileOutputStream(DB));
            out.writeObject(ex);
            out.close();
            System.out.println("Added "+ex);
        } catch (IOException e) {
            throw new AddException();
        }
}


@SuppressWarnings("deprecation")
public static void getAllExpenses() {
    if (DB.length() == 0) return;
    try {
        ObjectInputStream in = new ObjectInputStream(new FileInputStream(DB));
        try {
            Expense exp=(Expense)in.readObject();
            System.out.println(exp);
            in.close();
         } catch (ClassNotFoundException e) {}
    } catch (IOException e) {
        System.err.println("Error");
    }
}

what is the issue here

vikeng21
– vikeng21

2014-06-14 19:20:55 +00:00
Commented Jun 14, 2014 at 19:20 — vikeng21
– vikeng21, Commented Jun 14, 2014 at 19:20

Braj · Accepted Answer · 2014-06-14 19:24:17Z

1

When you open a file and then write to it, it will overwrite its contents. instead use new FileOutputStream(DB,true) to append to the end of the file

edited Jun 14, 2014 at 19:24

Braj

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answered Jun 14, 2014 at 19:20

vandale

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7 Comments

Airwavezx Over a year ago

Did you mean FileOutputStream?

Airwavezx Over a year ago

Yep it worked :) How about the InputStream? Can I get it to loop over the objects or do I have to import them all at once?

vandale Over a year ago

@Airwavezx readObject() will only return one object at a time so you will have to loop to get them all.

Boris the Spider Over a year ago

This won't really work. Each ObjectOutputStream will output its header before writing the object. This will result is a file that consists of gibberish and can only be read by spawning ObjectInputStream in a loop, one for each object. The correct way to do this is to persist a Collection or use a structured representation like XML.

Airwavezx Over a year ago

Exactly as Boris said, I tried Expense exp=(Expense)in.readObject(); while (exp != null){ which did not work. If I insist in doing it in a loop and not using a Collection how do I do that?

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