Reading/Writing objects to file returning null

Question

I'm trying to read and write objects into a file. Reading the output into a new object works, but every value is null.

Here's the code:

public void read() throws Exception
    {
        try
        {
            FileInputStream fIn = new FileInputStream(file);
            ObjectInputStream in = new ObjectInputStream(fIn);

            Object obj = in.readObject();

            System.out.println(obj);

public void save() throws Exception
    {
        FileOutputStream fOut = new FileOutputStream(file.toString());
        ObjectOutputStream out = new ObjectOutputStream(fOut);

        out.writeObject(this);
        out.flush();
        out.close();
    }

Here is the file output: (image of output)

I'd like to receive the values I previously wrote to the file in the new object created, however all I get is null for all values.

Edit: since people are asking for the entire class, and I have no idea what code could be causing what, here's the entire UserFile class: https://pastebin.com/Gr1tcGsg

Is the new object of the right class? Also show the source of the class that you have serialized. You may have mistakes in it. — Erwin Bolwidt, Apr 08 '19 at 05:00
NB You don't need `file.toString()`. Just `file` will do. You also don't need to flush before closing. — user207421, Apr 08 '19 at 05:24
@user207421 close calls flush, and ye, he is flushing before closing anyway. — Antoniossss, Apr 08 '19 at 05:28
OK i read that he doesn't flush. As a good practice with object stream its good to flush after write to push headers. But that is general rule of particular stream implementation. — Antoniossss, Apr 08 '19 at 05:31
No they are not. That is general rule for ObjectOutputStream. — Antoniossss, Apr 08 '19 at 05:32
@Antoniossss No what are not what? If you're still trying to educate me about how `FilterOutputStream.close()` works in Java, don't bother. I learnt it in 1997, and it's all documented anyway. — user207421, Apr 08 '19 at 05:33
@Antoniossss I don't know what you're trying to contribute here that hasn't already been said. Don't make personal remarks here. — user207421, Apr 08 '19 at 07:04

OughtToPrevail · Answer 1 · 2019-04-08T14:57:24.517

I have ran that code and it works which means you most likely read before you write or you got an exception such as InvalidClassException: no valid constructor which would make sense in your case.

The code I ran:

public class SavedObject implements Serializable
{
    public static void main(String[] args) throws IOException, ClassNotFoundException
    {
        new SavedObject();
    }

    private final int random;

    private SavedObject() throws IOException, ClassNotFoundException
    {
        random = ThreadLocalRandom.current().nextInt();
        File file = new File("Object.txt");
        save(file);
        read(file);
    }

    private void save(File file) throws IOException
    {
        FileOutputStream fileOutput = new FileOutputStream(file);
        ObjectOutputStream objectOutput = new ObjectOutputStream(fileOutput);

        objectOutput.writeObject(this);
        objectOutput.close();
        fileOutput.close();

        System.out.println(this);
    }

    private void read(File file) throws IOException, ClassNotFoundException
    {
        FileInputStream fileInput = new FileInputStream(file);
        ObjectInputStream objectInput = new ObjectInputStream(fileInput);

        Object obj = objectInput.readObject();

        System.out.println(obj);

        objectInput.close();
        fileInput.close();
    }

    public String toString()
    {
        return "SavedObject(Random: " + random + ")";
    }
}

Which prints:

SavedObject(Random: -2145716528)
SavedObject(Random: -2145716528)

Also a few tips for you:

Don't have a try-catch if you throws
Have more readable variable names
Send more code in your next question
ObjectOutputStream is not recommended, if you can, you should write the values as they are
Don't use throws "Exception" instead use throws "
Put the file in instead of file.toString()

score 0 · Answer 2 · answered Apr 08 '19 at 16:58

0

I found the problem, it was that in my constructor I wasn't correctly applying the retrieved info from the file. Thanks for everyone's help.

answered Apr 08 '19 at 16:58

geger42 _

15
5

Reading/Writing objects to file returning null

2 Answers2