ObjectOutputStream read in other language

I used to save items in Android using Android Studio with Java. But now I want to read in exactly that data in my rewritten app in Flutter SDK (Dart language). But it seems that the files that are written are very Java specific. This is the save and load method in Java.

public void save(Item item)
{
    String json = gson.toJson(item);
    FileOutputStream fos = null;
    try {
        fos = context.openFileOutput("item_"+ item.getUid(), Context.MODE_PRIVATE);
        ObjectOutputStream os = new ObjectOutputStream(fos);
        os.writeObject(json);
        os.close();
        fos.close();

    } catch (FileNotFoundException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }
}


private Item loadItem(String fileName)
{
    FileInputStream fis = null;
    try {
        fis = context.openFileInput(fileName);
        ObjectInputStream is = new ObjectInputStream(fis);
        Object object = is.readObject();
        Item item = null;
        String json = (String)object;
        item = gson.fromJson(json, Item.class);
        is.close();
        fis.close();
        return item;
    }
    catch (Exception e)
    {
        e.printStackTrace();
    }
    return null;
}

And now for reading it in Dart

Uint8List bytes = f.readAsBytesSync();
var data = bytes.buffer.asByteData();
// HOW TO DETECT JAVA
if (?SOMETHING?) { }

The length of bytes is quite large. It is an array of length = 3 245 500.
The length of the json string should be between 740 and 840 bytes long (i.e. a relatively short json).

How could I read in only the json part and not the entire Java 'overhead'/'java serialization format'

What you are currently doing is:

1. Serialize to JSON
2. Serialize the Java String with JSON data using Java serialization

Instead, change your code to:

1. Serialize to JSON (directly to file, or first to string, then to file using OutputStreamWriter wrapping a FileOutputStream).

Trying to reimplement Java serialization in another language is extremely painful, and your current usage of Java serialization makes little sense, and it is unnecessary overhead and only makes your life harder. Remove it from the mix and write the JSON directly to file.

It will simplify your code, and removes the need to implement Java serialization in Dart.

To be able to handle current files written in this combination of Java serialization wrapping JSON data, I would recommend using some extra code in your Java application to detect if a file is Java serialization and if so do a one-off conversion to store the JSON directly.

Alternatively, you can read up on the Java serialization protocol in Java Object Serialization Specification.