How I can load a file and save this in a list of objects in Android?

Question

Hey I have a Problem with my Android App. I'm a rookie in Android Development and try a little Android App. I want to save input in a file and load this. I want to save a object (or list of objects) serialize in the file. My Idea is to load this file if this exist and fill my ListView with them else I have a text message.

I debug my Application a look in LogCat and Android Monitor. I have a little Idea where is the Error.

The Problem is, if I start my App and the onStart Method begin. I execute a function that give me a list of records back. But for this I load the file in this list.(Here is the error I think).

Here is my Error in LogCat: (A part of this)

04-22 19:41:29.547 13928-13928/de.tarasov.wladimir.leistungen E/AndroidRuntime: FATAL EXCEPTION: main
                                                                                Process: de.tarasov.wladimir.leistungen, PID: 13928


  java.lang.RuntimeException: Unable to start activity ComponentInfo{de.tarasov.wladimir.leistungen/de.tarasov.wladimir.leistungen.persistence.RecordsActivity}: java.lang.ClassCastException: de.tarasov.wladimir.leistungen.models.Record cannot be cast to java.util.List

Here is my MainActivity:

@Override
protected void onStart(){
    super.onStart();

    List<Record> records = RecordDAO.findAll(this);

    ArrayAdapter<Record> adapter =
            new ArrayAdapter<Record>(this,
                    android.R.layout.simple_list_item_1,records);

    mRecordsListView.setAdapter(adapter);

}  

Here is the RecordDAO:

public static List<Record> findAll(Context ctx){

        List<Record> result;
        Object obj = null;

        File f = ctx.getFileStreamPath("records.obj");

        if(f.exists()){

            try(FileInputStream in = ctx.openFileInput("records.obj")){

                ObjectInputStream objIn = new ObjectInputStream(in);
                obj = objIn.readObject();

            }catch(Exception e){
                e.printStackTrace();
            }

        }

        if(obj != null){
            result = (List<Record>) obj;
        }else{
            result = new ArrayList<>();
        }

        return result;
    }

UPDATE : here is my method for saving the Object.

public static void persistAll(Context ctx, List<Record> records){

    try(FileOutputStream out = ctx.openFileOutput("records.obj",Context.MODE_PRIVATE)){
        ObjectOutputStream objOut = new ObjectOutputStream(out);
        objOut.writeObject(records);
    }catch(Exception e){
        e.printStackTrace();
    }
}

Answer 1

It would be great if you could attach the code where you write the objects into a file.
As the error message says, you are probably writing a Record in the file and trying to read it as a List. Instead you should have written it as an ArrayList and read it as a List.

Answer 2

It appears that your application is crashing on this line:

result = (List<Record>) obj;

Try checking the type of obj . It looks like it may be of type Record . If that is the case, try adding obj to result .

List<Record> result = new ArrayList();
if (obj instanceof Record) {
    result.add(obj);
}