How can I save an ArrayList to a file?

Question

How can I save an ArrayList to a file? What am I doing wrong?

I have used this SO question to help me with Serializable objects.:

how to serialize ArrayList on android

and I used this SO question on how to write an array list:

Java - How Can I Write My ArrayList to a file, and Read (load) that file to the original ArrayList?

However when I attempt to write the to the file I get the error:

java.io.NotSerializableException: at

java.io.ObjectOutputStream.writeObject at

com.mycompany.MyClass.saveData

Here is MyClass that attempts to save the file

private ArrayList < MyCustomObject > arrayList;
private File dataFile;
private String FILE_NAME = "FILE_DATA.dat";

public void init(final Context context) {
    this.appContext = context;

    dataFile = new File(appContext.getFilesDir(), FILE_NAME);

    if (dataFile.exists()) {
        loadData();

    } else {

        try {
            dataFile.createNewFile();
        } catch (IOException e) {
            e.printStackTrace();
        }

        arrayList = new ArrayList < MyCustomObject > ();
saveData();
   }

}


private void saveData() {
    FileOutputStream fos = null;

    try {
        fos = new FileOutputStream(dataFile);
    } catch (FileNotFoundException e) {
        e.printStackTrace();
    } finally {

        if (fos != null) {

            ObjectOutputStream oos = null;
            try {

                oos = new ObjectOutputStream(fos);

                if (oos != null) {

                    oos.writeObject(arrayList);

                }
                assert oos != null;
                oos.close();

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


}

private void loadData() {

    FileInputStream fis = null;
    try {
        fis = new FileInputStream(dataFile);
    } catch (FileNotFoundException e) {
        e.printStackTrace();
    } finally {

        if (fis != null) {

            ObjectInputStream ois = null;
            try {
                ois = new ObjectInputStream(fis);

                if (ois != null) {

                    try {

                        arrayList = (ArrayList < MyCustomObject > ) ois.readObject();
                    } catch (ClassNotFoundException e) {
                        e.printStackTrace();
                    }
                } else {

                }
                assert ois != null;
                ois.close();

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

}

Here is MyCustomObject

public class MyCustomObject implements Serializable {

    public String myname = "";
    public String someOtherItem = "";
    public int aNumber = 0;
    public MyCustomObject getCustomObject() {
        return this;
    }
}

What are you trying to serialise - the `arrayList` member variable of a `MyClass` instance, or the `MyClass` instance itself? — clownba0t, Nov 11 '16 at 17:20
I guess I should be serializing my ArrayList since that is what needs to be saved. — user-44651, Nov 11 '16 at 18:14
Indeed - `MyClass` seems to have a `Context` member (see `this.appContext` in the `init` method) which is certainly _not_ serialisable! — clownba0t, Nov 11 '16 at 18:16
Since MyCustomObject IS Serializable, wouldn't the ArrayList also be Serializable through inheritance? It is literally a list of Serializable objects. — user-44651, Nov 11 '16 at 18:18
Yes, but you're trying to serialise an instance of `MyClass`, right? Otherwise, you wouldn't be seeing `com.mycompany.MyClass.saveData` in the exception stack trace. Simply implementing the `Serializable` interface doesn't magically make a class serialisable - this is exactly what the `NotSerializableException` is for. The stack trace you posted is missing data, but I expect you received it because Android was attempting to serialize the `Context` member variable of `MyClass`, which as I said is not serialisable. — clownba0t, Nov 11 '16 at 18:30
As an aside, you shouldn't need the special handling code in `MyClass` to serialise the `ArrayList` of `MyCustomObject` instances, because `ArrayList` itself implements the `Serializable` interface and the `ArrayList` in question holds `MyCustomObject` instances which are also serialisable. Take a look at https://developer.android.com/reference/java/io/Serializable.html for more details. — clownba0t, Nov 11 '16 at 18:31

alainlompo · Accepted Answer · 2016-11-11T19:06:06.450

Replace this method

 public MyCustomObject MyCustomObject() {
        return this;
 }

in your MyCustomObject class and your code should work fine. Use something like

 public MyCustomObject getMyCustomObject() {
        return this;
 }

Because the way you name your method is conflicting wit the default constructor that java creates for MyCustomObject class when you do not provide a constructor yourself. I assume that you are using this method to be able to add an instance of MyCustomObject to your array list: you don't really need such a method but with the proper naming you can still use it.

You should also put sample datas in your ArrayList before saving it to the disk by calling the saveData() method.

Here is an illustration from your code that works. I am not sure what your Context object is exactly but your are using it to get access to the file path, so to get things going I just used a particular file path.

public class MyClass {
    private ArrayList < MyCustomObject > arrayList;
    private File dataFile;
    private String FILE_NAME = "FILE_DATA.dat";

    public void init(final Context context) {


        dataFile = new File("C:\\lompo\\file1.txt");

        if (dataFile.exists()) {
            loadData();

        } else {

            try {
                dataFile.createNewFile();
            } catch (IOException e) {
                e.printStackTrace();
            }

            arrayList = new ArrayList < MyCustomObject > ();
            MyCustomObject obj1 = new MyCustomObject();
            obj1.aNumber = 125;
            obj1.myname = "HIS NAME";
            arrayList.add(obj1);
            saveData();
       }

    }

    public static void main(String[] args) {
        MyClass myClazz = new MyClass();
        myClazz.init(null);

        System.out.println("Arraylist has " + myClazz.arrayList.size() + " elements");
    }

    private void saveData() {
        FileOutputStream fos = null;

        try {
            fos = new FileOutputStream(dataFile);
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } finally {

            if (fos != null) {

                ObjectOutputStream oos = null;
                try {

                    oos = new ObjectOutputStream(fos);

                    if (oos != null) {

                        oos.writeObject(arrayList);

                    }
                    assert oos != null;
                    oos.close();

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


    }

    private void loadData() {

        FileInputStream fis = null;
        try {
            fis = new FileInputStream(dataFile);
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } finally {

            if (fis != null) {

                ObjectInputStream ois = null;
                try {
                    ois = new ObjectInputStream(fis);

                    if (ois != null) {

                        try {

                            arrayList = (ArrayList < MyCustomObject > ) ois.readObject();
                        } catch (ClassNotFoundException e) {
                            e.printStackTrace();
                        }
                    } else {

                    }
                    assert ois != null;
                    ois.close();

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

    }
}

As you can see in the code, the first run of the main method will save the file on disk with an arrayList populated by one object. The second run reads from the file and then I printed the number of elements and the infos that I have saved before: the picture illustrates the result

Thanks. Let me work thought this with your suggestions and I'll get back with you — user-44651, Nov 11 '16 at 18:35
No I still get `java.io.NotSerializableException: W/System.err: at java.io.ObjectOutputStream.writeNewObject` on saving — user-44651, Nov 11 '16 at 18:55
Then once the file is created, if the app is restarted I get `Read an exception; java.io.NotSerializableException: W/System.err: at java.io.ObjectInputStream.readNonPrimitiveConten` — user-44651, Nov 11 '16 at 18:56
Really? Have you changed the name of MyCustomObject MyCustomObject() method in MyCustomObject class? — alainlompo, Nov 11 '16 at 18:57
Yes, I updated the code in my question to reflect the changes I made based upon your suggestions. — user-44651, Nov 11 '16 at 18:58
There wont alwasy be data stored in the file. Sometimes the file will be empty. — user-44651, Nov 11 '16 at 19:02
Yes, I updated my answer. are you putting sample MyCustomObject instances into the list before saving it? Your post does not show it — alainlompo, Nov 11 '16 at 19:04
yes, there will be instance in which there will be nothing in the file, so I need to save it with just an empty ArrayList — user-44651, Nov 11 '16 at 19:05
It is still working for me even wthout the sample datas. Can you share your actual code with me on a sharing platform if possible: I would like to have a closer look — alainlompo, Nov 11 '16 at 19:09
Let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/127922/discussion-between-alainlompo-and-user-44651). — alainlompo, Nov 11 '16 at 19:12

How can I save an ArrayList to a file?

1 Answers1