Внешнее хранилище в Android

Я пытаюсь сохранить ArrayList на внешнем хранилище в приложении для Android, и, похоже, он терпит неудачу каждый раз, но я не уверен, почему. Я следил за онлайн-учебниками об использовании внешнего хранилища, и я не вижу ничего другого, кроме кода, который у меня есть. Вот моя функция чтения:

public static ArrayList<MapRoute> readFile(Context context, String filename) {

        ObjectInputStream ois = null;
        ArrayList<MapRoute> result = null;

        if (!externalStorageAvailable || !externalStorageWritable) {
            Log.d("READ FAILED", Environment.getExternalStorageState());
            return null;
        }

        try {
            File file = new File(context.getExternalFilesDir(null), filename);
            FileInputStream fis = new FileInputStream(file);
            ois = new ObjectInputStream(fis);
            result = (ArrayList<MapRoute>)ois.readObject();
            ois.close();
        } catch (Exception e) {
            Log.d("FAILED", "READ FAILED");
        } finally {
            try {
                if (ois != null) {
                    ois.close();
                }
            } catch (IOException e) {
                Log.d("ENDREAD", "FAIL");
            }
        }

        return result;
    }

И написать:

public static boolean saveFile(Context context, ArrayList<MapRoute> routes, String filename) {

        if (!externalStorageAvailable && !externalStorageWritable) {
            Log.d("FAILED", "FAIL");
            return false;
        }

        File file = new File(context.getExternalFilesDir(null), filename);
        ObjectOutputStream oos = null;
        boolean success = false;

        try {
            OutputStream os = new FileOutputStream(file);
            oos = new ObjectOutputStream(os);
            oos.writeObject(routes);
            success = true;
        } catch (IOException e) {
            Log.d("OOS", "FAILED");
        } finally {
            try {
                if (oos != null) {
                    oos.close();
                }
            } catch (IOException e) {
                Log.d("OOS", "FAILED2");
            }
        }

        return success;
    }

В моем сообщении logcat выводится сообщение FAILED READ FAILED из блока try внутри функции readFile, и Im не уверен, что это связано с тем, как я пытаюсь прочитать список массивов.

Любая помощь по этому вопросу будет принята с благодарностью.

EDIT: Вот трассировка стека

    03-13 22:20:33.690: W/System.err(25353): java.io.WriteAbortedException: Read an      exception; java.io.NotSerializableException: mark.es3.mapRoute.MapRoute
03-13 22:20:33.690: W/System.err(25353):    at java.io.ObjectInputStream.readNonPrimitiveContent(ObjectInputStream.java:933)
03-13 22:20:33.690: W/System.err(25353):    at java.io.ObjectInputStream.readObject(ObjectInputStream.java:2262)
03-13 22:20:33.690: W/System.err(25353):    at java.io.ObjectInputStream.readObject(ObjectInputStream.java:2217)
03-13 22:20:33.690: W/System.err(25353):    at java.util.ArrayList.readObject(ArrayList.java:665)
03-13 22:20:33.690: W/System.err(25353):    at java.lang.reflect.Method.invokeNative(Native Method)
03-13 22:20:33.690: W/System.err(25353):    at java.lang.reflect.Method.invoke(Method.java:507)
03-13 22:20:33.690: W/System.err(25353):    at java.io.ObjectInputStream.readObjectForClass(ObjectInputStream.java:1520)
03-13 22:20:33.690: W/System.err(25353):    at java.io.ObjectInputStream.readHierarchy(ObjectInputStream.java:1443)
03-13 22:20:33.695: W/System.err(25353):    at java.io.ObjectInputStream.readNewObject(ObjectInputStream.java:2112)
03-13 22:20:33.695: W/System.err(25353):    at java.io.ObjectInputStream.readNonPrimitiveContent(ObjectInputStream.java:916)
03-13 22:20:33.695: W/System.err(25353):    at java.io.ObjectInputStream.readObject(ObjectInputStream.java:2262)
03-13 22:20:33.695: W/System.err(25353):    at java.io.ObjectInputStream.readObject(ObjectInputStream.java:2217)
03-13 22:20:33.695: W/System.err(25353):    at mark.es3.activities.CreateSessionActivity$FavouriteRoutesTab.readFile(CreateSessionActivity.java:245)
03-13 22:20:33.695: W/System.err(25353):    at mark.es3.activities.CreateSessionActivity$FavouriteRoutesTab.onCreate(CreateSessionActivity.java:148)
03-13 22:20:33.695: W/System.err(25353):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
03-13 22:20:33.695: W/System.err(25353):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1615)
03-13 22:20:33.695: W/System.err(25353):    at android.app.ActivityThread.startActivityNow(ActivityThread.java:1491)
03-13 22:20:33.695: W/System.err(25353):    at android.app.LocalActivityManager.moveToState(LocalActivityManager.java:127)
03-13 22:20:33.700: W/System.err(25353):    at android.app.LocalActivityManager.startActivity(LocalActivityManager.java:339)
03-13 22:20:33.700: W/System.err(25353):    at android.widget.TabHost$IntentContentStrategy.getContentView(TabHost.java:691)
03-13 22:20:33.700: W/System.err(25353):    at android.widget.TabHost.setCurrentTab(TabHost.java:341)
03-13 22:20:33.700: W/System.err(25353):    at android.widget.TabHost$2.onTabSelectionChanged(TabHost.java:140)
03-13 22:20:33.700: W/System.err(25353):    at android.widget.TabWidget$TabClickListener.onClick(TabWidget.java:456)
03-13 22:20:33.700: W/System.err(25353):    at android.view.View.performClick(View.java:2538)
03-13 22:20:33.700: W/System.err(25353):    at android.view.View$PerformClick.run(View.java:9152)
03-13 22:20:33.700: W/System.err(25353):    at android.os.Handler.handleCallback(Handler.java:587)
03-13 22:20:33.700: W/System.err(25353):    at android.os.Handler.dispatchMessage(Handler.java:92)
03-13 22:20:33.700: W/System.err(25353):    at android.os.Looper.loop(Looper.java:123)
03-13 22:20:33.720: W/System.err(25353):    at android.app.ActivityThread.main(ActivityThread.java:3691)
03-13 22:20:33.720: W/System.err(25353):    at java.lang.reflect.Method.invokeNative(Native Method)
03-13 22:20:33.720: W/System.err(25353):    at java.lang.reflect.Method.invoke(Method.java:507)
03-13 22:20:33.720: W/System.err(25353):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:847)
03-13 22:20:33.720: W/System.err(25353):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:605)
03-13 22:20:33.720: W/System.err(25353):    at dalvik.system.NativeStart.main(Native Method)
03-13 22:20:33.720: W/System.err(25353): Caused by: java.io.NotSerializableException: mark.es3.mapRoute.MapRoute
03-13 22:20:33.720: W/System.err(25353):    at java.io.ObjectOutputStream.writeNewObject(ObjectOutputStream.java:1535)
03-13 22:20:33.720: W/System.err(25353):    at java.io.ObjectOutputStream.writeObjectInternal(ObjectOutputStream.java:1847)
03-13 22:20:33.720: W/System.err(25353):    at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:1689)
03-13 22:20:33.725: W/System.err(25353):    at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:1653)
03-13 22:20:33.725: W/System.err(25353):    at java.util.ArrayList.writeObject(ArrayList.java:651)
03-13 22:20:33.725: W/System.err(25353):    at java.lang.reflect.Method.invokeNative(Native Method)
03-13 22:20:33.725: W/System.err(25353):    at java.lang.reflect.Method.invoke(Method.java:507)
03-13 22:20:33.725: W/System.err(25353):    at java.io.ObjectOutputStream.writeHierarchy(ObjectOutputStream.java:1219)
03-13 22:20:33.725: W/System.err(25353):    at java.io.ObjectOutputStream.writeNewObject(ObjectOutputStream.java:1575)
03-13 22:20:33.725: W/System.err(25353):    at java.io.ObjectOutputStream.writeObjectInternal(ObjectOutputStream.java:1847)
03-13 22:20:33.725: W/System.err(25353):    at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:1689)
03-13 22:20:33.725: W/System.err(25353):    at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:1653)
03-13 22:20:33.725: W/System.err(25353):    at mark.es3.activities.CreateSessionActivity$FavouriteRoutesTab.saveFile(CreateSessionActivity.java:224)
03-13 22:20:33.725: W/System.err(25353):    at mark.es3.activities.CreateSessionActivity$FavouriteRoutesTab.saveRoute(CreateSessionActivity.java:207)
03-13 22:20:33.730: W/System.err(25353):    at mark.es3.activities.PlotRouteActivity.onHandleActionBarItemClick(PlotRouteActivity.java:94)
03-13 22:20:33.730: W/System.err(25353):    at greendroid.app.GDMapActivity$1.onActionBarItemClicked(GDMapActivity.java:247)
03-13 22:20:33.730: W/System.err(25353):    at greendroid.widget.ActionBar$1.onClick(ActionBar.java:396)
03-13 22:20:33.730: W/System.err(25353):    ... 11 more

Мой класс MapRoute реализует Serializable, поэтому я не уверен, что это значит.