java.lang.RuntimeException: Parcel android.os.Parc

I seem to be getting a strange error in my app (see GitHub), which occurs when I pass objects to different activities that implement Parcelable.

I have checked other questions and answers here on Stack Overflow, but I was unable to find a solution. I've tried the answer here, for example - here it is for reference:

-keepclassmembers class * implements android.os.Parcelable {
    static ** CREATOR;
}

I've also made sure that the method calls in writeToParcel are in order. Most other questions on Stack Overflow about this issue don't have answers.

Moreover, the reason I am asking a new question is because I think my problem is caused because of how I have used interfaces in my app (I will expand on this point later on). Other questions on Stack Overflow would not suit my particular scenario.

In the following, I have provided links to the code via GitHub, so that you can explore more of the code if required.

When I click on a button to launch a new activity (passing an object that implements Parcelable), there is a crash:

Process: com.satsuware.flashcards, PID: 4664
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.satsuware.flashcards/com.satsumasoftware.flashcards.ui.FlashCardActivity}: java.lang.RuntimeException: Parcel android.os.Parcel@d2219e4: Unmarshalling unknown type code 6815860 at offset 200
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
    ...
 Caused by: java.lang.RuntimeException: Parcel android.os.Parcel@d2219e4: Unmarshalling unknown type code 6815860 at offset 200
at android.os.Parcel.readValue(Parcel.java:2319)
at android.os.Parcel.readListInternal(Parcel.java:2633)
at android.os.Parcel.readArrayList(Parcel.java:1914)
at android.os.Parcel.readValue(Parcel.java:2264)
at android.os.Parcel.readArrayMapInternal(Parcel.java:2592)
at android.os.BaseBundle.unparcel(BaseBundle.java:221)
at android.os.Bundle.getParcelable(Bundle.java:786)
at android.content.Intent.getParcelableExtra(Intent.java:5377)
at com.satsumasoftware.flashcards.ui.FlashCardActivity.onCreate(FlashCardActivity.java:71)
at android.app.Activity.performCreate(Activity.java:6237)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
...

I call the aforementioned activity like so (also see GitHub):

Intent intent = new Intent(TopicDetailActivity.this, FlashCardActivity.class);
intent.putExtra(FlashCardActivity.EXTRA_TOPIC, mTopic);
intent.putExtra(FlashCardActivity.EXTRA_NUM_CARDS, mSelectedNumCards);
intent.putExtra(FlashCardActivity.EXTRA_CARD_LIST, mFilteredCards);
startActivity(intent);

The main part to consider is when I pass mTopic. This is a Topic interface that I created.

However, the Topic interface extends Parcelable and so the objects that implement Topic also include the constructor, CREATOR field, and the methods that a class implementing Parcelable would normally have to have.

You can view the relevant classes via the GitHub links, but I will provide the relevant parts of these classes below. Here is the Topic interface:

public interface Topic extends Parcelable {

    int getId();

    String getIdentifier();

    String getName();

    Course getCourse();


    ArrayList<FlashCard> getFlashCards(Context context);


    class FlashCardsRetriever {

        public static ArrayList<FlashCard> filterStandardCards(ArrayList<FlashCard> flashCards, @StandardFlashCard.ContentType int contentType) {
            ArrayList<FlashCard> filteredCards = new ArrayList<>();
            for (FlashCard flashCard : flashCards) {
                boolean isPaper2 = ((StandardFlashCard) flashCard).isPaper2();
                boolean condition;
                switch (contentType) {
                    case StandardFlashCard.PAPER_1:
                        condition = !isPaper2;
                        break;
                    case StandardFlashCard.PAPER_2:
                        condition = isPaper2;
                        break;
                    case StandardFlashCard.ALL:
                        condition = true;
                        break;
                    default:
                        throw new IllegalArgumentException("content type '" + contentType + "' is invalid");
                }
                if (condition) filteredCards.add(flashCard);
            }
            return filteredCards;
        }

        ...
    }

}

A class (object) that implements Topic:

public class CourseTopic implements Topic {

    ...

    public CourseTopic(int id, String identifier, String name, Course course) {
        ...
    }

    @Override
    public int getId() {
        return mId;
    }

    @Override
    public String getIdentifier() {
        return mIdentifier;
    }

    ...


    protected CourseTopic(Parcel in) {
        mId = in.readInt();
        mIdentifier = in.readString();
        mName = in.readString();
        mCourse = in.readParcelable(Course.class.getClassLoader());
    }

    public static final Parcelable.Creator<CourseTopic> CREATOR = new Parcelable.Creator<CourseTopic>() {
        @Override
        public CourseTopic createFromParcel(Parcel in) {
            return new CourseTopic(in);
        }

        @Override
        public CourseTopic[] newArray(int size) {
            return new CourseTopic[size];
        }
    };

    @Override
    public int describeContents() {
        return 0;
    }

    @Override
    public void writeToParcel(Parcel dest, int flags) {
        dest.writeInt(mId);
        dest.writeString(mIdentifier);
        dest.writeString(mName);
        dest.writeParcelable(mCourse, flags);
    }

}

In one of the last lines of the code above, you can see I pass mCourse, which is a Course object I created. Here it is:

public class Course implements Parcelable {

    ...

    public Course(String subject, String examBoard, @FlashCard.CourseType String courseType,
              String revisionGuide) {
        ...
    }


    public String getSubjectIdentifier() {
        return mSubjectIdentifier;
    }

    public String getExamBoardIdentifier() {
        return mBoardIdentifier;
    }

    public ArrayList<Topic> getTopics(Context context) {
        ArrayList<Topic> topics = new ArrayList<>();
        String filename = mSubjectIdentifier + "_" + mBoardIdentifier + "_topics.csv";
        CsvParser parser = CsvUtils.getMyParser();
        try {
            List<String[]> allRows = parser.parseAll(context.getAssets().open(filename));
            for (String[] line : allRows) {
                int id = Integer.parseInt(line[0]);
                topics.add(new CourseTopic(id, line[1], line[2], this));
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
        return topics;
    }

    ...


    protected Course(Parcel in) {
        mSubjectIdentifier = in.readString();
        mBoardIdentifier = in.readString();
        mCourseType = in.readString();
        mRevisionGuide = in.readString();
    }

    public static final Creator<Course> CREATOR = new Creator<Course>() {
        @Override
        public Course createFromParcel(Parcel in) {
            return new Course(in);
        }

        @Override
        public Course[] newArray(int size) {
            return new Course[size];
        }
    };

    @Override
    public int describeContents() {
        return 0;
    }

    @Override
    public void writeToParcel(Parcel dest, int flags) {
        dest.writeString(mSubjectIdentifier);
        dest.writeString(mBoardIdentifier);
        dest.writeString(mCourseType);
        dest.writeString(mRevisionGuide);
    }

}

I suspect something here may be causing the problem, and is the reason my scenario is different from those in other questions.

To be honest, I'm not exactly sure what may be causing the error, so explanations and guidance in answers would be much appreciated.

Edit:

After David Wasser's suggestions, I have updated parts of my code like so:

FlashCardActivity.java - onCreate(...):

Bundle extras = getIntent().getExtras();
extras.setClassLoader(Topic.class.getClassLoader());
mTopic = extras.getParcelable(EXTRA_TOPIC);

Course.java - writeToParcel(...):

dest.writeString(mSubjectIdentifier);
dest.writeString(mBoardIdentifier);
dest.writeString(mCourseType);
dest.writeInt(mRevisionGuide == null ? 0 : 1);
if (mRevisionGuide != null) dest.writeString(mRevisionGuide);

Course.java - Course(Parcel in):

mSubjectIdentifier = in.readString();
mBoardIdentifier = in.readString();
mCourseType = in.readString();
if (in.readInt() != 0) mRevisionGuide = in.readString();

I've added log messages using Log.d(...) to see if any variables are null when being passed in writeToParcel(...) and used David Wasser's method to properly handle this.

I still get the same error message, however.

Your problem is in LanguagesFlashCard. Here are your parcel/unparcel methods:

protected LanguagesFlashCard(Parcel in) {
    mId = in.readInt();
    mEnglish = in.readString();
    mAnswerPrefix = in.readString();
    mAnswer = in.readString();
    mTier = in.readInt();
    mTopic = in.readParcelable(Topic.class.getClassLoader());
}

As you can see, they don't match. The second item you write to the Parcel is an int, the second item you read from the Parcel is a String.

@Override
public void writeToParcel(Parcel dest, int flags) {
    dest.writeInt(mId);
    dest.writeInt(mTier);
    dest.writeString(mEnglish);
    dest.writeString(mAnswerPrefix);
    dest.writeString(mAnswer);
    dest.writeParcelable(mTopic, flags);
}