Pass parcelable object containing Map between Activity

Question

My class ExpenseFB, which implements Parcelable, contains a Map of UserFB (which implements Parcelable too):

ExpenseFB:

public class ExpenseFB implements Parcelable {

private String id;
private String name;
private String description;
private String whopaidID;
private String whopaidName;
private Double amount;
private Map<String, UserFB> partecipants;
// setters and getters...
@Override
public int describeContents() {
    return 0;
}

@Override
public void writeToParcel(Parcel dest, int flags) {
    dest.writeString(id);
    dest.writeString(name);
    dest.writeString(description);
    dest.writeString(whopaidID);
    dest.writeString(whopaidName);
    dest.writeMap(partecipants);
}


protected ExpenseFB(Parcel in) {
    id = in.readString();
    name = in.readString();
    description = in.readString();
    whopaidID = in.readString();
    whopaidName = in.readString();
    in.readMap(partecipants,UserFB.class.getClassLoader());
}

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

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

UserFB:

public class UserFB implements Parcelable{

private String id;
private String name;
private String email;
private Map<String, GroupFB> groups;
private Map<String, UserFB> friends;
// setters and getters
@Override
public int describeContents() {
    return 0;
}

@Override
public void writeToParcel(Parcel dest, int flags) {
    dest.writeString(id);
    dest.writeString(name);
    dest.writeString(email);
    dest.writeMap(groups);
    dest.writeMap(friends);
}

protected UserFB(Parcel in) {
    id = in.readString();
    name = in.readString();
    email = in.readString();
    in.readMap(groups,GroupFB.class.getClassLoader());
    in.readMap(friends,UserFB.class.getClassLoader());
}

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

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

I want to pass an ExpenseFB object between two Activities by adding the object ExpenseFB to the intent:

intent.putExtra("id", expenseFB);

When, in debug mode, I execute getIntent().getParcelableExtra("id") in the second activity it raises the following exception when tries to do the readMap() method on the partecipants map:

 ... Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'java.lang.Object java.util.Map.put(java.lang.Object, java.lang.Object)' on a null object reference

I see that the partecipants map in the first activity is filled: I think that the problem is in the writeMap() method.
Does exist a standard or better way to pass a Parcelable object containing a Map?
Have I to call another method to parcel the Map?
I don't want to use Serializable object because I read that they make worse performances.

Answer 1

The problem is that readMap() is used to read data from a Parcel into and existing Map. You haven't created the Map before calling readMap(), so you get the NullPointerException.

You can solve this by initializing the map when you declare it:

private Map<String, GroupFB> groups = new HashMap<String, GroupFB>();
private Map<String, UserFB> friends = new HashMap<String, UserFB>();

Or, you can create the empty Map in the UserFB constructor, like this:

protected UserFB(Parcel in) {
    id = in.readString();
    name = in.readString();
    email = in.readString();
    groups = new HashMap<String, GroupFB>();
    in.readMap(groups,GroupFB.class.getClassLoader());
    friends = new HashMap<String, UserFB>()
    in.readMap(friends,UserFB.class.getClassLoader());
}

Answer 2

You got the point but I think you need to know how to write Map<> into parcelable

Pasting writeParcel() method

@Override
public void writeToParcel(Parcel dest, int flags) {
    dest.writeInt(this.groups.size());
    for (Map.Entry<String, GroupFb> entry : this.groups.entrySet()) {
        dest.writeString(entry.getKey());
        dest.writeParcelable(entry.getValue(), flags);
    }
}

protected UserFB (Parcel in) {
    int groupsSize = in.readInt();
    this.groups = new HashMap<String, GroupFb>(groupsSize);
    for (int i = 0; i < groupsSize; i++) {
        String key = in.readString();
        GroupFb value = in.readParcelable(GroupFb.class.getClassLoader());
        this.groups.put(key, value);
    }
}

Do the same for another Map<> too.