What's the best approach to parcel/unparcel a HashMap of custom objects?

Question

I have an Event class that defines a private HashMap like this:

private Map<String, Object> data = new HashMap<String, Object>();

The Event class is a wrapped for any kind of "event". This HashMap can contain any object instance which is referenced by a key. The class receiving an Event instance knows what class is related to each key, so it can safely cast the Object to its corresponding subclass.

The problem arises when I try to pass an Event instance between 2 processes. Event implements Parcelable so it can be sent through a Message:

Bundle bundle = new Bundle();
bundle.putParcelable(Event.BUNDLE_KEY, event);

// Make the message with the bundle
Message message = new Message();
message.setData(bundle);

When unmarshalling:

public void readFromParcel(Parcel in) {

    idEvent = in.readInt();
    priority = in.readInt();
    idSource = in.readInt();
    idDestination = in.readInt();
    action = in.readInt();

    Bundle mapBundle = in.readBundle();

    if (mapBundle.getSerializable(MAP_KEY) instanceof HashMap) {
        data = (Map<String, Object>) mapBundle.getSerializable(MAP_KEY);
    } else {
        Log.e(TAG, "Parcel reading error: not a HashMap");
    }
}

The problem is this won't work since I need to specify to mapBundle which ClassLoader to use, e.g. mapBundle.setClassLoader(Entity.class.getClassLoader());. But I don't know what Object subclasses the HashMap will have...

This is what I thought:

• Write a ClassLoader that loads any of these classes. The problem is that I cannot get a way to obtain the byte[] representing the object since it's inside the HashMap. And I cannot use mapBundle.getSerializable() to get it because it precisely throws ClassNotFound exception.

• Pass some extra information so I can know what classes are on the HashMap. Besides this looking as redundant information, still no go because if I set one classloader on the Bundle, it will still throw a ClassNotFound exception on the other classes...

I would really appreciate some help on this issue. Thanks in advance!

Answer 1

Actually this answers my question. I have to get a reference to my ClassLoader and then set it up on de-serialization, which I think it's done through another thread, probably a system thread, and thus it doesn't know about my classes.

Answer 2

I think the best solution is to rethink your plan to have a hashmap of objects and casting them to real objects depending on the key in the hashmap.

By doing that you will not really utilize the typesafety that java provides for you, and you will also get extra overhead from the HashMap.

I would instead recommend to implement the Event as a interface, and have each type of Event implement that interface.

That way the "data" variable in the classes can be renamed with a descriptive name (that makes the code easier to read), and you don't have to store any redundant data.