Question

I am trying to create a JSONMessage class that can parse a json string and add the objects contained in the message to the List<T> t.

The object contained in the message implements the interface JSONSerialisation and implements the two methods toJSON() and fromJSON().

The code below does not work because I cannot instantiate the Type T and I get an error on the row t2.fromJSON... (as t2 has not been initialized).

I am not sure if my approach here is correct and if what I try to is achievable (create a generic JSONMessage) that I can use to encode/parse different type of objects. If that approach is not possible, I would appreciate hints of how I could achieve a similar result.

Regards

Interface

public interface JSONSerialisation {
    public JSONObject toJSON();
    public void fromJSON(JSONObject jsonObject);
}

Class

public class JSONMessage<T extends JSONSerialisation> {

    private List<T> t;

    public JSONMessage(String json) {
        parseJSONMessage(json);
    }

    public void parseJSONMessage(String json) {
        try {
            this.t = new ArrayList<T>();
            JSONObject jsonObject;
            JSONArray lineItems;
            jsonObject = new JSONObject(json);
            this.messageHeader = new MessageHeader(jsonObject.getJSONObject("Header"));

            lineItems = jsonObject.getJSONArray("Data");
            int size = lineItems.length();

            for (int i = 0; i < size; i++) {
                T t2;
                t2.fromJSON(lineItems.getJSONObject(i));
                t.add(t2);
            }
        } catch (JSONException e) {
            e.printStackTrace();
        }
    }

}

Answer 1

Looks like you are trying to replace the functionality available from most JSON deserialization libraries. You will notice that most of them require a parametized class, as a solution to the very problem you are facing. It’s not the most elegant solution, but it will work. I would drop the generic parameter from the deserialization class, and the stateful variables as well:

public class JSONMessage {
    public static <T extends JSONSerialisation> Collection<T> parseJSONMessage(Class<T> clazz, String json) {
        try {
            final JSONObject jsonObject = new JSONObject(json);
            final JSONArray lineItems = jsonObject.getJSONArray("Data");

            Collection<T> results = new ArrayList<T>(lineItems.size());

            for (final JSONElement elem: lineItems) {
                T result = clazz.newInstance();
                result.fromJSON(elem);
                results.add(result);
            }
        } catch (JSONException e) {
            e.printStackTrace();
        }
    }

    public JSONMessage() {
        super();
    }
}

The code is obviously missing some error handling, and I made up a bogus JSONElement type in the enhanced for loop, but you get the general idea. Good luck.