Arraylist is of type LinkedTreeMap instead of Brewer

Question

I am trying to make my answer class generic. The problem is that it returns a list with type com.google.gson.internal.LinkedTreeMap instead of type Brewer.

Function that runs it all:

List<Brewer> brewerList = null; try { Response<Brewer> responseHandler = new Response<Brewer>(); brewerList = responseHandler.getAll(response); functionCallBack = Constants.FUNCTION_DB_BREWER; list = brewerList.toArray(new Brewer[brewerList.size()]); } catch (IOException e) { Tools.LOG_ERROR(e); } 

This is my answer class:

 public class Response<T> { public List<T> getAll(JSONObject response) throws IOException { List<T> brewerList = null; // Map JSON to JAVA Objects Type listResponse = new TypeToken<ListResponse<T>>() { }.getType(); ListResponse<T> responseObject = Shared.gson.fromJson(response.toString(), listResponse); if (responseObject != null) { // Status, Message, Data int status = responseObject.getStatus(); Tools.LOG_DEBUG("Response - getAll, Status: " + status); if (responseObject.getData() != null && responseObject.getStatus() == 200) { brewerList = responseObject.getData(); } else { Tools.LOG_DEBUG("Couldn't find the webservices!"); } } return brewerList; } } 

And this is my data class that I give for analysis from json

 public class ListResponse<T> { private int status; private List<T> data; public ListResponse() { } public int getStatus() { return status; } public void setStatus(int status) { this.status = status; } public List<T> getData() { return data; } public void setData(List<T> data) { this.data = data; } } 

An exception:

 04-11 10:54:55.994 31660-31660/be.appmax.ktsjjt E/AndroidRuntime﹕ FATAL EXCEPTION: main java.lang.ArrayStoreException: source[0] of type com.google.gson.internal.LinkedTreeMap cannot be stored in destination array of type be.appmax.ktsjjt.models.Brewer[] at java.lang.System.arraycopy(Native Method) at java.util.ArrayList.toArray(ArrayList.java:523) at be.appmax.ktsjjt.database.SyncHandler.onRestTaskCompleted(SyncHandler.java:96) at be.appmax.ktsjjt.webservices.WSBrewer$1.onSuccess(WSBrewer.java:36) at com.loopj.android.http.JsonHttpResponseHandler$1$1.run(JsonHttpResponseHandler.java:125) at android.os.Handler.handleCallback(Handler.java:730) at android.os.Handler.dispatchMessage(Handler.java:92) at android.os.Looper.loop(Looper.java:176) at android.app.ActivityThread.main(ActivityThread.java:5419) at java.lang.reflect.Method.invokeNative(Native Method) at java.lang.reflect.Method.invoke(Method.java:525) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1046) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:862) at dalvik.system.NativeStart.main(Native Method) 

Answer 1

Unfortunately, you cannot do what you are trying to do due to erasing styles in Java.

The TypeToken destruction used by various libraries that do deserialization tries to get around this, but you cannot use it with the type parameter of the type you are trying to do. Type T is removed at runtime, and Gson can no longer determine the actual type, so it returns a Map .

Edit to add:. To get around this, you need to go into the TypeToken , and not try to create it in the method. This allows you to find out the type. The hard bit in your case is you want to return List<T> , but the TypeToken is actually ListResponse<T> . Because of this, you need to understand Generics a bit and deduce the TypeToken type with a restriction:

 public <V extends ListResponse<T>> List<T> getAll(JSONObject response, TypeToken<V> token) throws IOException { ... V responseObject = Shared.gson.fromJson(response.toString(), token.getType()); ... } 

When you call this, you need to pass an instance of TypeToken , but then it will work.

 Response<Brewer> responseHandler = new Response<Brewer>(); TypeToken<ListResponse<Brewer>> token = new TypeToken<ListResponse<Brewer>>(){}; brewerList = responseHandler.getAll(response, token);