Converting JSON objects in Android with gson

Question

I am new to Java and Android so please bear with me. I am trying to create a function which calls a wcf service and converts the returned result from JSON to a Java object (I pass the type as the object t ), but it's throwing a null pointer exception on t , which must be null as I just want to pass an object of the correct type so that it becomes filled when converted. Kindly help me with it.

    public static String Post(String serviceURL, Map<String, String> entites,
        Class<?> t) {
    String responseString = "";
    StrictMode.ThreadPolicy policy = new StrictMode.ThreadPolicy.Builder()
            .permitAll().build();
    StrictMode.setThreadPolicy(policy);


            Gson gson = new GsonBuilder().create();
            //now we will try to convert the class to the specified type.
            t = (Class<?>) gson.fromJson(responseString, t);



    } catch (Exception e) {

        responseString = e.toString();

    }
    return responseString;

Thanks a lot.

After some tries, I ended up with this code but I am still facing a null pointer exception.

    MemberInfo mem = new MemberInfo();

    TypeToken<MemberInfo> m = null ;
    ServiceCaller.Post(getString(R.string.LoginService), values , m);


            Gson gson = new GsonBuilder().create();
            //now we will try to convert the class to the specified type.
            t = (TypeToken<T>) gson.fromJson(responseString, (Type) t);

Answer 1

As far as I know, this is impossible. In order for one to do this, gson would have to be able to detect the correct class only from the serialized string . However, a single string can be interpreted in many valid ways. For instance, take an example on the gson website:

class BagOfPrimitives {
  private int value1 = 1;
  private String value2 = "abc";
  BagOfPrimitives() {
    // no-args constructor
  }
}

Using Gson.toJson on an instance of this class results in the string:

{"value1":1,"value2":"abc"}

However, if I made another class identical to the first in all respects but name:

class SackOfPrimitives {
  private int value1 = 1;
  private String value2 = "abc";
  SackOfPrimitives() {
    // no-args constructor
  }
}

Then this would also serialize to the same string:

{"value1":1,"value2":"abc"}

My point is that given a single string like {"value1":1,"value2":"abc"} , there is no way for gson to determine whether it should deserialize it into an object of type BagOfPrimitives or type SackOfPrimitives . Therefore, you always have to provide gson with the correct type, because it wouldn't be possible for gson to figure it out by itself.