No-args constructor for class XXX does not exist

Question

I am new to Android development. Here, I am making a GET call like this -

    protected String doInBackground(String... params) {
        List<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>(1);
        nameValuePairs.add(new BasicNameValuePair("email", "guest@example.com"));

        JSONHttpClient jsonHttpClient = new JSONHttpClient();
        ProductDetail[] products  = jsonHttpClient.Get(ServiceUrl.PRODUCT, nameValuePairs, ProductDetail[].class);

        return null;
    }

This is the GET call in JSONHttpClient file -

 public <T> T Get(String url, List<NameValuePair> params, final Class<T> objectClass) {
    DefaultHttpClient defaultHttpClient = new DefaultHttpClient();
    String paramString = URLEncodedUtils.format(params, "utf-8");
    url += "?" + paramString;
    HttpGet httpGet = new HttpGet(url);


        httpGet.setHeader("Accept", "application/json");
        httpGet.setHeader("Accept-Encoding", "gzip");
        httpGet.setHeader("Authorization", "Bearer <code>");

        HttpResponse httpResponse = defaultHttpClient.execute(httpGet);
        HttpEntity httpEntity = httpResponse.getEntity();
        if (httpEntity != null) {
            InputStream inputStream = httpEntity.getContent();
            Header contentEncoding = httpResponse.getFirstHeader("Content-Encoding");
            if (contentEncoding != null && contentEncoding.getValue().equalsIgnoreCase("gzip")) {
                inputStream = new GZIPInputStream(inputStream);
            }

            String resultString = convertStreamToString(inputStream);
            inputStream.close();
            return new GsonBuilder().create().fromJson(resultString, objectClass);

        }

    return null;
}

And this is my ProductDetail class -

public class ProductDetail {

    public int Id;
    public String Name;

}

On running this, I am getting below error -

No-args constructor for class com.compa.ProductDetail does not exist. Register an InstanceCreator with Gson for this type to fix this problem.

This is thrown on this line in JSONHttpClient file -

return new GsonBuilder().create().fromJson(resultString, objectClass);

Can anyone help on this?

In my web api, I am creating json like this (proddetails is a C# IEnumerable object) -

json = JsonConvert.SerializeObject(proddetails);
var response = this.Request.CreateResponse(HttpStatusCode.OK);
response.Content = new StringContent(json, Encoding.UTF8, "application/json");
return response;

The structure of response json is -

[
   {
       "Id": 1,
       "Name": "First"
   },
   {
       "Id": 2,
       "Name": "Second"
   }          
] 

Answer 1

The Gson user guide (https://sites.google.com/site/gson/gson-user-guide) tells you that a well behaved class (meant for serialization and deserialization) should have a no argument constructor. If this is not there, it advises you to use InstanceCreator.

Even if you do not have a constructor, Gson will create an ObjectConstructor for your class. But this is not safe always and has it's own limitations. This question on SO goes more into the details: Is default no-args constructor mandatory for Gson?

NOTE: Please see that if this is an inner class, then it MUST have a constructor as explained in the documentation.

EDIT: Your json is an array. So you need to have the specified number of array objects in the containing class. So you can do the following and then cast:

public class ProductDetailArray {

    public ProductDetailArray[] array;

    public static ProductDetail {
        public ProductDetail() {} // You can also make the constructor private if you don't want anyone to instantiate this

        public int Id;
        public String Name; 
    }
}

Once you cast your json similarly as before:

ProductDetailArray obj = GsonBuilder.create().fromJson(response, ProductDetailArray.class);

ProductDetail one = obj.array[0];
ProductDetail two = obj.array[1];

And then you can do your manipulation.. also you should probably be using Gson.fromJson() rather than the GsonBuilder