从后端解析JSON的问题

Question

As I have been working on an Android project in which there is a need to receive JSON from the backend, I prepared the JSON in Java using Spring Boot 2.1.7.

From the backend side it is ok, works. But from the Android side, I cannot parse the variable. When I try to parse the variable using construction:

JSONArray jsonArray = new JSONArray(variable);

then I recieve the information that JSONArray() in JSONArray cannot be applied to com.package.Model . I have also tried different approach to this problem:

Backend

@RequestMapping( value = "/approachh", method = RequestMethod.GET )
@ResponseBody
public List<Categories> getList() {
    List<Categories> categoriesList = new ArrayList<>();
    categoriesList = categoriesRepository.findAll();
    return categoriesList;
}

JSON response

{
  "List":[
    {
      "id":1,
      "categoryName":"Categories1",
      "subCategories":[
        {
          "id":1,
          "subCategoriesName":"lkjasdf"
        }
      ]
    },
    {
      "id":2,
      "categoryName":"Categories2",
      "subCategories":[
        {
          "id":2,
          "subCategoriesName":"lkjasdasdfasf"
        }
      ]
    }
  ]
}

Client (Android)

JSONArray jsonArray = new JSONArray(variable); 
// then I recieve the information that JSONArray() in JSONArray cannot be applied to com.package.Model. I have also tried different approach to this problem:

String json = "";
Gson gson = new GsonBuilder().create();
Type showType = new TypeToken<Collection<Approach>>(){}.getType();
ArrayList<Approach> approaches = gson.fromJson(json, showType);
String catName = String.valueOf(approaches.get(0).getCategoryName());

// but when I tried to receive information about the catName, there was 0.
Log.d("catName","catName" + catName);

Approach.class

public class Approach {


@SerializedName("categoryName")
@Expose
private String categoryName;

@SerializedName("subCategories")
@Expose
private List<SubCategories> subCategories;

public Approach(String categoryName, List<SubCategories> subCategories) {
    this.categoryName = categoryName;
    this.subCategories = subCategories;
}

public String getCategoryName() {
    return categoryName;
}

public void setCategoryName(String categoryName) {
    this.categoryName = categoryName;
}

public List<SubCategories> getSubCategories() {
    return subCategories;
}

public void setSubCategories(List<SubCategories> subCategories) {
    this.subCategories = subCategories;
}

}

SubCategories.class

public class SubCategories {


@SerializedName("id")
@Expose
private int id;

@SerializedName("subCategoriesName")
@Expose
private String subCategoriesName;

public int getId() {
    return id;
}

public void setId(int id) {
    this.id = id;
}

public String getSubCategoriesName() {
    return subCategoriesName;
}

public void setSubCategoriesName(String subCategoriesName) {
    this.subCategoriesName = subCategoriesName;
}

}

MainActivity.class Android

method in which I connect with retrofit to localhost to retrieve information

private void fetchhhhData() {

    compositeDisposable.add(iMyAPI.getAll()
    .subscribeOn(Schedulers.io())
    .subscribeOn(AndroidSchedulers.mainThread())
    .subscribe(new Consumer<Approach>() {
        @RequiresApi(api = Build.VERSION_CODES.KITKAT)
        @Override
        public void accept(Approach approach) throws Exception {


            showData(Collections.singletonList(approach));

        }
    }));

}

MainActivity.class Android

showData()

  private void showData(List<Approach> approaches) {

    JSONArray jsonArray = new JSONArray(approaches);



    for (int i=1; i<= jsonArray.length() - 1; i++) {

        try {
            JSONObject jsonObject = jsonArray.getJSONObject(i);

            String categ = jsonObject.get("List").toString();

             Log.d("categ","categ" + categ);

         //   String catName = String.valueOf(jsonObject.get("List"));


     //       String catName = jsonObject.getString("categoryName");

      //      Log.d("catName","catName" + catName);


        } catch (JSONException e) {
            e.printStackTrace();
        }

    }



}

Could you please point me out, what I am doing wrong?

Thanks.

Answer 1

Here you have working example. It is much simpler if you introduce one more class CategoriesResponse (see below).

Then make sure that the json argument passed to gson.fromJson(json, CategoriesResponse.class); is actual json String or Reader or JsonObject retrieved from your endpoint.

    @Test
    public void test() {
        String json = "{\n" + "  \"List\":[\n" + "    {\n" + "      \"id\":1,\n" + "      \"categoryName"
                + "\":\"Categories1\",\n"
                + "      \"subCategories\":[\n" + "        {\n" + "          \"id\":1,\n"
                + "          \"subCategoriesName\":\"lkjasdf\"\n" + "        }\n" + "      ]\n" + "    },\n" + "    {\n"
                + "      \"id\":2,\n" + "      \"categoryName\":\"Categories2\",\n" + "      \"subCategories\":[\n"
                + "        {\n" + "          \"id\":2,\n" + "          \"subCategoriesName\":\"lkjasdasdfasf\"\n"
                + "        }\n" + "      ]\n" + "    }\n" + "  ]\n" + "}";

        Gson gson = new Gson();
        CategoriesResponse gsonResponse = gson.fromJson(json, CategoriesResponse.class);

        assertEquals(2, gsonResponse.categories.size());

    }

    public class Approach {

        @SerializedName("categoryName") @Expose private String categoryName;
        @SerializedName("subCategories") @Expose private List<SubCategories> subCategories;

        public Approach(String categoryName, List<SubCategories> subCategories) {
            this.categoryName = categoryName;
            this.subCategories = subCategories;
        }
    }

    public class SubCategories {

        @SerializedName("id") @Expose private int id;
        @SerializedName("subCategoriesName") @Expose private String subCategoriesName;
    }

    public class CategoriesResponse {

        @SerializedName("List") @Expose private List<Approach> categories;
    }