How to solve com.google.gson.JsonArray cannot be cast while parsing Json file in java

Question

I am parsing json file in java first time. Where my json file structure is fixed one (I can not change this). I have gone through different answers, but didn't help with my json. I want list of all Group Name in one arrylist and Unit Name in another arraylist Please help to resolve this error, Thanks I used below code and tried to read from json, but failed to parsed. JSON FILE:

[
    {"No":"1","Component":"Amazon","group_id":"1","Group Name":"shop","Unit Name":"UN","PM":"NULL"},
    {"No":"2","Component":"Amazon","group_id":"1","Group Name":"shop","Unit Name":"UM","PM":"NULL"},
    {"No":"3","Component":"Amazon","group_id":"1","Group Name":"shop","Unit Name":"UP","PM":"NULL"},
    {"No":"4","Component":"Amazon","group_id":"1","Group Name":"shop","Unit Name":"SO","PM":"NULL"},
    {"No":"5","Component":"Amazon","group_id":"1","Group Name":"cart","Unit Name":"SP","PM":"NULL"},
    {"No":"6","Component":"Amazon","group_id":"2","Group Name":"payment","Unit Name":"NZ","PM":"TRUE"}
]

Code Trial 1 :

JsonArray jsonArray = new JsonArray();
            Object obj = new JsonParser().parse(new FileReader(JSON_FILE_PATH));
            JsonObject jsonObject = jsonArray.getAsJsonObject();
            String groupName = jsonObject.get("Group Name").toString();
            

ERROR: Exception in Test Case: (IllegalStateException: Not a JSON Object: [])
java.lang.IllegalStateException: Not a JSON Object: []

Code Trial 2:

Object obj = new JsonParser().parse(new FileReader(JSON_FILE_PATH));
JsonObject jsonObject = (JsonObject) obj;            
String firstName = jsonObject.get("Group Name").toString();

 
ERROR: Exception in Test Case: (ClassCastException: com.google.gson.JsonArray cannot be cast to com.google.gson.JsonObject)
java.lang.ClassCastException: com.google.gson.JsonArray cannot be cast to com.google.gson.JsonObject

I have solved this issue with below code :

JsonArray jsonArray = new JsonParser().parse(new FileReader(fileName)).getAsJsonArray();
          
            ArrayList<String> pmGrpListFromWebOm = new ArrayList<>();
            for (JsonElement jsonObjectElement : jsonArray) {
                JsonObject jsonObject = jsonObjectElement.getAsJsonObject();
                String grpname= jsonObject.get("Group Name").toString();
                pmGrpListFromWebOm.add(grpname);
            }

Answer 1

The JSON file contains array of objects so you should be able to read the JsonArray immediately using parser.parse method and explicit casting.

Then you should iterate the array and access its JsonObject items using get method.

JsonParser parser = new JsonParser();
JsonArray jsonArray = (JsonArray) parser.parse(new FileReader(JSON_FILE_PATH));

for (int i = 0; i < jsonArray.size(); i++) {
    JsonObject item = (JsonObject) jsonArray.get(i);
    String groupName = item.get("Group Name").getAsString();
    System.out.printf("%d group=%s%n", i, groupName); // item index and groupName
}

The output will be:

0 group=shop
1 group=shop
2 group=shop
3 group=shop
4 group=cart
5 group=payment

---

However, Gson JsonParser is deprecated and it is recommended to use Gson object mapper which allows to read/write objects in more type-safe way.

Each item in the example JSON can be considered as a map of key-value pairs, where both key and value are String.

Gson gson = new Gson();

// prepare type information
Type itemListType = new TypeToken<ArrayList<Map<String, String>>>(){}.getType();

// read the list
List<Map<String, String>> list = gson.fromJson(json, itemListType);

// process items in the list
for (Map<String, String> item : list) {
    String no = item.get("No");
    String groupName = item.get("Group Name");
    System.out.printf("no=%s group=%s%n", no, groupName);
}

output:

no=1 group=shop
no=2 group=shop
no=3 group=shop
no=4 group=shop
no=5 group=cart
no=6 group=payment