How to get array of objects from JSON response using GSON

Question

I am writing a REST client using RestTemplate and GSON. Below is sample of my JSON response

{
  "value": [
    {
      "properties": {
        "vmId": "f7f953fb-d853-4373-b564-",
        "hardwareProfile": {
          "vmSize": "Standard_D2"
        },

        },
        "name": "A",
        "Id": ""
    },

    {
      "properties": {
        "vmId": "f7f953fb-d853-4373-b564-",
        "hardwareProfile": {
          "vmSize": "Standard_D2"
        },

        },
        "name": "B",
        "Id": ""
    },

    {
      "properties": {
        "vmId": "f7f953fb-d853-4373-b564-",
        "hardwareProfile": {
          "vmSize": "Standard_D2"
        },

        },
        "name": "C",
        "Id": ""
    }
    ]
}

What I want to is that I want to get only the values for the property --> "name"

So I created a simple POJO that has only name as the member field.

public class VMNames {
    public String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

}

and I am trying to use the GSON like this to get a array of this POJO. Here, the response is my JSON response object.

Gson gson = new Gson();
VMNames[] vmNamesArray = gson.fromJson(response.getBody(), VMNames[].class);
System.out.println(vmNamesArray.length);

But when I do this, I get an error i.e. as below:

java.lang.IllegalStateException: Expected BEGIN_ARRAY but was BEGIN_OBJECT at line 1 column 2 path $

Please note that I don't want to create a POJO that has exactly same structure as my JSON object because I want to get only one attribute out of my JSON object. I am hoping that I won't have to really create a POJO with the same structure as my JSON response because, in reality, it's a huge response and I don't control it, so it can also change tomorrow.

Your JSON is an object and not an array. Well there is a property value that is of type array but also containing objects. — Sir Rufo
– Sir Rufo, Commented Sep 2, 2016 at 5:41
Sumit - this is sample JSON, not proper response. But what I am trying to do is a right approach ? — sunny arya
– sunny arya, Commented Sep 2, 2016 at 16:48

user3145373 ツ · Accepted Answer · 2016-09-02 05:00:38Z

2

Can you try this:

public class VMNames {
    @SerializedName("name")
    public String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

Type collectionType = new TypeToken<Collection<VMNames>>(){}.getType();
Collection<VMNames> vmNamesArray = gson.fromJson(response.getBody(), collectionType);
System.out.println(vmNamesArray.length);

or try:

VMNames[] vmNamesArray = gson.fromJson(response.getBody(), VMNames[].class);

edited Sep 2, 2016 at 5:00

answered Sep 2, 2016 at 4:54

user3145373 ツ

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2 Comments

sunny arya Over a year ago

Tried your first code snippet, Same error still...And I am already using the second snippet where I am getting above error.

user3145373 ツ Over a year ago

hello @sunnyarya. please check stackoverflow.com/questions/20835536/… you have same problem. You need to fetch only specified sub elements so you have to read from array and have to create sub class if required.

user3145373 ツ · Accepted Answer · 2016-09-09 10:12:30Z

So I could get this done. Posting the answer to this so that someone can be benefited tomorrow :)

First thing, i stopped using GSON and started using JSON. And below is the code that helped.

    RestTemplate restTemplate = new RestTemplate();
    response = (ResponseEntity<String>) restTemplate.exchange(url, HttpMethod.GET, entity, String.class);

    JSONObject jsonObj = new JSONObject(response.getBody().toString());

    JSONArray c = jsonObj.getJSONArray("value");

    for (int i = 0; i < c.length(); i++) {
        JSONObject obj = c.getJSONObject(i);
        String VMName = obj.getString("name");
        VMNames vmnames = new VMNames();
        vmnames.setName(VMName);
        vmNames.add(vmnames);
    }

    return vmNames;

And i get a list of all the value against the attribute name in form of a json array.

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