Converting Json to java objects using Google's Gson

Question

I am using Spring Social FqlQuery to get data's from facebook. Here is the JSON response I am getting from facebook. My controller where i am getting Json output is here,

fql = "SELECT work FROM user WHERE uid = me()";
facebook.fqlOperations().query(fql, new FqlResultMapper<Object>() {
    public Object mapObject(FqlResult result) {
        List list = (List) result.getObject("work");
        for (Object object : list) {
           JsonHelper jsonHelper = new JsonHelper();
           Gson gson = new GsonBuilder().setPrettyPrinting().create();
           String jsonOutput = gson.toJson(object);
           System.out.println(jsonOutput);
           gson.fromJson(jsonOutput, JsonHelper.class);
        }

System.out.println inside for loop Outputs multiple json as below.:

{
  "employer": {
    "id": 129843057436,
    "name": "www.metroplots.com"
  },
  "location": {
    "id": 102186159822587,
    "name": "Chennai, Tamil Nadu"
  },
  "position": {
    "id": 108480125843293,
    "name": "Web Developer"
  },
  "start_date": "2012-10-01",
  "end_date": "2013-05-31"
}
{
  "employer": {
    "id": 520808381292985,
    "name": "Federation of Indian Blood Donor Organizations"
  },
  "start_date": "0000-00",
  "end_date": "0000-00"
}

Here is my Helper Class:

import java.util.List;

public class JsonHelper {

class Employer{
    private int id;
    private String name;
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }

}
class Location{
    private int id;
    private String name;
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }   

}
class Position{
    private int id;
    private String name;
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
}
//Edited After here
private String start_Date;
private String end_Date;
private Employer employer;
private Location location;
private Position position;
public String getStart_Date() {
    return start_Date;
}
public void setStart_Date(String start_Date) {
    this.start_Date = start_Date;
}
public String getEnd_Date() {
    return end_Date;
}
public void setEnd_Date(String end_Date) {
    this.end_Date = end_Date;
}
public Employer getEmployer() {
    return employer;
}
public void setEmployer(Employer employer) {
    this.employer = employer;
}
public Location getLocation() {
    return location;
}
public void setLocation(Location location) {
    this.location = location;
}
public Position getPosition() {
    return position;
}
public void setPosition(Position position) {
    this.position = position;
}
}

When I try to convert the json objects to java object as done above I am getting this exception.

HTTP Status 500 - Request processing failed; nested exception is com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected BEGIN_ARRAY but was BEGIN_OBJECT at line 6 column 16

Can any one help me where I am wrong. Please help me converting json to java objects. Hope my question is clear. Thanks in advance.

EDIT MADE TO CONTROLLER:

facebook.fqlOperations().query(fql, new FqlResultMapper<Object>() {
public Object mapObject(FqlResult result) {
List<JsonHelper> json = new ArrayList<JsonHelper>();
List list = (List) result.getObject("work");

for (Object object : list) {
    Gson gson = new GsonBuilder().setPrettyPrinting().create();
    String jsonOutput = gson.toJson(object);
    System.out.println(jsonOutput);
    JsonHelper jsonHelper = gson.fromJson(jsonOutput, JsonHelper.class);
    json.add(jsonHelper);
    System.out.println(jsonHelper.getStart_Date());
}

for (JsonHelper jsonHelper : json) {
    System.out.println(jsonHelper.getStart_Date());
}

return list;
}

});

Sulphuric

The json you have mentioned in the example above is actually a jsonOject and not jsonArray, And you are trying to cast it into list, thats only possible for jsonArray, not jsonObject – Cultism

Since in for loop you are getting jsonObject, so in your JsonHelper class, remove List or create json array in your for loop and parse the whole array together. – Cultism

@Jhanvi: I edited my POJO class as you said. I am not getting exceptions now. So its binding the values I think. after adding i am putting the JsonHelper object inside arrayList and then when i try to access the values from inside objects i am getting "null" as output. So can you figure out where its wrong? – Sulphuric

Answer 1

Since i am not having the actual api access, so i am trying it with static value in the example. Firstly in your JsonHelper class, replace all int by long , as the values mentioned in the json are of type long and String. Then try it like mentioned below:

            String str = "{\n"
            + "  \"employer\": {\n"
            + "    \"id\": 129843057436,\n"
            + "    \"name\": \"www.metroplots.com\"\n"
            + "  },\n"
            + "  \"location\": {\n"
            + "    \"id\": 102186159822587,\n"
            + "    \"name\": \"Chennai, Tamil Nadu\"\n"
            + "  },\n"
            + "  \"position\": {\n"
            + "    \"id\": 108480125843293,\n"
            + "    \"name\": \"Web Developer\"\n"
            + "  },\n"
            + "  \"start_date\": \"2012-10-01\",\n"
            + "  \"end_date\": \"2013-05-31\"\n"
            + "}";


    List<JsonHelper> json = new ArrayList<JsonHelper>();
    Gson gson = new Gson();
    JsonHelper users = gson.fromJson(str, JsonHelper.class);
    json.add(users);


    for (JsonHelper js_obj : json) {
        System.out.println(js_obj.getEmployer().getId());
        System.out.println(js_obj.getEmployer().getName());
    }

Cultism

Thankyou for your help. I got answer. Json variable and my pojo variable should have same name. If it has same name only its getting values. Else null is printing... – Sulphuric

@VigneshGopalakrishnan Yes, in Gson, the names of variables must be same and even the dataType should be correct otherwise it creates problem. – Cultism