将pojo映射到json时为nullpointerexception

Question

{
   "App":[{
        "AppID": 0,
        "AppTime": "08:00 AM"
    }],
   "Rows": 0
}

Here is my JSON Response, but when i map to my POJO class and access i get a NullPointerException .

ListResponse value = new Gson().fromJson(result, ListResponse.class);

import java.util.List;
public class ListResponse{

    public List<AppList> appList;

    public List<AppList> getAppList() {
        return appList;
    }
}

When i try to access my result in my activity class, like this i get an error.

public void accessData(ListResponse result){
    System.out.println(result.getAppList()); // I am getting null here.
}

Update:

My AppList Class contains, AppID, AppTime and Rows.

Answer 1

I think you are storing your response in value

 ListResponse value = new Gson().fromJson(result, ListResponse.class);

Hence try calling value.getAppList() :

 public void accessData(ListResponse result){
    System.out.println(value.getAppList()); // shouldn't get NullPointer
 }

Also its good idea to put a null check, if the value could possibly be null as:

 public void accessData(ListResponse result){
    if(value != null){
        System.out.println(value.getAppList()); // shouldn't get NullPointer
     }else{
        System.out.println("value is null");
     }
 }

Answer 2

When you use GSon to translate a String to an Object (in your case the ListResponse object), the names of the attributes in this class need to be exactly the same value in the keys of the json string.

The problem in your case: The key in json is "App":

"App":[{
    "AppID": 0,
    "AppTime": "08:00 AM"
}],

And the attribute name is appList . Try put appList in the json, or change to public List<AppList> App; in the class. This should solve the null.