在Java中使用GSON序列化JSON文件

Question

I want to convert this JSON into objects in java:

{
    "mapping": [
        {
            "boardPosition": {
                "row": 1,
                "col": 1
            },
            "nodeId": 3242324
        },
        {
            "boardPosition": {
                "row": 1,
                "col": 2
            },
            "nodeId": 432423
        },
        {
            "boardPosition": {
                "row": 1,
                "col": 3
            },
            "nodeId": 424324132
        }
    ]
}

this is how I created my java classes

class MapeoWumpus {
    public mapp mapping;

    }

class mapp{
    public boardP boardPosition;
    public String nodeId;
}

class boardP{
    public int row;
    public int col;

}

and then when I try to convert my file like this

MapeoWumpus mapa=new MapeoWumpus();
mapa=gson.fromJson(filetext, MapeoWumpus.class);

I get a null object

What can I do?

EDIT: This is my entire code:

package parserjson;

import java.io.FileNotFoundException;
import java.util.*;
import com.google.gson.*;

public class Main {

    /**
     * @param args
     */
    public static void main(String[] args) throws FileNotFoundException {
            String filetext;
            ParserJson parser=new ParserJson();
            Gson gson=new Gson();
            MapeoWumpus mapa=new MapeoWumpus();
            filetext=parser.leerArchivo("b1.json");
            mapa=gson.fromJson(filetext, MapeoWumpus.class);
    }

}

"leerArchivo" is just a method to get the json file, as you can see my json file is in a string variable

Answer 1

You should define instance variable mapp as array. Because your JSON data seems to contain mapping array.

class MapeoWumpus {
    public mapp[] mapping;

}

Creating new MapeoWumpus in the below code is unnecessary

MapeoWumpus mapa=new MapeoWumpus();
mapa=gson.fromJson(filetext, MapeoWumpus.class);

Just change it as follows

MapeoWumpus mapa=gson.fromJson(filetext, MapeoWumpus.class);