Accessing nested Lists in Json with Gson

user_14568260 0
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Hi i am currently trying to access a json file using gson but i am having trouble reading the values when they are nested into a list, i am able to read the outer list and values but cannot read the drive values that are nested in pos

my json file looks like this

{
    "pos":[
       {
         "x": 1.0,
         "y": 1.0,
         "theta": 1.0,
         "drive":[{
            "p": 1.0,
            "i": 0.5
         }]
       },
       {
         "x": 2.0,
         "y": 2.0,
         "theta": 2.0,
         "drive":[{
            "p": 1.0,
            "i": 0.5
         }]
          
       },
       {
         "x": 3.0,
         "y": 3.0,
         "theta": 3.0,
         "drive":[{
            "p": 1.0,
            "i": 0.5
         }]
          
       }
    ]
 }

I can get all the x, y, and theta values inside pos but when I try accessing the nested values in drive i get a null pointer exception

Exception in thread "main" java.lang.NullPointerException
        at frc.robot.files.take2.JsonManager.main(JsonManager.java:63)

This is how I am looping to retrieve the values pls note I am excluding the setter and getters for the values in the lists

    @SerializedName("pos")
    private List<PosData> pos = null;

    public List<PosData> getPositions() {
        return pos;
    }
    public void setPositions(List<PosData> pos) {
        this.pos = pos;
    }

    @SerializedName("drive")
    private List<PidData> drive = null;

    public List<PidData> getDrive() {
        return drive;
    }
    public void setDrive(List<PidData> drive) {
        this.drive = drive;
    }

        JsonManager manager = new Gson().fromJson(json, new TypeToken<JsonManager>() {}.getType());

        List<PosData> posList = manager.getPositions();
        List<Double> xList = new ArrayList<>();
        List<Double> yList = new ArrayList<>();
        List<Double> thetaList = new ArrayList<>();
        
        List<PidData> driveList = manager.getDrive();
        // List<List<Double>> driveList = new ArrayList<>();
        List<Double> drivePID = new ArrayList<>();
        //make parent list list for drivePID that isnt driveList

        System.out.println(drivePID);

        for (PosData pos : posList) {
            xList.add(pos.getX());
            yList.add(pos.getY());
            thetaList.add(pos.getTheta());
            for(PidData pid : driveList){
                drivePID.add(pid.getP());
            }

        }

        System.out.println(xList);
        System.out.println(yList);
        System.out.println(thetaList);
        System.out.println(drivePID);
    }

I have double checked that it works properly when they are not nested by just having my drive list in the json file and trying to read its values

{
   "drive":[{
      "p": 1.0,
      "i": 0.5
   }]
 }
 

for(PidData pid : driveList){
   drivePID.add(pid.getP());
}

Thanks for any help in advance!

gson json java
1 Answers

I don't think parse List<PidData> directly in JsonManager will work. You should include it in your PosData and change driveList to pos.getDrive().

public class JsonManager {
    @SerializedName("pos")
    private List<PosData> pos = null;

    public List<PosData> getPositions() {
        return pos;
    }
    public void setPositions(List<PosData> pos) {
        this.pos = pos;
    }
}


public class PosData {
    private Double x;
    private Double y;
    private Double theta;
    // @SerializedName("drive")
    private List<PidData> drive = null;
    
    // your getter/setter
}

            for(PidData pid : pos.getDrive()){
                drivePID.add(pid.getP());
            }
user_104008700
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