Parse JSON nested object to Java POJO with GSON

Question

I would like to parse JSON string with nested array to POJO with Gson but it seems that it does not work.

My JSON string is as follow:

{
   "magdeburg":{
      "average":"21.40",
      "maximum":"22.33",
      "minimum":"19.06"
   },
   "frankfurt":{
      "average":"16.41",
      "maximum":"16.57",
      "minimum":"16.09"
   }
}

with my POJO class as follow:

public class PingResult {

    public double average;
    public double maximum;
    public double minimum;

    public PingResult(double average, double maximum, double minimum) {
        this.average= average;
        this.maximum =  maximum;
        this.minimum= minimum;
    }

    public String toString() {
        return "average:" + this.average + ",maximum:"+this.maximum+",minimum:"+this.minimum;
    }
}

but when I want to parse it to POJO with the code like this:

List<PingResult> pr =  (List<PingResult>) gson.fromJson(q, PingResult.class);
        for(PingResult p:pr) {
            System.out.println(p.toString());
        }

it gave error of

java.lang.ClassCastException: PingResult cannot be cast to java.util.List at DelayBasedMeasurement.main(DelayBasedMeasurement.java:380).

I have tried to parse to PingResult[] but still it does not work.

Any advice?

Shadov · Accepted Answer · 2018-09-07 14:20:39Z

1

Not sure how to do it with a List, but this is how to do it with Map:

Map<String, Ping> map = gson.fromJson(json, new TypeToken<Map<String, Ping>>(){}.getType());

Then you can just map.values() if you want all the values.

answered Sep 7, 2018 at 14:20

Shadov

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OneCricketeer · Accepted Answer · 2018-09-07 14:20:01Z

1

First, gson.fromJson(q, PingResult.class) won't return a List, only a single object. You need to use a TypeToken.

Secondly, you don't have a list of JSON objects, you have a map. There are no [] characters, and each ping result is the value of the city string key.

Try TreeMap<String, PingResult> as your deserialization type

Type pingType =  new TypeToken<TreeMap<String, PingResult>>(){}.getType();

TreeMap<String, PingResult> pingMap = gson.fromJson(json, pingType);

edited Sep 7, 2018 at 14:20

answered Sep 7, 2018 at 14:14

OneCricketeer

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