Gson to append json object within an array in a json file in java

Question

I am trying to store a game result in a JSON file. The results are stored when the game is over, and are supposed to be displayed afterwards in a table.

I have this function that creates the game score and adds it to json file using GSON

 private void createGameScoreGson () throws Exception {
        var gson = new GsonBuilder().setPrettyPrinting().create();
        var score = new ScoreResult();
        score.setPlayerName(name);
        score.setSteps(steps);
        score.setTime(time);
        score.setSolved(solved);

        // writing to a json file
        File file = new File("src\\main\\resources\\results.json");
        try (var writer = new FileWriter("resources\\results.json", true)) {
            gson.toJson(score, writer);
        }
}

That method creates a JSON file that looks like this:

{
    "playerName": "a",
    "steps": 1,
    "time": "00:00:11",
    "solved": false
}

The problem is when I try to add another game result to the file, it appears like this:

{
    "playerName": "a",
    "steps": 1,
    "time": "00:00:11",
    "solved": false
}
{
    "playerName": "b",
    "steps": 2,
    "time": "00:00:20",
    "solved": false
}

Which is not a valid JSON file and so it is not read properly when I try to show the results later. How can I use Gson (or anything else really) to show the results in the JSON file like this:

[
 {
    "playerName": "a",
    "steps": 1,
    "time": "00:00:11",
    "solved": false
 },
 {
    "playerName": "b",
    "steps": 2,
    "time": "00:00:20",
    "solved": false
 }
]

Any suggestion will be helpful!

Answer 1

this should work for you, but if there are a lot of entries in the file, then I think it will be necessary to change the approach.

add maven dependecy:

        <dependency>
            <groupId>com.googlecode.json-simple</groupId>
            <artifactId>json-simple</artifactId>
            <version>1.1.1</version>
        </dependency>

public void createGameScoreGson () throws Exception {
        ScoreResult score = new ScoreResult();
        Gson gson = new GsonBuilder().setPrettyPrinting().create();
        JSONArray jsonArray = parseFromFile();
        
        // You can create file if you need
        try (FileWriter writer = new FileWriter("src/main/resources/results.json", false)) {
            String gs = gson.toJson(score);
            Map map = gson.fromJson(gson.toJson(score), Map.class);
            jsonArray.add(map);
            String jsonPretty = gson.toJson(jsonArray);
            writer.write(jsonPretty);
        }
    }

public JSONArray parseFromFile() throws IOException {
        JSONParser parser = new JSONParser();
        try {
            return (JSONArray) parser.parse(new FileReader("src/main/resources/results.json"));
        }
        catch (ParseException e){
            return new JSONArray();
        }
        catch (FileNotFoundException e){
            return null; // any code
        }
    }