GSON Adding java object to json array

Question

I have json {"name": "John", "tests":["apple"]} . In my Java code I want to update this json throught gson - add string field to json array and save to file to have this json - {"name": "John", "tests":["apple", "pinapple"]} . So is there a way how ? Java class for my json look like this:

public class Test{
    private String name;
    private List<String> tests;

    // Getters/Setters
}

Reading json in java like this:

Gson gson = new Gson();

try (Reader reader = new FileReader("D:\\file.json")) {
    Test js = gson.fromJson(reader, Test.class);
    System.out.println(js.getName());   
} catch (IOException e) {
     e.printStackTrace();
}

Doing this my new json file is empty:

Gson gson = new Gson();
try (Reader reader = new FileReader("D:\\file.json")) {
        Test test = gson.fromJson(reader, Test.class);
        System.out.println(test.getName());
        test.getTests().add("pinapple");
        String newJson = gson.toJson(test);
        System.out.println(newJson);
        gson.toJson(test, new FileWriter("D:\\file.json"));
    } catch (IOException e) {
        e.printStackTrace();
    }

Answer 1

Test test = gson.fromJson(reader, Test.class);

Once you have your Test object do:

test.getTests().add("pineapple");

Now you have the pineapple in your array. You can make your JSON string:

String newJson = gson.toJson(test);
System.out.println(newJson);

or write it back in the same file

gson.toJson(test, new FileWriter("D:\\file.json"));

EDIT : Whole code

String fileName = "D:\\file.json";
Gson gson = new Gson();
Test test = null;
try (Reader reader = new FileReader(fileName)) {
    test = gson.fromJson(reader, Test.class);
} catch (IOException e) {
    e.printStackTrace();
}

test.getTests().add("pineapple");
String newJson = gson.toJson(test);
System.out.println(newJson);

try (Writer writer = new FileWriter(fileName)) {
    gson.toJson(test, writer);
}