How to concatenate / append a JsonObject to a JsonArray - json

Question

Given the following code which gets called variously in the code, and then gets written to a file (separate method), I simple need to append the next value into the array. This is probably a simple Collections question but wasn't sure if there was a specialized Gson way of doing this:
public static void metricAsJSON(String testName, long testTime) {
Date date = new Date();
Timestamp ts = new Timestamp(date.getTime());
JSONObject obj = new JSONObject();
obj.put("testname", testName);
obj.put("Duration", testTime);
obj.put("Timestamp", ts.toString());
Gson gson = new GsonBuilder().setPrettyPrinting().create();
JsonParser jp = new JsonParser();
JsonElement je = jp.parse(obj.toJSONString());
jsoncontent = gson.toJson(je);
JsonArray jsonArray = new JsonArray();
jsonArray.add(je);
jsoncontent = gson.toJson(jsonArray);
}
public static JsonArray appendMetricAsJson(String jsoncontent){
JsonArray jsonContentList = new JsonArray();
jsonContentList.add(jsoncontent);
return jsonContentList;
FileWriter fw1 = new FileWriter(f1, true);
PrintWriter pw1 = new PrintWriter(fw1);
if (f1.exists() && f1.isFile()) {
pw1.println(jsonContentList);
pw1.flush();
pw1.close();
fw1.close();
}
}
Current output:
[
{
"Duration": 30,
"testname": "Shopping link click to dropdown display time:",
"Timestamp": "2019-10-15 09:47:53.804"
}
]
Desired output:
[
{
"Duration": 30,
"testname": "Shopping link click to dropdown display time:",
"Timestamp": "2019-10-15 09:45:13.334"
},
{
"Duration": 16,
"testname": "Clothing link click to dropdown display time",
"Timestamp": "2019-10-15 09:44:34.356"
},
{
"Duration": 24,
"testname": "Toys link click to dropdown display time",
"Timestamp": "2019-10-15 09:46:33.453"
},
{
"Duration": 34,
"testname": "Electrics link click to dropdown display time",
"Timestamp": "2019-10-15 09:47:53.566"
}
]