目的

{
  "Last Access" : "2020/3/35",
  "value" : [ {
    "Time" : "2020/3/24",
    "sentence" : "kosuei",
    "bunsyou" : "こうせい"
  }, {
    "Time" : "2020/3/25",
    "sentence" : "The exsecutive meeting took placed last night.",
    "bunsyou" : "昨夜、役員会議が開かれました"
  } ]
}

こんな感じのJsonファイルを作りたいで。

現状

java
1class UserData{
2    public String date;
3    public String sentence;
4    public String bunsyou;
5    
6    UserData(String date,String sentence,String bunsyou){
7        this.date = date;
8        this.sentence = sentence;
9        this.bunsyou = bunsyou;
10    }
11}
12
13
14public class JsonTry2{
15    public static String date;
16    JsonTry2(){
17        Calendar cal = Calendar.getInstance();
18        int y = cal.get(Calendar.YEAR);
19        int m = cal.get(Calendar.MONTH);
20        int d = cal.get(Calendar.DATE);
21        date = y+"/"+m+"/"+d;
22    }
23
24    public static void main(String[] args){
25        File per = new File("Sample.json");
26        File memo = new File("UserData.json");
27        UserData data1 = new UserData(JsonTry2.date,"I am a hero","私はヒーローです");
28        UserData data2 = new UserData(JsonTry2.date,"The exsecutive meeting took placed last night.","昨夜、役員会議が開かれました");
29        List<UserData> uDatas = Arrays.asList(data1,data2);
30
31        ObjectMapper mapper = new ObjectMapper()
32            .enable(SerializationFeature.INDENT_OUTPUT);
33        ObjectNode root = mapper.createObjectNode();
34        ArrayNode array = mapper.createArrayNode();
35
36        try{
37            JsonNode tree = mapper.readTree(per);
38            ObjectNode item = mapper.createObjectNode();
39            ObjectNode format = mapper.createObjectNode();
40            format.put("Last Access",JsonTry2.date);
41            for (UserData ud :uDatas){
42                item.put("Time",ud.date);
43                item.put("sentence",ud.sentence);
44                item.put("bunsyou",ud.bunsyou);
45            }
46            array.add(tree);
47            array.add(item);
48            root.set("value",array);
49            mapper.writeValue(memo,root);
50        }catch(IOException e){
51            e.printStackTrace();
52        }
53    }
54}

こんな感じにプログラムを作って

{
  "value" : [ {
    "Time" : "2020/3/24",
    "sentence" : "kosuei",
    "bunsyou" : "こうせい"
  }, {
    "Time" : "2020/3/25",
    "sentence" : "The exsecutive meeting took placed last night.",
    "bunsyou" : "昨夜、役員会議が開かれました"
  } ]
}

ここまでできました。
#質問
最後に"Last Access" : "2020/3/35",を追加するために、root.set("Last Access","2020/3/25")ってやればいいのかなと思ったのですが、2020/3/35の部分はJsonNode型にしなくてはいけません。いろいろ探したのですがStringをJsonNodeに変換するものはなく、どうしてもできません。

どうやったら、Last Access" : "2020/3/35これを追加できますか？