How to convert map and nested map to string and vise versa in Kotlin?

Question

I have no idea how to convert HashMap and nested Hashmap to string and vise versa:

I use Room to save data in a local database. The "users " hashmap is a parameter of the data class. To save this parameter "users " in the Room database, we must convert it to a primitive such as string, boolen, Integer, etc. Room does not support object references, so type converters are required.

val users = HashMap<String, HashMap<String, Any>>()

    val user1 = HashMap<String, Any>()
    user1["filter"] = 0
    user1["group"] = "groupId"
    user1["labelsVisibility"] = true
    user1["sort"] = 1
    user1["view"] = 3
    users["user1"] = user1

    val user2 = HashMap<String, Any>()
    user2["filter"] = 0
    user2["group"] = "groupId"
    user2["labelsVisibility"] = true
    user2["sort"] = 1
    user2["view"] = 3
    users["user2"] = user2

Please clarify what do you mean by "convert HashMap to String" and/or provide expected output. — Nikolai Shevchenko, Feb 16 '22 at 05:50

score 1 · Accepted Answer · answered Feb 17 '22 at 10:01

As I said earlier in the comments, it could be easily done with Gson library:

// Configure Gson
val gson = GsonBuilder()
    // Using this strategy Gson first tries converting number to Long. 
    // If it fails (number is float), then it tries converting to Double
    .setObjectToNumberStrategy(ToNumberPolicy.LONG_OR_DOUBLE) 
    .create()

// Convert to JSON string
val jsonString = gson.toJson(users)
println("JSON => $jsonString")

// Store at Room as a string...
// Fetch later from Room as a string...

val type = object : TypeToken<HashMap<String, HashMap<String, Any>>>() {}.type
val fromJsonMap: HashMap<String, HashMap<String, Any>> = gson.fromJson(jsonString, type)

println("ORIGINAL MAP =>\n$users")
println("RESTORED MAP =>\n$fromJsonMap")

Output:

JSON => {
  "user1": {
    "filter": 0,
    "view": 3,
    "labelsVisibility": true,
    "sort": 1,
    "group": "groupId"
  },
  "user2": {
    "filter": 0,
    "view": 3,
    "labelsVisibility": true,
    "sort": 1,
    "group": "groupId"
  }
}

ORIGINAL MAP =>
{user1={filter=0, view=3, labelsVisibility=true, sort=1, group=groupId}, user2={filter=0, view=3, labelsVisibility=true, sort=1, group=groupId}}
RESTORED MAP =>
{user1={filter=0, view=3, labelsVisibility=true, sort=1, group=groupId}, user2={filter=0, view=3, labelsVisibility=true, sort=1, group=groupId}}

As you can see both original Map users and restored Map fromJsonMap are identical.

Yeah, this is really good solution, thank you. – Zheentoro Feb 18 '22 at 12:24 — Zheentoro, Feb 18 '22 at 12:24
But how to do it with `kotlinx.serialization`? – Danon Jun 17 '23 at 16:37 — Danon, Jun 17 '23 at 16:37

Zheentoro · Answer 2 · 2022-02-16T11:56:12.713

Solution

There may be a better way to do this, but it works

converting a HashMap<String, HashMap<String, Any>> to a string using special symbols * + (a symbols must be unique in string)

 fun fromUsersPreferences(users: HashMap<String, HashMap<String, Any>>): String {
   var usersString = ""
   users.forEach {
     usersString = usersString.plus("${it.key}*${it.value}+")
   }
   usersString = usersString.dropLast(1)
   return usersString
 }

output:

"user1*{filter=0, view=3, labelsVisibility=true, sort=1, group=groupId}+user2*{filter=1, view=1, labelsVisibility=false, sort=0, group=groupId}"

converting a string to a HashMap<String, HashMap<String, Any>>

fun toUsersPreferences(users: String): HashMap<String, HashMap<String, Any>> {
 val usersMap = users.split("+").associateTo(HashMap()){
     val list = it.split("*")
     val userKey = list[0]
     val userValue = list[1].removeSurrounding("{", "}").split(", ").associateTo(HashMap()){
         val (key, value) = it.split("=")
         if (key.equals("group")) {
             Pair<String, Any>(key, value)
         } else if (key.equals("labelsVisibility")) {
             Pair<String, Any>(key, value.toBoolean())
         } else {
             Pair<String, Any>(key, value.toInt())
         }
     }

     Pair(userKey, userValue)
 }

 return usersMap
}

output:

{user1={filter=0, view=3, labelsVisibility=true, sort=1, group=groupId}, user2={filter=1, view=1, labelsVisibility=false, sort=0, group=groupId}}

Well, that looks bit overkill. Why don't you just use `Gson` library to convert Map to JSON (string) and back? — Nikolai Shevchenko, Feb 16 '22 at 12:39
@NikolaiShevchenko I think you're right, but I don't know how to do it with Gson, post your solution if you can, thanks. — Zheentoro, Feb 16 '22 at 16:26

How to convert map and nested map to string and vise versa in Kotlin?

2 Answers2