Gson - convert from Json to a typed ArrayList T

Question

Using the Gson library  how do I convert a JSON string to an ArrayList of a custom class JsonLog  Basically  JsonLog is an interface implemented by different kinds of logs made by my Android app--SMS logs  call logs  data logs--and this ArrayList is a collection of all of them  I keep getting an error in line 6   public static void log File destination  JsonLog log        Collection lt JsonLog gt  logs   null      if  destination exists              Gson gson   new Gson            BufferedReader br   new BufferedReader new FileReader destination            logs   gson fromJson br  ArrayList lt JsonLog gt  class      line 6            logs add log              serialize  logs  again           It seems the compiler doesn t understand I m referring to a typed ArrayList  What do I do

User · Answer

If you want to use Arrays  it s pretty simple   logs   gson fromJson br  JsonLog   class      line 6   Provide the JsonLog as an array JsonLog   class

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You have a string like this       id  2550  cityName   Langkawi   hotelName   favehotel Cenang Beach - Langkawi   hotelId   H1266070      id  2551  cityName   Kuala Lumpur   hotelName   Metro Hotel Bukit Bintang   hotelId   H835758       Then you can covert it to ArrayList via Gson like  var hotels   Gson   fromJson historyItem hotels  Array lt HotelInfo gt   class java  toList     Your HotelInfo class should like this   import com squareup moshi Json  data class HotelInfo        Json name  cityName       val cityName  String    null        Json name  id       val id  Int    null        Json name  hotelId       val hotelId  String    null        Json name  hotelName       val hotelName  String    null

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Kotlin   data class Player val name   String  val surname  String   val json              name    name 1        surname    surname 1                name    name 2        surname    surname 2                name    name 3        surname    surname 3         val typeToken   object   TypeToken lt List lt Player gt  gt       type val playerArray   Gson   fromJson lt List lt Player gt  gt  json  typeToken    OR  val playerArray   Gson   fromJson json  Array lt Player gt   class java

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You may use TypeToken to load the json string into a custom object   logs   gson fromJson br  new TypeToken lt List lt JsonLog gt  gt      getType       Documentation      Represents a generic type T        Java doesn t yet provide a way to represent generic types  so this class does  Forces clients to create a subclass of this class which enables retrieval the type information even at runtime       For example  to create a type literal for List lt String gt   you can create an empty anonymous inner class       TypeToken lt List lt String gt  gt  list   new TypeToken lt List lt String gt  gt             This syntax cannot be used to create type literals that have wildcard parameters  such as Class lt   gt  or List lt   extends CharSequence gt     Kotlin    If you need to do it in Kotlin you can do it like this   val myType   object   TypeToken lt List lt JsonLong gt  gt       type val logs   gson fromJson lt List lt JsonLong gt  gt  br  myType    Or you can see this answer for various alternatives

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Why nobody wrote this simple way of converting JSON string in List    List lt Object gt  list   Arrays asList new GsonBuilder   create   fromJson jsonString  Object   class

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I am not sure about gson but this is how you do it with Jon sample hope there must be similar way using gson             Players                      player 1                    player 2                    player 3                    player 4                    player 5                                                                    import java io FileReader  import java util List   import org json simple JSONObject  import org json simple parser JSONParser   public class JosnFileDemo        public static void main String   args  throws Exception               String jsonfile   fileloaction fileName json                     FileReader reader   null                  JSONObject jsb   null                  try                       reader   new FileReader jsonfile                       JSONParser jsonParser   new JSONParser                         jsb    JSONObject  jsonParser parse reader                     catch  Exception e                        throw new Exception e                     finally                       if  reader    null                          reader close                                      List lt String gt  Players  List lt String gt   jsb get  Players                    for  String player   Players                        System out println player

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Your JSON sample is          status    ok        comment            result          id   276       firstName    mohamed        lastName    hussien        players                  player 1                player 2                player 3                player 4                player 5            so if you want to save arraylist of modules in your SharedPrefrences so    1- will convert your returned arraylist for json format using this method  public static String toJson Object jsonObject        return new Gson   toJson jsonObject       2- Save it in shared prefreneces  PreferencesUtils getInstance context  setString  players   toJson       ArrayList you want to convert            3- to retrieve it at any time get JsonString from Shared preferences like that  String playersString  PreferencesUtils getInstance this  getString  players      4- convert it again to array list   public static Object fromJson String jsonString  Type type        return new Gson   fromJson jsonString  type      ArrayList lt String gt  playersList   ArrayList lt String gt   fromJson playersString                      new TypeToken lt ArrayList lt String gt  gt                            getType       this solution also doable if you want to parse ArrayList of Objects Hope it s help you by using Gson Library

User · Answer

If you want convert from Json to a typed ArrayList   it s wrong to specify the type of the object contained in the list  The correct syntax is as follows   Gson gson   new Gson      List lt MyClass gt  myList   gson fromJson inputString  ArrayList class

[java] Gson - convert from Json to a typed ArrayList<T>

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