Google Gson - deserialize list class object generic type

Question

I want to transfer a list object via Google Gson  but I don t know how to deserialize generic types    What I tried after looking at this  BalusC s answer    MyClass mc   new Gson   fromJson result  new List lt MyClass gt      getClass       but then I get an error in eclipse saying  The type new List     must implement the inherited abstract method     and if I use a quick fix I get a monster of over 20 method stubs    I am pretty sure that there is an easier solution  but I seem unable to find it   Edit   Now I have   Type listType   new TypeToken lt List lt MyClass gt  gt                                        getType     MyClass mc   new Gson   fromJson result  listType     However  I do get the following exception at the  fromJson  line   java lang NullPointerException at org apache harmony luni lang reflect ListOfTypes length ListOfTypes java 47  at org apache harmony luni lang reflect ImplForType toString ImplForType java 83  at java lang StringBuilder append StringBuilder java 203  at com google gson JsonDeserializerExceptionWrapper deserialize JsonDeserializerExceptionWrapper java 56  at com google gson JsonDeserializationVisitor invokeCustomDeserializer JsonDeserializationVisitor java 88  at com google gson JsonDeserializationVisitor visitUsingCustomHandler JsonDeserializationVisitor java 76  at com google gson ObjectNavigator accept ObjectNavigator java 106  at com google gson JsonDeserializationContextDefault fromJsonArray JsonDeserializationContextDefault java 64  at com google gson JsonDeserializationContextDefault deserialize JsonDeserializationContextDefault java 49  at com google gson Gson fromJson Gson java 568  at com google gson Gson fromJson Gson java 515  at com google gson Gson fromJson Gson java 484  at com google gson Gson fromJson Gson java 434    I do catch JsonParseExceptions and  result  is not null   I checked listType with the debugger and got the following    list Type    args   ListOfTypes   list   null resolvedTypes   Type  1    loader   PathClassLoader ownerType0   null ownerTypeRes   null rawType   Class  java util ArrayList  rawTypeName    java util ArrayList     so it seems the  getClass  invocation didn t work properly  Any suggestions      Edit2  I v checked on the Gson User Guide  It mentions a Runtime Exception that should happen during parsing a generic type to Json  I did it  wrong   not shown above   just as in the example  but didn t get that exception at all  So I changed the serialization as in the user guide suggested  Didn t help  though   Edit3   Solved  see my answer below

User · Answer

I have created GsonUtils lib for this case  I add this into maven central repository  Map lt String  SimpleStructure gt  expected   new HashMap lt  gt     expected put  quot foo quot   new SimpleStructure  quot peperoni quot      String json   GsonUtils writeValue expected    Map lt String  SimpleStructure gt  actual   GsonUtils readMap json  String class  SimpleStructure class

User · Answer

Wep  another way to achieve the same result  We use it for its readability   Instead of doing this hard-to-read sentence   Type listType   new TypeToken lt ArrayList lt YourClass gt  gt      getType    List lt YourClass gt  list   new Gson   fromJson jsonArray  listType     Create a empty class that extends a List of your object   public class YourClassList extends ArrayList lt YourClass gt       And use it when parsing the JSON   List lt YourClass gt  list   new Gson   fromJson jsonArray  YourClassList class

User · Answer

Another way is to use an array as a type  e g    MyClass   mcArray   gson fromJson jsonString  MyClass   class     This way you avoid all the hassle with the Type object  and if you really need a list you can always convert the array to a list by   List lt MyClass gt  mcList   Arrays asList mcArray     IMHO this is much more readable   And to make it be an actual list  that can be modified  see limitations of Arrays asList    then just do the following   List lt MyClass gt  mcList   new ArrayList lt  gt  Arrays asList mcArray

User · Answer

public static final  lt T gt  List lt T gt  getList final Class lt T   gt  clazz  final String json        final T   jsonToObject   new Gson   fromJson json  clazz        return Arrays asList jsonToObject       Example   getList MyClass   class

User · Answer

I liked the answer from kays1 but I couldn t implement it  So I built my own version using his concept   public class JsonListHelper      public static final  lt T gt  List lt T gt  getList String json  throws Exception           Gson gson   new GsonBuilder   setDateFormat  yyyy-MM-dd HH mm ss   create            Type typeOfList   new TypeToken lt List lt T gt  gt      getType            return gson fromJson json  typeOfList             Usage   List lt MyClass gt  MyList  JsonListHelper getList jsonArrayString

User · Answer

Here is a solution that works with a dynamically defined type   The trick is creating the proper type of of array using Array newInstance         public static  lt T gt  List lt T gt  fromJsonList String json  Class lt T gt  clazz        Object    array    Object   java lang reflect Array newInstance clazz  0       array   gson fromJson json  array getClass         List lt T gt  list   new ArrayList lt T gt         for  int i 0   i lt array length   i            list add clazz cast array i         return list

User · Answer

Refer to example 2 for  Type  class understanding of Gson   Example 1  In this deserilizeResturant we used Employee   array and get the details  public static void deserializeResturant            String empList       name     Ram     empId   1     name     Surya     empId   2     name     Prasants     empId   3            Gson gson   new Gson           Employee   emp   gson fromJson empList  Employee   class          int numberOfElementInJson   emp length           System out println  Total JSON Elements    numberOfElementInJson          for Employee e  emp              System out println e getName                System out println e getEmpId                     Example 2     Above deserilizeResturant used Employee   array but what if we need to use List lt Employee gt  public static void deserializeResturantUsingList         String empList       name     Ram     empId   1     name     Surya     empId   2     name     Prasants     empId   3         Gson gson   new Gson            Additionally we need to se the Type then only it accepts List lt Employee gt  which we sent here empTypeList     Type empTypeList   new TypeToken lt ArrayList lt Employee gt  gt      getType          List lt Employee gt  emp   gson fromJson empList  empTypeList       int numberOfElementInJson   emp size        System out println  Total JSON Elements    numberOfElementInJson       for Employee e  emp           System out println e getName             System out println e getEmpId

User · Answer

Since Gson 2 8  we can create util function like  public  lt T gt  List lt T gt  getList String jsonArray  Class lt T gt  clazz        Type typeOfT   TypeToken getParameterized List class  clazz  getType        return new Gson   fromJson jsonArray  typeOfT       Example using  String jsonArray       List lt User gt  user   getList jsonArray  User class

User · Answer

Method to deserialize generic collection   import java lang reflect Type  import com google gson reflect TypeToken        Type listType   new TypeToken lt ArrayList lt YourClass gt  gt      getType    List lt YourClass gt  yourClassList   new Gson   fromJson jsonArray  listType     Since several people in the comments have mentioned it  here s an explanation of how the TypeToken class is being used  The construction new TypeToken lt     gt       getType   captures a compile-time type  between the  lt  and  gt   into a runtime java lang reflect Type object  Unlike a Class object  which can only represent a raw  erased  type  the Type object can represent any type in the Java language  including a parameterized instantiation of a generic type   The TypeToken class itself does not have a public constructor  because you re not supposed to construct it directly  Instead  you always construct an anonymous subclass  hence the     which is a necessary part of this expression    Due to type erasure  the TypeToken class is only able to capture types that are fully known at compile time   That is  you can t do new TypeToken lt List lt T gt  gt       getType   for a type parameter T    For more information  see the documentation for the TypeToken class

User · Answer

For Kotlin simply    import java lang reflect Type import com google gson reflect TypeToken     val type   object   TypeToken lt List lt T gt  gt       type   or  here is a useful function   fun  lt T gt  typeOfList    Type       return object   TypeToken lt List lt T gt  gt       type     Then  to use   val type   typeOfList lt YourMagicObject gt

User · Answer

I want to add for one more possibility  If you don t want to use TypeToken and want to convert json objects array to an ArrayList  then you can proceed like this   If your json structure is like       results                    a   100           b    value1            c   true                       a   200           b    value2            c   false                       a   300           b    value3            c   true              and your class structure is like   public class ClassName implements Parcelable        public ArrayList lt InnerClassName gt  results   new ArrayList lt InnerClassName gt         public static class InnerClassName           int a          String b          boolean c                  then you can parse it like   Gson gson   new Gson    final ClassName className   gson fromJson data  ClassName class   int currentTotal   className results size      Now you can access each element of className object

User · Answer

Refer to this post  Java Type Generic as Argument for GSON  I have better solution for this  Here s the wrapper class for list so the wrapper can store the exactly type of list   public class ListOfJson lt T gt  implements ParameterizedType     private Class lt   gt  wrapped     public ListOfJson Class lt T gt  wrapper          this wrapped   wrapper          Override   public Type   getActualTypeArguments             return new Type     wrapped            Override   public Type getRawType           return List class          Override   public Type getOwnerType           return null          And then  the code can be simple   public static  lt T gt  List lt T gt  toList String json  Class lt T gt  typeClass        return sGson fromJson json  new ListOfJson lt T gt  typeClass

User · Answer

As it answers my original question  I have accepted doc 180 s answer  but if someone runs into this problem again  I will answer the 2nd half of my question as well   The NullPointerError I described had nothing to do with the List itself  but with its content    The  MyClass  class didn t have a  no args  constructor  and neither had its superclass one  Once I added a simple  MyClass    constructor to MyClass and its superclass  everything worked fine  including the List serialization and deserialization as suggested by doc 180

User · Answer

In My case  uncaught exceptions s answer didn t work  I had to use List class instead of java lang reflect Type   String jsonDuplicatedItems   request getSession   getAttribute  jsonDuplicatedItems   toString    List lt Map Entry lt Product  Integer gt  gt  entries   gson fromJson jsonDuplicatedItems  List class

[java] Google Gson - deserialize list<class> object? (generic type)

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