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Gson - Nested Object inside a CustomObject - java.lang.ClassCastException: com.google.gson.internal.LinkedTreeMap cannot be cast to java.util.List

I googled and saw many question and answers but none of it are helping me. Here is the issue. I have a Class
public class ResponseData {
public static transient final int SUCCESS = 1;
public static transient final int FAILED = 0;
public String id;
public int status;
public Object data;
// Constructor, Getters and Setters
}
I'm using ResponseData as the common return object for my server and the server has many APIs. In one of the API, it is setting the data parameter to ArrayList. Then converting as json using Gson (2.8.0).
And then sending back to caller. (It's not HTTP)
public class MyServerClass {
private final Gson gson;
public MyServerClass() {
GsonBuilder builder = new GsonBuilder();
gson = builder.serializeNulls().create();
}
public String someAPI() {
ResponseData responseData = new Response("myid", ResponseData.SUCCESS, Arrays.asList("Some string value", ArrayList<MyCustomObject>, "Some other Value"));
String json = gson.toJson(response)
}
}
And the MyCustomClass is a plain POJO class with some set of attributes.
public class MyCustomClass {
private String name;
private String id;
private String createdTime;
//Constructor, Getters & Setters
}
At the receiving side I have below code.
private Gson gson = null;
GsonBuilder builder = new GsonBuilder();
gson = builder.create();
///
ResponseData response = gson.fromJson(eventData, ResponseData.class);
ArrayList list = (ArrayList) respone.getData();
String val = (String) list.get(0);
List rwData = (List) list.get(1);
for(List<List<String>> entry: rwData) { // Exception is thrown Here. How to get it as List?
//Coverting Data
}
Exception is thrown when trying to get the data as List<List<String>>. How to convert the json string properly here? I cannot use the MyCustomClass at my client layer. That's why trying list
Exception occured:com.google.gson.internal.LinkedTreeMap cannot be cast to java.util.List
java.lang.ClassCastException: com.google.gson.internal.LinkedTreeMap cannot be cast to java.util.List

How does Polymorphism work with Gson (Retrofit)

Here is my Retrofit instance:
#Provides
#Singleton
ApiManager provideApiManager() {
RxJava2CallAdapterFactory rxAdapter = RxJava2CallAdapterFactory.create();
OkHttpClient okHttpClient = new OkHttpClient.Builder()
.addNetworkInterceptor(new StethoInterceptor())
.build();
Gson gson = new GsonBuilder().create();
GsonConverterFactory converterFactory = GsonConverterFactory.create(gson);
Retrofit retrofit = new Retrofit.Builder()
.baseUrl(AppConstants.BASE_URL)
.addConverterFactory(converterFactory)
.addCallAdapterFactory(rxAdapter)
.client(okHttpClient)
.build();
return retrofit.create(ApiManager.class);
}
Model:
class AbstractMessage {
String id;
}
class TextMessage extends AbstractMessage {
String textMessage;
}
class ImageMessage extends AbstractMessage {
String url;
String text;
}
Request:
#GET("direct/messages")
Observable<List<AbstractMessage>> getMessages(#Header("Authorization") String authHeader, #Body RequestObject request);
Executing request:
apiManager.getMessages(authHeader, requestObject)
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(new Consumer<List<AbstractMessage>>() {
#Override
public void accept(List<AbstractMessage> messages) throws Exception {
...
}
});
When I execute a request I receive a collection of AbstractMessage objects. The JSON can contain both text and image messages. In my case JSON converter creates AbstractMessage and maps only the id field. How can I make converter to create TextMessage and ImageMessage objects map all matching fields and then cast it to AbstractMessage. Or there may be some other solution.

You must create a RuntimeTypeAdapterFactory for the objects AbstractMessage, TextMessage and ImageMessage and then you must set it into the gson instance.
Suppose that you have those objects:
public class Animal {
protected String name;
protected String type;
public Animal(String name, String type) {
this.name = name;
this.type = type;
}
}
public class Dog extends Animal {
private boolean playsCatch;
public Dog(String name, boolean playsCatch) {
super(name, "dog");
this.playsCatch = playsCatch;
}
}
public class Cat extends Animal {
private boolean chasesLaser;
public Cat(String name, boolean chasesLaser) {
super(name, "cat");
this.chasesLaser = chasesLaser;
}
}
This below is the RuntimeTypeAdapter that you need in order to deserialize (and serialize) correctly those objects:
RuntimeTypeAdapterFactory<Animal> runtimeTypeAdapterFactory = RuntimeTypeAdapterFactory
.of(Animal.class, "type")
.registerSubtype(Dog.class, "dog")
.registerSubtype(Cat.class, "cat");
Gson gson = new GsonBuilder()
.registerTypeAdapterFactory(runtimeTypeAdapterFactory)
.create();
The class RuntimeTypeAdapterFactory.java is not shipped with the Gson package, so you have to download it manually.
You can read more about the runtime adapter here and here
Please note that the title of your question should be "Polymorphism with Gson"
I hope it helps.

Gson parse class with generic item

I have base class WebAnswer,
public class WebAnswer<T> {
private int id;
private T result;
private ErrorModel error;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public T getResult() {
return result;
}
public void setResult(T result) {
this.result = result;
}
public ErrorModel getError() {
return error;
}
public void setError(ErrorModel error) {
this.error = error;
}
}
and inherited class RequestAnsModel
public class RequestAnsModel extends WebAnswer<RequestModel> {
}
I need to deserialize JSON object with Gson, in code I do it like this:
Gson gson = new Gson();
WebAnswer<RequestModel> data = new WebAnswer<RequestModel>();
data = gson.fromJson(response.toString(),data.getClass());
and in Field result I get array of objects com.google.gson.internal.LinkedTreeMap$Node#. But when I do something like this
WebAnswer<RequestModel> data = new RequestAnsModel();
I get correct model where result is object of RequestModel. Are there any ways to desirialize in gson somthing like this correct with Generic without create inherited class?

You should use TypeToken like in the code below:
Gson gson = new Gson();
Type type = new TypeToken<WebAnswer<RequestModel>>() {
}.getType()
WebAnswer<RequestModel> data = gson.fromJson(response.toString(), type);

Custom Conversion from Java Object to JSON Object

I have the following code
Gson gson = new Gson();
String json = gson.toJson(criteria.list()); // list is passed by Hibernate
The result would be something like this:
{creationTime:0, enabled:true, id:1, loginDuration:0, online:false, userName:someone}
I would like to add new attribute (DT_RowId which has the same value as id) within the JSON response. The end result should be like this:
{creationTime:0, enabled:true, id:1, loginDuration:0, online:false, userName:someone, DT_RowId=1}
UPDATED
I created a field with #Transient annotation on the entity in order to solve this issue.
...
#Transient
private long DT_RowId;
public void setId(long id) {
this.id = id;
this.DT_RowId=id;
}
...
However the setId function was never been called. Can someone enlighten me on this ?

GSON won't call your getters and setters. It accesses member vars directly via reflection. To accomplish what you are trying to do, you will need to use a GSON custom serializer/deserializer. The GSON docs on custom serializers/deserializers provide some examples for how to do this.
Here is a working example with a passing JUnit test that demonstrates how to do it:
Entity.java
public class Entity {
protected long creationTime;
protected boolean enabled;
protected long id;
protected long loginDuration;
protected boolean online;
protected String userName;
protected long DT_RowId;
}
EntityJsonSerializer.java
import java.lang.reflect.Type;
import com.google.gson.*;
public class EntityJsonSerializer implements JsonSerializer<Entity> {
#Override
public JsonElement serialize(Entity entity, Type typeOfSrc, JsonSerializationContext context) {
entity.DT_RowId = entity.id;
Gson gson = new Gson();
return gson.toJsonTree(entity);
}
}
JSONTest.java
import static org.junit.Assert.*;
import org.junit.Test;
import com.google.gson.*;
public class JSONTest {
#Test
public final void testSerializeWithDTRowId() {
Entity entity = new Entity();
entity.creationTime = 0;
entity.enabled = true;
entity.id = 1;
entity.loginDuration = 0;
entity.online = false;
entity.userName = "someone";
GsonBuilder builder = new GsonBuilder();
builder.registerTypeAdapter(Entity.class, new EntityJsonSerializer());
Gson gson = builder.create();
String json = gson.toJson(entity);
String expectedJson = "{\"creationTime\":0,\"enabled\":true,\"id\":1,\"loginDuration\":0,\"online\":false,\"userName\":\"someone\",\"DT_RowId\":1}";
assertEquals(expectedJson, json);
}
}

How to expose a method using GSon?

Using Play Framework, I serialize my models via GSON. I specify which fields are exposed and which aren't.
This works great but I'd also like to #expose method too. Of course, this is too simple.
How can I do it ?
Thanks for your help !
public class Account extends Model {
#Expose
public String username;
#Expose
public String email;
public String password;
#Expose // Of course, this don't work
public String getEncodedPassword() {
// ...
}
}

The best solution I came with this problem was to make a dedicated serializer :
public class AccountSerializer implements JsonSerializer<Account> {
#Override
public JsonElement serialize(Account account, Type type, JsonSerializationContext context) {
JsonObject root = new JsonObject();
root.addProperty("id", account.id);
root.addProperty("email", account.email);
root.addProperty("encodedPassword", account.getEncodedPassword());
return root;
}
}
And to use it like this in my view:
GsonBuilder gson = new GsonBuilder();
gson.registerTypeAdapter(Account.class, new AccountSerializer());
Gson parser = gson.create();
renderJSON(parser.toJson(json));
But having #Expose working for a method would be great: it would avoid making a serializer just for showing methods!

Check out Gson on Fire: https://github.com/julman99/gson-fire
It's a library I made that extends Gson to handle cases like exposing method, results Post-serialization, Post-deserialization and many other things that I've needed over time with Gson.
This library is used in production in our company Contactive (http://goo.gl/yueXZ3), on both Android and the Java Backend

Gson's #Expose seem to only be supported on fields. There is an issue registered on this: #Expose should be used with methods.

Couple different options based on Cyril's answer:
Custom serializer with a shortcut:
public static class Sample
{
String firstName = "John";
String lastName = "Doe";
public String getFullName()
{
return firstName + " " + lastName;
}
}
public static class SampleSerializer implements JsonSerializer<Sample>
{
public JsonElement serialize(Sample src, Type typeOfSrc, JsonSerializationContext context)
{
JsonObject tree = (JsonObject)new Gson().toJsonTree(src);
tree.addProperty("fullName", src.getFullName());
return tree;
}
}
public static void main(String[] args) throws Exception
{
GsonBuilder gson = new GsonBuilder();
gson.registerTypeAdapter(Sample.class, new SampleSerializer());
Gson parser = gson.create();
System.out.println(parser.toJson(new Sample()));
}
-OR-
Annotation based serializer
public static class Sample
{
String firstName = "John";
String lastName = "Doe";
#ExposeMethod
public String getFullName()
{
return firstName + " " + lastName;
}
}
public static class MethodSerializer implements JsonSerializer<Object>
{
public JsonElement serialize(Object src, Type typeOfSrc, JsonSerializationContext context)
{
Gson gson = new Gson();
JsonObject tree = (JsonObject)gson.toJsonTree(src);
try
{
PropertyDescriptor[] properties = Introspector.getBeanInfo(src.getClass()).getPropertyDescriptors();
for (PropertyDescriptor property : properties)
{
if (property.getReadMethod().getAnnotation(ExposeMethod.class) != null)
{
Object result = property.getReadMethod().invoke(src, (Object[])null);
tree.add(property.getName(), gson.toJsonTree(result));
}
}
}
catch (Exception ex)
{
ex.printStackTrace();
}
return tree;
}
}
#Retention(RetentionPolicy.RUNTIME)
#Target(ElementType.METHOD) //can use in method only.
public static #interface ExposeMethod {}
public static void main(String[] args) throws Exception
{
GsonBuilder gson = new GsonBuilder();
gson.registerTypeAdapter(Sample.class, new MethodSerializer());
Gson parser = gson.create();
System.out.println(parser.toJson(new Sample()));
}