Gson does not support inheritance out of the box.

Let’s say we have the following class hierarchy:

public class BaseClass {
    int a;
 
    public int getInt() {
        return a;
   }
}
 
public class DerivedClass1 extends BaseClass {
     int b;
 
     @Override
     public int getInt() {
         return b;
     }
 }
 
public class DerivedClass2 extends BaseClass {
    int c;
 
    @Override
    public int getInt() {
        return c;
    }
}

And now we want to serialize an instance of DerivedClass1 to a JSON string

DerivedClass1 derivedClass1 = new DerivedClass1();
derivedClass1.b = 5;
derivedClass1.a = 10;
 
Gson gson = new Gson();
String derivedClass1Json = gson.toJson(derivedClass1);

Now, in another place, we receive this json string and want to deserialize it - but in compile time we only know it is supposed to be an instance of BaseClass:

BaseClass maybeDerivedClass1 = gson.fromJson(derivedClass1Json, BaseClass.class);
System.out.println(maybeDerivedClass1.getInt());

But GSON does not know derivedClass1Json was originally an instance of DerivedClass1, so this will print out 10.

How to solve this?

You need to build your own JsonDeserializer, that handles such cases. The solution is not perfectly clean, but I could not come up with a better one.

First, add the following field to your base class

@SerializedName("type")
private String typeName;

And initialize it in the base class constructor

public BaseClass() {
   typeName = getClass().getName();
}

Now add the following class:

public class JsonDeserializerWithInheritance<T> implements JsonDeserializer<T> {
 
 @Override
 public T deserialize(
     JsonElement json, Type typeOfT, JsonDeserializationContext context)
     throws JsonParseException {
     JsonObject jsonObject = json.getAsJsonObject();
     JsonPrimitive classNamePrimitive = (JsonPrimitive) jsonObject.get("type");
 
     String className = classNamePrimitive.getAsString();
 
     Class<?> clazz;
     try {
     clazz = Class.forName(className);
     } catch (ClassNotFoundException e) {
     throw new JsonParseException(e.getMessage());
     }
     return context.deserialize(jsonObject, clazz);
 }
}

All there is left to do is hook everything up -

GsonBuilder builder = new GsonBuilder();
 builder
 .registerTypeAdapter(BaseClass.class, new JsonDeserializerWithInheritance<BaseClass>());
 Gson gson = builder.create();

And now, running the following code-

DerivedClass1 derivedClass1 = new DerivedClass1();
derivedClass1.b = 5;
derivedClass1.a = 10;
String derivedClass1Json = gson.toJson(derivedClass1);

BaseClass maybeDerivedClass1 = gson.fromJson(derivedClass1Json, BaseClass.class);
System.out.println(maybeDerivedClass1.getInt());