Allow TypeAdapters for Object

Type type = new TypeToken>()
{
}.getType();

System.out.println(list);
System.out.println("--------------");

Gson gson = new GsonBuilder().registerTypeAdapterFactory(runtimeTypeApatper)
.setPrettyPrinting()
.create();

String json = gson.toJson(list, type);

System.out.println(json);
System.out.println("--------------");

System.out.println(list2);
System.out.println("--------------");
System.out.println(list.equals(list2));

But to add also C to the list, I need a RuntimeTypeAdapterFactory of the Object class, see:

System.out.println("Test Base Object");
System.out.println("##############");
System.out.println("--------------");

List list = new ArrayList<>();
list.add(new A("aa"));
list.add(new AB("aaa", "bbb"));
list.add(new C("cc", new D("dd")));

Type type = new TypeToken>()
{
}.getType();

System.out.println(list);
System.out.println("--------------");

RuntimeTypeAdapterFactory runtimeTypeApatper = RuntimeTypeAdapterFactory
.of(Object.class);
runtimeTypeApatper.registerSubtype(A.class);
runtimeTypeApatper.registerSubtype(AB.class);
runtimeTypeApatper.registerSubtype(C.class);

Gson gson = new GsonBuilder().registerTypeAdapterFactory(runtimeTypeApatper)
.setPrettyPrinting()
.create();

String json = gson.toJson(list, type);

System.out.println(json);
System.out.println("--------------");

List list2 = gson.fromJson(json, type);

System.out.println(list2);
System.out.println("--------------");
System.out.println(list.equals(list2));

But this does not work. I looked at your code and found the problem in com.google.gson.Gson.Gson(…)

List factories = new ArrayList();

// built-in type adapters that cannot be overridden
factories.add(TypeAdapters.JSON_ELEMENT_FACTORY);
factories.add(ObjectTypeAdapter.FACTORY);

// the excluder must precede all adapters that handle user-defined types
factories.add(excluder);

// users' type adapters
factories.addAll(factoriesToBeAdded);

// type adapters for basic platform types
factories.add(TypeAdapters.STRING_FACTORY);
...

I we reorder the lines, so that the ObjectTypeAdapter is added after the user defined factories, my example works!

List factories = new ArrayList();

// built-in type adapters that cannot be overridden
factories.add(TypeAdapters.JSON_ELEMENT_FACTORY);
// the excluder must precede all adapters that handle user-defined types
factories.add(excluder);

// users' type adapters
factories.addAll(factoriesToBeAdded);

factories.add(ObjectTypeAdapter.FACTORY);

// type adapters for basic platform types
factories.add(TypeAdapters.STRING_FACTORY);