cannot select Parameterized Type

I want to create a rest to communicate between server and client.

The constructor given below:

public class RestHelper<I, R> { public RestHelper(String url, I input, Class<R> output){ ResponseEntity<R> responseEntity = restTemplate.exchange(url, HttpMethod.POST, requestEntity, output ); } } 

For normal type, I can do:

RestHelper<User, Result> helper = new RestHelper<>(url, user, Result.class); 

How can I pass a generic type, like:

ResultContainData<Boolean> 

The code below is not working:

 ResultContainData<Boolean> result = new ResultContainData<>(); RestHelper<User, ResultContainData<Boolean>> helper = new RestHelper<>(url, user, (Class<ResultContainData<Boolean>>) ((ParameterizedType) result.getClass().getGenericSuperclass()).getActualTypeArguments()[0]); 

I got a runtime error: cannot cast to ParameterizedType.

2 Answers

I got the solution.

ResultContainData<Boolean> result = new ResultContainData<>(); RestHelper<User, ResultContainData<Boolean>> helper = new RestHelper<>(url, user, (Class<ResultContainData<Boolean>>)result.getClass()); 

It's working for me. I am still looking for a better solution.

You can only learn the value of I and R by capturing them in a subclass definition - otherwise they are erased at runtime. Ex:

class MyStringRestHelper extends RestHelper<String, String> { 

Then using something like my TypeTools you can resolve the values of I and R:

Class<?>[] typeArgs = TypeResolver.resolveRawArguments(RestHelper.class, MyStringRestHelper.class); Class<?> i = typeArgs[0]; Class<?> r = typeArgs[1]; assert i == r == String.class;