cannot select Parameterized Type

Question

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.

Instead of telling us it "is not working" it would probably be better to post the compiler error. I reckon the problem is with `ResultContainData.class`, right? — John, Nov 18 '14 at 17:27
As @John wrote, there is no such thing as **ResultContainData.class**, try **ResultContainData.class**. — Grzesuav, Nov 18 '14 at 17:30

score 14 · Answer 1 · answered Nov 18 '14 at 17:47

14

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.

answered Nov 18 '14 at 17:47

Michael

425
1
4
14

10

Mind to [explain your solution](http://stackoverflow.com/help/how-to-answer) a bit? After you have done so, feel free to accept it. – Markus W Mahlberg Nov 09 '15 at 15:36

score 9 · Answer 2 · edited Sep 20 '22 at 18:13

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;

cannot select Parameterized Type

2 Answers2

Linked