Class<Result> cannot be converted to Class<Result<Integer>>

Question

Suppose I have and API method declared as following

public class Dummy {

    public static class Result<ValueT extends Comparable> {

        public ValueT value;

    }



    public static <ValueT extends Comparable> Result<ValueT> 

            getResult(Class<? extends Result<ValueT>> ofType) throws Exception {

        return ofType.newInstance();

    }

}

Now I'd like to invoke it relying on Java compile-time type verification and just can't find correct syntax to do this:

Attempts:

getResult(Result<Integer>.class)

<-- expected syntax

public static void invokeGetResult() {

    Result<Integer> intResult = getResult(Result<Integer>.class);

}

results in

error: <identifier> expected

  Result<Integer> intResult = getResult(Result<Integer>.class);

                                                        ^

getResult(Result.class)

<-- just to try

public static void invokeGetResult() {

    Result<Integer> intResult = getResult(Result.class);

}

results in

error: method getResult in class Dummy cannot be applied to given types;

    Result<Integer> intResult = getResult(Result.class);

                                ^

  required: Class<? extends Result<ValueT>>

  found: Class<Result>

  reason: cannot infer type-variable(s) ValueT

    (argument mismatch; Class<Result> cannot be converted to Class<? extends Result<ValueT>>)

  where ValueT is a type-variable:

    ValueT extends Comparable declared in method <ValueT>getResult(Class<? extends Result<ValueT>>)

getResult((Class<Result<Integer>>)Result.class)

<-- just to try

public static void invokeGetResult() {

    Result<Integer> intResult = getResult((Class<Result<Integer>>)Result.class);

}

results in

error: incompatible types: Class<Result> cannot be converted to Class<Result<Integer>>

    Result<Integer> intResult = getResult((Class<Result<Integer>>)Result.class);

                                                                        ^

Answer 1

EDIT: I did not understand clearly the original intentions. This example should work for your call, however it is a bit hacky and not so recommended.

public class Dummy {
    public static class Result<ValueT extends Comparable> {
        public ValueT value;
    }

    public static <ValueT extends Comparable> Result<ValueT>
        getResult(Class<? extends Result<ValueT>> ofType) {
        return null;
    }
}

You can call it like this:

Dummy.Result<Integer> result = Dummy.getResult(new Result<Integer>(){}.getClass());

The compile time verification should work with this example, as you are creating an anonymous empty class and it's type information can be retrieved successfully.

You could also do a type cast as @Jorn suggested in comments, although your compiler will warn you about the fact it is an "unchecked cast".

Class<Result> cannot be converted to Class<Result<Integer>>

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