Java Question
Class<Result> cannot be converted to Class<Result<Integer>>
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 syntaxpublic 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 trypublic 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 trypublic 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);
^
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Answer Source
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".
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