CODEWITHSUNDEEP
javagenericswildcardjavacecj
stonar96
stonar96

Reputation: 1469

Eclipse compiler and OpenJDK javac behave differently when assigning generic types with wildcards

The following code compiles and runs just fine with Eclipse 2022-12 (4.26.0).

public static void main(String[] args) {
    List<List<?>> list = Arrays.asList(Arrays.asList("test"));
    System.out.println(test(list));
}

public static <T> T test(List<? extends List<T>> list) {
    return list.get(0).get(0);
}

However, when compiling with OpenJDK javac 19.0.2 I get the following error.

error: method test in class Test cannot be applied to given types;
        System.out.println(test(list));
                           ^
  required: List<? extends List<T>>
  found:    List<List<?>>
  reason: cannot infer type-variable(s) T
    (argument mismatch; List<List<?>> cannot be converted to List<? extends List<T>>)
  where T is a type-variable:
    T extends Object declared in method <T>test(List<? extends List<T>>)

Which compiler is correct in this case, and why? Is there a JLS that specifies that this assignment is (not) allowed? I have also read this answer. However, this doesn't help me to figure out if the assignment is allowed or not because the case of a generic type parameter T is not treated there.

Here is an example where this is relevant.

ExecutorService executorService = /* ... */;
List<Callable<?>> callables = /* ... */;
executorService.invokeAll(callables);

This code compiles with Eclipse but not with OpenJDK javac.

Upvotes: 1

Views: 46

Answers (0)

Related Questions