Java：通配符类型不匹配导致编译错误

Question

    import java.util.List;

    public class GenTest {
        public static void main(String[] args) {
            String test1 = "";
            IRandomType<String> test2 = null;
            IAnotherRandomType<?> test3 = null;

            IManager<String> test1Manager = Factory.createManager(test1);
            IManager<IRandomType<String>> test2Manager = Factory.createManager(test2);
            IManager<IAnotherRandomType<?>> test3Manager = Factory.createManager(test3); // Doesn't compile. Why?

            // Work around?
            IManager<?> test3ManagerTmp = Factory.createManager(test3);
            IManager<IAnotherRandomType<?>> test3Manager2 = (IManager<IAnotherRandomType<?>>) test3ManagerTmp;
        }

        public interface IRandomType<T> {}
        public interface IAnotherRandomType<T> {}
        public interface IManager<T> {}

        public static class Factory {
            public static <T> IManager<T> createManager(T object) {
                return null;
            }
        }
    }

    Type mismatch: cannot convert from GenTest.IManager<GenTest.IAnotherRandomType<capture#1-of ?>> to GenTest.IManager<GenTest.IAnotherRandomType<?>>

• 与有界通配符相关的编译器错误
• 应兼容的不兼容通配符类型

Answer 1

使用以下方法：

IManager<IAnotherRandomType<?>> test3Manager =
        Factory.<IAnotherRandomType<?>>createManager(test3);

这只是编译器的类型推断失败的一种情况，因此有必要显式地为t提供类型参数。

更严格地说：

null