Java 编译时出现一般错误 : "type argument is not within bounds of type-variable T"

Question

我正在开发一个代理适配器，以使用 Java 中的泛型将请求/响应转换为另一个请求/响应。

我有一个界面:

public interface ProxyAdapter<FROM, TO> {

    TO adapt(FROM o);
}

适配器的实现(仅以请求案例为例):

public interface RequestProxyAdapter<FROM, TO> extends ProxyAdapter<RequestEntity<FROM>, RequestEntity<TO>> { }

抽象类:

public abstract class ProxyAdapterResolver<U extends HttpEntity<?>, T extends ProxyAdapter<U, U>> {

   private final Map<String, T> adapters;

   private final T defaultAdapter;

   public ProxyAdapterResolver(Collection<T> adapters, T defaultAdapter) {
       this.adapters = adapters.stream()
            .collect(Collectors.toMap(this::proxyNameResolver, Function.identity()));

       this.defaultAdapter = defaultAdapter;
   }
   // other methods...
}

具体类:

@Component
public class RequestProxyAdapterResolver extends ProxyAdapterResolver<RequestEntity<?>, RequestProxyAdapter<?, ?>> {
    @Autowired
    public RequestProxyAdapterResolver(Collection<RequestProxyAdapter<?, ?>> allAdapters) {
       super(allAdapters, a -> a);
    }
}

异常发生在编译时，进入具体类RequestProxyAdapterResolver。这是堆栈跟踪:

Error:(11, 108) java: type argument br.com.afferolab.attendancelist.core.proxy.service.RequestProxyAdapter<?,?> is not within bounds of type-variable T

obs:类RequestEntity来自org.springframework.http包

异常很明显，我确实在堆栈溢出中阅读了有关此问题的一些问题。但是我找不到适合我的情况的解决方案。例如:

Java generics issue: Class "not within bounds of type-variable" error.

Type argument T is not within bounds of type-variable T(Java Generic)

Inferring a generic type from a generic type in Java (compile time error)

我花了大约三天的时间来解决这个问题。有任何想法吗？谢谢。

Answer 1

我认为问题在于 RequestEntity<?>作为第一个参数和 RequestEntity<?> 给出通配符参数的隐含上限为 RequestProxyAdapater不被视为完全相同的类型，但您要求它们全部为 U 类型.

我发现当我在让泛型协同工作时遇到困难时， ? extends或? super经常会修复它。试试这个:

public abstract class ProxyAdapterResolver<U extends HttpEntity<?>, T extends ProxyAdapter<? extends U, ? extends U>> {