与基类和子类"not applicable for the arguments"相关的 Java 泛型

Question

我确实发现了一些关于一般问题的类似帖子，但没有一篇以我能理解的方式捕捉到我的问题。我希望有人能帮助我解决这个问题。

我尝试了很多方法，比如从不同的地方删除"？extends"，玩"？super"，看看另一个错误是否会给我指明解决方案的方向。

以下代码：

final ResponseBase response = executor.execute(myRequest);

出现以下编译器错误："CommandExecutor类型中的方法execute（捕获#6-of？extends RequestBase）不适用于参数（RequestBase）"

完整的代码列表：

public class MainClass {
    private final static Map<Class<? extends RequestBase>, CommandExecutor<? extends RequestBase, ? extends ResponseBase>> MAP = new HashMap<>();
    public static void main(String[] args) {
        final DummyCommandExecutor dummyCommandExecutor = new DummyCommandExecutor();
        MAP.put(MyRequest.class, dummyCommandExecutor);
        final RequestBase myRequest = new MyRequest();
        myRequest.setRequestString("this is my request");
        final CommandExecutor<? extends RequestBase, ? extends ResponseBase> executor = MAP.get(myRequest.getClass());
        final ResponseBase response = executor.execute(myRequest);
        System.out.println(response.getResponseString());
    }
}
interface CommandExecutor<T, R> {
    R execute(T object);
}
class DummyCommandExecutor implements CommandExecutor<MyRequest, MyResponse> {
    @Override
    public MyResponse execute(MyRequest request) {
        final MyResponse response = new MyResponse();
        response.setResponseString(request.getRequestString());
        return response;
    }
}
class MyResponse extends ResponseBase {
}
class ResponseBase {
    String responseString;
    public String getResponseString() {
        return this.responseString;
    }
    public void setResponseString(String responseString) {
        this.responseString = responseString;
    }
}
class MyRequest extends RequestBase {
}
class RequestBase {
    String requestString;
    public String getRequestString() {
        return this.requestString;
    }
    public void setRequestString(String requestString) {
        this.requestString = requestString;
    }
}

Answer 1

如果不进行强制转换，就无法做到这一点。虽然您知道映射会为给定类型返回正确的命令执行器，但编译器不会，因此您需要告诉编译器不要在意：

final CommandExecutor<RequestBase,ResponseBase> executor = (CommandExecutor) MAP.get(myRequest.getClass());

Answer 2

您不能直接匹配吗？带T

一个选项就是：

final CommandExecutor executor = MAP.get(myRequest.getClass());

当然，在哪个选项中，如果映射返回smth，则可以获得运行时异常