Java泛型類型不兼容

Question

我有2個抽象類是被擴展，並使用它們其他類：

-----基地-----

public abstract class HttpRequest<RESPONSE_TYPE extends HttpResponse> 
{...} 
public abstract class HttpResponse<DATA_TYPE> 
{...} 

-----擴展-----

public class MockyTextRequest extends HttpRequest<MockyTextResponse> 
{...} 
public class MockyTextResponse extends HttpResponse<TextWithTitle> 
{...} 

---- ------實用

public class HttpClient 
{... 
    public <REQUEST_TYPE extends HttpRequest<RESPONSE_TYPE>, RESPONSE_TYPE extends HttpResponse<RESULT_TYPE>, RESULT_TYPE> RESPONSE_TYPE synchronicRequest(@NonNull final REQUEST_TYPE httpRequest, @Nullable final Object tag) throws IOException, HttpException, ParseException 
    {...} 
...} 

---

當我編譯，我得到：

error: incompatible types: inference variable RESPONSE_TYPE has incompatible upper bounds HttpResponse,MockyTextResponse

上

MockyTextResponse httpResponse = HttpClient.get().synchronicRequest(new MockyTextRequest(), null); 

什麼想法？

---

施坦格事實上，這個調用

public <REQUEST_TYPE extends HttpRequest<RESPONSE_TYPE>, RESPONSE_TYPE extends HttpResponse<RESULT_TYPE>, RESULT_TYPE> void request(@NonNull final REQUEST_TYPE httpRequest, @Nullable final Object tag, @Nullable final Listener<REQUEST_TYPE, RESPONSE_TYPE> listener) 
{...} 

稱爲

HttpClient.get().request(new MockyTextRequest(), MainActivity.this); 

這裏工作確定

全部代碼github

Answer 1

推理算法見ms無法推斷出RESPONSE_TYPE的類型。它看起來像所有三種類型參數應該由MockyTextRequest給出。

我不是究竟是當然爲什麼這是規範的情況，但它與「嵌套」有關。

錯誤可以通過以下方式複製：

// same error on this line 
// C (String) is not inferred 
m(new ArrayList<List<String>>()); 

// the compiler is able to infer B from A 
// but not C from A 
static <A extends List<B>, B extends List<C>, C> void m(A a) {} 

這是一個已經固定的Java 8中的東西類型：

• Ideone Java 7 compilation
• Ideone Java 8 compilation

你可能提供證人：

MockyTextResponse httpResponse = 
    HttpClient.get(). 
     <MockyTextRequest, MockyTextResponse, TextWithTitle> 
     synchronicRequest(new MockyTextRequest(), null); 

還是有一些方法來放鬆的類型參數：

1.

public < 
    RESPONSE_TYPE extends HttpResponse<RESULT_TYPE>, 
    RESULT_TYPE 
> RESPONSE_TYPE synchronicRequest(
    final HttpRequest<RESPONSE_TYPE> httpRequest, final Object tag) 

2.

public < 
    REQUEST_TYPE extends HttpRequest<RESPONSE_TYPE>, 
    RESPONSE_TYPE extends HttpResponse<?> 
> RESPONSE_TYPE synchronicRequest(
    final REQUEST_TYPE httpRequest, final Object tag) 

3。

public < 
    REQUEST_TYPE extends HttpRequest<RESPONSE_TYPE>, 
    RESPONSE_TYPE extends HttpResponse<? extends RESULT_TYPE>, 
    RESULT_TYPE 
> RESPONSE_TYPE synchronicRequest(
    final REQUEST_TYPE httpRequest, final Object tag) 

從您的描述看，似乎＃1是最好的。 （＃2，＃3基本上是相同的，因爲RESULT_TYPE沒有得到推斷反正。）

---

另一種方法是定義HttpRequest類似如下：

class HttpRequest< 
    RESPONSE_TYPE extends HttpResponse<RESULT_TYPE>, 
    RESULT_TYPE 
> 

然後：

class MockyTextRequest 
extends HttpRequest<MockyTextResponse, TextWithTitle> 

public < 
    REQUEST_TYPE extends HttpRequest<RESPONSE_TYPE, RESULT_TYPE>, 
    RESPONSE_TYPE extends HttpResponse<RESULT_TYPE>, 
    RESULT_TYPE 
> RESPONSE_TYPE synchronicRequest(
    final REQUEST_TYPE httpRequest, final Object tag) 

但是，這可能會讓你的一些代碼更加冗長。

---

作爲一個側面說明，你應該avoid raw types時，你可以：

//             vvv 
class HttpRequest<RESPONSE_TYPE extends HttpResponse<?>>