如何在Java中调用参数化方法？

Question

I have such a method:

private static <T extends HomerMessage> HomerMessage postRequest(String path, HomerMessage json) throws IOException, HomerDoh
{
    RequestBody body = RequestBody.create(JSON, toJson(json));
    Request request = new Request.Builder().url("http://" + path).post(body).build();

    String response = new OkHttpClient().newCall(request).execute().body().string();
    System.out.println(response);
    JsonNode responseNode = new ObjectMapper().readValue(response, JsonNode.class);
    if(!"200".equals(responseNode.get("status")))
    {
        throw readData(response, new TypeReference<HomerDoh>() {});
    }
    return readData(response, new TypeReference<T>() {});
}

private static <T> T readData(String is, TypeReference<T> ref) throws IOException
{
    return mapper.readValue(is, ref);
}

All works fine, but I could not figure out how to call it... I have tried:

     AuthResponse ar = HomerClient.postRequest(url + "/api/v1/session", auth);

The last expression does not compile.

How to call parametrized method in Java?

AuthResponse extends HomerMessage

Answer 1

您需要提供定义static方法的类的名称：

Exception ar = SomeClass.<Exception>getException();

Answer 2

Your code doesn't compile because it's not returning an AuthResponse : it is returning a HomerMessage .

You can make the return type AuthResponse if you change the return type of the method to:

private static <T extends HomerMessage> T postRequest(
    String path, HomerMessage json, TypeReference<T> typeRef)
        throws IOException, HomerDoh

which is the return type of your only normally-completing code path.

---

As noted by @SLaks, you can't use TypeReference with generics:

new TypeReference<T>() {}

Because of erasure, this will be equivalent at runtime to:

new TypeReference<Object>() {}

which is almost certainly not what you want - otherwise you could just have used that, and not had an issue calling the generic method in the first place.

You need to actually pass in the concrete TypeReference as a parameter:

private static <T extends HomerMessage> T postRequest(
    String path, HomerMessage json, TypeReference<T> typeRef)
        throws IOException, HomerDoh

then you can call this simply as:

AuthResponse response = postRequest(
    url + "/api/v1/session", auth,
    new TypeReference<AuthResponse>() {});

and the type T is inferred from the third parameter.

Answer 3

The expression Exception ar = <Exception>getException(); cannot compile because the generic method parametrization idiom requires a class name (or variable name if the method wasn't static) prior to parametrization.

For instance:

Exception ar = MyClass.<Exception>getException();

For instance methods:

• Exception ar = theObject.<Exception>getException();
• Exception ar = this.<Exception>getException();