将方法的泛型作为泛型类泛型

Question

I'm querying JSON object from a service, where pagination is being used. I wanted to create a generic class, and a generic method where the values are decoded to the corresponding type, then I ran into a problem where I couldn't pass the method's generic as another generic type's generic.

Consider the following classes and methods:

PageResponse.class

public class PageResponse<T> {
    @JsonProperty("values")
    private List<T> values;
    @JsonProperty("next")
    private String next;
}

and the method

public void fetchAllFoos(String url, AsyncResp resp) {
    this.<Foo>fetchAllPage(url, new ArrayList<Foo>(), resp);
}

public void fetchAllBars(String url, AsyncResp resp) {
    this.<Bar>fetchAllPage(url, new ArrayList<Bar>(), resp);
}

private <T> void fetchAllPage(String next, List<T> results, AsyncResp resp) {
    TypeReference<PageResponse<T>> valueTypeRef = new TypeReference<PageResponse<T>>() {
    };
    client.makeGetRequest(next, e -> {
        PageResponse<T> page = Json.mapper.readValue(e.toString(), valueTypeRef);
        results.addAll(page.getValues());
        if (page.getNext() != null) {
            fetchAllPage(page.getNext(), results, resp);
        } else {
            resp.returnValue(results);
        }
    });
}

The code compiles fine, and it works except that, the TypeReference 's generic type is PageRequest<T> instead of PageRequest<Foo> or PageRequest<Bar> and that leads me to PageResponse.values treated as List<Object> .

I'm not looking for other kind of implementation for the problem, I'm just curious about this behavior. Is the generics evaulated in compile time, or what causes this behavior, and can it be solved?

Answer 1

What you see here is type erasure . In a nutshell, generic types are for compile time only. After compilation, the byte code won't have this information anymore.

Your solution is perfect. It's called a type token . You can find it in java lib too, eg at EnumMap :

EnumMap<MyEnum, String> map = new EnumMap<>(MyEnum.class);