龙目岛@Builder class 和 generics

Question

Is there a way to use Lombok's @Builder with Java generics?

@Builder
public class Response<T> {
    private String status;
    private long total;
    private List<T> page;
}

Later in a service that handles requests from controller:

long total = getTotalItemsCount();
List<Article> page = getPage();

Response<Article> response =
    Response.builder()
        .status("ok")
        .total(total)
        .page(page); // error on this line

It's not possible to call page method:

The method page(List<Object>) in the type Response.ResponseBuilder<Object> is not applicable for the arguments ( List<Article> )

Any ideas?

Answer 1

You'll need to specify a "type witness", the 2nd <Article> part below:

Response<Article> response =
        Response.<Article>builder() // here
            .status("ok")
            .total(total)
            .page(page)
            .build();

This is really nothing to do with Lombok, but a shortcoming of Java's type inference. It cannot infer through the chain of method calls that builder() needs to create a Response of type Article, even though it's ultimately assigned to a variable of that type.

The type witness is a way of overriding the inference and specifying the generic type parameter explicitly.