using generic type in static class of generic class

Question

In a restful project, I'm trying to use a generic response. In this generic response, there is a static responseBuilder. But the build method in responseBuilder cannot accept generic type. Code :

public class RestResponse<T>{

private int status;

private String message;

private T entity;

/**
 * 
 */
public static class RestResponseBuilder {

    private final RestResponse restResponse;

    public RestResponseBuilder(RestResponse resp) {
        this.restResponse = resp;
    }

    public static RestResponseBuilder ok() {

        return status(HttpServletResponse.SC_OK).msg("ok");
    }

    public static RestResponseBuilder status(int status) {
        final RestResponse resp = new RestResponse();
        resp.setStatus(status);

        return new RestResponseBuilder(resp);
    }

    public RestResponseBuilder msg(String msg) {
        this.restResponse.setMessage(msg);
        return this;
    }

    public RestResponseBuilder entity(Object entity) {
        this.restResponse.setEntity(entity);
        return this;
    }

    public RestResponse build() {

        return restResponse;
    }
}

}

When I use like this : RestResponseBuilder.ok().entity(null).build(); There a warning : Type safety: The expression of type RestResponse needs unchecked conversion to conform to

My question is, how can I add generic type in RestResponseBuilder to avoid this warning? Thanks

Why not just make RestResponseBuilder generic as well, using the same type parameter? (class RestResponseBuilder<T> {, public RestResponseBuilder(RestResponse<T> resp) {, etc.)? — MTCoster
– MTCoster, Commented Nov 17, 2018 at 11:50

JB Nizet · Accepted Answer · 2018-11-17 11:54:02Z

Don't use raw types. Make your builder class generic, too, and its static methods generic, too:

public class RestResponse<T> {

    private int status;

    private String message;

    private T entity;

    /**
     *
     */
    public static class RestResponseBuilder<T> {

        private final RestResponse<T> restResponse;

        public RestResponseBuilder(RestResponse<T> resp) {
            this.restResponse = resp;
        }

        public static <T> RestResponseBuilder<T> ok() {

            return RestResponseBuilder.<T>status(200).msg("ok");
        }

        public static <T> RestResponseBuilder<T> status(int status) {
            final RestResponse<T> resp = new RestResponse<T>();
            resp.status = status;

            return new RestResponseBuilder<T>(resp);
        }

        public RestResponseBuilder<T> msg(String msg) {
            this.restResponse.message = msg;
            return this;
        }

        public RestResponseBuilder<T> entity(T entity) {
            this.restResponse.entity = entity;
            return this;
        }

        public RestResponse<T> build() {

            return restResponse;
        }
    }

}

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