关于类中定义接口

请问老师为什么将排序的HashSet定义在接口中
这样做的目的是什么
或者说有什么性能上的提升

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@Component
public class Constant {
public static final String SALT = "chenyuechuan";

    public static final String IMOOC_MALL_USER = "imooc_mall_user";

    public static String FILE_UPLOAD_DIR;

    @Value("${file.upload.dir}")
public void setFileUploadDir (String fileUploadDir) {
FILE_UPLOAD_DIR = fileUploadDir;
    }

public interface ProductListOrderBy {
        Set<String> PRICE_ASC_DESC = Sets.newHashSet("price desc", "price asc");
    }
}