实现文章上一篇下一篇

@GetMapping(value = "newsDetail")
    public String newsDetail(Model model, @RequestParam(value = "nId") String nId)
    {
        News n = newsService.findOne(Long.valueOf(nId.trim()));
        if (n != null) {
            List<News> newsList = newsService.listAll();
　　　　　　　　//判断元素是否存在 contains()
            if(newsList.size()!=0){
                int index = newsList.indexOf(n);
                //当前新闻为第一个时，上一篇为最后一篇
                News upperNews = newsList.get(index==0?newsList.size()-1:index-1);
                //当前新闻为最后一个时，下一篇为第一篇
                News lowerNews = newsList.get(index==newsList.size()-1?0:index+1);
                model.addAttribute("upperNews",upperNews);
                model.addAttribute("lowerNews",lowerNews);
                model.addAttribute("news", n);
                return this.render("newsDetail");
            }
        }
        return this.render("index");
    }