Null pointer access: The variable marketcommends can only be null at this location_null pointer access: the variable can only be null-优快云博客

本文链接：https://blog.youkuaiyun.com/change_on/article/details/50878316

本文讨论了Java编程中出现的NullPointerAccess错误，特别是对于市场推荐列表的处理方式。通过实例分析，展示了如何避免错误的初始化方法，正确使用List的clear()方法来清空列表，以及在循环中安全地操作列表元素。文章提供了防止空指针异常的实用技巧，帮助开发者提升代码质量和稳定性。

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Null pointer access: The variable marketcommends can only be null at this location

源代码:

List meunList=backServiceInter.getmeunBypage(9,1);
		List meunrealList=new ArrayList();
		Meun meun=null;
		Photo photo=null;
		List<Marketmeun> marketlist=new ArrayList<Marketmeun>();
		marketlist=null;
		List<Market> marketcommends=new ArrayList<Market>();
		Market market=null;
		Marketmeun marketmeun=null;
		MeunQueryVo meunQueryVo=new MeunQueryVo();
		for(int i=0;i<meunList.size();i++)
		{
			meun=(Meun)meunList.get(i);
			photo=(Photo)backServiceInter.getPhoto("2",meun.getId());
			marketlist=backServiceInter.getMarketmeun(meun.getId());
			marketcommends.clear();
			/*错误的写法*/  
			//marketcommends=null;
			for(int j=0;j<marketlist.size();j++)
			{
				marketmeun=(Marketmeun)marketlist.get(j);
				market=(Market)backServiceInter.findById(Market.class,marketmeun.getMarket().getId());
				marketcommends.add(j,market);
				System.out.println("market="+market.getName());
			}
			meunQueryVo.setCommendmarkets(marketcommends);
			//System.out.println("marketcommends="+marketcommends.size());
			meunQueryVo.setMeun(meun);
			meunQueryVo.setBigImage(photo.getBigphoto());
			meunrealList.add(i,meunQueryVo);
		}

在一般的for循环时，为了避免过多的new 对象，常常采用上面的写法，即把对象先在对象外创建好，然后=null，但是List要注意，