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业务开发中会运用到很多的List<T>集合数据类型，存放我们需要的业务数据，比如说有个产品产量信息表T，业务需要用柱状图展示每个产品每个月的产量图示，那我们定义了一个实体类ProductVO来传递这些数据,包含了月份month,总数count,产品pro，通过dao.xml中对产量信息表进行SQL编程，将产量数按月份和产品分组求和，最终返回在一个List<ProductVo>中。
在service层进行调用得到这个list集合，一般我们都是通过for循环或者迭代器进行遍历，做判断，现在java8新特性Stream能帮我们简洁且高效的编码完成该循环。不需要再写冗杂沉重的循环了
代码如下：
//调用dao层方法，得到将产量分组好的集合数据 List<ProductVO > list= productDao.getProduct();
//通过stream流 将月份等于5的元素取出来，findFirst取第一个元素 ProductVO vo =
list.stream().filter(f->Integer.parseInt(f.getMonth())=5).findFirst().get();
Stream的findFirst方法在此流中查找第一个元素作为Optional。 get方法则返回处理的元素。
如果findFirst选择的元素为null，它将抛出NullPointerException。
可能会遇到的问题：
NoSuchElementException：空指针的话可以用这个方法进行过滤 orElse方法：
将.get()替换为.orElse(null)，这样就会返回null值了。不会再报错
