java中分组统计的三种实现方式

//下面是初始化的数据

        List<Student> list = new ArrayList<Student>();

         Student student1 = new Student( "李四1" , "女" , "一班" );

         Student student2 = new Student( "李四2" , "女" , "一班" );

         Student student3 = new Student( "李四3" , "女" , "一班" );

         Student student4 = new Student( "李四4" , "男" , "一班" );

         Student student5 = new Student( "李四5" , "男" , "一班" );

         Student student6 = new Student( "李四6" , "男" , "二班" );

         Student student7 = new Student( "李四7" , "男" , "二班" );

         Student student8 = new Student( "李四8" , "男" , "二班" );

         Student student9 = new Student( "李四9" , "男" , "二班" );

         list.add(student1);

         list.add(student2);

         list.add(student3);

         list.add(student4);

         list.add(student5);

         list.add(student6);

         list.add(student7);

         list.add(student8);

         list.add(student9);

for (Student  stu : list) {

            if (!map.containsKey(stu.getProvinceCode())) {

                ArrayList<ArrearageDeal> al = new ArrayList<ArrearageDeal>();

                map.put(stu.getProvinceCode(),  al.add(stu));

            } else {

                map.get(stu.getProvinceCode()).add(stu);

            }

        }

Multimap<String, Student> mulMap = ArrayListMultimap.create();

for (Student  stu : list) {

        mulMap.put(stu.getGrade,stu);  

}

//一行就可以解决

Map<String, List<Student  >> collect = list.stream().collect(Collectors.groupingBy(ArrearageDeal::getGrade));

// 分组统计

Map<String, Long> countMap = records.stream().collect(Collectors.groupingBy(o -> o.getProductType() + "_" + o.getCountry(), Collectors.counting()));

  List<Record> countRecords = countMap.keySet().stream().map(key -> {

     String[] temp = key.split( "_" );

     String productType = temp[ 0 ];

     String country = temp[ 1 ];    

     Record record = new Record();

     record.set( "device_type" , productType);

     record.set( "location" , country;

     record.set( "count" , countMap.get(key).intValue());

     return record;

}).collect(Collectors.toList());