JAVA8

@Test
    public void testGroup2() {
        //1.分組計數 更具classId分組，計算個數
        List<Student> list1 = Arrays.asList(
                new Student(1L, "one", "oneS1"),
                new Student(2L, "one", "oneS2"),
                new Student(3L, "one", "oneS3"),
                new Student(4L, "tow", "towS1"),
                new Student(5L, "tow", "towS2"),
                new Student(6L, "three", "sun"));


        //[1, 2, 3, 4, 5, 6]
        List<Long> sId = list1.stream().map(item -> item.getId()).collect(Collectors.toList());
        System.out.println(sId);



        //{
        //  one=[Student(id=1, classId=one, name=oneS1), Student(id=2, classId=one, name=oneS2), Student(id=3, classId=one, name=oneS3)],
        //  three=[Student(id=6, classId=three, name=sun)],
        //  tow=[Student(id=4, classId=tow, name=towS1), Student(id=5, classId=tow, name=towS2)]
        //}
        Map<String, List<Student>> collect = list1.stream().collect(Collectors.groupingBy(Student::getClassId));
        System.out.println(collect);


        //1.1根據某個屬性分組計數
        //{one=3, three=1, tow=2}
        Map<String, Long> result1 = list1.stream().collect(Collectors.groupingBy(Student::getClassId, Collectors.counting()));
        System.out.println(result1);





        //1.3根據分組的key值對結果進行排序、放進另外一個map中並輸出
        //comparingByKey
        //{one=3, tow=2, three=1}
        Map<String,Long> xMap=new HashMap<>();
        result1.entrySet().stream().sorted(Map.Entry.<String,Long>comparingByValue().reversed()).forEachOrdered(x->xMap.put(x.getKey(),x.getValue()));
        System.out.println(xMap);



        //2.分組，並統計其中一個屬性值得sum或者avg:id總和
        /**
         {one=LongSummaryStatistics{count=3, sum=6, min=1, average=2.000000, max=3},
         three=LongSummaryStatistics{count=1, sum=6, min=6, average=6.000000, max=6},
         tow=LongSummaryStatistics{count=2, sum=9, min=4, average=4.500000, max=5}}
         */
        Map<String, LongSummaryStatistics> summaryStatisticsMap = list1.stream().collect(Collectors.groupingBy(Student::getClassId, Collectors.summarizingLong(Student::getId)));
        System.out.println(summaryStatisticsMap);


    }