How to implement the java grouping statistics function

Let’s take a look at the following case:

//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);

1. Reasonable use of map operations

In actual development, reasonable use of map’s own methods can solve many problems

 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);
            }
        }

2. Use guava’s Multimap

Multimap<String, Student> mulMap = ArrayListMultimap.create();
for (Student  stu : list) {
       mulMap.put(stu.getGrade,stu);   
}

3. Use the new features of jdk8 – don’t reject new things

After all, java14 has come out, and you still need to know more about the new features of java8

//一行就可以解决
Map<String, List<Student  >> collect = list.stream().collect(Collectors.groupingBy(ArrearageDeal::getGrade));

From the perspective of code volume among the above three, java8 was the simplest. However, in actual development, both 2 and 3 are good choices based on specific scenarios.

Java8 Multiple Field Group Statistics

// 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());

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