Three Realization Methods of Grouping Statistics in java

In normal work, it is often used to group data, for example, a student object with class, name, gender, score, etc., which needs to be counted according to class groups. How to operate? A reasonable algorithm can improve a lot of efficiency.

Let's look at the following case:

// The following is the initialized data 
       List<Student> list = new ArrayList<Student>();
        Student student1 = new Student(" Li 41", " Female ", "1 Class ");
        Student student2 = new Student(" Li 42", " Female ", "1 Class ");
        Student student3 = new Student(" Li 43", " Female ", "1 Class ");
        Student student4 = new Student(" Li 44", " Male ", "1 Class ");
        Student student5 = new Student(" Li 45", " Male ", "1 Class ");
        Student student6 = new Student(" Li 46", " Male ", "2 Class ");
        Student student7 = new Student(" Li 47", " Male ", "2 Class ");
        Student student8 = new Student(" Li 48", " Male ", "2 Class ");
        Student student9 = new Student(" Li 49", " Male ", "2 Class ");
        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 operation

Reasonable use of map's own methods in actual development 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. Multimap using guava

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

3. Use the new features of jdk8 without excluding new things

After all, java14 has come out, and the new features of java8 still need to be known more

//1 OK, it can be solved 
Map<String, List<Student  >> collect = list.stream().collect(Collectors.groupingBy(ArrearageDeal::getGrade)); 

The above three kinds of code at that time, java8 is the most concise. However, in actual development, combined with specific scenarios, 2 and 3 are both good choices.

Java8 Grouping statistics of multiple fields

//  Group statistics 
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());