You are using groupingBy一个参数的方法，该方法只能传参一个Function as the mapping function of Key

But in factCollectors.groupingBy there are methods with at most three parameters. The other two parameters and one parameter are just overloads of the three-parameter method. So if you want to process grouped Value data, you need to understand the fundamentals. What capabilities does the above three parameter method provide?

Collector<T, ?, M> groupingBy(Function<? super T, ? extends K> classifier,
                                  Supplier<M> mapFactory,
                                  Collector<? super T, A, D> downstream) {

and it is also a collector. That is to say, these three parameters are actually to solve the grouping problemFunction了，第二个参数是一个map工厂，也就是最终结果的容器，一般默认的是采用的HashMap::new，最后一个参数很重要是一个downstream，类型是Collector

1. The first parameter: what classification the grouping is based on

2. Second parameter: What container is used to save the final group and return it

3. The third parameter: After classifying according to the first parameter, how to collect the corresponding classification results

As mentioned before, the second parameter defaults to HashMap::new, so You can use the
two-parameter method, as you can see below, the second parameter is HashMap::newHashMap::new，所以你就可以采用Collectors.groupingBy两个参数的方法，如下可以看到，第二个参数是HashMap::new

public static <T, K, A, D>
    Collector<T, ?, Map<K, D>> groupingBy(Function<? super T, ? extends K> classifier,
                                          Collector<? super T, A, D> downstream) {
        return groupingBy(classifier, HashMap::new, downstream);
    }

method actually defaults to Collectors.toList, so the final type returned by what you wrote above is Map<String, List>Collectors.groupingBy方法的第三个参数收集器其实默认是Collectors.toList，所以你上面写的返回最终的类型是Map<String, List>

Student
String type
int score
String name

List<Student> students = new ArrayList<>()

// 每个学生类型的总分数
Map<String, Integer> map1 = 
students.stream.collect(Collectors.groupingBy(Student::getType, Collectors.summingInt(Student::getScore))));

// 每个学生类型中的得分最高的那个学生
Map<String, Optional<Student>> map2 = students.stream.collect(Collectors.groupingBy(Student::getType, Collectors.maxBy(Comparator.comparing(Student::getScore))));

. If it doesn’t work, you can customize the collector. I hope it can help youCollectors