Group before getting the maximum value in stream operation

Group by an attribute , And take the maximum according to another attribute

A list of students , From multiple classes , There are several students in each class ; Take out the oldest student in each class , Form a new list ;

return new ArrayList<>(studentList.stream()
   .collect(Collectors.toMap(Student::getClassId, Function.identity(),
      (c1, c2) -> c1.getAge() > c2.getAge() ? c1 : c2))
   .values());

1. First, turn to Map, In class ID by key, The oldest in the class Student The object is value;
2. Take out the Map Of values;
3. Plug into the ArrayList And in return ;