如何根据一个字段对数据列表进行分组

Question

I have a list of custom class data, I want to group them based on one field and the value should be another field. Take the example below.

List<Student> list = new ArrayList<>();
list.add(new Student(1001, "Class 1", "Jon"));
list.add(new Student(1002, "Class 1", "Eliana"));
list.add(new Student(1003, "Class 2", "Piter"));
list.add(new Student(1004, "Class 3", "Joy"));
list.add(new Student(1005, "Class 2", "Anderson"));
list.add(new Student(1006, "Class 1", "Ram"));
list.add(new Student(1007, "Class 1", "Mohan"));

Now I would like to group this data based on class. The expected output should be a map containing key as the class and value as the list of student names.

{"Class 1" -> ["Jon", "Eliana", "Ram", "Mohan"]}
{"Class 2" -> ["Piter", "Anderson"]}
{"Class 3" -> ["Joy"]}

I want to get this using java 8 stream feature. I tried doing this and I could group the whole object into a map as the key being the class name and value is a list of students. But my requirement is to get value as the only a list of student names.

My pice of code is something like this:

list.stream().collect(Collectors.groupingBy(Students::getClass));

Answer 1

From the above comments section, it can be done in this way:

Map<EligibilityRequest, List<List<String>>> result = 
  list.stream()
    .collect(Collectors.groupingBy(Student::getClass,
      Collectors.mapping(Student::getName,  toList())
    ));