Group map values but keys are same

Question

I have a map like this. Map<long,List<Student>> studentMap

Key is a number 1,2,3,4... Student object is :

public class Student {
 private long addressNo;
 private String code;
 private BigDecimal tax;
 private String name;
 private String city;

 // getter and setters` 
}

What i want to do is to convert it Map<long,List<StudentInfo>> studentInfoMap object and group id, addressNo and code fields.I want key are same for both maps.

I can group the map by using these codes but summingDouble is not working for BigDecimal.Also I cannot convert my studentMap to studentInfoMap.:(

 studentInfoMap.values().stream()
            .collect(
                     Collectors.groupingBy(StudentInfo::getCode, 
                     Collectors.groupingBy(StudentInfo::getAddressNo, 
                     Collectors.summingDouble(StudentInfo::getTax))));

My studentInfo object is :

public class StudentInfo {
  private long addressNo;
  private String code;
  private BigDecimal tax;

  // getter and setters` 
}

Answer 1

Map<Integer, Object> map = new HashMap<>();
map.put(1, studentInfoMap.values().stream().map(
    student -> student.getAddressNo()
  ).collect(Collectors.toList()));
map.put(2, studentInfoMap.values().stream().map(
  student -> student.getCode()
).collect(Collectors.toList()));
// and so ...

Answer 2

To convert the map from Student to StudentInfo, whilst keeping the same keys, you could do something like this:

Set<Long> keys = studentMap.keySet();
List<Long> keylist = new ArrayList<>(keys);
Map<Long, List<StudentInfo>> studentInfoMap = new HashMap<>();
for(int i = 0; i < keys.size(); i++){
    long key = keylist.get(i);
    List<Student> list = studentMap.get(key);
    List<StudentInfo> result = new ArrayList<>();
    // Create the new StudentInfo object with your values
    list.forEach(s -> result.add(new StudentInfo(s.name())));
    // Put them into the new Map with the same keys
   studentInfoMap.put(key, result);
}