将 map 值转换为另一个 map object

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>> studentMap object and group id, addressNo and code fields.

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

If you want to convert a Map<Long,List<Student>> to a Map<Long,List<StudentInfo>> with the keys being the same, try the following:

Map<Long,List<StudentInfo>> convertedMap = 
  studentInfoMap.entrySet() //get the entry set to access keys and values
    .stream() 
    .collect(Collectors.toMap( //collect the entries into another map
        entry -> entry.getKey(), //use the keys as they are
        entry -> entry.getValue() //get the value list
                      .stream() /
                      .map(student -> new StudentInfo(/*pass in the student parameters you need*/) //create a StudentInfo out of a Student - and entry.getKey() if needed
                      .toList() //collect the StudentInfo elements into a list
     ));