Comparator.comparing(…) of a nested field

Question

Suppose I have a domain model like this:

class Lecture {
     Course course;
     ... // getters
}

class Course {
     Teacher teacher;
     int studentSize         


        

Answer 1

import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.function.Function;

  class Person {
      String name ;
      PersonalDetail pDetail;
    public Person(String name, PersonalDetail pDetail) {
        super();
        this.name = name;
        this.pDetail = pDetail;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public PersonalDetail getpDetail() {
        return pDetail;
    }
    public void setpDetail(PersonalDetail pDetail) {
        this.pDetail = pDetail;
    }




  }

  class PersonalDetail{
      BirthDate birthDate;

    public BirthDate getBirthDate() {
        return birthDate;
    }

    public void setBirthDate(BirthDate birthDate) {
        this.birthDate = birthDate;
    }

    public PersonalDetail(BirthDate birthDate) {
        super();
        this.birthDate = birthDate;
    }


  }

    class BirthDate {
        public String getBirthdate() {
            return birthdate;
        }

        public void setBirthdate(String birthdate) {
            this.birthdate = birthdate;
        }

    String birthdate;

        public BirthDate(String birthdate) {
            super();

            this.birthdate = birthdate;
        }  
  }

  public class Test1 {
       public static void main(String[] args) {
           BirthDate b1 = new BirthDate("2019-08-08");
           BirthDate b2 = new BirthDate("2025-09-09");
           BirthDate b3 = new BirthDate("2025-09-08");
           BirthDate b4 = new BirthDate("2024-09-08");

           PersonalDetail pd1  = new PersonalDetail(b1);
           PersonalDetail pd2  = new PersonalDetail(b2);
           PersonalDetail pd3  = new PersonalDetail(b3);
           PersonalDetail pd4  = new PersonalDetail(b4);

           Person p1  = new Person("P1",pd1);
           Person p2  = new Person("P2",pd2);
           Person p3  = new Person("P3",pd3);
           Person p4  = new Person("P4",pd4);

           List persons = new ArrayList();
           persons.add(p1);
           persons.add(p2);
           persons.add(p3);
           persons.add(p4);

           Function getCourse = Person::getpDetail;  

           Person minByAge = persons.stream()
                      .max(Comparator.comparing(getCourse.andThen(PersonalDetail::getBirthDate).andThen(BirthDate::getBirthdate))).get();


          System.out.println(maxByAge.getName());

       }

    }