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

Unfortunately there is no nice syntax in java for that.

If you want to reuse parts of comparator I can see 2 ways:

• by composing comparators

  return comparing(Lecture::getCourse, comparing(Course::getTeacher, comparing(Teacher::getAge)))
         .thenComparing(Lecture::getCourse, comparing(Course::getStudentSize));
  
  // or with separate comparators
  Comparator byAge = comparing(Teacher::getAge);
  Comparator byTeacherAge = comparing(Course::getTeacher, byAge);
  Comparator byStudentsSize = comparing(Course::getStudentSize);
  return comparing(Lecture::getCourse, byTeacherAge).thenComparing(Lecture::getCourse, byStudentsSize);
  

• by composing getter functions

  Function getCourse = Lecture::getCourse;            
  return comparing(getCourse.andThen(Course::getTeacher).andThen(Teacher::getAge))
         .thenComparing(getCourse.andThen(Course::getStudentSize));
  
  // or with separate getters
  Function getCourse = Lecture::getCourse;
  Function teacherAge = getCourse.andThen(Course::getTeacher).andThen(Teacher::getAge);
  Function studentSize = getCourse.andThen(Course::getStudentSize);
  return comparing(teacherAge).thenComparing(studentSize);