Comparator.comparing（...）嵌套場

Question

假設我有一個領域模型是這樣的：

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

class Course { 
    Teacher teacher; 
    int studentSize; 
    ... // getters 
} 

class Teacher { 
    int age; 
    ... // getters 
} 

現在，我可以創造一個老師比較喜歡這樣的：

return Comparator 
      .comparing(Teacher::getAge); 

但如何我會比較一下嵌套字段的講座嗎？

return Comparator 
      .comparing(Lecture::getCourse::getTeacher:getAge) 
      .thenComparing(Lecture::getCourse::getStudentSize); 

我不能在模型上添加一個方法Lecture.getTeacherAge()。

Answer 1

您不能嵌套方法引用。您可以使用lambda表達式來代替：

return Comparator 
     .comparing(l->l.getCourse().getTeacher().getAge(), Comparator.reverseOrder()) 
     .thenComparing(l->l.getCourse().getStudentSize()); 

Answer 2

不幸的是，在java中的沒有很好的語法。

如果你想重用比較的部分，我可以看到2種方式：

• 通過組合比較

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

• 通過組合吸氣功能

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