Question

假设我有一个这样的域模型：

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

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

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

现在我可以像这样创建一个教师比较器：

    return Comparator
            .comparing(Teacher::getAge);

但是如何在嵌套字段上比较Lecture，像这样？

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

我无法在模型上添加方法Lecture.getTeacherAge()。

Answer 1

您无法嵌套方法引用。您可以改为使用lambda表达式：

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

如果不需要逆向订单，它就会更加冗长：

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

注意：在某些情况下，您需要明确说明泛型类型。例如，下面的代码在Java 8中没有<FlightAssignment, LocalDateTime> comparing(...)之前就无法工作。

flightAssignmentList.sort(Comparator
        .<FlightAssignment, LocalDateTime>comparing(a -> a.getFlight().getDepartureUTCDateTime())
        .thenComparing(a -> a.getFlight().getArrivalUTCDateTime())
        .thenComparing(FlightAssignment::getId));

较新的java版本具有更好的自动类型检测功能，可能不需要。

Answer 2

不幸的是，java中没有很好的语法。

如果你想重用部分比较器我可以看到两种方式：

通过编写比较器

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);

通过撰写getter函数

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);

Answer 3

npm config set msvs_version 2015 --global
npm install -g node-sass

Comparator.comparing（...）嵌套字段

3 个答案: