Comparator.comparing（...）嵌套字段 [英] Comparator.comparing(...) of a nested field

问题描述

假设我有这样的域名模型：

Suppose I have a domain model like this:

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

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

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

现在我可以创建一名教师这样的比较器：

Now I can create a Teacher Comparator like this:

    return Comparator
            .comparing(Teacher::getAge);

但是如何在嵌套字段上比较Lecture，像这样？

But how do I compare Lecture's on nested fields, like this?

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

我无法添加方法 Lecture.getTeacherAge（）在模型上。

I can't add a method Lecture.getTeacherAge() on the model.

推荐答案

您不能嵌套方法引用。您可以改为使用lambda表达式：

You can't nest method references. You can use lambda expressions instead:

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

---

无需逆向订单，它的详细程度更低：

---

Without the need for reverse order it's even less verbose:

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，在比较（...）< FlightAssignment，LocalDateTime> ，下面的代码将无效>在Java 8中。

Note: in some cases you need to explicitly state the generic types. For example, the code below won't work without the <FlightAssignment, LocalDateTime> before comparing(...) in Java 8.

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。

Newer java version have better auto type detection and might not require that.

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