Java 8 Comparator keyExtractor [英] Java 8 Comparator keyExtractor

问题描述

在Java 8 Comparator中，我们可以按如下方式创建一个比较器。

In Java 8 Comparator, we can create a comparator as follows.

Comparator.comparing(keyExtractor);

目前我有一个课程如下

class Employee {
    String name;
    Department dept;
}

class Department {
    String departmentName;
}

现在，如果我想为Employee类创建一个比较器来对记录进行排序根据部门名称，我如何编写我的密钥提取器？

Now, if I want to create a comparator for Employee class which sorts the records based on the department name, how can I write my key extractor?

尝试下面的代码，但没有用。

Tried the below code, but did not work.

Comparator.comparing(Employee::getDept::getDepartmentName);

推荐答案

这里的诀窍是方法引用不是对象而是没有会员可以访问。所以你不能这样做：

The trick here is method references are not objects and don't have members to access. So you can't do this:

Employee::getDept.getDepartmentName

此外，方法引用不是类，因此您无法从它们获取另一个方法引用。所以这也失败了。

Moreover, method references are not classes, so you can't get another method reference from them. So this also fails.

Employee::getDept::getDepartmentName

最后，我们留下的唯一选择就是这个。

Finally, the only option that is left with us is this.

e -> e.getDept().getDepartmentName()

试试这个，

Employee empOne = new Employee("Mark", new Department("Accounts"));
Employee empTwo = new Employee("Melissa", new Department("Sales"));
List<Employee> employees = Arrays.asList(empOne, empTwo);
employees.sort(Comparator.comparing(e -> e.getDept().getDepartmentName()));
employees.forEach(System.out::println);

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