Java 中比较器的问题

Question

我有一个员工类，如下所示

public class Employee {
private String name;
private String jobTitle;
private int age;
private int salary;
public Employee(String name, String jobTitle, int age, int salary) {
this.name = name;
this.jobTitle = jobTitle;
this.age = age;
this.salary = salary;
}

然后我有一个 Util 类来对员工进行排序，例如

public class EmployeeComparator {
private EmployeeComparator(){}
public static List<Employee> sortEmployees(List<Employee> employeeList, String secondarySortField) {
Comparator<Employee> compareByName = Comparator
.comparing(Employee::getName).thenComparing(Employee::getAge);
List<Employee> sortedEmployees = employeeList.stream()
.sorted(compareByName)
.collect(Collectors.toList());
return sortedEmployees;
}
}

这将首先根据员工的姓名对员工进行排序，然后根据年龄对员工进行排序，并且工作正常。现在我希望辅助排序是动态的，所以我做了一个更改，比如

public static List<Employee> sortEmployees(List<Employee> employeeList, String secondarySortField) {
Comparator<Employee> compareByName = Comparator
.comparing(Employee::getName);

if(secondarySortField.equals("age")){
compareByName.thenComparing(Employee::getAge);
}
List<Employee> sortedEmployees = employeeList.stream()
.sorted(compareByName)
.collect(Collectors.toList());
return sortedEmployees;
}

现在年龄排序不起作用，知道为什么会发生这种情况吗？

Answer 1

compareByName.thenComparing()返回一个新的Comparator，你不保留它的引用，所以你永远不会使用该Comparator。

如果应该是：

if(secondarySortField.equals("age")){
compareByName = compareByName.thenComparing(Employee::getAge);
}

Answer 2

同时使用第二个比较器(compareByName.thenComparing(Employee::getAge)(：

public static List<Employee> sortEmployees(List<Employee> employeeList, String secondarySortField) {
Comparator<Employee> dyanmicComparator = Comparator
.comparing(Employee::getName);

if(secondarySortField.equals("age")){
dyanmicComparator = dyanmicComparator.thenComparing(Employee::getAge); // store the new comparator
}
List<Employee> sortedEmployees = employeeList.stream()
.sorted(dyanmicComparator)
.collect(Collectors.toList());
return sortedEmployees;
}