# Java 8 带有 lambda 的`Comparator`示例 > 原文： [https://howtodoinjava.com/java8/comparator-example-lambda/](https://howtodoinjava.com/java8/comparator-example-lambda/) 当我们想要对可以相互比较的对象的[集合](//howtodoinjava.com/java/collections/useful-java-collection-interview-questions/ "Useful java collection interview questions")进行排序时，使用[比较器](https://docs.oracle.com/javase/10/docs/api/java/util/Comparator.html)。 也可以使用`Comparable`接口完成此比较，但是它限制了您只能以一种特定的方式比较这些对象。 如果要基于多个条件/字段对该集合进行排序，则仅需使用`Comparator`。 > **快速参考：** > > ```java > //Compare by Id > Comparator<Employee> compareById_1 = Comparator.comparing(e -> e.getId()); > > Comparator<Employee> compareById_2 = (Employee o1, Employee o2) -> o1.getId().compareTo( o2.getId() ); > > //Compare by firstname > Comparator<Employee> compareByFirstName = Comparator.comparing(e -> e.getFirstName()); > > //how to use comparator > Collections.sort(employees, compareById); > ``` ## 1）概述 为了演示该概念，我将使用具有四个属性的类`Employee`。 我们将使用它来理解各种用例。 ```java public class Employee { private Integer id; private String firstName; private String lastName; private Integer age; public Employee(Integer id, String firstName, String lastName, Integer age){ this.id = id; this.firstName = firstName; this.lastName = lastName; this.age = age; } //Other getter and setter methods @Override public String toString() { return "\n["+this.id+","+this.firstName+","+this.lastName+","+this.age+"]"; } } ``` 另外，我编写了一种方法，该方法始终以未排序的顺序返回`Employees`的列表。 ```java private static List<Employee> getEmployees(){ List<Employee> employees = new ArrayList<>(); employees.add(new Employee(6,"Yash", "Chopra", 25)); employees.add(new Employee(2,"Aman", "Sharma", 28)); employees.add(new Employee(3,"Aakash", "Yaadav", 52)); employees.add(new Employee(5,"David", "Kameron", 19)); employees.add(new Employee(4,"James", "Hedge", 72)); employees.add(new Employee(8,"Balaji", "Subbu", 88)); employees.add(new Employee(7,"Karan", "Johar", 59)); employees.add(new Employee(1,"Lokesh", "Gupta", 32)); employees.add(new Employee(9,"Vishu", "Bissi", 33)); employees.add(new Employee(10,"Lokesh", "Ramachandran", 60)); return employees; } ``` ## 2）按名字排序 基本用例，其中将根据员工的名字对员工列表进行排序。 ```java List<Employee> employees = getEmployees(); //Sort all employees by first name employees.sort(Comparator.comparing(e -> e.getFirstName())); //OR you can use below employees.sort(Comparator.comparing(Employee::getFirstName)); //Let's print the sorted list System.out.println(employees); Output: //Names are sorted by first name [ [3,Aakash,Yaadav,52], [2,Aman,Sharma,28], [8,Balaji,Subbu,88], [5,David,Kameron,19], [4,James,Hedge,72], [7,Karan,Johar,59], [1,Lokesh,Gupta,32], [10,Lokesh,Ramachandran,60], [9,Vishu,Bissi,33], [6,Yash,Chopra,25] ] ``` ## 3）按名字逆序排序 如果我们想按姓氏排序但又受尊敬的顺序怎么办。 这真的很容易； 使用`reversed()`方法。 ```java List<Employee> employees = getEmployees(); //Sort all employees by first name; And then reversed Comparator<Employee> comparator = Comparator.comparing(e -> e.getFirstName()); employees.sort(comparator.reversed()); //Let's print the sorted list System.out.println(employees); Output: //Names are sorted by first name [[6,Yash,Chopra,25], [9,Vishu,Bissi,33], [1,Lokesh,Gupta,32], [10,Lokesh,Ramachandran,60], [7,Karan,Johar,59], [4,James,Hedge,72], [5,David,Kameron,19], [8,Balaji,Subbu,88], [2,Aman,Sharma,28], [3,Aakash,Yaadav,52]] ``` ## 4）按姓氏排序 我们也可以使用类似的代码对姓氏进行排序。 ```java List<Employee> employees = getEmployees(); //Sort all employees by first name employees.sort(Comparator.comparing(e -> e.getLastName())); //OR you can use below employees.sort(Comparator.comparing(Employee::getLastName)); //Let's print the sorted list System.out.println(employees); Output: //Names are sorted by first name [[9,Vishu,Bissi,33], [6,Yash,Chopra,25], [1,Lokesh,Gupta,32], [4,James,Hedge,72], [7,Karan,Johar,59], [5,David,Kameron,19], [10,Lokesh,Ramachandran,60], [2,Aman,Sharma,28], [8,Balaji,Subbu,88], [3,Aakash,Yaadav,52]] ``` ## 5）在多个字段上排序 – `thenComparing()` 在这里，我们首先按员工的名字对他们的名单进行排序，然后再对姓氏的列表进行再次排序。 就像我们对 SQL 语句应用排序一样。 这实际上是一个非常好的特性。 现在，您无需始终对 SQL `select`语句中的多个字段使用排序，也可以在 Java 中对其进行排序。 ```java List<Employee> employees = getEmployees(); //Sorting on multiple fields; Group by. Comparator<Employee> groupByComparator = Comparator.comparing(Employee::getFirstName) .thenComparing(Employee::getLastName); employees.sort(groupByComparator); System.out.println(employees); Output: [3,Aakash,Yaadav,52], [2,Aman,Sharma,28], [8,Balaji,Subbu,88], [5,David,Kameron,19], [4,James,Hedge,72], [7,Karan,Johar,59], [1,Lokesh,Gupta,32], //These both employees are [10,Lokesh,Ramachandran,60], //sorted on last name as well [9,Vishu,Bissi,33], [6,Yash,Chopra,25] ``` ## 5）并行排序（具有多个线程） 您还可以使用多个线程并行地对对象集合进行排序。 如果集合足够大，可以容纳数千个对象，那么它将非常快。 对于少量对象，常规排序就足够了，建议使用。 ```java //Parallel Sorting Employee[] employeesArray = employees.toArray(new Employee[employees.size()]); //Parallel sorting Arrays.parallelSort(employeesArray, groupByComparator); System.out.println(employeesArray); Output: [3,Aakash,Yaadav,52], [2,Aman,Sharma,28], [8,Balaji,Subbu,88], [5,David,Kameron,19], [4,James,Hedge,72], [7,Karan,Johar,59], [1,Lokesh,Gupta,32], //These both employees are [10,Lokesh,Ramachandran,60], //sorted on last name as well [9,Vishu,Bissi,33], [6,Yash,Chopra,25] ``` 这就是使用带有`Comparator`的 lambda 对对象进行排序的全部。 如果您了解有关此概念的更多技术，请与我们所有人共享。 学习愉快！