过尽千帆皆不是，斜晖脉脉水悠悠，肠断白频洲。
当我们要对可以相互比较的对象集合进行排序时，使用比较器。也可以使用Comparable接口完成此比较，但是它限制了你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，则仅需使用比较器。
//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);
概述
为了演示这个概念，我使用具有四个属性的Employee类。我们将使用它来理解各种用例。
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，我还写了一个方法，它总是以未排序的顺序返回雇员列表
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;
}
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。
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] ]
按名字排序-倒序
如果我们想按名字排序但顺序相反该怎么办。这真的很容易；使用reverse（）方法。
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]]
按姓氏排序
我们也可以使用类似的代码对姓氏进行排序。
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]]
在多个字段上排序– thenComparing（）
在这里，我们首先按员工的名字对员工列表进行排序，然后对姓氏列表进行再次排序。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功能。
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。
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]
并行排序（多个线程）
您也可以使用多个线程并行排序对象集合。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。
//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]
这就是将lambda与Comparator结合使用来对对象进行排序的原因。
原文链接：Java Comparator with Lambda