JAVA8系列教程-基于lambda的Java 8 Comparator示例

Comparator用于我们要排序并且可相互比较的对象集合。也可以使用Comparableinterface 进行此比较，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Comparator仅需使用。

快速参考：

//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

为了演示这个概念，我使用Employee具有四个属性的类。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+"]"; 
    }
}

此外，我编写了一种方法，该方法始终返回Employees未排序顺序的列表。

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;
}

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。

	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）按名字排序-“逆序”

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。

	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。

	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，我们先按员工的名字对员工列表进行排序，然后再对姓氏列表进行排序。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，您不必始终对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]

5）并行排序（具有多个线程）

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]

这就是使用lambda和Comparator来对对象进行排序的全部内容。如果您了解有关此概念的更多技术，请与我们所有人共享。