Collection 需要排序的时候可以使用Comparator和Comparable实现。在Java 8中，同样可以使用Comparator对Stream进行排序，本节中将使用Employee作为示例
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+"]";
}
}获取Employee的一个列表
private static List<Employee> getEmployees(){
List<Employee> employees = new ArrayList<>();
employees.add(new Employee(1,"Lokesh", "Gupta", 32));
employees.add(new Employee(2,"Aman", "Sharma", 28));
employees.add(new Employee(3,"Aakash", "Yaadav", 52));
employees.add(new Employee(4,"James", "Hedge", 72));
employees.add(new Employee(5,"David", "Kameron", 19));
employees.add(new Employee(6,"Yash", "Chopra", 25));
employees.add(new Employee(7,"Karan", "Johar", 59));
employees.add(new Employee(8,"Balaji", "Subbu", 88));
employees.add(new Employee(9,"Vishu", "Bissi", 33));
employees.add(new Employee(10,"Lokesh", "Ramachandran", 60));
return employees;
}根据firstName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);输出
[[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]]排序反转
使用Comparator.reversed()可以反转一个排序
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);输出
[[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]]通过lastName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);输出
[[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]]先根据firstName排序，再根据lastName排序
使用thenComparing()
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);并行排序
只需要使用 parallelSort() 即可进行并行排序。
//Parallel Sorting
Employee[] employeesArray = employees.toArray(new Employee[employees.size()]);
//Parallel sorting
Arrays.parallelSort(employeesArray, groupByComparator);
System.out.println(employeesArray);输出
[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]使用count() 和 counting() 统计数量
count()和counting()的区别是，count()是一个终端操作，对Stream计数并返回一个long，counting() 则是Collector的收集方法,用法区别如下：
使用count()
public static void main(String[] args)
{
long count = Stream.of("how","to","do","in","java").count();
System.out.printf("There are %d elements in the stream %n", count);
count = IntStream.of(1,2,3,4,5,6,7,8,9).count();
System.out.printf("There are %d elements in the stream %n", count);
count = LongStream.of(1,2,3,4,5,6,7,8,9).filter(i -> i%2 == 0).count();
System.out.printf("There are %d elements in the stream %n", count);
}使用counting()
public static void main(String[] args)
{
long count = Stream.of("how","to","do","in","java").collect(Collectors.counting());
System.out.printf("There are %d elements in the stream %n", count);
count = Stream.of(1,2,3,4,5,6,7,8,9).collect(Collectors.counting());
System.out.printf("There are %d elements in the stream %n", count);
count = Stream.of(1,2,3,4,5,6,7,8,9).filter(i -> i%2 == 0).collect(Collectors.counting());
System.out.printf("There are %d elements in the stream %n", count);
}