// java.util.stream.Collectors 类的主要作用就是辅助进行各类有用的 reduction 操作，例如转变输出为 Collection，把 Stream 元素进行归组。

public final class Collectors

extends Object

Implementations of Collector that implement various useful reduction operations, such as accumulating elements into collections, summarizing elements according to various criteria, etc.

The following are examples of using the predefined collectors to perform common mutable reduction tasks:

     // Accumulate names into a List

     List<String> list = people.stream().map(Person::getName).collect(Collectors.toList());

     // Accumulate names into a TreeSet

     Set<String> set = people.stream().map(Person::getName).collect(Collectors.toCollection(TreeSet::new));

     // Convert elements to strings and concatenate them, separated by commas

     String joined = things.stream()

                           .map(Object::toString)

                           .collect(Collectors.joining(", "));

     // Compute sum of salaries of employee

     int total = employees.stream()

                          .collect(Collectors.summingInt(Employee::getSalary)));

     // Group employees by department

     Map<Department, List<Employee>> byDept

         = employees.stream()

                    .collect(Collectors.groupingBy(Employee::getDepartment));

     // Compute sum of salaries by department

     Map<Department, Integer> totalByDept

         = employees.stream()

                    .collect(Collectors.groupingBy(Employee::getDepartment,

                                                   Collectors.summingInt(Employee::getSalary)));

     // Partition students into passing and failing

     Map<Boolean, List<Student>> passingFailing =

         students.stream()

                 .collect(Collectors.partitioningBy(s -> s.getGrade() >= PASS_THRESHOLD));