Java--java.util.stream.Collectors文档实例

// 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));