本文完整测试代码见文末。

测试数据是List里的4个员工对象实例：

根据员工所在的城市进行分组：

结果分成了三组：

第一组的员工在上海：

第二组的员工在成都：

统计每组员工个数：

把员工进行分组，得分大于101分的在一组，小于等于101的在另一组：

分组结果：

package java8;

import java.util.ArrayList;

import java.util.List;

import java.util.Map;

import java.util.Map.Entry;

import java.util.function.Consumer;

import java.util.stream.Collectors;

class Employee {

private String city;

private String name;

private int score;

public Employee(String name, String city, int score){

this.city = city;

this.name = name;

this.score = score;

}

public String getCity(){

System.out.println("city: " + this.city);

return this.city;

}

public String getName() {

return this.name;

}

public int getScore() {

return this.score;

}

@Override

public String toString() {

return String.format("Employee: " + this.name + " city: " + this.city);

}

}

class Person {

private String name;

private int age;

Person(String name, int age) {

this.name = name;

this.age = age;

}

@Override

public String toString() {

return String.format("Person{name='%s', age=%d}", name, age);

}

}

// Jerry 2016-01-15 20:51PM ? 多用于extends generic的type，接受所有Object的sub class

public class StreamTest {

private static void printMap(Map extends Object, ? extends Object> map) {

for(Entry extends Object, ? extends Object> entry:map.entrySet()) {

System.out.println("key = " + entry.getKey() + " , Value = " + entry.getValue());

}

}

public static void main(String[] args) {

ArrayList employees = new ArrayList();

employees.add(new Employee("A", "Shanghai",100));

employees.add(new Employee("B", "Chengdu",101));

employees.add(new Employee("C", "Shenzhen",102));

employees.add(new Employee("D", "Chengdu",104));

// group by City

Map> employeesByCity =

employees.stream().collect( Collectors.groupingBy(Employee::getCity));

// default void forEach(Consumer super T> action) {

for(Map.Entry> entry:employeesByCity.entrySet()) {

System.out.println("key= " + entry.getKey() + " , Value = " + entry.getValue());

entry.getValue().forEach(System.out::println);

}

// 2016-01-15 20:37PM

Consumer aa = a -> { System.out.println("Employee: " + a.getName() + " : " + a.getScore()); };

List chengduEmployee = employeesByCity.get("Chengdu");

chengduEmployee.forEach(aa);

// test for counting

Map employeesByCity2 =

employees.stream().collect( Collectors.groupingBy(Employee::getCity, Collectors.counting()));

printMap(employeesByCity2);

// calculate average score

Map employeesByCity3 =

employees.stream().collect( Collectors.groupingBy(Employee::getCity,

Collectors.averagingInt(Employee::getScore)));

printMap(employeesByCity3);

/*Stream people = Stream.of(new Person("Paul", 24), new Person("Mark", 30), new Person("Will", 28));

Map> peopleByAge = people.collect(groupingBy(p -> p.age, mapping((Person p) -> p.name, toList())));

System.out.println(peopleByAge);*/

/*

* 分区是一种特殊的分组，结果 map 至少包含两个不同的分组——一个true，一个false。

* 例如，如果想找出最优秀的员工，你可以将所有雇员分为两组，一组销售量大于 N，

* 另一组小于 N，使用 partitioningBy 收集器：

*/

System.out.println("partition result");

Map> partitioned =

employees.stream().collect(Collectors.partitioningBy(e -> e.getScore() > 101));

printMap(partitioned);

/*

* 你也可以将 groupingBy 收集器传递给 partitioningBy 收集器来将联合使用分区和分组。例如，你可以统计每个分区中的每个城市的雇员人数：

Map> result =

employees.stream().collect(partitioningBy(e -> e.getNumSales() > 150,

groupingBy(Employee::getCity, counting())));

这样会生成一个二级 Map:

{false={London=1}, true={New York=1, Hong Kong=1, London=1}}

*/

}

}

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