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;}@Overridepublic 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;}@Overridepublic String toString() {return String.format("Person{name='%s', age=%d}", name, age);}}// Jerry -01-15 20:51PM ? 多用于extends generic的type，接受所有Object的sub classpublic 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<Employee> employees = new ArrayList<Employee>();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 CityMap<String, List<Employee>> employeesByCity =employees.stream().collect( Collectors.groupingBy(Employee::getCity));// default void forEach(Consumer<? super T> action) {for(Map.Entry<String, List<Employee>> entry:employeesByCity.entrySet()) {System.out.println("key= " + entry.getKey() + " , Value = " + entry.getValue());entry.getValue().forEach(System.out::println);}// -01-15 20:37PM Consumer<Employee> aa = a -> {System.out.println("Employee: " + a.getName() + " : " + a.getScore()); };List<Employee> chengduEmployee = employeesByCity.get("Chengdu");chengduEmployee.forEach(aa);// test for countingMap<String, Long> employeesByCity2 = employees.stream().collect( Collectors.groupingBy(Employee::getCity, Collectors.counting()));printMap(employeesByCity2);// calculate average scoreMap<String, Double> employeesByCity3 = employees.stream().collect( Collectors.groupingBy(Employee::getCity,Collectors.averagingInt(Employee::getScore)));printMap(employeesByCity3);/*Stream<Person> people = Stream.of(new Person("Paul", 24), new Person("Mark", 30), new Person("Will", 28));Map<Integer, List<String>> 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<Boolean, List<Employee>> 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<Boolean, Map<String, Long>> 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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