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}}
*/
}
}
要获取更多Jerry的原创文章，请关注公众号"汪子熙":
