java8分组聚合

package com.stream;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.Random;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.stream.Collectors;

/**
* Java 8 Stream 之groupingBy 分组讲解
*
* @author zzg
*
*/
public class Java8GroupBy {

List<Employee> employees = new ArrayList<Employee>();

/**
* 数据初始化
*/
public void init() {
List<String> citys = Arrays.asList("湖南", "湖北", "江西", "广西 ");
for (int i = 0; i < 10; i++) {
Random random = new Random();
Integer index = random.nextInt(4);
Employee employee = new Employee(citys.get(index), "姓名" + i, (random.nextInt(4) * 10 - random.nextInt(4)),
(random.nextInt(4) * 1000 - random.nextInt(4)));
employees.add(employee);
}
}

/**
* 使用java8 stream groupingBy操作,按城市分组list
*/
public void groupingByCity() {
Map<String, List<Employee>> map = employees.stream().collect(Collectors.groupingBy(Employee::getCity));

map.forEach((k, v) -> {
System.out.println(k + " = " + v);
});
}

/**
* 使用java8 stream groupingBy操作,按城市分组list统计count
*/
public void groupingByCount() {
Map<String, Long> map = employees.stream()
.collect(Collectors.groupingBy(Employee::getCity, Collectors.counting()));

map.forEach((k, v) -> {
System.out.println(k + " = " + v);
});
}

/**
* 使用java8 stream groupingBy操作,按城市分组list并计算分组年龄平均值
*/
public void groupingByAverage() {
Map<String, Double> map = employees.stream()
.collect(Collectors.groupingBy(Employee::getCity, Collectors.averagingInt(Employee::getAge)));

map.forEach((k, v) -> {
System.out.println(k + " = " + v);
});
}

/**
* 使用java8 stream groupingBy操作,按城市分组list并计算分组销售总值
*/
public void groupingBySum() {
Map<String, Long> map = employees.stream()
.collect(Collectors.groupingBy(Employee::getCity, Collectors.summingLong(Employee::getAmount)));

map.forEach((k, v) -> {
System.out.println(k + " = " + v);
});

}

/**
* 使用java8 stream groupingBy操作,按城市分组list并通过join操作连接分组list中的对象的name 属性使用逗号分隔
*/
public void groupingByString() {
Map<String, String> map = employees.stream().collect(Collectors.groupingBy(Employee::getCity,
Collectors.mapping(Employee::getName, Collectors.joining(", ", "Names: [", "]"))));

map.forEach((k, v) -> {
System.out.println(k + " = " + v);
});
}

/**
* 使用java8 stream groupingBy操作,按城市分组list,将List转化为name的List
*/
public void groupingByList() {
Map<String, List<String>> map = employees.stream().collect(
Collectors.groupingBy(Employee::getCity, Collectors.mapping(Employee::getName, Collectors.toList())));

map.forEach((k, v) -> {
System.out.println(k + " = " + v);
v.stream().forEach(item -> {
System.out.println("item = " + item);
});
});
}

/**
* 使用java8 stream groupingBy操作,按城市分组list,将List转化为name的Set
*/
public void groupingBySet() {
Map<String, Set<String>> map = employees.stream().collect(
Collectors.groupingBy(Employee::getCity, Collectors.mapping(Employee::getName, Collectors.toSet())));

map.forEach((k, v) -> {
System.out.println(k + " = " + v);
v.stream().forEach(item -> {
System.out.println("item = " + item);
});
});
}

/**
* 使用java8 stream groupingBy操作,通过Object对象的成员分组List
*/
public void groupingByObject() {
Map<Manage, List<Employee>> map = employees.stream().collect(Collectors.groupingBy(item -> {
return new Manage(item.getName());
}));

map.forEach((k, v) -> {
System.out.println(k + " = " + v);
});
}

/**
* 使用java8 stream groupingBy操作, 基于city 和name 实现多次分组
*/
public void groupingBys() {
Map<String, Map<String, List<Employee>>> map = employees.stream()
.collect(Collectors.groupingBy(Employee::getCity, Collectors.groupingBy(Employee::getName)));

map.forEach((k, v) -> {
System.out.println(k + " = " + v);
v.forEach((i, j) -> {
System.out.println(i + " = " + j);
});
});
}

/**
* 使用java8 stream groupingBy操作, 基于Distinct 去重数据
*/
public void groupingByDistinct() {
List<Employee> list = employees.stream().filter(distinctByKey(Employee :: getCity))
.collect(Collectors.toList());;

list.stream().forEach(item->{
System.out.println("city = " + item.getCity());
});


}

/**
* 自定义重复key 规则
* @param keyExtractor
* @return
*/
private static <T> Predicate<T> distinctByKey(Function<? super T, ?> keyExtractor) {
Set<Object> seen = ConcurrentHashMap.newKeySet();
return t -> seen.add(keyExtractor.apply(t));
}

public static void main(String[] args) {
// TODO Auto-generated method stub
Java8GroupBy instance = new Java8GroupBy();
instance.init();
instance.groupingByCity();
instance.groupingByCount();
instance.groupingByAverage();
instance.groupingBySum();
instance.groupingByString();
instance.groupingByList();
instance.groupingBySet();
instance.groupingByObject();
instance.groupingBys();
instance.groupingByDistinct();

}

class Employee {
private String city;
private String name;
private Integer age;
private Integer amount;

public String getCity() {
return city;
}

public void setCity(String city) {
this.city = city;
}

public String getName() {
return name;
}

public void setName(String name) {
this.name = name;
}

public Integer getAge() {
return age;
}

public void setAge(Integer age) {
this.age = age;
}

public Integer getAmount() {
return amount;
}

public void setAmount(Integer amount) {
this.amount = amount;
}

public Employee(String city, String name, Integer age, Integer amount) {
super();
this.city = city;
this.name = name;
this.age = age;
this.amount = amount;
}

public Employee() {
super();
}
}

class Manage {
private String name;

public String getName() {
return name;
}

public void setName(String name) {
this.name = name;
}

public Manage(String name) {
super();
this.name = name;
}

public Manage() {
super();
}
}

}

标签：Collectors,聚合,stream,groupingBy,public,分组,Employee,java8,name
来源： https://www.cnblogs.com/wangdonghua/p/16143194.html