Java8 Stream --groupingBy 分组讲解

本文主要讲解:Java 8 Stream之Collectors.groupingBy()分组示例

Collectors.groupingBy() 分组之常见用法

功能代码:

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/**
     * 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);
        });
    }

Collectors.groupingBy() 分组之统计每个分组的count

功能代码:

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/**
     * 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);
        });
    }

Collectors.groupingBy() 分组之统计分组平均值

功能代码:

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/**
     * 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);
        });
    }

Collectors.groupingBy() 分组之统计分组总值

功能代码:

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/**
     * 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);
        });
        // 对Map按照分组销售总值逆序排序
        Map<String, Long> sortedMap = new LinkedHashMap<>();
        map.entrySet().stream().sorted(Map.Entry.<String, Long> comparingByValue().reversed())
                .forEachOrdered(e -> sortedMap.put(e.getKey(), e.getValue()));
        sortedMap.forEach((k, v) -> {
            System.out.println(k + " = " + v);
        });
    }

Collectors.groupingBy() 分组之Join分组List

功能代码:

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/**
     * 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);
        });
    } 

Collectors.groupingBy() 分组之转换分组结果List -> List

功能代码:

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/**
     * 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);
            });
        });
    }

Collectors.groupingBy() 分组之转换分组结果List -> Set

功能代码:

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/**
     * 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);
            });
        });
    }

Collectors.groupingBy() 分组之使用对象分组List

功能代码:

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/**
     * 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);
        });
    }

Collectors.groupingBy() 分组之使用两个成员分组List

功能代码:

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/**
     * 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);
            });
        });
    }

Collectors.groupingBy() 分组之按月分组

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//根据日期字段的 yyyy-MM 进行分组
Map<String, List<SomeEntity>> monthMap = someEntityList.stream().collect(Collectors.groupingBy(p -> cn.hutool.core.date.DateUtil.format(p.getOrderTime(), "yyyy-MM")));

自定义Distinct对结果去重

功能代码

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/**
     * 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));
    }
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完整源代码:
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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);
        });
        // 对Map按照分组销售总值逆序排序
        Map<String, Long> sortedMap = new LinkedHashMap<>();
        map.entrySet().stream().sorted(Map.Entry.<String, Long> comparingByValue().reversed())
    .forEachOrdered(e -> sortedMap.put(e.getKey(), e.getValue()));
sortedMap.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();
    }
    }
}

 

posted @   jason饼干大怪兽  阅读(1760)  评论(0编辑  收藏  举报
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