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:
/*** 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:
/*** 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:
/*** 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:
/*** 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:
/*** 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:
/*** 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:
/*** 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:
/*** 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:
/*** 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);});});}
自定义Distinct对结果去重
功能代码
/*** 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));}
完整源代码：
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 stubJava8GroupBy 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();}}}
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