简单分组

• 按照年龄对 Person 对象进行分组：

1. 代码示例

import java.util.*;
import java.util.stream.Collectors;public class SimpleGrouping {public static void main(String[] args) {List<Person> people = Arrays.asList(new Person("Alice", 30),new Person("Bob", 25),new Person("Charlie", 30),new Person("David", 25),new Person("Eve", 35));// 通过对age字段进行分组，得到相同年龄的people对象listMap<Integer, List<Person>> groupedByAge = people.stream().collect(Collectors.groupingBy(Person::getAge));groupedByAge.forEach((age, personList) -> {System.out.println("Age: " + age);personList.forEach(person -> System.out.println(person));});}
}

2. 输出结果

Age: 25
Person{name='Bob', age=25}
Person{name='David', age=25}
Age: 30
Person{name='Alice', age=30}
Person{name='Charlie', age=30}
Age: 35
Person{name='Eve', age=35}

分组并统计

• 按年龄分组并统计每个年龄段的人数：

1. 代码示例

import java.util.*;
import java.util.stream.Collectors;public class GroupingAndCounting {public static void main(String[] args) {List<Person> people = Arrays.asList(new Person("Alice", 30),new Person("Bob", 25),new Person("Charlie", 30),new Person("David", 25),new Person("Eve", 35));// 通过对年龄字段进行分组，然后对分组后的数据进行统计Map<Integer, Long> countByAge = people.stream().collect(Collectors.groupingBy(Person::getAge,Collectors.counting()));countByAge.forEach((age, count) -> {System.out.println("Age: " + age + ", Count: " + count);});}
}

2. 数据结果

Age: 25, Count: 2
Age: 30, Count: 2
Age: 35, Count: 1

分组并进行数据转换

• 按年龄分组，并将每组中的 Person 对象转换为姓名列表：

1. 代码示例

import java.util.*;
import java.util.stream.Collectors;public class GroupingAndMapping {public static void main(String[] args) {List<Person> people = Arrays.asList(new Person("Alice", 30),new Person("Bob", 25),new Person("Charlie", 30),new Person("David", 25),new Person("Eve", 35));Map<Integer, List<String>> namesByAge = people.stream().collect(Collectors.groupingBy(Person::getAge,Collectors.mapping(Person::getName, Collectors.toList())));namesByAge.forEach((age, names) -> {System.out.println("Age: " + age + ", Names: " + names);});}
}

2. 输出结果

Age: 25, Names: [Bob, David]
Age: 30, Names: [Alice, Charlie]
Age: 35, Names: [Eve]

多级分组

• 先按年龄分组，再按姓名的首字母进行分组：

1. 示例代码

import java.util.*;
import java.util.stream.Collectors;public class MultiLevelGrouping {public static void main(String[] args) {List<Person> people = Arrays.asList(new Person("Alice", 30),new Person("Bob", 25),new Person("Charlie", 30),new Person("David", 25),new Person("Eve", 35));Map<Integer, Map<Character, List<Person>>> multiLevelGrouping = people.stream().collect(Collectors.groupingBy(Person::getAge,Collectors.groupingBy(person -> person.getName().charAt(0))));multiLevelGrouping.forEach((age, group) -> {System.out.println("Age: " + age);group.forEach((initial, persons) -> {System.out.println("  Initial: " + initial);persons.forEach(person -> System.out.println("    " + person));});});}
}

2. 输出结果

Age: 25Initial: BPerson{name='Bob', age=25}Initial: DPerson{name='David', age=25}
Age: 30Initial: APerson{name='Alice', age=30}Initial: CPerson{name='Charlie', age=30}
Age: 35Initial: EPerson{name='Eve', age=35}

分组并收集到不同的集合

• 按年龄分组，并将每组中的 Person 对象收集到 Set 中：

1. 代码示例

import java.util.*;
import java.util.stream.Collectors;public class GroupingToSet {public static void main(String[] args) {List<Person> people = Arrays.asList(new Person("Alice", 30),new Person("Bob", 25),new Person("Charlie", 30),new Person("David", 25),new Person("Eve", 35));Map<Integer, Set<Person>> groupedByAgeToSet = people.stream().collect(Collectors.groupingBy(Person::getAge,Collectors.toSet()));groupedByAgeToSet.forEach((age, personSet) -> {System.out.println("Age: " + age);personSet.forEach(person -> System.out.println(person));});}
}

2. 输出结果

Age: 25
Person{name='Bob', age=25}
Person{name='David', age=25}
Age: 30
Person{name='Alice', age=30}
Person{name='Charlie', age=30}
Age: 35
Person{name='Eve', age=35}

自定义收集器

• 按年龄分组，并计算每组的平均年龄：

1. 代码示例

import java.util.*;
import java.util.stream.Collectors;public class GroupingWithCustomCollector {public static void main(String[] args) {List<Person> people = Arrays.asList(new Person("Alice", 30),new Person("Bob", 25),new Person("Charlie", 30),new Person("David", 25),new Person("Eve", 35));Map<Integer, Double> averageAgeByGroup = people.stream().collect(Collectors.groupingBy(Person::getAge,Collectors.averagingInt(Person::getAge)));averageAgeByGroup.forEach((age, avgAge) -> {System.out.println("Age: " + age + ", Average Age: " + avgAge);});}
}

2. 输出结果

Age: 25, Average Age: 25.0
Age: 30, Average Age: 30.0
Age: 35, Average Age: 35.0