Java中如何通过分组收集集合中的数据

import java.util.*;
import java.util.stream.Collectors;

class Employee {
    private String name;
    private String department;

    public Employee(String name, String department) {
        this.name = name;
        this.department = department;
    }

    // getter 方法
    public String getDepartment() { return department; }
    public String getName() { return name; }
}

public class GroupExample {
    public static void main(String[] args) {
        List<Employee> employees = Arrays.asList(
            new Employee("Alice", "HR"),
            new Employee("Bob", "IT"),
            new Employee("Charlie", "IT"),
            new Employee("David", "HR")
        );

        Map<String, List<Employee>> grouped = employees.stream()
            .collect(Collectors.groupingBy(Employee::getDepartment));

        grouped.forEach((dept, empList) -> {
            System.out.println(dept + ": " + empList.size() + " 人");
        });
    }
}

Map<String, Long> countByDept = employees.stream()
    .collect(Collectors.groupingBy(Employee::getDepartment, Collectors.counting()));

// 输出：{HR=2, IT=2}

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Map<String, Map<Integer, List<Employee>>> nestedGroup = employees.stream()
    .collect(Collectors.groupingBy(
        Employee::getDepartment,
        Collectors.groupingBy(emp -> emp.getName().length())
    ));

Map<Character, List<Employee>> groupByFirstLetter = employees.stream()
    .collect(Collectors.groupingBy(emp -> emp.getName().charAt(0)));

// 输出如：A=[Alice], B=[Bob], C=[Charlie], D=[David]