如何使用Java 8 streams生成嵌套采集的汇总报告？

import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class ReportGenerator {
    public static void main(String[] args) {
        // 假设有一个包含多个部门的员工列表
        List<Employee> employees = Arrays.asList(
                new Employee("John", "Sales", 1000),
                new Employee("Jane", "Sales", 1500),
                new Employee("Dave", "Marketing", 2000),
                new Employee("Alice", "Marketing", 2500),
                new Employee("Bob", "HR", 3000),
                new Employee("Eve", "HR", 3500)
        );

        // 使用流按部门进行分组，并计算每个部门的总工资
        Map<String, Integer> departmentSalarySum = employees.stream()
                .collect(Collectors.groupingBy(Employee::getDepartment, Collectors.summingInt(Employee::getSalary)));

        // 输出每个部门的总工资
        departmentSalarySum.forEach((department, salarySum) -> {
            System.out.println("Department: " + department);
            System.out.println("Total Salary: " + salarySum);
            System.out.println();
        });
    }

    static class Employee {
        private String name;
        private String department;
        private int salary;

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

        public String getName() {
            return name;
        }

        public String getDepartment() {
            return department;
        }

        public int getSalary() {
            return salary;
        }
    }
}