Generate Reports from Collections

public Map<String, Integer> getHeadcountByDepartment() {
    Map<String, Integer> headcount = new Map<String, Integer>();

    for (Employee emp : employeesById.values()) {
        String dept = emp.getDepartment();
        if (! headcount.containsKey(dept)) {
            headcount.put(dept, 0);
        }
        headcount.put(dept, headcount.get(dept) + 1);
    }

    return headcount;
}

1. public Map<String, Integer> getHeadcountByDepartment() { — returns a count per department.
2. Map<String, Integer> headcount = new Map<String, Integer>(); — starts empty.
3. (blank line)
4. for (Employee emp : employeesById.values()) { — loops over every employee.
5. String dept = emp.getDepartment(); — reads their department.
6. if (! headcount.containsKey(dept)) { — the first time a department is seen...
7. headcount.put(dept, 0); — ...its count starts at 0.
8. } — closes the if.
9. headcount.put(dept, headcount.get(dept) + 1); — increments the existing count either way.
10. } — closes the loop.
11. (blank line)
12. return headcount; — a single pass over every employee produces a complete report, no separate query or loop per department needed.
13. } — closes the method.

public Map<String, List<Employee>> getEmployeesByDepartment() {
    Map<String, List<Employee>> grouped = new Map<String, List<Employee>>();

    for (Employee emp : employeesById.values()) {
        String dept = emp.getDepartment();
        if (! grouped.containsKey(dept)) {
            grouped.put(dept, new List<Employee>());
        }
        grouped.get(dept).add(emp);
    }

    return grouped;
}

1. public Map<String, List<Employee>> getEmployeesByDepartment() { — this is Module 16's "Nested Collections" lesson — a Map<String, List<Employee>> — put to real use.
2. Map<String, List<Employee>> grouped = new Map<String, List<Employee>>(); — starts empty.
3. (blank line)
4. for (Employee emp : employeesById.values()) { — loops over every employee.
5. String dept = emp.getDepartment(); — reads their department.
6. if (! grouped.containsKey(dept)) { — the first time a department is seen...
7. grouped.put(dept, new List<Employee>()); — ...creates an empty list for it.
8. } — closes the if.
9. grouped.get(dept).add(emp); — adds this employee to their department's list.
10. } — closes the loop.
11. (blank line)
12. return grouped; — every department's full employee list, generated from the same underlying data as the headcount report above, no re-querying needed.
13. } — closes the method.

Both reports loop over employeesById.values() once and build their result in memory — no second database query, no re-fetching data that's already sitting in the directory. This mirrors the bulkification instinct from Module 7: do the necessary work in one clean pass over data you already have, rather than repeating expensive operations per report.