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;
}

For each employee, this either starts a department's count at 0 (the first time it's seen) or increments the existing count — a single pass over every employee produces a complete report, no separate query or loop per department needed.

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;
}

This is Module 16's "Nested Collections" lesson — a Map<String, List<Employee>> — put to real use: every department's full employee list, generated from the same underlying data as the headcount report above, no re-querying needed.

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.