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Coding problem · Medium · Language Idioms

Reporting with Streams and Collectors

Medium Language practicals Most askedSeen at Amazon, Goldman Sachs, JPMorgan, Atlassian

Given record Employee(String name, String dept, double salary), implement a Reports class with static methods:

  1. Map<String, Long> countByDepartment(List<Employee> employees)
  2. Map<String, Double> averageSalaryByDepartment(List<Employee> employees)
  3. List<String> topEarners(List<Employee> employees, int n) — names of the n highest salaries, highest first
  4. Map<String, List<String>> namesByDepartment(List<Employee> employees) — names sorted alphabetically per department

Use streams and Collectors rather than manual loops.

Hints

Hint 1

Collectors.groupingBy(Employee::dept, Collectors.counting())

Hint 2

Downstream collectors: averagingDouble, mapping(..., toList()), collectingAndThen to sort.

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Solution and explanation

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Time O(n log n) where sorting is involved · Space O(n)

groupingBy(classifier, downstream) is the workhorse: the downstream collector decides what each group becomes — a count, an average, or a mapped list. collectingAndThen post-processes each group (here, sorting names). Stream.toList() (Java 16+) returns an unmodifiable list. Records give you constructors, accessors, equals/hashCode and toString for free.

Reference solution

record Employee(String name, String dept, double salary) {}

class Reports {
    static Map<String, Long> countByDepartment(List<Employee> employees) {
        return employees.stream().collect(Collectors.groupingBy(Employee::dept, Collectors.counting()));
    }

    static Map<String, Double> averageSalaryByDepartment(List<Employee> employees) {
        return employees.stream().collect(Collectors.groupingBy(Employee::dept, Collectors.averagingDouble(Employee::salary)));
    }

    static List<String> topEarners(List<Employee> employees, int n) {
        return employees.stream()
            .sorted(Comparator.comparingDouble(Employee::salary).reversed())
            .limit(n)
            .map(Employee::name)
            .toList();
    }

    static Map<String, List<String>> namesByDepartment(List<Employee> employees) {
        return employees.stream().collect(Collectors.groupingBy(
            Employee::dept,
            Collectors.collectingAndThen(
                Collectors.mapping(Employee::name, Collectors.toList()),
                names -> names.stream().sorted().toList())));
    }
}

Common mistakes

  • Using Comparator.comparing(Employee::salary) on a primitive without comparingDouble (boxing).
  • Assuming groupingBy returns a sorted map — wrap with TreeMap::new if order matters.

Follow-up questions

  • Partition by salary threshold with partitioningBy.
  • Use a TreeMap supplier for sorted departments.