Java Streams: group a list by field

Group a list of employees by department using Collectors.groupingBy.

Code

Map<String, List<Employee>> byDept = employees.stream()
        .collect(Collectors.groupingBy(Employee::getDepartment));
byDept.forEach((dept, list) ->
        System.out.println(dept + " -> " + list.size()));

Output

Engineering -> 12
Sales -> 7
HR -> 3

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Published 2026-07-25