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groupingBy classifier collector

Data​

    List<Employee> employees = List.of(
            new Employee(1, "Alex", 30, new Department(1, "HR")),
            new Employee(2, "Alex", 36, new Department(1, "HR")),
            new Employee(3, "David", 30, new Department(1, "HR")),
            new Employee(4, "Andrew", 30, new Department(2, "Finance")),
            new Employee(5, "Edward", 31, new Department(2, "Finance")),
            new Employee(6, "Nathan",42, new Department(3, "ADMIN")),
            new Employee(7, "Frank",23 , new Department(3, "ADMIN")),
            new Employee(8, "David", 30, new Department(1, "HR")));

Syntax:


 groupingBy(Function<? super T,? extends K> classifier, Collector<? super T,A,D> downstream)

Examples​

groupingByCounting()​

    // counting
    public void groupingByCounting() {
        Map<Employee, Long> nameCount = employees.stream().collect(Collectors
                .groupingBy(string -> string, Collectors.counting()));

        nameCount.forEach((name, count) -> { System.out.println(name + ":" + count); });
    }