initialiseMap

GroupByClassifier.java

        final Collection<Employee> employees = List.of(
                new Employee("Karen Smith", 51200.0, 29, Employee.Sex.FEMALE),
                new Employee("John Smith", 24000.0, 32, Employee.Sex.MALE),
                new Employee("Anthony Jackson", 44000.0, 33, Employee.Sex.MALE),
                new Employee("Alyson Palmer", 34320.0, 36, Employee.Sex.FEMALE),
                new Employee("Jessica Sanders", 64320.0, 34, Employee.Sex.FEMALE)
        );

        final Map<Employee.Sex, List<Employee>> existingMap = new HashMap<>() {
            {
                put(Employee.Sex.MALE, new ArrayList<>() {{add(new Employee("Peter Parker", 28100.0, 33, Employee.Sex.MALE));}});
                put(Employee.Sex.FEMALE, new ArrayList<>() {{add(new Employee("Sarah Williams", 18100.0, 19, Employee.Sex.FEMALE));}});
            }
        };

        final Map<Employee.Sex, List<Employee>> employeesOverThirtyBySex = employees.stream()
                .collect(groupingBy(Employee::getSex, () -> existingMap, filtering(employee -> employee.getAge() > 30, toList())));

        System.out.println(employeesOverThirtyBySex);