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Add examples of groupingBy Stream API usage
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‎src/main/java/by/andd3dfx/stream/GroupingByExamples.java‎

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package by.andd3dfx.stream;
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import lombok.AllArgsConstructor;
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import lombok.Data;
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import java.util.List;
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import java.util.Map;
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import java.util.Set;
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import java.util.stream.Collectors;
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/**
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* Examples of groupingBy Stream API usage
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* <p>
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* Based on <a href="https://habr.com/ru/articles/348536/">article</a>
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*/
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public class GroupingByExamples {
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@Data
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@AllArgsConstructor
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public static class Worker {
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private String name;
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private int age;
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private int salary;
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private String position;
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}
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/**
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* 1. Группировка списка рабочих по их должности (деление на списки)
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*/
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public static Map<String, List<Worker>> groupWorkersByPositionLists(List<Worker> workers) {
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return workers.stream()
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.collect(Collectors.groupingBy(Worker::getPosition));
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}
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/**
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* 2. Группировка списка рабочих по их должности (деление на множества)
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*/
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public static Map<String, Set<Worker>> groupWorkersByPositionSets(List<Worker> workers) {
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return workers.stream().collect(
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Collectors.groupingBy(Worker::getPosition, Collectors.toSet())
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);
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}
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/**
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* 3. Подсчет количества рабочих, занимаемых конкретную должность
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*/
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public static Map<String, Long> calculateAmountOfWorkersForEachPosition(List<Worker> workers) {
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return workers.stream().collect(
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Collectors.groupingBy(Worker::getPosition, Collectors.counting())
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);
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}
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/**
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* 4. Группировка списка рабочих по их должности, при этом нас интересуют только имена
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*/
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public static Map<String, Set<String>> groupWorkersNamesByPosition(List<Worker> workers) {
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return workers.stream().collect(
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Collectors.groupingBy(Worker::getPosition, Collectors.mapping(Worker::getName, Collectors.toSet()))
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);
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}
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/**
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* 5. Расчет средней зарплаты для данной должности
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*/
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public static Map<String, Double> calculateAverageSalaryForEachPosition(List<Worker> workers) {
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return workers.stream().collect(
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Collectors.groupingBy(Worker::getPosition, Collectors.averagingInt(Worker::getSalary))
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);
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}
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/**
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* 6. Группировка списка рабочих по их должности, рабочие представлены только именами единой строкой
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*/
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public static Map<String, String> groupWorkersByPositionWhenWorkerRepresentedByString(List<Worker> workers) {
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return workers.stream().collect(
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Collectors.groupingBy(
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Worker::getPosition, Collectors.mapping(Worker::getName, Collectors.joining(", ", "{", "}"))
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));
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}
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/**
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* 7. Группировка списка рабочих по их должности и по возрасту.
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*/
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public static Map<String, Map<Integer, List<Worker>>> groupWorkersByPositionAndAge(List<Worker> workers) {
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return workers.stream().collect(
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Collectors.groupingBy(Worker::getPosition, Collectors.groupingBy(Worker::getAge))
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);
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}
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}

‎src/test/java/by/andd3dfx/stream/GroupingByExamplesTest.java‎

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package by.andd3dfx.stream;
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import org.junit.Test;
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import java.util.List;
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import java.util.Map;
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import java.util.Set;
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import static org.assertj.core.api.Assertions.assertThat;
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public class GroupingByExamplesTest {
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private static final GroupingByExamples.Worker WORKER1 = new GroupingByExamples.Worker("Иван", 30, 50000, "Разработчик");
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private static final GroupingByExamples.Worker WORKER2 = new GroupingByExamples.Worker("Петр", 25, 45000, "Разработчик");
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private static final GroupingByExamples.Worker WORKER3 = new GroupingByExamples.Worker("Мария", 28, 60000, "Менеджер");
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private static final GroupingByExamples.Worker WORKER4 = new GroupingByExamples.Worker("Анна", 32, 55000, "Менеджер");
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private static final GroupingByExamples.Worker WORKER5 = new GroupingByExamples.Worker("Сергей", 27, 40000, "Тестировщик");
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private static final GroupingByExamples.Worker WORKER6 = new GroupingByExamples.Worker("Иван", 35, 70000, "Разработчик");
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private List<GroupingByExamples.Worker> buildWorkersList() {
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return List.of(WORKER1, WORKER2, WORKER3, WORKER4, WORKER5, WORKER6);
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}
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@Test
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public void groupWorkersByPositionLists() {
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List<GroupingByExamples.Worker> workers = buildWorkersList();
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Map<String, List<GroupingByExamples.Worker>> result = GroupingByExamples.groupWorkersByPositionLists(workers);
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assertThat(result).hasSize(3);
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assertThat(result.get("Разработчик")).hasSize(3);
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assertThat(result.get("Разработчик")).containsExactlyInAnyOrder(WORKER1, WORKER2, WORKER6);
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assertThat(result.get("Менеджер")).hasSize(2);
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assertThat(result.get("Менеджер")).containsExactlyInAnyOrder(WORKER3, WORKER4);
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assertThat(result.get("Тестировщик")).hasSize(1);
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assertThat(result.get("Тестировщик")).containsExactly(WORKER5);
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}
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@Test
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public void groupWorkersByPositionListsForEmptyList() {
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List<GroupingByExamples.Worker> workers = List.of();
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Map<String, List<GroupingByExamples.Worker>> result = GroupingByExamples.groupWorkersByPositionLists(workers);
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assertThat(result).isEmpty();
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}
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@Test
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public void groupWorkersByPositionSets() {
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List<GroupingByExamples.Worker> workers = buildWorkersList();
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Map<String, Set<GroupingByExamples.Worker>> result = GroupingByExamples.groupWorkersByPositionSets(workers);
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assertThat(result).hasSize(3);
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assertThat(result.get("Разработчик")).hasSize(3);
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assertThat(result.get("Разработчик")).containsExactlyInAnyOrder(WORKER1, WORKER2, WORKER6);
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assertThat(result.get("Менеджер")).hasSize(2);
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assertThat(result.get("Менеджер")).containsExactlyInAnyOrder(WORKER3, WORKER4);
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assertThat(result.get("Тестировщик")).hasSize(1);
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assertThat(result.get("Тестировщик")).containsExactly(WORKER5);
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}
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@Test
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public void groupWorkersByPositionSetsForEmptyList() {
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List<GroupingByExamples.Worker> workers = List.of();
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Map<String, Set<GroupingByExamples.Worker>> result = GroupingByExamples.groupWorkersByPositionSets(workers);
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assertThat(result).isEmpty();
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}
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@Test
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public void calculateAmountOfWorkersForEachPosition() {
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List<GroupingByExamples.Worker> workers = buildWorkersList();
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Map<String, Long> result = GroupingByExamples.calculateAmountOfWorkersForEachPosition(workers);
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assertThat(result).hasSize(3);
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assertThat(result.get("Разработчик")).isEqualTo(3L);
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assertThat(result.get("Менеджер")).isEqualTo(2L);
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assertThat(result.get("Тестировщик")).isEqualTo(1L);
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}
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@Test
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public void calculateAmountOfWorkersForEachPositionForEmptyList() {
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List<GroupingByExamples.Worker> workers = List.of();
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Map<String, Long> result = GroupingByExamples.calculateAmountOfWorkersForEachPosition(workers);
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assertThat(result).isEmpty();
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}
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@Test
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public void groupWorkersNamesByPosition() {
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List<GroupingByExamples.Worker> workers = buildWorkersList();
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Map<String, Set<String>> result = GroupingByExamples.groupWorkersNamesByPosition(workers);
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assertThat(result).hasSize(3);
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// Set не содержит дубликатов, поэтому два "Иван" становятся одним
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assertThat(result.get("Разработчик")).hasSize(2);
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assertThat(result.get("Разработчик")).containsExactlyInAnyOrder("Иван", "Петр");
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assertThat(result.get("Менеджер")).hasSize(2);
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assertThat(result.get("Менеджер")).containsExactlyInAnyOrder("Мария", "Анна");
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assertThat(result.get("Тестировщик")).hasSize(1);
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assertThat(result.get("Тестировщик")).containsExactly("Сергей");
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}
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@Test
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public void groupWorkersNamesByPositionForEmptyList() {
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List<GroupingByExamples.Worker> workers = List.of();
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Map<String, Set<String>> result = GroupingByExamples.groupWorkersNamesByPosition(workers);
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assertThat(result).isEmpty();
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}
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@Test
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public void calculateAverageSalaryForEachPosition() {
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List<GroupingByExamples.Worker> workers = buildWorkersList();
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Map<String, Double> result = GroupingByExamples.calculateAverageSalaryForEachPosition(workers);
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assertThat(result).hasSize(3);
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// Разработчики: (50000 + 45000 + 70000) / 3 = 55000.0
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assertThat(result.get("Разработчик")).isEqualTo(55000.0);
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// Менеджеры: (60000 + 55000) / 2 = 57500.0
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assertThat(result.get("Менеджер")).isEqualTo(57500.0);
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// Тестировщик: 40000 / 1 = 40000.0
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assertThat(result.get("Тестировщик")).isEqualTo(40000.0);
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}
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@Test
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public void calculateAverageSalaryForEachPositionForEmptyList() {
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List<GroupingByExamples.Worker> workers = List.of();
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Map<String, Double> result = GroupingByExamples.calculateAverageSalaryForEachPosition(workers);
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assertThat(result).isEmpty();
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}
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@Test
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public void groupWorkersByPositionWhenWorkerRepresentedByString() {
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List<GroupingByExamples.Worker> workers = buildWorkersList();
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Map<String, String> result = GroupingByExamples.groupWorkersByPositionWhenWorkerRepresentedByString(workers);
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assertThat(result).hasSize(3);
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assertThat(result.get("Разработчик")).isEqualTo("{Иван, Петр, Иван}");
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assertThat(result.get("Менеджер")).isEqualTo("{Мария, Анна}");
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assertThat(result.get("Тестировщик")).isEqualTo("{Сергей}");
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}
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@Test
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public void groupWorkersByPositionWhenWorkerRepresentedByStringForEmptyList() {
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List<GroupingByExamples.Worker> workers = List.of();
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Map<String, String> result = GroupingByExamples.groupWorkersByPositionWhenWorkerRepresentedByString(workers);
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assertThat(result).isEmpty();
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}
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@Test
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public void groupWorkersByPositionAndAge() {
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List<GroupingByExamples.Worker> workers = buildWorkersList();
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Map<String, Map<Integer, List<GroupingByExamples.Worker>>> result = GroupingByExamples.groupWorkersByPositionAndAge(workers);
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assertThat(result).hasSize(3);
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// Проверяем группировку разработчиков по возрасту
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Map<Integer, List<GroupingByExamples.Worker>> developers = result.get("Разработчик");
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assertThat(developers).hasSize(3);
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assertThat(developers.get(30)).hasSize(1).containsExactly(WORKER1);
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assertThat(developers.get(25)).hasSize(1).containsExactly(WORKER2);
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assertThat(developers.get(35)).hasSize(1).containsExactly(WORKER6);
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// Проверяем группировку менеджеров по возрасту
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Map<Integer, List<GroupingByExamples.Worker>> managers = result.get("Менеджер");
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assertThat(managers).hasSize(2);
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assertThat(managers.get(28)).hasSize(1).containsExactly(WORKER3);
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assertThat(managers.get(32)).hasSize(1).containsExactly(WORKER4);
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// Проверяем группировку тестировщиков по возрасту
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Map<Integer, List<GroupingByExamples.Worker>> testers = result.get("Тестировщик");
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assertThat(testers).hasSize(1);
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assertThat(testers.get(27)).hasSize(1).containsExactly(WORKER5);
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}
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@Test
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public void groupWorkersByPositionAndAgeForEmptyList() {
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List<GroupingByExamples.Worker> workers = List.of();
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Map<String, Map<Integer, List<GroupingByExamples.Worker>>> result = GroupingByExamples.groupWorkersByPositionAndAge(workers);
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assertThat(result).isEmpty();
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}
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@Test
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public void groupWorkersByPositionAndAgeWithSameAge() {
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GroupingByExamples.Worker worker1 = new GroupingByExamples.Worker("Иван", 30, 50000, "Разработчик");
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GroupingByExamples.Worker worker2 = new GroupingByExamples.Worker("Петр", 30, 45000, "Разработчик");
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List<GroupingByExamples.Worker> workers = List.of(worker1, worker2);
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Map<String, Map<Integer, List<GroupingByExamples.Worker>>> result = GroupingByExamples.groupWorkersByPositionAndAge(workers);
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assertThat(result).hasSize(1);
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Map<Integer, List<GroupingByExamples.Worker>> developers = result.get("Разработчик");
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assertThat(developers).hasSize(1);
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assertThat(developers.get(30)).hasSize(2);
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assertThat(developers.get(30)).containsExactlyInAnyOrder(worker1, worker2);
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}
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}

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