Введение
Идея, которая лежит в упражнениях, приведенных ниже, это изучить Streaming Java 8, используя подход test-driven development (пишем имплементацию для первого теста, убеждаемся, что она работает и двигаемся к следующему).
Каждая секция начинается с темы в форме тестов, которые докажут, что имплементация, которую мы напишем, будет корректно работать. Каждый из тестов сопровождается имплементацией на Java 7 и на Java 8, используя Stream API. Таким способом мы сравним некоторые особенности Java 8 с эквивалентом из Java 7.
Java 8 map
Приведите элементы коллекции в вид с верхним регистром.
Тест
public class ToUpperCaseSpec {
@Test
public void transformShouldConvertCollectionElementsToUpperCase() {
List<String> collection = asList("My", "name", "is", "John", "Doe");
List<String> expected = asList("MY", "NAME", "IS", "JOHN", "DOE");
assertThat(transform(collection)).hasSameElementsAs(expected);
assertThat(transform7(collection)).hasSameElementsAs(expected);
}
}
Имплементации на Java 7 и Java 8
public class ToUpperCase {
public static List<String> transform7(List<String> collection) {
List<String> coll = new ArrayList<>();
for (String element : collection) {
coll.add(element.toUpperCase());
}
return coll;
}
public static List<String> transform(List<String> collection) {
return collection.stream() // Convert collection to Stream
.map(String::toUpperCase) // Convert each element to upper case
.collect(toList()); // Collect results to a new list
}
}
Java 8 filter
Отфильтруйте коллекцию так что вернуться только элементы с менее 4 символами.
Тест
public class FilterCollectionSpec {
@Test
public void transformShouldFilterCollection() {
List<String> collection = asList("My", "name", "is", "John", "Doe");
List<String> expected = asList("My", "is", "Doe");
assertThat(transform(collection)).hasSameElementsAs(expected);
assertThat(transform7(collection)).hasSameElementsAs(expected);
}
}
Имплементации на Java 7 и Java 8
public class FilterCollection {
public static List<String> transform7(List<String> collection) {
List<String> newCollection = new ArrayList<>();
for (String element : collection) {
if (element.length() < 4) {
newCollection.add(element);
}
}
return newCollection;
}
public static List<String> transform(List<String> collection) {
return collection.stream() // Convert collection to Stream
.filter(value -> value.length() < 4) // Filter elements with length smaller than 4 characters
.collect(toList()); // Collect results to a new list
}
}
Java 8 flatMap
Делаем плоской многоуровневую колекцию
Тест
public class FlatMapSpec {
@Test
public void transformShouldFlattenCollection() {
List<List<String>> collection = asList(asList("Viktor", "Farcic"), asList("John", "Doe", "Third"));
List<String> expected = asList("Viktor", "Farcic", "John", "Doe", "Third");
assertThat(FlatCollection.transform(collection)).hasSameElementsAs(expected);
assertThat(FlatCollection.transform7(collection)).hasSameElementsAs(expected);
}
}
Имплементации на Java 7 и Java 8
public class FlatCollection {
public static List<String> transform7(List<List<String>> collection) {
List<String> newCollection = new ArrayList<>();
for (List<String> subCollection : collection) {
for (String value : subCollection) {
newCollection.add(value);
}
}
return newCollection;
}
public static List<String> transform(List<List<String>> collection) {
return collection.stream() // Convert collection to Stream
.flatMap(value -> value.stream()) // Replace list with stream
.collect(Collectors.toList()); // Collect results to a new list
}
}
Java 8 max и comparator
Получить самого старшего человека из коллекции
Тест
public class OldestPersonSpec {
@Test
public void getOldestPersonShouldReturnOldestPerson() {
Person sara = new Person("Sara", 4);
Person viktor = new Person("Viktor", 40);
Person eva = new Person("Eva", 42);
List<Person> collection = asList(sara, eva, viktor);
assertThat(getOldestPerson(collection)).isEqualToComparingFieldByField(eva);
assertThat(getOldestPerson7(collection)).isEqualToComparingFieldByField(eva);
}
}
Имплементации на Java 7 и Java 8
public class OldestPerson {
public static Person getOldestPerson7(List<Person> people) {
Person oldestPerson = new Person("", 0);
for (Person person : people) {
if (person.getAge() > oldestPerson.getAge()) {
oldestPerson = person;
}
}
return oldestPerson;
}
public static Person getOldestPerson(List<Person> people) {
return people.stream() // Convert collection to Stream
.max(Comparator.comparing(Person::getAge)) // Compares people ages
.get(); // Gets stream result
}
}
Java 8 sum и reduce
Получить сумму всех элементов коллекции
Тест
public class SumSpec {
@Test
public void transformShouldConvertCollectionElementsToUpperCase() {
List<Integer> numbers = asList(1, 2, 3, 4, 5);
assertThat(calculate(numbers)).isEqualTo(1 + 2 + 3 + 4 + 5);
}
}
Имплементации на Java 7 и Java 8
public class Sum {
public static int calculate7(List<Integer> numbers) {
int total = 0;
for (int number : numbers) {
total += number;
}
return total;
}
public static int calculate(List<Integer> people) {
return people.stream() // Convert collection to Stream
.reduce(0, (total, number) -> total + number); // Sum elements with 0 as starting value
}
}
Java 8 filter и map
Получить имена всех детей (до 18 лет)
Тест
public class KidsSpec {
@Test
public void getKidNameShouldReturnNamesOfAllKidsFromList() {
Person sara = new Person("Sara", 4);
Person viktor = new Person("Viktor", 40);
Person eva = new Person("Eva", 42);
Person anna = new Person("Anna", 5);
List<Person> collection = asList(sara, eva, viktor, anna);
assertThat(getKidNames(collection))
.contains("Sara", "Anna")
.doesNotContain("Viktor", "Eva");
assertThat(getKidNames7(collection))
.contains("Sara", "Anna")
.doesNotContain("Viktor", "Eva");
}
}
Имплементации на Java 7 и Java 8
public class Kids {
public static Set<String> getKidNames7(List<Person> people) {
Set<String> kids = new HashSet<>();
for (Person person : people) {
if (person.getAge() < 18) {
kids.add(person.getName());
}
}
return kids;
}
public static Set<String> getKidNames(List<Person> people) {
return people.stream()
.filter(p -> p.getAge() < 18) // Filter kids (under age of 18)
.map(Person::getName) // Map Person elements to names
.collect(toSet()); // Collect values to a Set
}
}
Java 8 summaryStatisctics
Получить стастику по людям: средний возраст, количество, максимальный возраст, минимальный возраст и сумма всех возрастов.
Тест
public class PeopleStatsSpec {
Person sara = new Person("Sara", 4);
Person viktor = new Person("Viktor", 40);
Person eva = new Person("Eva", 42);
List<Person> collection = asList(sara, eva, viktor);
@Test
public void getStatsShouldReturnAverageAge() {
assertThat(getStats(collection).getAverage())
.isEqualTo((double)(4 + 40 + 42) / 3);
assertThat(getStats7(collection).getAverage())
.isEqualTo((double)(4 + 40 + 42) / 3);
}
@Test
public void getStatsShouldReturnNumberOfPeople() {
assertThat(getStats(collection).getCount())
.isEqualTo(3);
assertThat(getStats7(collection).getCount())
.isEqualTo(3);
}
@Test
public void getStatsShouldReturnMaximumAge() {
assertThat(getStats(collection).getMax())
.isEqualTo(42);
assertThat(getStats7(collection).getMax())
.isEqualTo(42);
}
@Test
public void getStatsShouldReturnMinimumAge() {
assertThat(getStats(collection).getMin())
.isEqualTo(4);
assertThat(getStats7(collection).getMin())
.isEqualTo(4);
}
@Test
public void getStatsShouldReturnSumOfAllAges() {
assertThat(getStats(collection).getSum())
.isEqualTo(40 + 42 + 4);
assertThat(getStats7(collection).getSum())
.isEqualTo(40 + 42 + 4);
}
}
Имплементации на Java 7 и Java 8
public class PeopleStats {
public static Stats getStats7(List<Person> people) {
long sum = 0;
int min = people.get(0).getAge();
int max = 0;
for (Person person : people) {
int age = person.getAge();
sum += age;
min = Math.min(min, age);
max = Math.max(max, age);
}
return new Stats(people.size(), max, min, sum);
}
public static IntSummaryStatistics getStats(List<Person> people) {
return people.stream()
.mapToInt(Person::getAge)
.summaryStatistics();
}
}
Java 8 partitioningBy
Необходимо разделить взрослых и детей
Тест
public class PartitioningSpec {
@Test
public void partitionAdultsShouldSeparateKidsFromAdults() {
Person sara = new Person("Sara", 4);
Person viktor = new Person("Viktor", 40);
Person eva = new Person("Eva", 42);
List<Person> collection = asList(sara, eva, viktor);
Map<Boolean, List<Person>> result = partitionAdults(collection);
Map<Boolean, List<Person>> result2 = partitionAdults7(collection);
assertThat(result.get(true)).hasSameElementsAs(asList(viktor, eva));
assertThat(result.get(false)).hasSameElementsAs(asList(sara));
assertThat(result2.get(true)).hasSameElementsAs(asList(viktor, eva));
assertThat(result2.get(false)).hasSameElementsAs(asList(sara));
}
}
Имплементации на Java 7 и Java 8
public class Partitioning {
public static Map<Boolean, List<Person>> partitionAdults7(List<Person> people) {
Map<Boolean, List<Person>> map = new HashMap<>();
map.put(true, new ArrayList<>());
map.put(false, new ArrayList<>());
for (Person person : people) {
map.get(person.getAge() >= 18).add(person);
}
return map;
}
// Partition stream of people into adults (age => 18) and kids
public static Map<Boolean, List<Person>> partitionAdults(List<Person> people) {
return people.stream()
.collect(partitioningBy(p -> p.getAge() >= 18));
}
}
Java 8 groupingBy
Нужно сгруппировать людей по национальности
Тест
public class GroupungSpec {
@Test
public void groupByNationatilityShouldSeparatePeopleByNationality() {
Person sara = new Person("Sara", 4, "Norwegian");
Person viktor = new Person("Viktor", 40, "Serbian");
Person eva = new Person("Eva", 42, "Norwegian");
List<Person> collection = asList(sara, eva, viktor);
Map<String, List<Person>> result = groupByNationality(collection);
assertThat(result.get("Norwegian")).hasSameElementsAs(asList(sara, eva));
assertThat(result.get("Serbian")).hasSameElementsAs(asList(viktor));
result = groupByNationality7(collection);
assertThat(result.get("Norwegian")).hasSameElementsAs(asList(sara, eva));
assertThat(result.get("Serbian")).hasSameElementsAs(asList(viktor));
}
}
Имплементации на Java 7 и Java 8
public class Grouping {
public static Map<String, List<Person>> groupByNationality7(List<Person> people) {
Map<String, List<Person>> map = new HashMap<>();
for (Person person : people) {
if (!map.containsKey(person.getNationality())) {
map.put(person.getNationality(), new ArrayList<>());
}
map.get(person.getNationality()).add(person);
}
return map;
}
public static Map<String, List<Person>> groupByNationality(List<Person> people) {
return people.stream() // Convert collection to Stream
.collect(groupingBy(Person::getNationality)); // Group people by nationality
}
}
Java 8 joining
Необходимо вернуть имена людей, разделенные запятой
Тест
public class JoiningSpec {
@Test
public void toStringShouldReturnPeopleNamesSeparatedByComma() {
Person sara = new Person("Sara", 4);
Person viktor = new Person("Viktor", 40);
Person eva = new Person("Eva", 42);
List<Person> collection = asList(sara, viktor, eva);
assertThat(namesToString7(collection))
.isEqualTo("Names: Sara, Viktor, Eva.");
assertThat(namesToString(collection))
.isEqualTo("Names: Sara, Viktor, Eva.");
}
}
Имплементации на Java 7 и Java 8
public class Joining {
public static String namesToString7(List<Person> people) {
String label = "Names: ";
StringBuilder sb = new StringBuilder(label);
for (Person person : people) {
if (sb.length() > label.length()) {
sb.append(", ");
}
sb.append(person.getName());
}
sb.append(".");
return sb.toString();
}
public static String namesToString(List<Person> people) {
return people.stream() // Convert collection to Stream
.map(Person::getName) // Map Person to name
.collect(joining(", ", "Names: ", ".")); // Join names
}
}
Исходный код
Весь исходный код размещен на GitHub. Помимо тестов и имплементаций, репозиторий включает в себя pom.xml, который может быть полезен для скачивания зависимости Assertj и запуска тестов.
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