Java Streams API: функциональное программирование

1// Из коллекции
2List<String> names = List.of("Alice", "Bob", "Charlie");
3Stream<String> stream = names.stream();
4
5// Из массива
6int[] numbers = {1, 2, 3, 4, 5};
7IntStream arrayStream = Arrays.stream(numbers);
8
9// Из значений
10Stream<String> valueStream = Stream.of("a", "b", "c");
11
12// Генерация
13Stream<Integer> infiniteStream = Stream.iterate(0, n -> n + 2);
14Stream<Double> randomStream = Stream.generate(Math::random);

1List<String> names = List.of("alice", "bob", "charlie");
2
3List<String> upperNames = names.stream()
4    .map(String::toUpperCase)
5    .collect(Collectors.toList());
6// [ALICE, BOB, CHARLIE]

1List<Integer> numbers = List.of(1, 2, 3, 4, 5, 6, 7, 8);
2
3List<Integer> evenNumbers = numbers.stream()
4    .filter(n -> n % 2 == 0)
5    .collect(Collectors.toList());
6// [2, 4, 6, 8]

1List<List<Integer>> lists = List.of(
2    List.of(1, 2, 3),
3    List.of(4, 5, 6),
4    List.of(7, 8, 9)
5);
6
7List<Integer> flat = lists.stream()
8    .flatMap(Collection::stream)
9    .collect(Collectors.toList());
10// [1, 2, 3, 4, 5, 6, 7, 8, 9]

1List<String> sorted = names.stream()
2    .sorted()
3    .collect(Collectors.toList());
4
5// С пользовательским компаратором
6List<User> sortedByAge = users.stream()
7    .sorted(Comparator.comparing(User::getAge))
8    .collect(Collectors.toList());

1List<Integer> unique = List.of(1, 2, 2, 3, 3, 3).stream()
2    .distinct()
3    .collect(Collectors.toList());
4// [1, 2, 3]

1List<String> result = names.stream()
2    .filter(name -> name.length() > 3)
3    .peek(name -> System.out.println("After filter: " + name))
4    .map(String::toUpperCase)
5    .peek(name -> System.out.println("After map: " + name))
6    .collect(Collectors.toList());

1// В список
2List<String> list = stream.collect(Collectors.toList());
3
4// В Set
5Set<String> set = stream.collect(Collectors.toSet());
6
7// В Map
8Map<String, Integer> map = stream.collect(
9    Collectors.toMap(User::getName, User::getAge));
10
11// Группировка
12Map<String, List<User>> byDepartment = users.stream()
13    .collect(Collectors.groupingBy(User::getDepartment));
14
15// Разделение
16Map<Boolean, List<User>> partitioned = users.stream()
17    .collect(Collectors.partitioningBy(User::isActive));

1// Сумма чисел
2int sum = numbers.stream()
3    .reduce(0, Integer::sum);
4
5// Конкатенация строк
6String result = words.stream()
7    .reduce("", (a, b) -> a.isEmpty() ? b : a + ", " + b);
8
9// Максимальный элемент
10Optional<Integer> max = numbers.stream()
11    .reduce(Integer::max);

1names.stream()
2    .filter(name -> name.length() > 3)
3    .forEach(System.out::println);

1Optional<Integer> min = numbers.stream().min(Integer::compareTo);
2Optional<Integer> max = numbers.stream().max(Integer::compareTo);
3long count = numbers.stream().filter(n -> n > 5).count();

1boolean hasEven = numbers.stream().anyMatch(n -> n % 2 == 0);
2boolean allPositive = numbers.stream().allMatch(n -> n > 0);
3boolean noneNegative = numbers.stream().noneMatch(n -> n < 0);

1Optional<String> first = names.stream()
2    .filter(name -> name.startsWith("A"))
3    .findFirst();
4
5Optional<String> any = names.parallelStream()
6    .filter(name -> name.length() > 5)
7    .findAny();

1// Последовательный
2long sequential = numbers.stream()
3    .filter(n -> n > 5)
4    .count();
5
6// Параллельный
7long parallel = numbers.parallelStream()
8    .filter(n -> n > 5)
9    .count();

1List<String> emails = users.stream()
2    .filter(User::isActive)
3    .map(User::getEmail)
4    .filter(email -> email.endsWith("@company.com"))
5    .sorted()
6    .distinct()
7    .collect(Collectors.toList());

1Map<String, Double> avgSalaryByDept = employees.stream()
2    .collect(Collectors.groupingBy(
3        Employee::getDepartment,
4        Collectors.averagingDouble(Employee::getSalary)
5    ));

1Optional<Product> mostExpensive = products.stream()
2    .filter(p -> p.getCategory().equals("electronics"))
3    .max(Comparator.comparing(Product::getPrice));