The Stream.collect() Method

The Stream.collect() Method

List<Integer> numberList = Arrays.asList(11, 3, 2, 3, 8, 9, 0, 7, 2);
numberList.stream().collect(Collectors.toSet());

Collect metodu Stream türündeki nesneleri başka biçimdeki nesneye, veri yapısına dönüştürmek için kullanılır. Collector arayüzünden türeyen bir parametre beklemektedir. Bu parametre bilgisi ile istenilen türe dönüşüm sağlanabilir. Collector türünden arayüzler Collectors sınıfının içinde bulunan metodlar ile elde edilebilir.

Collector Interface

• yeni bir sonuç containerının oluşturulması (supplier())
• yeni bir elemanı sonuc containerına dahi etme (accumulator())
• iki sonuç containerını bir araya getirmek (combiner())
• containerda isteğe bağlı bir son dönüşüm gerçekleştirme (finisher())

.Collect(Collectors.) örnekleri

List givenList = Arrays.asList("a", "bb", "ccc", "dd");

• toList()

List<String> result = givenList.stream().collect(toList());

• Collectors.toUnmodifiableList()

List<String>result=givenList.stream()
.collect(toUnmodifiableList());

• toSet()

Set<String> result = givenList.stream()
.collect(toSet());

• Collectors.toUnmodifiableSet()

Set<String> result = givenList.stream()
.collect(toUnmodifiableSet());

• Collectors.toCollection()

List<String> result = givenList.stream()
.collect(toCollection(LinkedList::new))

• Collectors.toMap()

Map<String, Integer> result = givenList.stream()
.collect(toMap(Function.identity(), String::length))

identity fonksiyonu

static <T> Function<T, T> identity() {
        return t -> t;

duplicate bir veri gelirse

List<String> listWithDuplicates = 
Arrays.asList("a", "bb", "c", "d", "bb"); assertThatThrownBy(
() -> { listWithDuplicates.stream()
.collect(toMap(Function.identity(), String::length)); })
.isInstanceOf(IllegalStateException.class);

çözümü

 Map<String, Integer> result = 
givenList.stream()
.collect(toMap(Function.identity(),
 String::length,
(item, identicalItem) -> item));

• Collectors.toUnmodifiableMap()

Map<String, Integer> result = givenList.stream()
.collect(toUnmodifiableMap(Function.identity(),
String::length))

• Collectors.collectingAndThen()

List<String> result = givenList.stream()
.collect(collectingAndThen(toList(), ImmutableList::copyOf))

• Collectors.joining()

String result = givenList.stream(joining());

"abbcccdd"

String  result  = givenList.stream()
.collect(joining(" "));

"a bb ccc dd"

String  result  = givenList.stream()
.collect(joining(" ", "PRE-", "-POST"));

"PRE-a bb ccc dd-POST"

• Collectors.counting()

Long result = givenList.stream()
  .collect(counting());

• Collectors.summarizingDouble/Long/Int()

DoubleSummaryStatistics  result  = givenList.stream()
.collect(summarizingDouble(String::length));

assertThat(result.getAverage()).isEqualTo(2); assertThat(result.getCount()).isEqualTo(4); assertThat(result.getMax()).isEqualTo(3); assertThat(result.getMin()).isEqualTo(1); assertThat(result.getSum()).isEqualTo(8);

• Collectors.maxBy()/minBy()

Optional<String> result = givenList.stream()
.collect(maxBy(Comparator.naturalOrder()));

• Collectors.groupingBy()

Map<Department, List<Employee>> byDept =
employees.stream()
.collect(Collectors.
groupingBy(Employee::getDepartment));

 Map<Integer, Set<String>> result = givenList.stream()
.collect
(groupingBy(String::length, toSet()));

Map<Department, Integer> totalByDept  = employees.stream()
.collect(Collectors
.groupingBy(Employee::getDepartment
.Collectors.summingInt(Employee::getSalary)));

• Collectors.partitioningBy()

Map<Boolean, List<String>> result =
givenList.stream()
.collect(partitioningBy(s -> s.length() > 2))

{false=["a", "bb", "dd"], true=["ccc"]}

Map<Boolean, List<Student>> passingFailing =
students.stream().collect(Collectors
.partitioningBy
(s -> s.getGrade() >= PASS_THRESHOLD));

• Collectors.teeing()

List<Integer> numbers = Arrays.asList(42, 4, 2, 24);

Optional<Integer> min =
numbers.stream().collect(minBy(Integer::compareTo)); 

Optional<Integer> max =
numbers.stream().collect(maxBy(Integer::compareTo));

--java 12 sonrası

numbers.stream()
.collect(teeing( minBy(Integer::compareTo),
maxBy(Integer::compareTo),
(min, max) ->
));

HashMap<String, Employee> result =
employeeList.stream().collect(Collectors.teeing(
Collectors.maxBy(Comparator.
comparing(Employee::getSalary)).

Collectors.minBy(Comparator.
comparing(Employee::getSalary)),
 (e1, e2) -> {
  HashMap<String, Employee> map = new HashMap();
  map.put("MAX", e1.get());
  map.put("MIN", e2.get());
  return map;
  }
 ));