Java : Collectors with Examples
July 1, 2023 
August 23, 2022
Collectors (Java SE 18 & JDK 18) API Examples.
You will find code examples on most Collectors methods.
Summary
Implementations of Collector that implement various useful reduction operations, such as accumulating elements into collections, summarizing elements according to various criteria, etc.
record Item(String name, int value) {
}
final var stream = Stream.of(
new Item("aaa", 10),
new Item("bbb", 20),
new Item("ccc", 30)
);
final var ret = stream.collect(
Collectors.toUnmodifiableMap(
Item::name, Item::value));
System.out.println(ret); // {aaa=10, ccc=30, bbb=20}
final var stream = Stream.of("aaa", "bbb", "ccc");
final var ret = stream.collect(Collectors.joining("-"));
System.out.println(ret); // aaa-bbb-ccc
Methods
static <T> Collector<T,?,Double> averagingDouble (ToDoubleFunction<? super T> mapper)
Returns a Collector that produces the arithmetic mean of a double-valued function applied to the input elements.
final var stream = Stream.of(1.0, 2.0, 3.0, 4.0);
final var ret = stream.collect(Collectors.averagingDouble(value -> value));
System.out.println(ret); // 2.5
static <T> Collector<T,?,Double> averagingInt (ToIntFunction<? super T> mapper)
Returns a Collector that produces the arithmetic mean of an integer-valued function applied to the input elements.
final var stream = Stream.of(1, 2, 3, 4);
final var ret = stream.collect(Collectors.averagingInt(value -> value));
System.out.println(ret); // 2.5
static <T> Collector<T,?,Double> averagingLong (ToLongFunction<? super T> mapper)
Returns a Collector that produces the arithmetic mean of a long-valued function applied to the input elements.
final var stream = Stream.of(10000000000L, 20000000000L, 30000000000L, 40000000000L);
final var ret = stream.collect(Collectors.averagingLong(value -> value));
System.out.println(ret); // 2.5E10
static <T, A, R, RR> Collector<T,A,RR> collectingAndThen (Collector<T,A,R> downstream, Function<R,RR> finisher)
Adapts a Collector to perform an additional finishing transformation.
Please see also : toUnmodifiableList()
final var stream = Stream.of("aaa", "bbb", "ccc");
final var ret = stream.collect(Collectors.collectingAndThen(
Collectors.toList(),
Collections::unmodifiableList));
System.out.println(ret); // [aaa, bbb, ccc]
ret.add("xxx"); // java.lang.UnsupportedOperationException
static <T> Collector<T,?,Long> counting ()
Returns a Collector accepting elements of type T that counts the number of input elements.
final var stream = Stream.of("aaa", "bbb", "ccc");
final var ret = stream.collect(Collectors.counting());
System.out.println(ret); // 3
static <T, A, R> Collector<T,?,R> filtering (Predicate<? super T> predicate, Collector<? super T,A,R> downstream)
Adapts a Collector to one accepting elements of the same type T by applying the predicate to each input element and only accumulating if the predicate returns true.
final var stream = Stream.of(1, 20, 3, 40);
final var ret = stream.collect(Collectors.filtering(
value -> value >= 10,
Collectors.toUnmodifiableList()));
System.out.println(ret); // [20, 40]
final var stream = Stream.of(1, 20, 3, 40);
final var ret = stream.collect(
Collectors.groupingBy(
(value) -> value % 2 == 0 ? "even" : "odd",
Collectors.filtering(
value -> value >= 10,
Collectors.toUnmodifiableList())));
System.out.println(ret); // {even=[20, 40], odd=[]}
static <T, U, A, R> Collector<T,?,R> flatMapping (Function<? super T,? extends Stream<? extends U>> mapper, Collector<? super U,A,R> downstream)
Adapts a Collector accepting elements of type U to one accepting elements of type T by applying a flat mapping function to each input element before accumulation.
record Team(String name, List<String> members) {
}
final var stream = Stream.of(
new Team("aaa", List.of("a-1", "a-2")),
new Team("bbb", List.of("b-1", "b-2", "b-3"))
);
final var ret = stream.collect(
Collectors.flatMapping(
team -> team.members().stream(),
Collectors.toUnmodifiableList()));
System.out.println(ret); // [a-1, a-2, b-1, b-2, b-3]
record Team(String name, List<String> members) {
}
final var stream = Stream.of(
new Team("aaa", List.of("a-1", "a-2")),
new Team("aaa", List.of("a-3", "a-4")),
new Team("bbb", List.of("b-1", "b-2", "b-3"))
);
final var ret = stream.collect(
Collectors.groupingBy(
Team::name,
Collectors.flatMapping(
team -> team.members().stream(),
Collectors.toUnmodifiableList())));
System.out.println(ret); // {aaa=[a-1, a-2, a-3, a-4], bbb=[b-1, b-2, b-3]}
static <T, K> Collector<T,?,Map<K,List<T>>> groupingBy (Function<? super T,? extends K> classifier)
Returns a Collector implementing a "group by" operation on input elements of type T, grouping elements according to a classification function, and returning the results in a Map.
final var stream = Stream.of(1, 2, 3, 4, 5);
final var ret = stream.collect(
Collectors.groupingBy((value) -> value % 2 == 0 ? "even" : "odd"));
System.out.println(ret); // {even=[2, 4], odd=[1, 3, 5]}
static <T, K, D, A, M extends Map<K, D>> Collector<T,?,M> groupingBy (Function<? super T,? extends K> classifier, Supplier<M> mapFactory, Collector<? super T,A,D> downstream)
Returns a Collector implementing a cascaded "group by" operation on input elements of type T, grouping elements according to a classification function, and then performing a reduction operation on the values associated with a given key using the specified downstream Collector.
record Book(String category, String title, int price) {
}
final var stream = Stream.of(
new Book("Travel", "aaa", 10),
new Book("History", "bbb", 20),
new Book("Cooking", "ccc", 50),
new Book("Cooking", "ddd", 40),
new Book("History", "eee", 30),
new Book("History", "fff", 60)
);
final var ret = stream.collect(Collectors.groupingBy(
Book::category,
TreeMap::new,
Collectors.mapping(
Book::title,
Collectors.toUnmodifiableList())));
// {Cooking=[ccc, ddd], History=[bbb, eee, fff], Travel=[aaa]}
System.out.println(ret);
static <T, K, A, D> Collector<T,?,Map<K,D>> groupingBy (Function<? super T,? extends K> classifier, Collector<? super T,A,D> downstream)
Returns a Collector implementing a cascaded "group by" operation on input elements of type T, grouping elements according to a classification function, and then performing a reduction operation on the values associated with a given key using the specified downstream Collector.
record Book(String category, String title, int price) {
}
final var stream = Stream.of(
new Book("Travel", "aaa", 10),
new Book("History", "bbb", 20),
new Book("Cooking", "ccc", 50),
new Book("Cooking", "ddd", 40),
new Book("History", "eee", 30),
new Book("History", "fff", 60)
);
final var ret = stream.collect(Collectors.groupingBy(
Book::category,
Collectors.mapping(
Book::title,
Collectors.toUnmodifiableList())));
// {Travel=[aaa], Cooking=[ccc, ddd], History=[bbb, eee, fff]}
System.out.println(ret);
static <T, K> Collector<T,?,ConcurrentMap<K,List<T>>> groupingByConcurrent (Function<? super T,? extends K> classifier)
Returns a concurrent Collector implementing a "group by" operation on input elements of type T, grouping elements according to a classification function.
final var stream = Stream.of(1, 2, 3, 4, 5).parallel();
System.out.println(stream.isParallel()); // true
final var ret = stream.collect(
Collectors.groupingByConcurrent(
(value) -> value % 2 == 0 ? "even" : "odd"));
System.out.println(ret); // {even=[4, 2], odd=[3, 5, 1]}
static <T, K, A, D, M extends ConcurrentMap<K, D>> Collector<T,?,M> groupingByConcurrent (Function<? super T,? extends K> classifier, Supplier<M> mapFactory, Collector<? super T,A,D> downstream)
Returns a concurrent Collector implementing a cascaded "group by" operation on input elements of type T, grouping elements according to a classification function, and then performing a reduction operation on the values associated with a given key using the specified downstream Collector.
record Book(String category, String title, int price) {
}
final var stream = Stream.of(
new Book("Travel", "aaa", 10),
new Book("History", "bbb", 20),
new Book("Cooking", "ccc", 50),
new Book("Cooking", "ddd", 40),
new Book("History", "eee", 30),
new Book("History", "fff", 60)
).parallel();
System.out.println(stream.isParallel()); // true
final var ret = stream.collect(Collectors.groupingByConcurrent(
Book::category,
ConcurrentSkipListMap::new,
Collectors.mapping(
Book::title,
Collectors.toUnmodifiableList())));
// {Cooking=[ddd, ccc], History=[fff, eee, bbb], Travel=[aaa]}
System.out.println(ret);
static <T, K, A, D> Collector<T,?,ConcurrentMap<K,D>> groupingByConcurrent (Function<? super T,? extends K> classifier, Collector<? super T,A,D> downstream)
Returns a concurrent Collector implementing a cascaded "group by" operation on input elements of type T, grouping elements according to a classification function, and then performing a reduction operation on the values associated with a given key using the specified downstream Collector.
record Book(String category, String title, int price) {
}
final var stream = Stream.of(
new Book("Travel", "aaa", 10),
new Book("History", "bbb", 20),
new Book("Cooking", "ccc", 50),
new Book("Cooking", "ddd", 40),
new Book("History", "eee", 30),
new Book("History", "fff", 60)
).parallel();
System.out.println(stream.isParallel()); // true
final var ret = stream.collect(Collectors.groupingByConcurrent(
Book::category,
Collectors.mapping(
Book::title,
Collectors.toUnmodifiableList())));
// {Travel=[aaa], Cooking=[ddd, ccc], History=[eee, fff, bbb]}
System.out.println(ret);
static Collector<CharSequence,?,String> joining ()
Returns a Collector that concatenates the input elements into a String, in encounter order.
final var stream = Stream.of("aaa", "XXX", "bbb", "YYY");
final var ret = stream.collect(Collectors.joining());
System.out.println(ret); // aaaXXXbbbYYY
static Collector<CharSequence,?,String> joining (CharSequence delimiter)
Returns a Collector that concatenates the input elements, separated by the specified delimiter, in encounter order.
final var stream = Stream.of("aaa", "XXX", "bbb", "YYY");
final var ret = stream.collect(Collectors.joining("-"));
System.out.println(ret); // aaa-XXX-bbb-YYY
static Collector<CharSequence,?,String> joining (CharSequence delimiter, CharSequence prefix, CharSequence suffix)
Returns a Collector that concatenates the input elements, separated by the specified delimiter, with the specified prefix and suffix, in encounter order.
final var stream = Stream.of("aaa", "XXX", "bbb", "YYY");
final var ret = stream.collect(Collectors.joining("-", "[", "]"));
System.out.println(ret); // [aaa-XXX-bbb-YYY]
static <T, U, A, R> Collector<T,?,R> mapping (Function<? super T,? extends U> mapper, Collector<? super U,A,R> downstream)
Adapts a Collector accepting elements of type U to one accepting elements of type T by applying a mapping function to each input element before accumulation.
record Book(String category, String title, int price) {
}
final var stream = Stream.of(
new Book("Travel", "aaa", 10),
new Book("History", "bbb", 20),
new Book("Cooking", "ccc", 50),
new Book("Cooking", "ddd", 40),
new Book("History", "eee", 30)
);
final var ret = stream.collect(
Collectors.mapping(
Book::title,
Collectors.toUnmodifiableList()));
System.out.println(ret); // [aaa, bbb, ccc, ddd, eee]
record Book(String category, String title, int price) {
}
final var stream = Stream.of(
new Book("Travel", "aaa", 10),
new Book("History", "bbb", 20),
new Book("Cooking", "ccc", 50),
new Book("Cooking", "ddd", 40),
new Book("History", "eee", 30)
);
final var ret = stream.collect(Collectors.groupingBy(
Book::category,
Collectors.mapping(
Book::title,
Collectors.toUnmodifiableList())));
// {Travel=[aaa], Cooking=[ccc, ddd], History=[bbb, eee]}
System.out.println(ret);
static <T> Collector<T,?,Optional<T>> maxBy (Comparator<? super T> comparator)
Returns a Collector that produces the maximal element according to a given Comparator, described as an Optional<T>.
final var stream = Stream.of(1, 100, -200, 50, -30);
final var ret = stream.collect(Collectors.maxBy(Comparator.naturalOrder()));
System.out.println(ret); // Optional[100]
final var stream = Stream.of("Tree", "book", "stone", "Cake");
final var ret = stream.collect(Collectors.maxBy(String.CASE_INSENSITIVE_ORDER));
System.out.println(ret); // Optional[Tree]
final var stream = Stream.<String>empty();
final var ret = stream.collect(Collectors.maxBy(Comparator.naturalOrder()));
System.out.println(ret); // Optional.empty
static <T> Collector<T,?,Optional<T>> minBy (Comparator<? super T> comparator)
Returns a Collector that produces the minimal element according to a given Comparator, described as an Optional<T>.
final var stream = Stream.of(1, 100, -200, 50, -30);
final var ret = stream.collect(Collectors.minBy(Comparator.naturalOrder()));
System.out.println(ret); // Optional[-200]
final var stream = Stream.of("Tree", "book", "stone", "Cake");
final var ret = stream.collect(Collectors.minBy(String.CASE_INSENSITIVE_ORDER));
System.out.println(ret); // Optional[book]
final var stream = Stream.<String>of();
final var ret = stream.collect(Collectors.minBy(Comparator.naturalOrder()));
System.out.println(ret); // Optional.empty
static <T> Collector<T,?,Map<Boolean,List<T>>> partitioningBy (Predicate<? super T> predicate)
Returns a Collector which partitions the input elements according to a Predicate, and organizes them into a Map<Boolean, List<T>>.
final var stream = Stream.of(1, 2, 3, 4, 5);
// true = even, false = odd
final var ret = stream.collect(
Collectors.partitioningBy((value) -> value % 2 == 0));
System.out.println(ret); // {false=[1, 3, 5], true=[2, 4]}
static <T, D, A> Collector<T,?,Map<Boolean,D>> partitioningBy (Predicate<? super T> predicate, Collector<? super T,A,D> downstream)
Returns a Collector which partitions the input elements according to a Predicate, reduces the values in each partition according to another Collector, and organizes them into a Map<Boolean, D> whose values are the result of the downstream reduction.
record Item(Integer value, String name) {
}
final var stream = Stream.of(
new Item(1, "aaa"),
new Item(2, "bbb"),
new Item(3, "ccc"),
new Item(4, "ddd"),
new Item(5, "eee")
);
// true = even, false = odd
final var ret = stream.collect(
Collectors.partitioningBy(
(item) -> item.value() % 2 == 0,
Collectors.mapping(
Item::name,
Collectors.toUnmodifiableList()
)));
System.out.println(ret); // {false=[aaa, ccc, eee], true=[bbb, ddd]}
static <T> Collector<T,?,Optional<T>> reducing (BinaryOperator<T> op)
Returns a Collector which performs a reduction of its input elements under a specified BinaryOperator.
record Book(String category, String title, int price) {
}
final var stream = Stream.of(
new Book("Travel", "aaa", 10),
new Book("History", "bbb", 20),
new Book("Cooking", "ccc", 50),
new Book("Cooking", "ddd", 40),
new Book("History", "eee", 30)
);
final var comparator = Comparator.comparing(Book::price);
final var map = stream.collect(
Collectors.groupingBy(
Book::category,
Collectors.reducing(
BinaryOperator.maxBy(comparator)
)));
System.out.println("-- entry --");
for (final var entry : map.entrySet()) {
System.out.println(entry);
}
// Result
// ↓
//-- entry --
//Travel=Optional[Book[category=Travel, title=aaa, price=10]]
//Cooking=Optional[Book[category=Cooking, title=ccc, price=50]]
//History=Optional[Book[category=History, title=eee, price=30]]
static <T> Collector<T,?,T> reducing (T identity, BinaryOperator<T> op)
Returns a Collector which performs a reduction of its input elements under a specified BinaryOperator using the provided identity.
record Book(String category, String title, int price) {
}
final var stream = Stream.of(
new Book("Travel", "aaa", 10),
new Book("History", "bbb", 20),
new Book("Cooking", "ccc", 50),
new Book("Cooking", "ddd", 40),
new Book("History", "eee", 30)
);
final var identity = new Book("", "", 0);
final var comparator = Comparator.comparing(Book::price);
final var map = stream.collect(
Collectors.groupingBy(
Book::category,
Collectors.reducing(
identity,
BinaryOperator.maxBy(comparator)
)));
System.out.println("-- entry --");
for (final var entry : map.entrySet()) {
System.out.println(entry);
}
// Result
// ↓
//-- entry --
//Travel=Book[category=Travel, title=aaa, price=10]
//Cooking=Book[category=Cooking, title=ccc, price=50]
//History=Book[category=History, title=eee, price=30]
static <T, U> Collector<T,?,U> reducing (U identity, Function<? super T,? extends U> mapper, BinaryOperator<U> op)
Returns a Collector which performs a reduction of its input elements under a specified mapping function and BinaryOperator.
record Book(String category, String title, int price) {
}
final var stream = Stream.of(
new Book("Travel", "aaa", 10),
new Book("History", "bbb", 20),
new Book("Cooking", "ccc", 50),
new Book("Cooking", "ddd", 40),
new Book("History", "eee", 30)
);
final var identity = 0;
final var map = stream.collect(
Collectors.groupingBy(
Book::category,
Collectors.reducing(
identity,
Book::price,
BinaryOperator.maxBy(Comparator.naturalOrder())
)));
System.out.println("-- entry --");
for (final var entry : map.entrySet()) {
System.out.println(entry);
}
// Result
// ↓
//-- entry --
//Travel=10
//Cooking=50
//History=30
static <T> Collector<T,?,DoubleSummaryStatistics> summarizingDouble (ToDoubleFunction<? super T> mapper)
Returns a Collector which applies an double-producing mapping function to each input element, and returns summary statistics for the resulting values.
final var stream = Stream.of(1.0, 2.0, 3.0);
final var ret = stream.collect(Collectors.summarizingDouble(value -> value));
// DoubleSummaryStatistics{count=3, sum=6.000000, min=1.000000, average=2.000000, max=3.000000}
System.out.println(ret);
static <T> Collector<T,?,IntSummaryStatistics> summarizingInt (ToIntFunction<? super T> mapper)
Returns a Collector which applies an int-producing mapping function to each input element, and returns summary statistics for the resulting values.
final var stream = Stream.of(1, 2, 3);
final var ret = stream.collect(Collectors.summarizingInt(value -> value));
// IntSummaryStatistics{count=3, sum=6, min=1, average=2.000000, max=3}
System.out.println(ret);
static <T> Collector<T,?,LongSummaryStatistics> summarizingLong (ToLongFunction<? super T> mapper)
Returns a Collector which applies an long-producing mapping function to each input element, and returns summary statistics for the resulting values.
final var stream = Stream.of(10000000000L, 20000000000L, 30000000000L);
final var ret = stream.collect(Collectors.summarizingLong(value -> value));
// LongSummaryStatistics{count=3, sum=60000000000,
// min=10000000000, average=20000000000.000000, max=30000000000}
System.out.println(ret);
static <T> Collector<T,?,Double> summingDouble (ToDoubleFunction<? super T> mapper)
Returns a Collector that produces the sum of a double-valued function applied to the input elements.
final var stream = Stream.of(1.0, 2.0, 3.0);
final var ret = stream.collect(Collectors.summingDouble(value -> value));
System.out.println(ret); // 6.0
static <T> Collector<T,?,Integer> summingInt (ToIntFunction<? super T> mapper)
Returns a Collector that produces the sum of a integer-valued function applied to the input elements.
final var stream = Stream.of(1, 2, 3);
final var ret = stream.collect(Collectors.summingInt(value -> value));
System.out.println(ret); // 6
static <T> Collector<T,?,Long> summingLong (ToLongFunction<? super T> mapper)
Returns a Collector that produces the sum of a long-valued function applied to the input elements.
final var stream = Stream.of(10000000000L, 20000000000L, 30000000000L);
final var ret = stream.collect(Collectors.summingLong(value -> value));
System.out.println(ret); // 60000000000
static <T, R1, R2, R> Collector<T,?,R> teeing (Collector<? super T,?,R1> downstream1, Collector<? super T,?,R2> downstream2, BiFunction<? super R1,? super R2,R> merger)
Returns a Collector that is a composite of two downstream collectors.
final var stream = Stream.of("aaa", "bbb", "ccc");
final var ret = stream.collect(Collectors.teeing(
Collectors.joining("-"),
Collectors.joining("+"),
(s1, s2) -> "[" + s1 + ", " + s2 + "]"
));
System.out.println(ret); // [aaa-bbb-ccc, aaa+bbb+ccc]
static <T, C extends Collection<T>> Collector<T,?,C> toCollection (Supplier<C> collectionFactory)
Returns a Collector that accumulates the input elements into a new Collection, in encounter order.
final var stream = Stream.of("aaa", "bbb", "ccc");
final var ret = stream.collect(Collectors.toCollection(LinkedList::new));
System.out.println(ret); // [aaa, bbb, ccc]
System.out.println(ret.getClass()); // class java.util.LinkedList
static <T, K, U> Collector<T,?,ConcurrentMap<K,U>> toConcurrentMap (Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper)
Returns a concurrent Collector that accumulates elements into a ConcurrentMap whose keys and values are the result of applying the provided mapping functions to the input elements.
record Item(String name, int value) {
}
final var stream = Stream.of(
new Item("aaa", 10),
new Item("bbb", 20),
new Item("ccc", 30)
);
final var ret = stream.collect(Collectors.toConcurrentMap(
Item::name, Item::value));
System.out.println(ret); // {aaa=10, ccc=30, bbb=20}
static <T, K, U> Collector<T,?,ConcurrentMap<K,U>> toConcurrentMap (Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction)
Returns a concurrent Collector that accumulates elements into a ConcurrentMap whose keys and values are the result of applying the provided mapping functions to the input elements.
record Item(String name, int value) {
}
final var stream = Stream.of(
new Item("aaa", 1),
new Item("aaa", 2),
new Item("bbb", 30),
new Item("bbb", 40),
new Item("ccc", 500)
);
final var ret = stream.collect(Collectors.toConcurrentMap(
Item::name, Item::value, (value1, value2) -> value1 + value2));
System.out.println(ret); // {aaa=3, ccc=500, bbb=70}
static <T, K, U, M extends ConcurrentMap<K, U>> Collector<T,?,M> toConcurrentMap (Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction, Supplier<M> mapFactory)
Returns a concurrent Collector that accumulates elements into a ConcurrentMap whose keys and values are the result of applying the provided mapping functions to the input elements.
record Item(String name, int value) {
}
final var stream = Stream.of(
new Item("aaa", 1),
new Item("aaa", 2),
new Item("bbb", 30),
new Item("bbb", 40),
new Item("ccc", 500)
);
final var ret = stream.collect(Collectors.toConcurrentMap(
Item::name, Item::value, (value1, value2) -> value1 + value2,
ConcurrentSkipListMap::new));
System.out.println(ret); // {aaa=3, bbb=70, ccc=500}
System.out.println(ret.getClass()); // class java.util.concurrent.ConcurrentSkipListMap
static <T> Collector<T,?,List<T>> toList ()
Returns a Collector that accumulates the input elements into a new List.
final var stream = Stream.of("aaa", "bbb", "ccc");
final var ret = stream.collect(Collectors.toList());
System.out.println(ret); // [aaa, bbb, ccc]
ret.add("XXX");
System.out.println(ret); // [aaa, bbb, ccc, XXX]
static <T, K, U> Collector<T,?,Map<K,U>> toMap (Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper)
Returns a Collector that accumulates elements into a Map whose keys and values are the result of applying the provided mapping functions to the input elements.
record Item(String name, int value) {
}
final var stream = Stream.of(
new Item("aaa", 10),
new Item("bbb", 20),
new Item("ccc", 30)
);
final var ret = stream.collect(Collectors.toMap(
Item::name, Item::value));
System.out.println(ret); // {aaa=10, ccc=30, bbb=20}
ret.put("XXX", 999);
System.out.println(ret); // {aaa=10, ccc=30, bbb=20, XXX=999}
static <T, K, U> Collector<T,?,Map<K,U>> toMap (Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction)
Returns a Collector that accumulates elements into a Map whose keys and values are the result of applying the provided mapping functions to the input elements.
record Item(String name, int value) {
}
final var stream = Stream.of(
new Item("aaa", 1),
new Item("aaa", 2),
new Item("bbb", 30),
new Item("bbb", 40),
new Item("ccc", 500)
);
final var ret = stream.collect(Collectors.toMap(
Item::name, Item::value, (value1, value2) -> value1 + value2));
System.out.println(ret); // {aaa=3, ccc=500, bbb=70}
ret.put("XXX", 999);
System.out.println(ret); // {aaa=3, ccc=500, bbb=70, XXX=999}
static <T, K, U, M extends Map<K, U>> Collector<T,?,M> toMap (Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction, Supplier<M> mapFactory)
Returns a Collector that accumulates elements into a Map whose keys and values are the result of applying the provided mapping functions to the input elements.
record Item(String name, int value) {
}
final var stream = Stream.of(
new Item("aaa", 1),
new Item("aaa", 2),
new Item("bbb", 30),
new Item("bbb", 40),
new Item("ccc", 500)
);
final var ret = stream.collect(Collectors.toMap(
Item::name, Item::value, (value1, value2) -> value1 + value2,
TreeMap::new));
System.out.println(ret); // {aaa=3, bbb=70, ccc=500}
System.out.println(ret.getClass()); // class java.util.TreeMap
static <T> Collector<T,?,Set<T>> toSet ()
Returns a Collector that accumulates the input elements into a new Set.
final var stream = Stream.of("aaa", "bbb", "ccc");
final var ret = stream.collect(Collectors.toSet());
System.out.println(ret); // [aaa, ccc, bbb]
ret.add("XXX");
System.out.println(ret); // [aaa, ccc, bbb, XXX]
static <T> Collector<T,?,List<T>> toUnmodifiableList ()
Returns a Collector that accumulates the input elements into an unmodifiable List in encounter order.
final var stream = Stream.of("aaa", "bbb", "ccc");
final var ret = stream.collect(Collectors.toUnmodifiableList());
System.out.println(ret); // [aaa, bbb, ccc]
ret.add("XXX"); // java.lang.UnsupportedOperationException
static <T, K, U> Collector<T,?,Map<K,U>> toUnmodifiableMap (Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper)
Returns a Collector that accumulates the input elements into an unmodifiable Map, whose keys and values are the result of applying the provided mapping functions to the input elements.
record Item(String name, int value) {
}
final var stream = Stream.of(
new Item("aaa", 10),
new Item("bbb", 20),
new Item("ccc", 30)
);
final var ret = stream.collect(Collectors.toUnmodifiableMap(
Item::name, Item::value));
System.out.println(ret); // {aaa=10, ccc=30, bbb=20}
ret.put("XXX", 999); // java.lang.UnsupportedOperationException
static <T, K, U> Collector<T,?,Map<K,U>> toUnmodifiableMap (Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction)
Returns a Collector that accumulates the input elements into an unmodifiable Map, whose keys and values are the result of applying the provided mapping functions to the input elements.
record Item(String name, int value) {
}
final var stream = Stream.of(
new Item("aaa", 1),
new Item("aaa", 2),
new Item("bbb", 30),
new Item("bbb", 40),
new Item("ccc", 500)
);
final var ret = stream.collect(Collectors.toUnmodifiableMap(
Item::name, Item::value, (value1, value2) -> value1 + value2));
System.out.println(ret); // {aaa=3, ccc=500, bbb=70}
ret.put("XXX", 999); // java.lang.UnsupportedOperationException
static <T> Collector<T,?,Set<T>> toUnmodifiableSet ()
Returns a Collector that accumulates the input elements into an unmodifiable Set.
final var stream = Stream.of("aaa", "bbb", "ccc");
final var ret = stream.collect(Collectors.toUnmodifiableSet());
System.out.println(ret); // [aaa, ccc, bbb]
ret.add("XXX"); // java.lang.UnsupportedOperationException
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