Added in API level 24

Collectors

class Collectors

kotlin.Any
↳	java.util.stream.Collectors

Implementations of Collector that implement various useful reduction operations, such as accumulating elements into collections, summarizing elements according to various criteria, etc.

The following are examples of using the predefined collectors to perform common mutable reduction tasks:

<code>// Accumulate names into a List
  List&lt;String&gt; list = people.stream()
    .map(Person::getName)
    .collect(Collectors.toList());
 
  // Accumulate names into a TreeSet
  Set&lt;String&gt; set = people.stream()
    .map(Person::getName)
    .collect(Collectors.toCollection(TreeSet::new));
 
  // Convert elements to strings and concatenate them, separated by commas
  String joined = things.stream()
    .map(Object::toString)
    .collect(Collectors.joining(", "));
 
  // Compute sum of salaries of employee
  int total = employees.stream()
    .collect(Collectors.summingInt(Employee::getSalary));
 
  // Group employees by department
  Map&lt;Department, List&lt;Employee&gt;&gt; byDept = employees.stream()
    .collect(Collectors.groupingBy(Employee::getDepartment));
 
  // Compute sum of salaries by department
  Map&lt;Department, Integer&gt; totalByDept = employees.stream()
    .collect(Collectors.groupingBy(Employee::getDepartment,
                                   Collectors.summingInt(Employee::getSalary)));
 
  // Partition students into passing and failing
  Map&lt;Boolean, List&lt;Student&gt;&gt; passingFailing = students.stream()
    .collect(Collectors.partitioningBy(s -&gt; s.getGrade() &gt;= PASS_THRESHOLD));
 
  </code>

Summary

Public methods
static Collector<T, *, Double!>!	`averagingDouble(mapper: ToDoubleFunction<in T>!)` Returns a `Collector` that produces the arithmetic mean of a double-valued function applied to the input elements.
static Collector<T, *, Double!>!	`averagingInt(mapper: ToIntFunction<in T>!)` Returns a `Collector` that produces the arithmetic mean of an integer-valued function applied to the input elements.
static Collector<T, *, Double!>!	`averagingLong(mapper: ToLongFunction<in T>!)` Returns a `Collector` that produces the arithmetic mean of a long-valued function applied to the input elements.
static Collector<T, A, RR>!	`collectingAndThen(downstream: Collector<T, A, R>!, finisher: Function<R, RR>!)` Adapts a `Collector` to perform an additional finishing transformation.
static Collector<T, *, Long!>!	`counting()` Returns a `Collector` accepting elements of type `T` that counts the number of input elements.
static Collector<T, *, R>!	`filtering(predicate: Predicate<in T>!, downstream: Collector<in T, A, R>!)` 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`.
static Collector<T, *, R>!	`flatMapping(mapper: Function<in T, out Stream<out U>!>!, downstream: Collector<in U, A, R>!)` 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.
static Collector<T, *, MutableMap<K, MutableList<T>!>!>!	`groupingBy(classifier: Function<in T, out K>!)` 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`.
static Collector<T, *, MutableMap<K, D>!>!	`groupingBy(classifier: Function<in T, out K>!, downstream: Collector<in T, A, D>!)` 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`.
static Collector<T, *, M>!	`groupingBy(classifier: Function<in T, out K>!, mapFactory: Supplier<M>!, downstream: Collector<in T, A, D>!)` 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`.
static Collector<T, *, ConcurrentMap<K, MutableList<T>!>!>!	`groupingByConcurrent(classifier: Function<in T, out K>!)` Returns a concurrent `Collector` implementing a "group by" operation on input elements of type `T`, grouping elements according to a classification function.
static Collector<T, *, ConcurrentMap<K, D>!>!	`groupingByConcurrent(classifier: Function<in T, out K>!, downstream: Collector<in T, A, D>!)` 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`.
static Collector<T, *, M>!	`groupingByConcurrent(classifier: Function<in T, out K>!, mapFactory: Supplier<M>!, downstream: Collector<in T, A, D>!)` 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`.
static Collector<CharSequence!, *, String!>!	`joining()` Returns a `Collector` that concatenates the input elements into a `String`, in encounter order.
static Collector<CharSequence!, *, String!>!	`joining(delimiter: CharSequence!)` Returns a `Collector` that concatenates the input elements, separated by the specified delimiter, in encounter order.
static Collector<CharSequence!, *, String!>!	`joining(delimiter: CharSequence!, prefix: CharSequence!, suffix: CharSequence!)` Returns a `Collector` that concatenates the input elements, separated by the specified delimiter, with the specified prefix and suffix, in encounter order.
static Collector<T, *, R>!	`mapping(mapper: Function<in T, out U>!, downstream: Collector<in U, A, R>!)` 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.
static Collector<T, *, Optional<T>!>!	`maxBy(comparator: Comparator<in T>!)` Returns a `Collector` that produces the maximal element according to a given `Comparator`, described as an `Optional<T>`.
static Collector<T, *, Optional<T>!>!	`minBy(comparator: Comparator<in T>!)` Returns a `Collector` that produces the minimal element according to a given `Comparator`, described as an `Optional<T>`.
static Collector<T, *, MutableMap<Boolean!, MutableList<T>!>!>!	`partitioningBy(predicate: Predicate<in T>!)` Returns a `Collector` which partitions the input elements according to a `Predicate`, and organizes them into a `Map<Boolean, List<T>>`.
static Collector<T, *, MutableMap<Boolean!, D>!>!	`partitioningBy(predicate: Predicate<in T>!, downstream: Collector<in T, A, D>!)` 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.
static Collector<T, *, T>!	`reducing(identity: T, op: BinaryOperator<T>!)` Returns a `Collector` which performs a reduction of its input elements under a specified `BinaryOperator` using the provided identity.
static Collector<T, *, Optional<T>!>!	`reducing(op: BinaryOperator<T>!)` Returns a `Collector` which performs a reduction of its input elements under a specified `BinaryOperator`.
static Collector<T, *, U>!	`reducing(identity: U, mapper: Function<in T, out U>!, op: BinaryOperator<U>!)` Returns a `Collector` which performs a reduction of its input elements under a specified mapping function and `BinaryOperator`.
static Collector<T, *, DoubleSummaryStatistics!>!	`summarizingDouble(mapper: ToDoubleFunction<in T>!)` Returns a `Collector` which applies an `double`-producing mapping function to each input element, and returns summary statistics for the resulting values.
static Collector<T, *, IntSummaryStatistics!>!	`summarizingInt(mapper: ToIntFunction<in T>!)` Returns a `Collector` which applies an `int`-producing mapping function to each input element, and returns summary statistics for the resulting values.
static Collector<T, *, LongSummaryStatistics!>!	`summarizingLong(mapper: ToLongFunction<in T>!)` Returns a `Collector` which applies an `long`-producing mapping function to each input element, and returns summary statistics for the resulting values.
static Collector<T, *, Double!>!	`summingDouble(mapper: ToDoubleFunction<in T>!)` Returns a `Collector` that produces the sum of a double-valued function applied to the input elements.
static Collector<T, *, Int!>!	`summingInt(mapper: ToIntFunction<in T>!)` Returns a `Collector` that produces the sum of a integer-valued function applied to the input elements.
static Collector<T, *, Long!>!	`summingLong(mapper: ToLongFunction<in T>!)` Returns a `Collector` that produces the sum of a long-valued function applied to the input elements.
static Collector<T, *, R>!	`teeing(downstream1: Collector<in T, , R1>!, downstream2: Collector<in T, , R2>!, merger: BiFunction<in R1, in R2, R>!)` Returns a `Collector` that is a composite of two downstream collectors.
static Collector<T, *, C>!	`toCollection(collectionFactory: Supplier<C>!)` Returns a `Collector` that accumulates the input elements into a new `Collection`, in encounter order.
static Collector<T, *, ConcurrentMap<K, U>!>!	`toConcurrentMap(keyMapper: Function<in T, out K>!, valueMapper: Function<in T, out U>!)` 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.
static Collector<T, *, ConcurrentMap<K, U>!>!	`toConcurrentMap(keyMapper: Function<in T, out K>!, valueMapper: Function<in T, out U>!, mergeFunction: BinaryOperator<U>!)` 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.
static Collector<T, *, M>!	`toConcurrentMap(keyMapper: Function<in T, out K>!, valueMapper: Function<in T, out U>!, mergeFunction: BinaryOperator<U>!, mapFactory: Supplier<M>!)` 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.
static Collector<T, *, MutableList<T>!>!	`toList()` Returns a `Collector` that accumulates the input elements into a new `List`.
static Collector<T, *, MutableMap<K, U>!>!	`toMap(keyMapper: Function<in T, out K>!, valueMapper: Function<in T, out U>!)` 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.
static Collector<T, *, MutableMap<K, U>!>!	`toMap(keyMapper: Function<in T, out K>!, valueMapper: Function<in T, out U>!, mergeFunction: BinaryOperator<U>!)` 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.
static Collector<T, *, M>!	`toMap(keyMapper: Function<in T, out K>!, valueMapper: Function<in T, out U>!, mergeFunction: BinaryOperator<U>!, mapFactory: Supplier<M>!)` 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.
static Collector<T, *, MutableSet<T>!>!	`toSet()` Returns a `Collector` that accumulates the input elements into a new `Set`.
static Collector<T, *, MutableList<T>!>!	`toUnmodifiableList()` Returns a `Collector` that accumulates the input elements into an unmodifiable List in encounter order.
static Collector<T, *, MutableMap<K, U>!>!	`toUnmodifiableMap(keyMapper: Function<in T, out K>!, valueMapper: Function<in T, out U>!)` 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.
static Collector<T, *, MutableMap<K, U>!>!	`toUnmodifiableMap(keyMapper: Function<in T, out K>!, valueMapper: Function<in T, out U>!, mergeFunction: BinaryOperator<U>!)` 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.
static Collector<T, *, MutableSet<T>!>!	`toUnmodifiableSet()` Returns a `Collector` that accumulates the input elements into an unmodifiable Set.

Public methods

averagingDouble

Added in API level 24

static fun <T : Any!> averagingDouble(mapper: ToDoubleFunction<in T>!): Collector<T, *, Double!>!

Returns a Collector that produces the arithmetic mean of a double-valued function applied to the input elements. If no elements are present, the result is 0.

The average returned can vary depending upon the order in which values are recorded, due to accumulated rounding error in addition of values of differing magnitudes. Values sorted by increasing absolute magnitude tend to yield more accurate results. If any recorded value is a NaN or the sum is at any point a NaN then the average will be NaN.

Parameters
`<T>`	the type of the input elements
`mapper`	ToDoubleFunction<in T>!: a function extracting the property to be averaged

Return
`Collector<T, *, Double!>!`	a `Collector` that produces the arithmetic mean of a derived property

averagingInt

Added in API level 24

static fun <T : Any!> averagingInt(mapper: ToIntFunction<in T>!): Collector<T, *, Double!>!

Returns a Collector that produces the arithmetic mean of an integer-valued function applied to the input elements. If no elements are present, the result is 0.

Parameters
`<T>`	the type of the input elements
`mapper`	ToIntFunction<in T>!: a function extracting the property to be averaged

Return
`Collector<T, *, Double!>!`	a `Collector` that produces the arithmetic mean of a derived property

averagingLong

Added in API level 24

static fun <T : Any!> averagingLong(mapper: ToLongFunction<in T>!): Collector<T, *, Double!>!

Returns a Collector that produces the arithmetic mean of a long-valued function applied to the input elements. If no elements are present, the result is 0.

Parameters
`<T>`	the type of the input elements
`mapper`	ToLongFunction<in T>!: a function extracting the property to be averaged

Return
`Collector<T, *, Double!>!`	a `Collector` that produces the arithmetic mean of a derived property

collectingAndThen

Added in API level 24

static fun <T : Any!, A : Any!, R : Any!, RR : Any!> collectingAndThen(
    downstream: Collector<T, A, R>!, 
    finisher: Function<R, RR>!
): Collector<T, A, RR>!

Adapts a Collector to perform an additional finishing transformation. For example, one could adapt the toList() collector to always produce an immutable list with:

<code>List&lt;String&gt; list = people.stream().collect(
    collectingAndThen(toList(),
                      Collections::unmodifiableList));
  </code>

Parameters
`<T>`	the type of the input elements
`<A>`	intermediate accumulation type of the downstream collector
`<R>`	result type of the downstream collector
`<RR>`	result type of the resulting collector
`downstream`	Collector<T, A, R>!: a collector
`finisher`	Function<R, RR>!: a function to be applied to the final result of the downstream collector

Return
`Collector<T, A, RR>!`	a collector which performs the action of the downstream collector, followed by an additional finishing step

counting

Added in API level 24

static fun <T : Any!> counting(): Collector<T, *, Long!>!

Returns a Collector accepting elements of type T that counts the number of input elements. If no elements are present, the result is 0.

Parameters
`<T>`	the type of the input elements

Return
`Collector<T, *, Long!>!`	a `Collector` that counts the input elements

filtering

Added in API level 33

static fun <T : Any!, A : Any!, R : Any!> filtering(
    predicate: Predicate<in T>!, 
    downstream: Collector<in T, A, R>!
): Collector<T, *, R>!

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.

Parameters
`<T>`	the type of the input elements
`<A>`	intermediate accumulation type of the downstream collector
`<R>`	result type of collector
`predicate`	Predicate<in T>!: a predicate to be applied to the input elements
`downstream`	Collector<in T, A, R>!: a collector which will accept values that match the predicate

Return
`Collector<T, *, R>!`	a collector which applies the predicate to the input elements and provides matching elements to the downstream collector

flatMapping

Added in API level 33

static fun <T : Any!, U : Any!, A : Any!, R : Any!> flatMapping(
    mapper: Function<in T, out Stream<out U>!>!, 
    downstream: Collector<in U, A, R>!
): Collector<T, *, R>!

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. The flat mapping function maps an input element to a stream covering zero or more output elements that are then accumulated downstream. Each mapped stream is closed after its contents have been placed downstream. (If a mapped stream is null an empty stream is used, instead.)

Parameters
`<T>`	the type of the input elements
`<U>`	type of elements accepted by downstream collector
`<A>`	intermediate accumulation type of the downstream collector
`<R>`	result type of collector
`mapper`	Function<in T, out Stream<out U>!>!: a function to be applied to the input elements, which returns a stream of results
`downstream`	Collector<in U, A, R>!: a collector which will receive the elements of the stream returned by mapper

Return
`Collector<T, *, R>!`	a collector which applies the mapping function to the input elements and provides the flat mapped results to the downstream collector

groupingBy

Added in API level 24

static fun <T : Any!, K : Any!> groupingBy(classifier: Function<in T, out K>!): Collector<T, *, MutableMap<K, MutableList<T>!>!>!

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.

The classification function maps elements to some key type K. The collector produces a Map<K, List<T>> whose keys are the values resulting from applying the classification function to the input elements, and whose corresponding values are Lists containing the input elements which map to the associated key under the classification function.

There are no guarantees on the type, mutability, serializability, or thread-safety of the Map or List objects returned.

Parameters
`<T>`	the type of the input elements
`<K>`	the type of the keys
`classifier`	Function<in T, out K>!: the classifier function mapping input elements to keys

Return
`Collector<T, *, MutableMap<K, MutableList<T>!>!>!`	a `Collector` implementing the group-by operation

See Also

#groupingBy(Function, Collector)
#groupingBy(Function, Supplier, Collector)
#groupingByConcurrent(Function)

groupingBy

Added in API level 24

static fun <T : Any!, K : Any!, A : Any!, D : Any!> groupingBy(
    classifier: Function<in T, out K>!, 
    downstream: Collector<in T, A, D>!
): Collector<T, *, MutableMap<K, D>!>!

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.

The classification function maps elements to some key type K. The downstream collector operates on elements of type T and produces a result of type D. The resulting collector produces a Map<K, D>.

There are no guarantees on the type, mutability, serializability, or thread-safety of the Map returned.

For example, to compute the set of last names of people in each city:

<code>Map&lt;City, Set&lt;String&gt;&gt; namesByCity
    = people.stream().collect(
      groupingBy(Person::getCity,
                 mapping(Person::getLastName,
                         toSet())));
  </code>

Parameters
`<T>`	the type of the input elements
`<K>`	the type of the keys
`<A>`	the intermediate accumulation type of the downstream collector
`<D>`	the result type of the downstream reduction
`classifier`	Function<in T, out K>!: a classifier function mapping input elements to keys
`downstream`	Collector<in T, A, D>!: a `Collector` implementing the downstream reduction

Return
`Collector<T, *, MutableMap<K, D>!>!`	a `Collector` implementing the cascaded group-by operation

See Also

#groupingBy(Function)
#groupingBy(Function, Supplier, Collector)
#groupingByConcurrent(Function, Collector)

groupingBy

Added in API level 24

static fun <T : Any!, K : Any!, D : Any!, A : Any!, M : MutableMap<K, D>!> groupingBy(
    classifier: Function<in T, out K>!, 
    mapFactory: Supplier<M>!, 
    downstream: Collector<in T, A, D>!
): Collector<T, *, M>!

For example, to compute the set of last names of people in each city, where the city names are sorted:

<code>Map&lt;City, Set&lt;String&gt;&gt; namesByCity
    = people.stream().collect(
      groupingBy(Person::getCity,
                 TreeMap::new,
                 mapping(Person::getLastName,
                         toSet())));
  </code>

Parameters
`<T>`	the type of the input elements
`<K>`	the type of the keys
`<A>`	the intermediate accumulation type of the downstream collector
`<D>`	the result type of the downstream reduction
`<M>`	the type of the resulting `Map`
`classifier`	Function<in T, out K>!: a classifier function mapping input elements to keys
`downstream`	Collector<in T, A, D>!: a `Collector` implementing the downstream reduction
`mapFactory`	Supplier<M>!: a supplier providing a new empty `Map` into which the results will be inserted

Return
`Collector<T, *, M>!`	a `Collector` implementing the cascaded group-by operation

See Also

#groupingBy(Function, Collector)
#groupingBy(Function)
#groupingByConcurrent(Function, Supplier, Collector)

groupingByConcurrent

Added in API level 24

static fun <T : Any!, K : Any!> groupingByConcurrent(classifier: Function<in T, out K>!): Collector<T, *, ConcurrentMap<K, MutableList<T>!>!>!

Returns a concurrent Collector implementing a "group by" operation on input elements of type T, grouping elements according to a classification function.

This is a concurrent and unordered Collector.

The classification function maps elements to some key type K. The collector produces a ConcurrentMap<K, List<T>> whose keys are the values resulting from applying the classification function to the input elements, and whose corresponding values are Lists containing the input elements which map to the associated key under the classification function.

There are no guarantees on the type, mutability, or serializability of the ConcurrentMap or List objects returned, or of the thread-safety of the List objects returned.

Parameters
`<T>`	the type of the input elements
`<K>`	the type of the keys
`classifier`	Function<in T, out K>!: a classifier function mapping input elements to keys

Return
`Collector<T, *, ConcurrentMap<K, MutableList<T>!>!>!`	a concurrent, unordered `Collector` implementing the group-by operation

See Also

#groupingBy(Function)
#groupingByConcurrent(Function, Collector)
#groupingByConcurrent(Function, Supplier, Collector)

groupingByConcurrent

Added in API level 24

static fun <T : Any!, K : Any!, A : Any!, D : Any!> groupingByConcurrent(
    classifier: Function<in T, out K>!, 
    downstream: Collector<in T, A, D>!
): Collector<T, *, ConcurrentMap<K, D>!>!

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.

This is a concurrent and unordered Collector.

There are no guarantees on the type, mutability, or serializability of the ConcurrentMap returned.

For example, to compute the set of last names of people in each city, where the city names are sorted:

<code>ConcurrentMap&lt;City, Set&lt;String&gt;&gt; namesByCity
    = people.stream().collect(
      groupingByConcurrent(Person::getCity,
                           mapping(Person::getLastName,
                                   toSet())));
  </code>

Parameters
`<T>`	the type of the input elements
`<K>`	the type of the keys
`<A>`	the intermediate accumulation type of the downstream collector
`<D>`	the result type of the downstream reduction
`classifier`	Function<in T, out K>!: a classifier function mapping input elements to keys
`downstream`	Collector<in T, A, D>!: a `Collector` implementing the downstream reduction

Return
`Collector<T, *, ConcurrentMap<K, D>!>!`	a concurrent, unordered `Collector` implementing the cascaded group-by operation

See Also

#groupingBy(Function, Collector)
#groupingByConcurrent(Function)
#groupingByConcurrent(Function, Supplier, Collector)

groupingByConcurrent

Added in API level 24

static fun <T : Any!, K : Any!, A : Any!, D : Any!, M : ConcurrentMap<K, D>!> groupingByConcurrent(
    classifier: Function<in T, out K>!, 
    mapFactory: Supplier<M>!, 
    downstream: Collector<in T, A, D>!
): Collector<T, *, M>!

This is a concurrent and unordered Collector.

For example, to compute the set of last names of people in each city, where the city names are sorted:

<code>ConcurrentMap&lt;City, Set&lt;String&gt;&gt; namesByCity
    = people.stream().collect(
      groupingByConcurrent(Person::getCity,
                           ConcurrentSkipListMap::new,
                           mapping(Person::getLastName,
                                   toSet())));
  </code>

Parameters
`<T>`	the type of the input elements
`<K>`	the type of the keys
`<A>`	the intermediate accumulation type of the downstream collector
`<D>`	the result type of the downstream reduction
`<M>`	the type of the resulting `ConcurrentMap`
`classifier`	Function<in T, out K>!: a classifier function mapping input elements to keys
`downstream`	Collector<in T, A, D>!: a `Collector` implementing the downstream reduction
`mapFactory`	Supplier<M>!: a supplier providing a new empty `ConcurrentMap` into which the results will be inserted

Return
`Collector<T, *, M>!`	a concurrent, unordered `Collector` implementing the cascaded group-by operation

See Also

#groupingByConcurrent(Function)
#groupingByConcurrent(Function, Collector)
#groupingBy(Function, Supplier, Collector)

joining

Added in API level 24

static fun joining(): Collector<CharSequence!, *, String!>!

Returns a Collector that concatenates the input elements into a String, in encounter order.

Return
`Collector<CharSequence!, *, String!>!`	a `Collector` that concatenates the input elements into a `String`, in encounter order

joining

Added in API level 24

static fun joining(delimiter: CharSequence!): Collector<CharSequence!, *, String!>!

Returns a Collector that concatenates the input elements, separated by the specified delimiter, in encounter order.

Parameters
`delimiter`	CharSequence!: the delimiter to be used between each element

Return
`Collector<CharSequence!, *, String!>!`	A `Collector` which concatenates CharSequence elements, separated by the specified delimiter, in encounter order

joining

Added in API level 24

static fun joining(
    delimiter: CharSequence!, 
    prefix: CharSequence!, 
    suffix: CharSequence!
): Collector<CharSequence!, *, String!>!

Returns a Collector that concatenates the input elements, separated by the specified delimiter, with the specified prefix and suffix, in encounter order.

Parameters
`delimiter`	CharSequence!: the delimiter to be used between each element
`prefix`	CharSequence!: the sequence of characters to be used at the beginning of the joined result
`suffix`	CharSequence!: the sequence of characters to be used at the end of the joined result

Return
`Collector<CharSequence!, *, String!>!`	A `Collector` which concatenates CharSequence elements, separated by the specified delimiter, in encounter order

mapping

Added in API level 24

static fun <T : Any!, U : Any!, A : Any!, R : Any!> mapping(
    mapper: Function<in T, out U>!, 
    downstream: Collector<in U, A, R>!
): Collector<T, *, R>!

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.

Parameters
`<T>`	the type of the input elements
`<U>`	type of elements accepted by downstream collector
`<A>`	intermediate accumulation type of the downstream collector
`<R>`	result type of collector
`mapper`	Function<in T, out U>!: a function to be applied to the input elements
`downstream`	Collector<in U, A, R>!: a collector which will accept mapped values

Return
`Collector<T, *, R>!`	a collector which applies the mapping function to the input elements and provides the mapped results to the downstream collector

maxBy

Added in API level 24

static fun <T : Any!> maxBy(comparator: Comparator<in T>!): Collector<T, *, Optional<T>!>!

Returns a Collector that produces the maximal element according to a given Comparator, described as an Optional<T>.

Parameters
`<T>`	the type of the input elements
`comparator`	Comparator<in T>!: a `Comparator` for comparing elements

Return
`Collector<T, *, Optional<T>!>!`	a `Collector` that produces the maximal value

minBy

Added in API level 24

static fun <T : Any!> minBy(comparator: Comparator<in T>!): Collector<T, *, Optional<T>!>!

Returns a Collector that produces the minimal element according to a given Comparator, described as an Optional<T>.

Parameters
`<T>`	the type of the input elements
`comparator`	Comparator<in T>!: a `Comparator` for comparing elements

Return
`Collector<T, *, Optional<T>!>!`	a `Collector` that produces the minimal value

partitioningBy

Added in API level 24

static fun <T : Any!> partitioningBy(predicate: Predicate<in T>!): Collector<T, *, MutableMap<Boolean!, MutableList<T>!>!>!

Returns a Collector which partitions the input elements according to a Predicate, and organizes them into a Map<Boolean, List<T>>. The returned Map always contains mappings for both false and true keys. There are no guarantees on the type, mutability, serializability, or thread-safety of the Map or List returned.

Parameters
`<T>`	the type of the input elements
`predicate`	Predicate<in T>!: a predicate used for classifying input elements

Return
`Collector<T, *, MutableMap<Boolean!, MutableList<T>!>!>!`	a `Collector` implementing the partitioning operation

See Also

#partitioningBy(Predicate, Collector)

partitioningBy

Added in API level 24

static fun <T : Any!, D : Any!, A : Any!> partitioningBy(
    predicate: Predicate<in T>!, 
    downstream: Collector<in T, A, D>!
): Collector<T, *, MutableMap<Boolean!, D>!>!

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.

The returned Map always contains mappings for both false and true keys. There are no guarantees on the type, mutability, serializability, or thread-safety of the Map returned.

Parameters
`<T>`	the type of the input elements
`<A>`	the intermediate accumulation type of the downstream collector
`<D>`	the result type of the downstream reduction
`predicate`	Predicate<in T>!: a predicate used for classifying input elements
`downstream`	Collector<in T, A, D>!: a `Collector` implementing the downstream reduction

Return
`Collector<T, *, MutableMap<Boolean!, D>!>!`	a `Collector` implementing the cascaded partitioning operation

See Also

#partitioningBy(Predicate)

reducing

Added in API level 24

static fun <T : Any!> reducing(
    identity: T, 
    op: BinaryOperator<T>!
): Collector<T, *, T>!

Returns a Collector which performs a reduction of its input elements under a specified BinaryOperator using the provided identity.

Parameters
`<T>`	element type for the input and output of the reduction
`identity`	T: the identity value for the reduction (also, the value that is returned when there are no input elements)
`op`	BinaryOperator<T>!: a `BinaryOperator<T>` used to reduce the input elements

Return
`Collector<T, *, T>!`	a `Collector` which implements the reduction operation

See Also

#reducing(BinaryOperator)
#reducing(Object, Function, BinaryOperator)

reducing

Added in API level 24

static fun <T : Any!> reducing(op: BinaryOperator<T>!): Collector<T, *, Optional<T>!>!

Returns a Collector which performs a reduction of its input elements under a specified BinaryOperator. The result is described as an Optional<T>.

Parameters
`<T>`	element type for the input and output of the reduction
`op`	BinaryOperator<T>!: a `BinaryOperator<T>` used to reduce the input elements

Return
`Collector<T, *, Optional<T>!>!`	a `Collector` which implements the reduction operation

See Also

#reducing(Object, BinaryOperator)
#reducing(Object, Function, BinaryOperator)

reducing

Added in API level 24

static fun <T : Any!, U : Any!> reducing(
    identity: U, 
    mapper: Function<in T, out U>!, 
    op: BinaryOperator<U>!
): Collector<T, *, U>!

Returns a Collector which performs a reduction of its input elements under a specified mapping function and BinaryOperator. This is a generalization of reducing(java.lang.Object,java.util.function.BinaryOperator) which allows a transformation of the elements before reduction.

Parameters
`<T>`	the type of the input elements
`<U>`	the type of the mapped values
`identity`	U: the identity value for the reduction (also, the value that is returned when there are no input elements)
`mapper`	Function<in T, out U>!: a mapping function to apply to each input value
`op`	BinaryOperator<U>!: a `BinaryOperator<U>` used to reduce the mapped values

Return
`Collector<T, *, U>!`	a `Collector` implementing the map-reduce operation

See Also

#reducing(Object, BinaryOperator)
#reducing(BinaryOperator)

summarizingDouble

Added in API level 24

static fun <T : Any!> summarizingDouble(mapper: ToDoubleFunction<in T>!): Collector<T, *, DoubleSummaryStatistics!>!

Returns a Collector which applies an double-producing mapping function to each input element, and returns summary statistics for the resulting values.

Parameters
`<T>`	the type of the input elements
`mapper`	ToDoubleFunction<in T>!: a mapping function to apply to each element

Return
`Collector<T, *, DoubleSummaryStatistics!>!`	a `Collector` implementing the summary-statistics reduction

See Also

#summarizingLong(ToLongFunction)
#summarizingInt(ToIntFunction)

summarizingInt

Added in API level 24

static fun <T : Any!> summarizingInt(mapper: ToIntFunction<in T>!): Collector<T, *, IntSummaryStatistics!>!

Returns a Collector which applies an int-producing mapping function to each input element, and returns summary statistics for the resulting values.

Parameters
`<T>`	the type of the input elements
`mapper`	ToIntFunction<in T>!: a mapping function to apply to each element

Return
`Collector<T, *, IntSummaryStatistics!>!`	a `Collector` implementing the summary-statistics reduction

See Also

#summarizingDouble(ToDoubleFunction)
#summarizingLong(ToLongFunction)

summarizingLong

Added in API level 24

static fun <T : Any!> summarizingLong(mapper: ToLongFunction<in T>!): Collector<T, *, LongSummaryStatistics!>!

Returns a Collector which applies an long-producing mapping function to each input element, and returns summary statistics for the resulting values.

Parameters
`<T>`	the type of the input elements
`mapper`	ToLongFunction<in T>!: the mapping function to apply to each element

Return
`Collector<T, *, LongSummaryStatistics!>!`	a `Collector` implementing the summary-statistics reduction

See Also

#summarizingDouble(ToDoubleFunction)
#summarizingInt(ToIntFunction)

summingDouble

Added in API level 24

static fun <T : Any!> summingDouble(mapper: ToDoubleFunction<in T>!): Collector<T, *, Double!>!

Returns a Collector that produces the sum of a double-valued function applied to the input elements. If no elements are present, the result is 0.

The sum returned can vary depending upon the order in which values are recorded, due to accumulated rounding error in addition of values of differing magnitudes. Values sorted by increasing absolute magnitude tend to yield more accurate results. If any recorded value is a NaN or the sum is at any point a NaN then the sum will be NaN.

Parameters
`<T>`	the type of the input elements
`mapper`	ToDoubleFunction<in T>!: a function extracting the property to be summed

Return
`Collector<T, *, Double!>!`	a `Collector` that produces the sum of a derived property

summingInt

Added in API level 24

static fun <T : Any!> summingInt(mapper: ToIntFunction<in T>!): Collector<T, *, Int!>!

Returns a Collector that produces the sum of a integer-valued function applied to the input elements. If no elements are present, the result is 0.

Parameters
`<T>`	the type of the input elements
`mapper`	ToIntFunction<in T>!: a function extracting the property to be summed

Return
`Collector<T, *, Int!>!`	a `Collector` that produces the sum of a derived property

summingLong

Added in API level 24

static fun <T : Any!> summingLong(mapper: ToLongFunction<in T>!): Collector<T, *, Long!>!

Returns a Collector that produces the sum of a long-valued function applied to the input elements. If no elements are present, the result is 0.

Parameters
`<T>`	the type of the input elements
`mapper`	ToLongFunction<in T>!: a function extracting the property to be summed

Return
`Collector<T, *, Long!>!`	a `Collector` that produces the sum of a derived property

teeing

Added in API level 35

static fun <T : Any!, R1 : Any!, R2 : Any!, R : Any!> teeing(
    downstream1: Collector<in T, *, R1>!, 
    downstream2: Collector<in T, *, R2>!, 
    merger: BiFunction<in R1, in R2, R>!
): Collector<T, *, R>!

Returns a Collector that is a composite of two downstream collectors. Every element passed to the resulting collector is processed by both downstream collectors, then their results are merged using the specified merge function into the final result.

The resulting collector functions do the following:

supplier: creates a result container that contains result containers obtained by calling each collector's supplier
accumulator: calls each collector's accumulator with its result container and the input element
combiner: calls each collector's combiner with two result containers
finisher: calls each collector's finisher with its result container, then calls the supplied merger and returns its result.

The resulting collector is Collector.Characteristics#UNORDERED if both downstream collectors are unordered and Collector.Characteristics#CONCURRENT if both downstream collectors are concurrent.

Parameters
`<T>`	the type of the input elements
`<R1>`	the result type of the first collector
`<R2>`	the result type of the second collector
`<R>`	the final result type
`downstream1`	Collector<in T, *, R1>!: the first downstream collector
`downstream2`	Collector<in T, *, R2>!: the second downstream collector
`merger`	BiFunction<in R1, in R2, R>!: the function which merges two results into the single one

Return
`Collector<T, *, R>!`	a `Collector` which aggregates the results of two supplied collectors.

toCollection

Added in API level 24

static fun <T : Any!, C : MutableCollection<T>!> toCollection(collectionFactory: Supplier<C>!): Collector<T, *, C>!

Returns a Collector that accumulates the input elements into a new Collection, in encounter order. The Collection is created by the provided factory.

Parameters
`<T>`	the type of the input elements
`<C>`	the type of the resulting `Collection`
`collectionFactory`	Supplier<C>!: a supplier providing a new empty `Collection` into which the results will be inserted

Return
`Collector<T, *, C>!`	a `Collector` which collects all the input elements into a `Collection`, in encounter order

toConcurrentMap

Added in API level 24

static fun <T : Any!, K : Any!, U : Any!> toConcurrentMap(
    keyMapper: Function<in T, out K>!, 
    valueMapper: Function<in T, out U>!
): Collector<T, *, ConcurrentMap<K, U>!>!

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.

If the mapped keys contain duplicates (according to Object#equals(Object)), an IllegalStateException is thrown when the collection operation is performed. If the mapped keys may have duplicates, use toConcurrentMap(java.util.function.Function,java.util.function.Function,java.util.function.BinaryOperator) instead.

There are no guarantees on the type, mutability, or serializability of the ConcurrentMap returned.

Parameters
`<T>`	the type of the input elements
`<K>`	the output type of the key mapping function
`<U>`	the output type of the value mapping function
`keyMapper`	Function<in T, out K>!: the mapping function to produce keys
`valueMapper`	Function<in T, out U>!: the mapping function to produce values

Return
`Collector<T, *, ConcurrentMap<K, U>!>!`	a concurrent, unordered `Collector` which collects elements into a `ConcurrentMap` whose keys are the result of applying a key mapping function to the input elements, and whose values are the result of applying a value mapping function to the input elements

See Also

#toMap(Function, Function)
#toConcurrentMap(Function, Function, BinaryOperator)
#toConcurrentMap(Function, Function, BinaryOperator, Supplier)

toConcurrentMap

Added in API level 24

static fun <T : Any!, K : Any!, U : Any!> toConcurrentMap(
    keyMapper: Function<in T, out K>!, 
    valueMapper: Function<in T, out U>!, 
    mergeFunction: BinaryOperator<U>!
): Collector<T, *, ConcurrentMap<K, U>!>!

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.

If the mapped keys contain duplicates (according to Object#equals(Object)), the value mapping function is applied to each equal element, and the results are merged using the provided merging function.

There are no guarantees on the type, mutability, or serializability of the ConcurrentMap returned.

Parameters
`<T>`	the type of the input elements
`<K>`	the output type of the key mapping function
`<U>`	the output type of the value mapping function
`keyMapper`	Function<in T, out K>!: a mapping function to produce keys
`valueMapper`	Function<in T, out U>!: a mapping function to produce values
`mergeFunction`	BinaryOperator<U>!: a merge function, used to resolve collisions between values associated with the same key, as supplied to `Map#merge(Object, Object, BiFunction)`

Return
`Collector<T, *, ConcurrentMap<K, U>!>!`	a concurrent, unordered `Collector` which collects elements into a `ConcurrentMap` whose keys are the result of applying a key mapping function to the input elements, and whose values are the result of applying a value mapping function to all input elements equal to the key and combining them using the merge function

See Also

#toConcurrentMap(Function, Function)
#toConcurrentMap(Function, Function, BinaryOperator, Supplier)
#toMap(Function, Function, BinaryOperator)

toConcurrentMap

Added in API level 24

static fun <T : Any!, K : Any!, U : Any!, M : ConcurrentMap<K, U>!> toConcurrentMap(
    keyMapper: Function<in T, out K>!, 
    valueMapper: Function<in T, out U>!, 
    mergeFunction: BinaryOperator<U>!, 
    mapFactory: Supplier<M>!
): Collector<T, *, M>!

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.

This is a concurrent and unordered Collector.

Parameters
`<T>`	the type of the input elements
`<K>`	the output type of the key mapping function
`<U>`	the output type of the value mapping function
`<M>`	the type of the resulting `ConcurrentMap`
`keyMapper`	Function<in T, out K>!: a mapping function to produce keys
`valueMapper`	Function<in T, out U>!: a mapping function to produce values
`mergeFunction`	BinaryOperator<U>!: a merge function, used to resolve collisions between values associated with the same key, as supplied to `Map#merge(Object, Object, BiFunction)`
`mapFactory`	Supplier<M>!: a supplier providing a new empty `ConcurrentMap` into which the results will be inserted

Return
`Collector<T, *, M>!`	a concurrent, unordered `Collector` which collects elements into a `ConcurrentMap` whose keys are the result of applying a key mapping function to the input elements, and whose values are the result of applying a value mapping function to all input elements equal to the key and combining them using the merge function

See Also

#toConcurrentMap(Function, Function)
#toConcurrentMap(Function, Function, BinaryOperator)
#toMap(Function, Function, BinaryOperator, Supplier)

toList

Added in API level 24

static fun <T : Any!> toList(): Collector<T, *, MutableList<T>!>!

Returns a Collector that accumulates the input elements into a new List. There are no guarantees on the type, mutability, serializability, or thread-safety of the List returned; if more control over the returned List is required, use toCollection(java.util.function.Supplier).

Parameters
`<T>`	the type of the input elements

Return
`Collector<T, *, MutableList<T>!>!`	a `Collector` which collects all the input elements into a `List`, in encounter order

toMap

Added in API level 24

static fun <T : Any!, K : Any!, U : Any!> toMap(
    keyMapper: Function<in T, out K>!, 
    valueMapper: Function<in T, out U>!
): Collector<T, *, MutableMap<K, U>!>!

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.

If the mapped keys contain duplicates (according to Object#equals(Object)), an IllegalStateException is thrown when the collection operation is performed. If the mapped keys might have duplicates, use toMap(java.util.function.Function,java.util.function.Function,java.util.function.BinaryOperator) instead.

There are no guarantees on the type, mutability, serializability, or thread-safety of the Map returned.

Parameters
`<T>`	the type of the input elements
`<K>`	the output type of the key mapping function
`<U>`	the output type of the value mapping function
`keyMapper`	Function<in T, out K>!: a mapping function to produce keys
`valueMapper`	Function<in T, out U>!: a mapping function to produce values

Return
`Collector<T, *, MutableMap<K, U>!>!`	a `Collector` which collects elements into a `Map` whose keys and values are the result of applying mapping functions to the input elements

See Also

#toMap(Function, Function, BinaryOperator)
#toMap(Function, Function, BinaryOperator, Supplier)
#toConcurrentMap(Function, Function)

toMap

Added in API level 24

static fun <T : Any!, K : Any!, U : Any!> toMap(
    keyMapper: Function<in T, out K>!, 
    valueMapper: Function<in T, out U>!, 
    mergeFunction: BinaryOperator<U>!
): Collector<T, *, MutableMap<K, U>!>!

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.

There are no guarantees on the type, mutability, serializability, or thread-safety of the Map returned.

Parameters
`<T>`	the type of the input elements
`<K>`	the output type of the key mapping function
`<U>`	the output type of the value mapping function
`keyMapper`	Function<in T, out K>!: a mapping function to produce keys
`valueMapper`	Function<in T, out U>!: a mapping function to produce values
`mergeFunction`	BinaryOperator<U>!: a merge function, used to resolve collisions between values associated with the same key, as supplied to `Map#merge(Object, Object, BiFunction)`

Return
`Collector<T, *, MutableMap<K, U>!>!`	a `Collector` which collects elements into a `Map` whose keys are the result of applying a key mapping function to the input elements, and whose values are the result of applying a value mapping function to all input elements equal to the key and combining them using the merge function

See Also

#toMap(Function, Function)
#toMap(Function, Function, BinaryOperator, Supplier)
#toConcurrentMap(Function, Function, BinaryOperator)

toMap

Added in API level 24

static fun <T : Any!, K : Any!, U : Any!, M : MutableMap<K, U>!> toMap(
    keyMapper: Function<in T, out K>!, 
    valueMapper: Function<in T, out U>!, 
    mergeFunction: BinaryOperator<U>!, 
    mapFactory: Supplier<M>!
): Collector<T, *, M>!

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.

Parameters
`<T>`	the type of the input elements
`<K>`	the output type of the key mapping function
`<U>`	the output type of the value mapping function
`<M>`	the type of the resulting `Map`
`keyMapper`	Function<in T, out K>!: a mapping function to produce keys
`valueMapper`	Function<in T, out U>!: a mapping function to produce values
`mergeFunction`	BinaryOperator<U>!: a merge function, used to resolve collisions between values associated with the same key, as supplied to `Map#merge(Object, Object, BiFunction)`
`mapFactory`	Supplier<M>!: a supplier providing a new empty `Map` into which the results will be inserted

Return
`Collector<T, *, M>!`	a `Collector` which collects elements into a `Map` whose keys are the result of applying a key mapping function to the input elements, and whose values are the result of applying a value mapping function to all input elements equal to the key and combining them using the merge function

See Also

#toMap(Function, Function)
#toMap(Function, Function, BinaryOperator)
#toConcurrentMap(Function, Function, BinaryOperator, Supplier)

toSet

Added in API level 24

static fun <T : Any!> toSet(): Collector<T, *, MutableSet<T>!>!

Returns a Collector that accumulates the input elements into a new Set. There are no guarantees on the type, mutability, serializability, or thread-safety of the Set returned; if more control over the returned Set is required, use toCollection(java.util.function.Supplier).

This is an unordered Collector.

Parameters
`<T>`	the type of the input elements

Return
`Collector<T, *, MutableSet<T>!>!`	a `Collector` which collects all the input elements into a `Set`

toUnmodifiableList

Added in API level 33

static fun <T : Any!> toUnmodifiableList(): Collector<T, *, MutableList<T>!>!

Returns a Collector that accumulates the input elements into an unmodifiable List in encounter order. The returned Collector disallows null values and will throw NullPointerException if it is presented with a null value.

Parameters
`<T>`	the type of the input elements

Return
`Collector<T, *, MutableList<T>!>!`	a `Collector` that accumulates the input elements into an unmodifiable List in encounter order

toUnmodifiableMap

Added in API level 33

static fun <T : Any!, K : Any!, U : Any!> toUnmodifiableMap(
    keyMapper: Function<in T, out K>!, 
    valueMapper: Function<in T, out U>!
): Collector<T, *, MutableMap<K, U>!>!

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.

If the mapped keys contain duplicates (according to Object#equals(Object)), an IllegalStateException is thrown when the collection operation is performed. If the mapped keys might have duplicates, use toUnmodifiableMap(java.util.function.Function,java.util.function.Function,java.util.function.BinaryOperator) to handle merging of the values.

The returned Collector disallows null keys and values. If either mapping function returns null, NullPointerException will be thrown.

Parameters
`<T>`	the type of the input elements
`<K>`	the output type of the key mapping function
`<U>`	the output type of the value mapping function
`keyMapper`	Function<in T, out K>!: a mapping function to produce keys, must be non-null
`valueMapper`	Function<in T, out U>!: a mapping function to produce values, must be non-null

Return
`Collector<T, *, MutableMap<K, U>!>!`	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

Exceptions
`java.lang.NullPointerException`	if either keyMapper or valueMapper is null

See Also

#toUnmodifiableMap(Function, Function, BinaryOperator)

toUnmodifiableMap

Added in API level 33

static fun <T : Any!, K : Any!, U : Any!> toUnmodifiableMap(
    keyMapper: Function<in T, out K>!, 
    valueMapper: Function<in T, out U>!, 
    mergeFunction: BinaryOperator<U>!
): Collector<T, *, MutableMap<K, U>!>!

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.

The returned Collector disallows null keys and values. If either mapping function returns null, NullPointerException will be thrown.

Parameters
`<T>`	the type of the input elements
`<K>`	the output type of the key mapping function
`<U>`	the output type of the value mapping function
`keyMapper`	Function<in T, out K>!: a mapping function to produce keys, must be non-null
`valueMapper`	Function<in T, out U>!: a mapping function to produce values, must be non-null
`mergeFunction`	BinaryOperator<U>!: a merge function, used to resolve collisions between values associated with the same key, as supplied to `Map#merge(Object, Object, BiFunction)`, must be non-null

Return
`Collector<T, *, MutableMap<K, U>!>!`	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

Exceptions
`java.lang.NullPointerException`	if the keyMapper, valueMapper, or mergeFunction is null

See Also

#toUnmodifiableMap(Function, Function)

toUnmodifiableSet

Added in API level 33

static fun <T : Any!> toUnmodifiableSet(): Collector<T, *, MutableSet<T>!>!

Returns a Collector that accumulates the input elements into an unmodifiable Set. The returned Collector disallows null values and will throw NullPointerException if it is presented with a null value. If the input contains duplicate elements, an arbitrary element of the duplicates is preserved.

This is an unordered Collector.

Parameters
`<T>`	the type of the input elements

Return
`Collector<T, *, MutableSet<T>!>!`	a `Collector` that accumulates the input elements into an unmodifiable Set