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Class Collectors

java.lang.Object

java.util.stream.Collectors

public final class Collectors extends Object

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:


 // Accumulate names into a List
 List<String> list = people.stream()
   .map(Person::getName)
   .collect(Collectors.toList());

 // Accumulate names into a TreeSet
 Set<String> 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<Department, List<Employee>> byDept = employees.stream()
   .collect(Collectors.groupingBy(Employee::getDepartment));

 // Compute sum of salaries by department
 Map<Department, Integer> totalByDept = employees.stream()
   .collect(Collectors.groupingBy(Employee::getDepartment,
                                  Collectors.summingInt(Employee::getSalary)));

 // Partition students into passing and failing
 Map<Boolean, List<Student>> passingFailing = students.stream()
   .collect(Collectors.partitioningBy(s -> s.getGrade() >= PASS_THRESHOLD));

Since:: 1.8

Method Summary

Modifier and Type	Method	Description
`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.
`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.
`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.
`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.
`static <T> Collector<T,?,Long>`	`counting()`	Returns a `Collector` accepting elements of type `T` that counts the number of input elements.
`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`.
`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.
`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`.
`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`.
`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`.
`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.
`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`.
`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`.
`static Collector<CharSequence,?,String>`	`joining()`	Returns a `Collector` that concatenates the input elements into a `String`, in encounter order.
`static Collector<CharSequence,?,String>`	`joining(CharSequence delimiter)`	Returns a `Collector` that concatenates the input elements, separated by the specified delimiter, in encounter order.
`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.
`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.
`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>`.
`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>`.
`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>>`.
`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.
`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`.
`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.
`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`.
`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.
`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.
`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.
`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.
`static <T> Collector<T,?,Integer>`	`summingInt(ToIntFunction<? super T> mapper)`	Returns a `Collector` that produces the sum of an integer-valued function applied to the input elements.
`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.
`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.
`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.
`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.
`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.
`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.
`static <T> Collector<T,?,List<T>>`	`toList()`	Returns a `Collector` that accumulates the input elements into a new `List`.
`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.
`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.
`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.
`static <T> Collector<T,?,Set<T>>`	`toSet()`	Returns a `Collector` that accumulates the input elements into a new `Set`.
`static <T> Collector<T,?,List<T>>`	`toUnmodifiableList()`	Returns a `Collector` that accumulates the input elements into an unmodifiable List in encounter order.
`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.
`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.
`static <T> Collector<T,?,Set<T>>`	`toUnmodifiableSet()`	Returns a `Collector` that accumulates the input elements into an unmodifiable Set.

Methods declared in class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Method Details

toCollection

public 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. The Collection is created by the provided factory.

Type Parameters:: T - the type of the input elements; C - the type of the resulting Collection
Parameters:: collectionFactory - a supplier providing a new empty Collection into which the results will be inserted
Returns:: a Collector which collects all the input elements into a Collection, in encounter order

toList

public static <T> Collector<T,?,List<T>> toList()

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(Supplier).

Type Parameters:: T - the type of the input elements
Returns:: a Collector which collects all the input elements into a List, in encounter order

toUnmodifiableList

public static <T> Collector<T,?,List<T>> toUnmodifiableList()

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.

Type Parameters:: T - the type of the input elements
Returns:: a Collector that accumulates the input elements into an unmodifiable List in encounter order
Since:: 10

toSet

public static <T> Collector<T,?,Set<T>> toSet()

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(Supplier).

This is an unordered Collector.

Type Parameters:: T - the type of the input elements
Returns:: a Collector which collects all the input elements into a Set

toUnmodifiableSet

public static <T> Collector<T,?,Set<T>> toUnmodifiableSet()

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.

Type Parameters:: T - the type of the input elements
Returns:: a Collector that accumulates the input elements into an unmodifiable Set
Since:: 10

joining

public static Collector<CharSequence,?,String> joining()

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

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

joining

public static Collector<CharSequence,?,String> joining(CharSequence delimiter)

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

Parameters:: delimiter - the delimiter to be used between each element
Returns:: A Collector which concatenates CharSequence elements, separated by the specified delimiter, in encounter order

joining

public 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.

Parameters:: delimiter - the delimiter to be used between each element; prefix - the sequence of characters to be used at the beginning of the joined result; suffix - the sequence of characters to be used at the end of the joined result
Returns:: A Collector which concatenates CharSequence elements, separated by the specified delimiter, in encounter order

mapping

public 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.

API Note:

The mapping() collectors are most useful when used in a multi-level reduction, such as downstream of a groupingBy or partitioningBy. For example, given a stream of Person, to accumulate the set of last names in each city:


 Map<City, Set<String>> lastNamesByCity
   = people.stream().collect(
     groupingBy(Person::getCity,
                mapping(Person::getLastName,
                        toSet())));

Type 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

Parameters:

mapper - a function to be applied to the input elements

downstream - a collector which will accept mapped values

Returns:

a collector which applies the mapping function to the input elements and provides the mapped results to the downstream collector

flatMapping

public 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. 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.)

API Note:

The flatMapping() collectors are most useful when used in a multi-level reduction, such as downstream of a groupingBy or partitioningBy. For example, given a stream of Order, to accumulate the set of line items for each customer:


 Map<String, Set<LineItem>> itemsByCustomerName
   = orders.stream().collect(
     groupingBy(Order::getCustomerName,
                flatMapping(order -> order.getLineItems().stream(),
                            toSet())));

Type 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

Parameters:

mapper - a function to be applied to the input elements, which returns a stream of results

downstream - a collector which will receive the elements of the stream returned by mapper

Returns:

a collector which applies the mapping function to the input elements and provides the flat mapped results to the downstream collector

Since:

filtering

public 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.

API Note:

The filtering() collectors are most useful when used in a multi-level reduction, such as downstream of a groupingBy or partitioningBy. For example, given a stream of Employee, to accumulate the employees in each department that have a salary above a certain threshold:


 Map<Department, Set<Employee>> wellPaidEmployeesByDepartment
   = employees.stream().collect(
     groupingBy(Employee::getDepartment,
                filtering(e -> e.getSalary() > 2000,
                          toSet())));

A filtering collector differs from a stream's filter() operation. In this example, suppose there are no employees whose salary is above the threshold in some department. Using a filtering collector as shown above would result in a mapping from that department to an empty Set. If a stream filter() operation were done instead, there would be no mapping for that department at all.

Type Parameters:

T - the type of the input elements

A - intermediate accumulation type of the downstream collector

R - result type of collector

Parameters:

predicate - a predicate to be applied to the input elements

downstream - a collector which will accept values that match the predicate

Returns:

a collector which applies the predicate to the input elements and provides matching elements to the downstream collector

Since:

collectingAndThen

public 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. For example, one could adapt the toList() collector to always produce an immutable list with:


 List<String> list = people.stream().collect(
   collectingAndThen(toList(),
                     Collections::unmodifiableList));

Type 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
Parameters:: downstream - a collector; finisher - a function to be applied to the final result of the downstream collector
Returns:: a collector which performs the action of the downstream collector, followed by an additional finishing step

counting

public static <T> Collector<T,?,Long> counting()

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

Implementation Requirements:

This produces a result equivalent to:


     reducing(0L, e -> 1L, Long::sum)

Type Parameters:

T - the type of the input elements

Returns:

a Collector that counts the input elements

minBy

public 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>.

Implementation Requirements:

This produces a result equivalent to:


     reducing(BinaryOperator.minBy(comparator))

Type Parameters:

T - the type of the input elements

Parameters:

comparator - a Comparator for comparing elements

Returns:

a Collector that produces the minimal value

maxBy

public 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>.

Implementation Requirements:

This produces a result equivalent to:


     reducing(BinaryOperator.maxBy(comparator))

Type Parameters:

T - the type of the input elements

Parameters:

comparator - a Comparator for comparing elements

Returns:

a Collector that produces the maximal value

summingInt

public static <T> Collector<T,?,Integer> summingInt(ToIntFunction<? super T> mapper)

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

Type Parameters:: T - the type of the input elements
Parameters:: mapper - a function extracting the property to be summed
Returns:: a Collector that produces the sum of a derived property

summingLong

public 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. If no elements are present, the result is 0.

Type Parameters:: T - the type of the input elements
Parameters:: mapper - a function extracting the property to be summed
Returns:: a Collector that produces the sum of a derived property

summingDouble

public 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. 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.

Type Parameters:: T - the type of the input elements
Parameters:: mapper - a function extracting the property to be summed
Returns:: a Collector that produces the sum of a derived property

averagingInt

public 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. If no elements are present, the result is 0.

Type Parameters:: T - the type of the input elements
Parameters:: mapper - a function extracting the property to be averaged
Returns:: a Collector that produces the arithmetic mean of a derived property

averagingLong

public 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. If no elements are present, the result is 0.

Type Parameters:: T - the type of the input elements
Parameters:: mapper - a function extracting the property to be averaged
Returns:: a Collector that produces the arithmetic mean of a derived property

averagingDouble

public 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. 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.

Implementation Note:: The double format can represent all consecutive integers in the range -2⁵³ to 2⁵³. If the pipeline has more than 2⁵³ values, the divisor in the average computation will saturate at 2⁵³, leading to additional numerical errors.
Type Parameters:: T - the type of the input elements
Parameters:: mapper - a function extracting the property to be averaged
Returns:: a Collector that produces the arithmetic mean of a derived property

reducing

public 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.

API Note:

The reducing() collectors are most useful when used in a multi-level reduction, downstream of groupingBy or partitioningBy. To perform a simple reduction on a stream, use Stream.reduce(Object, BinaryOperator)} instead.

Type Parameters:

T - element type for the input and output of the reduction

Parameters:

identity - the identity value for the reduction (also, the value that is returned when there are no input elements)

op - a BinaryOperator<T> used to reduce the input elements

Returns:

a Collector which implements the reduction operation

See Also:

reducing

public 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. The result is described as an Optional<T>.

API Note:

For example, given a stream of Person, to calculate tallest person in each city:


 Comparator<Person> byHeight = Comparator.comparing(Person::getHeight);
 Map<City, Optional<Person>> tallestByCity
   = people.stream().collect(
     groupingBy(Person::getCity,
                reducing(BinaryOperator.maxBy(byHeight))));

Type Parameters:

T - element type for the input and output of the reduction

Parameters:

op - a BinaryOperator<T> used to reduce the input elements

Returns:

a Collector which implements the reduction operation

See Also:

reducing

public 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. This is a generalization of reducing(Object, BinaryOperator) which allows a transformation of the elements before reduction.

API Note:

The reducing() collectors are most useful when used in a multi-level reduction, downstream of groupingBy or partitioningBy. To perform a simple map-reduce on a stream, use Stream.map(Function) and Stream.reduce(Object, BinaryOperator) instead.

For example, given a stream of Person, to calculate the longest last name of residents in each city:


 Comparator<String> byLength = Comparator.comparing(String::length);
 Map<City, String> longestLastNameByCity
   = people.stream().collect(
     groupingBy(Person::getCity,
                reducing("",
                         Person::getLastName,
                         BinaryOperator.maxBy(byLength))));

Type Parameters:

T - the type of the input elements

U - the type of the mapped values

Parameters:

identity - the identity value for the reduction (also, the value that is returned when there are no input elements)

mapper - a mapping function to apply to each input value

op - a BinaryOperator<U> used to reduce the mapped values

Returns:

a Collector implementing the map-reduce operation

See Also:

groupingBy

public 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.

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.

Implementation Requirements:

This produces a result similar to:


     groupingBy(classifier, toList());

Implementation Note:

The returned Collector is not concurrent. For parallel stream pipelines, the combiner function operates by merging the keys from one map into another, which can be an expensive operation. If preservation of the order in which elements appear in the resulting Map collector is not required, using groupingByConcurrent(Function) may offer better parallel performance.

Type Parameters:

T - the type of the input elements

K - the type of the keys

Parameters:

classifier - the classifier function mapping input elements to keys

Returns:

a Collector implementing the group-by operation

See Also:

groupingBy

public 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.

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:


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

Implementation Note:

The returned Collector is not concurrent. For parallel stream pipelines, the combiner function operates by merging the keys from one map into another, which can be an expensive operation. If preservation of the order in which elements are presented to the downstream collector is not required, using groupingByConcurrent(Function, Collector) may offer better parallel performance.

Type 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

Parameters:

classifier - a classifier function mapping input elements to keys

downstream - a Collector implementing the downstream reduction

Returns:

a Collector implementing the cascaded group-by operation

See Also:

groupingBy

public 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)

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


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

Implementation Note:

The returned Collector is not concurrent. For parallel stream pipelines, the combiner function operates by merging the keys from one map into another, which can be an expensive operation. If preservation of the order in which elements are presented to the downstream collector is not required, using groupingByConcurrent(Function, Supplier, Collector) may offer better parallel performance.

Type Parameters:

T - the type of the input elements

K - the type of the keys

D - the result type of the downstream reduction

A - the intermediate accumulation type of the downstream collector

M - the type of the resulting Map

Parameters:

classifier - a classifier function mapping input elements to keys

mapFactory - a supplier providing a new empty Map into which the results will be inserted

downstream - a Collector implementing the downstream reduction

Returns:

a Collector implementing the cascaded group-by operation

See Also:

groupingByConcurrent

public 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.

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.

Implementation Requirements:

This produces a result similar to:


     groupingByConcurrent(classifier, toList());

Type Parameters:

T - the type of the input elements

K - the type of the keys

Parameters:

classifier - a classifier function mapping input elements to keys

Returns:

a concurrent, unordered Collector implementing the group-by operation

See Also:

groupingByConcurrent

public 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.

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:


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

Type 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

Parameters:

classifier - a classifier function mapping input elements to keys

downstream - a Collector implementing the downstream reduction

Returns:

a concurrent, unordered Collector implementing the cascaded group-by operation

See Also:

groupingByConcurrent

public 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)

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:


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

Type 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

Parameters:

classifier - a classifier function mapping input elements to keys

mapFactory - a supplier providing a new empty ConcurrentMap into which the results will be inserted

downstream - a Collector implementing the downstream reduction

Returns:

a concurrent, unordered Collector implementing the cascaded group-by operation

See Also:

partitioningBy

public 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>>. 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.

API Note:

If a partition has no elements, its value in the result Map will be an empty List.

Type Parameters:

T - the type of the input elements

Parameters:

predicate - a predicate used for classifying input elements

Returns:

a Collector implementing the partitioning operation

See Also:

partitioningBy

public 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.

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.

API Note:

If a partition has no elements, its value in the result Map will be obtained by calling the downstream collector's supplier function and then applying the finisher function.

Type Parameters:

T - the type of the input elements

D - the result type of the downstream reduction

A - the intermediate accumulation type of the downstream collector

Parameters:

predicate - a predicate used for classifying input elements

downstream - a Collector implementing the downstream reduction

Returns:

a Collector implementing the cascaded partitioning operation

See Also:

toMap

public 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.

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(Function, Function, BinaryOperator) instead.

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

API Note:

It is common for either the key or the value to be the input elements. In this case, the utility method Function.identity() may be helpful. For example, the following produces a Map mapping students to their grade point average:


 Map<Student, Double> studentToGPA
   = students.stream().collect(
     toMap(Function.identity(),
           student -> computeGPA(student)));

And the following produces a Map mapping a unique identifier to students:


 Map<String, Student> studentIdToStudent
   = students.stream().collect(
     toMap(Student::getId,
           Function.identity()));

Implementation Note:

The returned Collector is not concurrent. For parallel stream pipelines, the combiner function operates by merging the keys from one map into another, which can be an expensive operation. If it is not required that results are inserted into the Map in encounter order, using toConcurrentMap(Function, Function) may offer better parallel performance.

Type 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

Parameters:

keyMapper - a mapping function to produce keys

valueMapper - a mapping function to produce values

Returns:

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:

toUnmodifiableMap

public 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.

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

Type 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

Parameters:

keyMapper - a mapping function to produce keys, must be non-null

valueMapper - a mapping function to produce values, must be non-null

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

Throws:

NullPointerException - if either keyMapper or valueMapper is null

Since:

See Also:

toMap

public 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.

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, serializability, or thread-safety of the Map returned.

API Note:

There are multiple ways to deal with collisions between multiple elements mapping to the same key. The other forms of toMap simply use a merge function that throws unconditionally, but you can easily write more flexible merge policies. For example, if you have a stream of Person, and you want to produce a "phone book" mapping name to address, but it is possible that two persons have the same name, you can do as follows to gracefully deal with these collisions, and produce a Map mapping names to a concatenated list of addresses:


 Map<String, String> phoneBook
   = people.stream().collect(
     toMap(Person::getName,
           Person::getAddress,
           (s, a) -> s + ", " + a));

Implementation Note:

The returned Collector is not concurrent. For parallel stream pipelines, the combiner function operates by merging the keys from one map into another, which can be an expensive operation. If it is not required that results are merged into the Map in encounter order, using toConcurrentMap(Function, Function, BinaryOperator) may offer better parallel performance.

Type 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

Parameters:

keyMapper - a mapping function to produce keys

valueMapper - a mapping function to produce values

mergeFunction - a merge function, used to resolve collisions between values associated with the same key, as supplied to Map.merge(Object, Object, BiFunction)

Returns:

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:

toUnmodifiableMap

public 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.

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

Type 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

Parameters:

keyMapper - a mapping function to produce keys, must be non-null

valueMapper - a mapping function to produce values, must be non-null

mergeFunction - 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

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

Throws:

NullPointerException - if the keyMapper, valueMapper, or mergeFunction is null

Since:

See Also:

toMap

public 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.

Implementation Note:

The returned Collector is not concurrent. For parallel stream pipelines, the combiner function operates by merging the keys from one map into another, which can be an expensive operation. If it is not required that results are merged into the Map in encounter order, using toConcurrentMap(Function, Function, BinaryOperator, Supplier) may offer better parallel performance.

Type 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

Parameters:

keyMapper - a mapping function to produce keys

valueMapper - a mapping function to produce values

mergeFunction - a merge function, used to resolve collisions between values associated with the same key, as supplied to Map.merge(Object, Object, BiFunction)

mapFactory - a supplier providing a new empty Map into which the results will be inserted

Returns:

See Also:

toConcurrentMap

public 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.

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(Function, Function, BinaryOperator) instead.

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

API Note:

It is common for either the key or the value to be the input elements. In this case, the utility method Function.identity() may be helpful. For example, the following produces a ConcurrentMap mapping students to their grade point average:


 ConcurrentMap<Student, Double> studentToGPA
   = students.stream().collect(
     toConcurrentMap(Function.identity(),
                     student -> computeGPA(student)));

And the following produces a ConcurrentMap mapping a unique identifier to students:


 ConcurrentMap<String, Student> studentIdToStudent
   = students.stream().collect(
     toConcurrentMap(Student::getId,
                     Function.identity()));

This is a concurrent and unordered Collector.

Type 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

Parameters:

keyMapper - the mapping function to produce keys

valueMapper - the mapping function to produce values

Returns:

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:

toConcurrentMap

public 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.

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

API Note:

There are multiple ways to deal with collisions between multiple elements mapping to the same key. The other forms of toConcurrentMap simply use a merge function that throws unconditionally, but you can easily write more flexible merge policies. For example, if you have a stream of Person, and you want to produce a "phone book" mapping name to address, but it is possible that two persons have the same name, you can do as follows to gracefully deal with these collisions, and produce a ConcurrentMap mapping names to a concatenated list of addresses:


 ConcurrentMap<String, String> phoneBook
   = people.stream().collect(
     toConcurrentMap(Person::getName,
                     Person::getAddress,
                     (s, a) -> s + ", " + a));

This is a concurrent and unordered Collector.

Type 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

Parameters:

keyMapper - a mapping function to produce keys

valueMapper - a mapping function to produce values

mergeFunction - a merge function, used to resolve collisions between values associated with the same key, as supplied to Map.merge(Object, Object, BiFunction)

Returns:

See Also:

toConcurrentMap

public 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.

This is a concurrent and unordered Collector.

Type 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

Parameters:

keyMapper - a mapping function to produce keys

valueMapper - a mapping function to produce values

mergeFunction - a merge function, used to resolve collisions between values associated with the same key, as supplied to Map.merge(Object, Object, BiFunction)

mapFactory - a supplier providing a new empty ConcurrentMap into which the results will be inserted

Returns:

See Also:

summarizingInt

public 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.

Type Parameters:

T - the type of the input elements

Parameters:

mapper - a mapping function to apply to each element

Returns:

a Collector implementing the summary-statistics reduction

See Also:

summarizingLong

public 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.

Type Parameters:

T - the type of the input elements

Parameters:

mapper - the mapping function to apply to each element

Returns:

a Collector implementing the summary-statistics reduction

See Also:

summarizingDouble

public 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.

Type Parameters:

T - the type of the input elements

Parameters:

mapper - a mapping function to apply to each element

Returns:

a Collector implementing the summary-statistics reduction

See Also:

teeing

public 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. 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.

Type 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
Parameters:: downstream1 - the first downstream collector; downstream2 - the second downstream collector; merger - the function which merges two results into the single one
Returns:: a Collector which aggregates the results of two supplied collectors.
Since:: 12

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https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/stream/Collectors.html