added in API level 24

Collectors

public final class Collectors
extends Object

java.lang.Object
   ↳ 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:

// 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));

 

Summary

Public methods

static <T> Collector<T, ?, Double> averagingDouble(ToDoubleFunction<? super T> mapper)

Returns a Collector that produces the arithmetic mean of a double-valued function applied to the input elements.

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, 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, 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 <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 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 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()

Returns a Collector that concatenates the input elements into a String, 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<BooleanList<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, 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, ?, 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> 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 a 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, 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, 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> mapSupplier)

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> 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> 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, 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> mapSupplier)

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)

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.

Inherited methods

Public methods

averagingDouble

added in API level 24
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.
Parameters
mapper ToDoubleFunction: a function extracting the property to be summed

Returns
Collector<T, ?, Double> a Collector that produces the sum of a derived property

averagingInt

added in API level 24
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.

Parameters
mapper ToIntFunction: a function extracting the property to be summed

Returns
Collector<T, ?, Double> a Collector that produces the sum of a derived property

averagingLong

added in API level 24
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.

Parameters
mapper ToLongFunction: a function extracting the property to be summed

Returns
Collector<T, ?, Double> a Collector that produces the sum of a derived property

collectingAndThen

added in API level 24
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> people
         = people.stream().collect(collectingAndThen(toList(), Collections::unmodifiableList));
 

Parameters
downstream Collector: a collector

finisher Function: a function to be applied to the final result of the downstream collector

Returns
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
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)
     
Returns
Collector<T, ?, Long> a Collector that counts the input elements

groupingBy

added in API level 24
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.
Parameters
classifier Function: the classifier function mapping input elements to keys

Returns
Collector<T, ?, Map<K, List<T>>> a Collector implementing the group-by operation

groupingBy

added in API level 24
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. The Map produced by the Collector is created with the supplied factory function.

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>.

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.
Parameters
classifier Function: a classifier function mapping input elements to keys

mapFactory Supplier: a function which, when called, produces a new empty Map of the desired type

downstream Collector: a Collector implementing the downstream reduction

Returns
Collector<T, ?, M> a Collector implementing the cascaded group-by operation

groupingBy

added in API level 24
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.
Parameters
classifier Function: a classifier function mapping input elements to keys

downstream Collector: a Collector implementing the downstream reduction

Returns
Collector<T, ?, Map<K, D>> a Collector implementing the cascaded group-by operation

groupingByConcurrent

added in API level 24
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.

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>.

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())));
 

Parameters
classifier Function: a classifier function mapping input elements to keys

downstream Collector: a Collector implementing the downstream reduction

Returns
Collector<T, ?, ConcurrentMap<K, D>> a concurrent, unordered Collector implementing the cascaded group-by operation

groupingByConcurrent

added in API level 24
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 Map 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());
     
Parameters
classifier Function: a classifier function mapping input elements to keys

Returns
Collector<T, ?, ConcurrentMap<K, List<T>>> a concurrent, unordered Collector implementing the group-by operation

groupingByConcurrent

added in API level 24
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. The ConcurrentMap produced by the Collector is created with the supplied factory function.

This is a concurrent and unordered 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>.

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(groupingBy(Person::getCity, ConcurrentSkipListMap::new,
                                              mapping(Person::getLastName, toSet())));
 

Parameters
classifier Function: a classifier function mapping input elements to keys

mapFactory Supplier: a function which, when called, produces a new empty ConcurrentMap of the desired type

downstream Collector: a Collector implementing the downstream reduction

Returns
Collector<T, ?, M> a concurrent, unordered Collector implementing the cascaded group-by operation

joining

added in API level 24
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 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

Returns
Collector<CharSequence, ?, String> A Collector which concatenates CharSequence elements, separated by the specified delimiter, in encounter order

joining

added in API level 24
Collector<CharSequence, ?, String> joining (CharSequence delimiter)

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

Returns
Collector<CharSequence, ?, String> A Collector which concatenates CharSequence elements, separated by the specified delimiter, in encounter order

joining

added in API level 24
Collector<CharSequence, ?, String> joining ()

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

Returns
Collector<CharSequence, ?, String> a Collector that concatenates the input elements into a String, in encounter order

mapping

added in API level 24
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())));
     
Parameters
mapper Function: a function to be applied to the input elements

downstream Collector: a collector which will accept mapped values

Returns
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
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))
     
Parameters
comparator Comparator: a Comparator for comparing elements

Returns
Collector<T, ?, Optional<T>> a Collector that produces the maximal value

minBy

added in API level 24
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))
     
Parameters
comparator Comparator: a Comparator for comparing elements

Returns
Collector<T, ?, Optional<T>> a Collector that produces the minimal value

partitioningBy

added in API level 24
Collector<T, ?, Map<BooleanList<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>>. There are no guarantees on the type, mutability, serializability, or thread-safety of the Map returned.

Parameters
predicate Predicate: a predicate used for classifying input elements

Returns
Collector<T, ?, Map<BooleanList<T>>> a Collector implementing the partitioning operation

partitioningBy

added in API level 24
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.

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

Parameters
predicate Predicate: a predicate used for classifying input elements

downstream Collector: a Collector implementing the downstream reduction

Returns
Collector<T, ?, Map<Boolean, D>> a Collector implementing the cascaded partitioning operation

reducing

added in API level 24
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:
  • 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 reduce(BinaryOperator) instead.

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

    Comparator<Person> byHeight = Comparator.comparing(Person::getHeight);
         Map<City, Person> tallestByCity
             = people.stream().collect(groupingBy(Person::getCity, reducing(BinaryOperator.maxBy(byHeight))));
     
Parameters
op BinaryOperator: a BinaryOperator<T> used to reduce the input elements

Returns
Collector<T, ?, Optional<T>> a Collector which implements the reduction operation

reducing

added in API level 24
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 map(Function) and 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))));
     
Parameters
identity U: the identity value for the reduction (also, the value that is returned when there are no input elements)

mapper Function: a mapping function to apply to each input value

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

Returns
Collector<T, ?, U> a Collector implementing the map-reduce operation

reducing

added in API level 24
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 reduce(Object, BinaryOperator)} instead.
Parameters
identity T: the identity value for the reduction (also, the value that is returned when there are no input elements)

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

Returns
Collector<T, ?, T> a Collector which implements the reduction operation

summarizingDouble

added in API level 24
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.

Parameters
mapper ToDoubleFunction: a mapping function to apply to each element

Returns
Collector<T, ?, DoubleSummaryStatistics> a Collector implementing the summary-statistics reduction

summarizingInt

added in API level 24
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.

Parameters
mapper ToIntFunction: a mapping function to apply to each element

Returns
Collector<T, ?, IntSummaryStatistics> a Collector implementing the summary-statistics reduction

summarizingLong

added in API level 24
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.

Parameters
mapper ToLongFunction: the mapping function to apply to each element

Returns
Collector<T, ?, LongSummaryStatistics> a Collector implementing the summary-statistics reduction

summingDouble

added in API level 24
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.

Parameters
mapper ToDoubleFunction: a function extracting the property to be summed

Returns
Collector<T, ?, Double> a Collector that produces the sum of a derived property

summingInt

added in API level 24
Collector<T, ?, Integer> summingInt (ToIntFunction<? super T> mapper)

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

Parameters
mapper ToIntFunction: a function extracting the property to be summed

Returns
Collector<T, ?, Integer> a Collector that produces the sum of a derived property

summingLong

added in API level 24
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.

Parameters
mapper ToLongFunction: a function extracting the property to be summed

Returns
Collector<T, ?, Long> a Collector that produces the sum of a derived property

toCollection

added in API level 24
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.

Parameters
collectionFactory Supplier: a Supplier which returns a new, empty Collection of the appropriate type

Returns
Collector<T, ?, C> a Collector which collects all the input elements into a Collection, in encounter order

toConcurrentMap

added in API level 24
Collector<T, ?, M> toConcurrentMap (Function<? super T, ? extends K> keyMapper, 
                Function<? super T, ? extends U> valueMapper, 
                BinaryOperator<U> mergeFunction, 
                Supplier<M> mapSupplier)

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 contains duplicates (according to equals(Object)), the value mapping function is applied to each equal element, and the results are merged using the provided merging function. The ConcurrentMap is created by a provided supplier function.

This is a concurrent and unordered Collector.

Parameters
keyMapper Function: a mapping function to produce keys

valueMapper Function: a mapping function to produce values

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

mapSupplier Supplier: a function which returns a new, empty Map into which the results will be inserted

Returns
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

toConcurrentMap

added in API level 24
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.

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

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 deals with these collisions, and produce a Map mapping names to a concatenated list of addresses:
    Map<String, String> phoneBook
             people.stream().collect(toConcurrentMap(Person::getName,
                                                     Person::getAddress,
                                                     (s, a) -> s + ", " + a));
     

    This is a concurrent and unordered Collector.

Parameters
keyMapper Function: a mapping function to produce keys

valueMapper Function: a mapping function to produce values

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

Returns
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

toConcurrentMap

added in API level 24
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 contains duplicates (according to 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.

API Note:
  • It is common for either the key or the value to be the input elements. In this case, the utility method 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(Functions.identity(),
                                             student -> computeGPA(student)));
     
    And the following produces a Map mapping a unique identifier to students:
    Map<String, Student> studentIdToStudent
             students.stream().collect(toConcurrentMap(Student::getId,
                                                       Functions.identity());
     

    This is a concurrent and unordered Collector.

Parameters
keyMapper Function: the mapping function to produce keys

valueMapper Function: the mapping function to produce values

Returns
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

toList

added in API level 24
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).

Returns
Collector<T, ?, List<T>> a Collector which collects all the input elements into a List, in encounter order

toMap

added in API level 24
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 contains duplicates (according to equals(Object)), the value mapping function is applied to each equal element, and the results are merged using the provided merging function.

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 deals 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.
Parameters
keyMapper Function: a mapping function to produce keys

valueMapper Function: a mapping function to produce values

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

Returns
Collector<T, ?, Map<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

toMap

added in API level 24
Collector<T, ?, M> toMap (Function<? super T, ? extends K> keyMapper, 
                Function<? super T, ? extends U> valueMapper, 
                BinaryOperator<U> mergeFunction, 
                Supplier<M> mapSupplier)

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 contains duplicates (according to equals(Object)), the value mapping function is applied to each equal element, and the results are merged using the provided merging function. The Map is created by a provided supplier function.

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.
Parameters
keyMapper Function: a mapping function to produce keys

valueMapper Function: a mapping function to produce values

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

mapSupplier Supplier: a function which returns a new, empty Map into which the results will be inserted

Returns
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

toMap

added in API level 24
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 contains duplicates (according to equals(Object)), an IllegalStateException is thrown when the collection operation is performed. If the mapped keys may have duplicates, use toMap(Function, Function, BinaryOperator) instead.

API Note:
  • It is common for either the key or the value to be the input elements. In this case, the utility method 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(Functions.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,
                                             Functions.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.
Parameters
keyMapper Function: a mapping function to produce keys

valueMapper Function: a mapping function to produce values

Returns
Collector<T, ?, Map<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

toSet

added in API level 24
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.

Returns
Collector<T, ?, Set<T>> a Collector which collects all the input elements into a Set