Class Collectors

java.lang.Object
java.util.stream.Collectors
public final classCollectorsextendsObject
Implementations ofCollector 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 aCollector 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 aCollector 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 aCollector 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 aCollector to perform an additional finishing transformation.
    static <T> Collector<T,?,Long>
    Returns aCollector accepting elements of typeT 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 aCollector to one accepting elements of the same typeT by applying the predicate to each input element and only accumulating if the predicate returnstrue.
    static <T,U,A,R> Collector<T,?,R>
    flatMapping(Function<? super T, ? extendsStream<? extends U>> mapper,Collector<? super U, A, R> downstream)
    Adapts aCollector accepting elements of typeU to one accepting elements of typeT 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 aCollector implementing a "group by" operation on input elements of typeT, grouping elements according to a classification function, and returning the results in aMap.
    static <T, K, D, A, M extendsMap<K,D>>
    Collector<T,?,M>
    groupingBy(Function<? super T, ? extends K> classifier,Supplier<M> mapFactory,Collector<? super T, A, D> downstream)
    Returns aCollector implementing a cascaded "group by" operation on input elements of typeT, grouping elements according to a classification function, and then performing a reduction operation on the values associated with a given key using the specified downstreamCollector.
    static <T,K,A,D> Collector<T,?,Map<K,D>>
    groupingBy(Function<? super T, ? extends K> classifier,Collector<? super T, A, D> downstream)
    Returns aCollector implementing a cascaded "group by" operation on input elements of typeT, grouping elements according to a classification function, and then performing a reduction operation on the values associated with a given key using the specified downstreamCollector.
    static <T,K> Collector<T, ?,ConcurrentMap<K,List<T>>>
    groupingByConcurrent(Function<? super T, ? extends K> classifier)
    Returns a concurrentCollector implementing a "group by" operation on input elements of typeT, grouping elements according to a classification function.
    static <T, K, A, D, M extendsConcurrentMap<K,D>>
    Collector<T,?,M>
    groupingByConcurrent(Function<? super T, ? extends K> classifier,Supplier<M> mapFactory,Collector<? super T, A, D> downstream)
    Returns a concurrentCollector implementing a cascaded "group by" operation on input elements of typeT, grouping elements according to a classification function, and then performing a reduction operation on the values associated with a given key using the specified downstreamCollector.
    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 concurrentCollector implementing a cascaded "group by" operation on input elements of typeT, grouping elements according to a classification function, and then performing a reduction operation on the values associated with a given key using the specified downstreamCollector.
    Returns aCollector that concatenates the input elements into aString, in encounter order.
    joining(CharSequence delimiter)
    Returns aCollector that concatenates the input elements, separated by the specified delimiter, in encounter order.
    joining(CharSequence delimiter,CharSequence prefix,CharSequence suffix)
    Returns aCollector 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 aCollector accepting elements of typeU to one accepting elements of typeT by applying a mapping function to each input element before accumulation.
    static <T> Collector<T, ?,Optional<T>>
    maxBy(Comparator<? super T> comparator)
    Returns aCollector that produces the maximal element according to a givenComparator, described as anOptional<T>.
    static <T> Collector<T, ?,Optional<T>>
    minBy(Comparator<? super T> comparator)
    Returns aCollector that produces the minimal element according to a givenComparator, described as anOptional<T>.
    static <T> Collector<T, ?,Map<Boolean,List<T>>>
    partitioningBy(Predicate<? super T> predicate)
    Returns aCollector which partitions the input elements according to aPredicate, and organizes them into aMap<Boolean, List<T>>.
    static <T,D,A> Collector<T, ?,Map<Boolean,D>>
    partitioningBy(Predicate<? super T> predicate,Collector<? super T, A, D> downstream)
    Returns aCollector which partitions the input elements according to aPredicate, reduces the values in each partition according to anotherCollector, and organizes them into aMap<Boolean, D> whose values are the result of the downstream reduction.
    static <T> Collector<T, ?,Optional<T>>
    Returns aCollector which performs a reduction of its input elements under a specifiedBinaryOperator.
    static <T> Collector<T,?,T>
    reducing(T identity,BinaryOperator<T> op)
    Returns aCollector which performs a reduction of its input elements under a specifiedBinaryOperator using the provided identity.
    static <T,U> Collector<T,?,U>
    reducing(U identity,Function<? super T, ? extends U> mapper,BinaryOperator<U> op)
    Returns aCollector which performs a reduction of its input elements under a specified mapping function andBinaryOperator.
    Returns aCollector which applies andouble-producing mapping function to each input element, and returns summary statistics for the resulting values.
    summarizingInt(ToIntFunction<? super T> mapper)
    Returns aCollector which applies anint-producing mapping function to each input element, and returns summary statistics for the resulting values.
    summarizingLong(ToLongFunction<? super T> mapper)
    Returns aCollector which applies anlong-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 aCollector 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 aCollector 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 aCollector 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 aCollector that is a composite of two downstream collectors.
    static <T, C extendsCollection<T>>
    Collector<T,?,C>
    toCollection(Supplier<C> collectionFactory)
    Returns aCollector that accumulates the input elements into a newCollection, 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 concurrentCollector that accumulates elements into aConcurrentMap 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 concurrentCollector that accumulates elements into aConcurrentMap whose keys and values are the result of applying the provided mapping functions to the input elements.
    static <T, K, U, M extendsConcurrentMap<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 concurrentCollector that accumulates elements into aConcurrentMap whose keys and values are the result of applying the provided mapping functions to the input elements.
    static <T> Collector<T,?,List<T>>
    Returns aCollector that accumulates the input elements into a newList.
    static <T,K,U> Collector<T,?,Map<K,U>>
    toMap(Function<? super T, ? extends K> keyMapper,Function<? super T, ? extends U> valueMapper)
    Returns aCollector that accumulates elements into aMap 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 aCollector that accumulates elements into aMap whose keys and values are the result of applying the provided mapping functions to the input elements.
    static <T, K, U, M extendsMap<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 aCollector that accumulates elements into aMap whose keys and values are the result of applying the provided mapping functions to the input elements.
    static <T> Collector<T,?,Set<T>>
    Returns aCollector that accumulates the input elements into a newSet.
    static <T> Collector<T,?,List<T>>
    Returns aCollector that accumulates the input elements into anunmodifiable 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 aCollector that accumulates the input elements into anunmodifiable 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 aCollector that accumulates the input elements into anunmodifiable Map, whose keys and values are the result of applying the provided mapping functions to the input elements.
    static <T> Collector<T,?,Set<T>>
    Returns aCollector that accumulates the input elements into anunmodifiable Set.

  • Method Details

    • toCollection

      public static <T, C extendsCollection<T>>Collector<T,?,C> toCollection(Supplier<C> collectionFactory)
      Returns aCollector that accumulates the input elements into a newCollection, in encounter order. TheCollection is created by the provided factory.
      Type Parameters:
      T - the type of the input elements
      C - the type of the resultingCollection
      Parameters:
      collectionFactory - a supplier providing a new emptyCollection into which the results will be inserted
      Returns:
      aCollector which collects all the input elements into aCollection, in encounter order

    • toList

      public static <T> Collector<T,?,List<T>> toList()
      Returns aCollector that accumulates the input elements into a newList. There are no guarantees on the type, mutability, serializability, or thread-safety of theList returned; if more control over the returnedList is required, usetoCollection(Supplier).
      Type Parameters:
      T - the type of the input elements
      Returns:
      aCollector which collects all the input elements into aList, in encounter order

    • toUnmodifiableList

      public static <T> Collector<T,?,List<T>> toUnmodifiableList()
      Returns aCollector that accumulates the input elements into anunmodifiable List in encounter order. The returned Collector disallows null values and will throwNullPointerException if it is presented with a null value.
      Type Parameters:
      T - the type of the input elements
      Returns:
      aCollector that accumulates the input elements into anunmodifiable List in encounter order
      Since:
      10

    • toSet

      public static <T> Collector<T,?,Set<T>> toSet()
      Returns aCollector that accumulates the input elements into a newSet. There are no guarantees on the type, mutability, serializability, or thread-safety of theSet returned; if more control over the returnedSet is required, usetoCollection(Supplier).

      This is anunordered Collector.

      Type Parameters:
      T - the type of the input elements
      Returns:
      aCollector which collects all the input elements into aSet

    • toUnmodifiableSet

      public static <T> Collector<T,?,Set<T>> toUnmodifiableSet()
      Returns aCollector that accumulates the input elements into anunmodifiable Set. The returned Collector disallows null values and will throwNullPointerException if it is presented with a null value. If the input contains duplicate elements, an arbitrary element of the duplicates is preserved.

      This is anunordered Collector.

      Type Parameters:
      T - the type of the input elements
      Returns:
      aCollector that accumulates the input elements into anunmodifiable Set
      Since:
      10

    • joining

      public static Collector<CharSequence, ?,String> joining()
      Returns aCollector that concatenates the input elements into aString, in encounter order.
      Returns:
      aCollector that concatenates the input elements into aString, in encounter order

    • joining

      public static Collector<CharSequence, ?,String> joining(CharSequence delimiter)
      Returns aCollector that concatenates the input elements, separated by the specified delimiter, in encounter order.
      Parameters:
      delimiter - the delimiter to be used between each element
      Returns:
      ACollector 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 aCollector 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:
      ACollector 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 aCollector accepting elements of typeU to one accepting elements of typeT by applying a mapping function to each input element before accumulation.
      API Note:
      Themapping() collectors are most useful when used in a multi-level reduction, such as downstream of agroupingBy orpartitioningBy. For example, given a stream ofPerson, 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, ? extendsStream<? extends U>> mapper,Collector<? super U, A, R> downstream)
      Adapts aCollector accepting elements of typeU to one accepting elements of typeT by applying a flat mapping function to each input element before accumulation. The flat mapping function maps an input element to astream covering zero or more output elements that are then accumulated downstream. Each mapped stream isclosed after its contents have been placed downstream. (If a mapped stream isnull an empty stream is used, instead.)
      API Note:
      TheflatMapping() collectors are most useful when used in a multi-level reduction, such as downstream of agroupingBy orpartitioningBy. For example, given a stream ofOrder, 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:
      9

    • filtering

      public static <T,A,R> Collector<T,?,R> filtering(Predicate<? super T> predicate,Collector<? super T, A, R> downstream)
      Adapts aCollector to one accepting elements of the same typeT by applying the predicate to each input element and only accumulating if the predicate returnstrue.
      API Note:
      Thefiltering() collectors are most useful when used in a multi-level reduction, such as downstream of agroupingBy orpartitioningBy. For example, given a stream ofEmployee, 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'sfilter() 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 emptySet. If a streamfilter() 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:
      9

    • collectingAndThen

      public static <T,A,R,RR> Collector<T,A,RR> collectingAndThen(Collector<T,A,R> downstream,Function<R,RR> finisher)
      Adapts aCollector to perform an additional finishing transformation. For example, one could adapt thetoList() 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 aCollector accepting elements of typeT 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:
      aCollector that counts the input elements

    • minBy

      public static <T> Collector<T, ?,Optional<T>> minBy(Comparator<? super T> comparator)
      Returns aCollector that produces the minimal element according to a givenComparator, described as anOptional<T>.
      Implementation Requirements:
      This produces a result equivalent to:
           reducing(BinaryOperator.minBy(comparator))
      Type Parameters:
      T - the type of the input elements
      Parameters:
      comparator - aComparator for comparing elements
      Returns:
      aCollector that produces the minimal value

    • maxBy

      public static <T> Collector<T, ?,Optional<T>> maxBy(Comparator<? super T> comparator)
      Returns aCollector that produces the maximal element according to a givenComparator, described as anOptional<T>.
      Implementation Requirements:
      This produces a result equivalent to:
           reducing(BinaryOperator.maxBy(comparator))
      Type Parameters:
      T - the type of the input elements
      Parameters:
      comparator - aComparator for comparing elements
      Returns:
      aCollector that produces the maximal value

    • summingInt

      public static <T> Collector<T,?,Integer> summingInt(ToIntFunction<? super T> mapper)
      Returns aCollector 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:
      aCollector that produces the sum of a derived property

    • summingLong

      public static <T> Collector<T,?,Long> summingLong(ToLongFunction<? super T> mapper)
      Returns aCollector 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:
      aCollector that produces the sum of a derived property

    • summingDouble

      public static <T> Collector<T,?,Double> summingDouble(ToDoubleFunction<? super T> mapper)
      Returns aCollector 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 aNaN or the sum is at any point aNaN then the sum will beNaN.

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

    • averagingInt

      public static <T> Collector<T,?,Double> averagingInt(ToIntFunction<? super T> mapper)
      Returns aCollector 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:
      aCollector that produces the arithmetic mean of a derived property

    • averagingLong

      public static <T> Collector<T,?,Double> averagingLong(ToLongFunction<? super T> mapper)
      Returns aCollector 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:
      aCollector that produces the arithmetic mean of a derived property

    • averagingDouble

      public static <T> Collector<T,?,Double> averagingDouble(ToDoubleFunction<? super T> mapper)
      Returns aCollector 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 aNaN or the sum is at any point aNaN then the average will beNaN.

      Implementation Note:
      Thedouble 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:
      aCollector that produces the arithmetic mean of a derived property

    • reducing

      public static <T> Collector<T,?,T> reducing(T identity,BinaryOperator<T> op)
      Returns aCollector which performs a reduction of its input elements under a specifiedBinaryOperator using the provided identity.
      API Note:
      Thereducing() collectors are most useful when used in a multi-level reduction, downstream ofgroupingBy orpartitioningBy. To perform a simple reduction on a stream, useStream.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 - aBinaryOperator<T> used to reduce the input elements
      Returns:
      aCollector which implements the reduction operation
      See Also:

    • reducing

      public static <T> Collector<T, ?,Optional<T>> reducing(BinaryOperator<T> op)
      Returns aCollector which performs a reduction of its input elements under a specifiedBinaryOperator. The result is described as anOptional<T>.
      API Note:
      Thereducing() collectors are most useful when used in a multi-level reduction, downstream ofgroupingBy orpartitioningBy. To perform a simple reduction on a stream, useStream.reduce(BinaryOperator) instead.

      For example, given a stream ofPerson, 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 - aBinaryOperator<T> used to reduce the input elements
      Returns:
      aCollector 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 aCollector which performs a reduction of its input elements under a specified mapping function andBinaryOperator. This is a generalization ofreducing(Object, BinaryOperator) which allows a transformation of the elements before reduction.
      API Note:
      Thereducing() collectors are most useful when used in a multi-level reduction, downstream ofgroupingBy orpartitioningBy. To perform a simple map-reduce on a stream, useStream.map(Function) andStream.reduce(Object, BinaryOperator) instead.

      For example, given a stream ofPerson, 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 - aBinaryOperator<U> used to reduce the mapped values
      Returns:
      aCollector 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 aCollector implementing a "group by" operation on input elements of typeT, grouping elements according to a classification function, and returning the results in aMap.

      The classification function maps elements to some key typeK. The collector produces aMap<K, List<T>> whose keys are the values resulting from applying the classification function to the input elements, and whose corresponding values areLists 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 theMap orList objects returned.

      Implementation Requirements:
      This produces a result similar to:
           groupingBy(classifier, toList());
      Implementation Note:
      The returnedCollector is not concurrent. For parallel stream pipelines, thecombiner 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 resultingMap collector is not required, usinggroupingByConcurrent(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:
      aCollector 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 aCollector implementing a cascaded "group by" operation on input elements of typeT, grouping elements according to a classification function, and then performing a reduction operation on the values associated with a given key using the specified downstreamCollector.

      The classification function maps elements to some key typeK. The downstream collector operates on elements of typeT and produces a result of typeD. The resulting collector produces aMap<K, D>.

      There are no guarantees on the type, mutability, serializability, or thread-safety of theMap 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 returnedCollector is not concurrent. For parallel stream pipelines, thecombiner 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, usinggroupingByConcurrent(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 - aCollector implementing the downstream reduction
      Returns:
      aCollector implementing the cascaded group-by operation
      See Also:

    • groupingBy

      public static <T, K, D, A, M extendsMap<K,D>>Collector<T,?,M> groupingBy(Function<? super T, ? extends K> classifier,Supplier<M> mapFactory,Collector<? super T, A, D> downstream)
      Returns aCollector implementing a cascaded "group by" operation on input elements of typeT, grouping elements according to a classification function, and then performing a reduction operation on the values associated with a given key using the specified downstreamCollector. TheMap produced by the Collector is created with the supplied factory function.

      The classification function maps elements to some key typeK. The downstream collector operates on elements of typeT and produces a result of typeD. The resulting collector produces aMap<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 returnedCollector is not concurrent. For parallel stream pipelines, thecombiner 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, usinggroupingByConcurrent(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 resultingMap
      Parameters:
      classifier - a classifier function mapping input elements to keys
      mapFactory - a supplier providing a new emptyMap into which the results will be inserted
      downstream - aCollector implementing the downstream reduction
      Returns:
      aCollector 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 concurrentCollector implementing a "group by" operation on input elements of typeT, grouping elements according to a classification function.

      This is aconcurrent andunordered Collector.

      The classification function maps elements to some key typeK. The collector produces aConcurrentMap<K, List<T>> whose keys are the values resulting from applying the classification function to the input elements, and whose corresponding values areLists 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 theConcurrentMap orList objects returned, or of the thread-safety of theList 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, unorderedCollector 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 concurrentCollector implementing a cascaded "group by" operation on input elements of typeT, grouping elements according to a classification function, and then performing a reduction operation on the values associated with a given key using the specified downstreamCollector.

      This is aconcurrent andunordered Collector.

      The classification function maps elements to some key typeK. The downstream collector operates on elements of typeT and produces a result of typeD. The resulting collector produces aConcurrentMap<K, D>.

      There are no guarantees on the type, mutability, or serializability of theConcurrentMap 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 - aCollector implementing the downstream reduction
      Returns:
      a concurrent, unorderedCollector implementing the cascaded group-by operation
      See Also:

    • groupingByConcurrent

      public static <T, K, A, D, M extendsConcurrentMap<K,D>>Collector<T,?,M> groupingByConcurrent(Function<? super T, ? extends K> classifier,Supplier<M> mapFactory,Collector<? super T, A, D> downstream)
      Returns a concurrentCollector implementing a cascaded "group by" operation on input elements of typeT, grouping elements according to a classification function, and then performing a reduction operation on the values associated with a given key using the specified downstreamCollector. TheConcurrentMap produced by the Collector is created with the supplied factory function.

      This is aconcurrent andunordered Collector.

      The classification function maps elements to some key typeK. The downstream collector operates on elements of typeT and produces a result of typeD. The resulting collector produces aConcurrentMap<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,                          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 resultingConcurrentMap
      Parameters:
      classifier - a classifier function mapping input elements to keys
      mapFactory - a supplier providing a new emptyConcurrentMap into which the results will be inserted
      downstream - aCollector implementing the downstream reduction
      Returns:
      a concurrent, unorderedCollector implementing the cascaded group-by operation
      See Also:

    • partitioningBy

      public static <T>Collector<T, ?,Map<Boolean,List<T>>> partitioningBy(Predicate<? super T> predicate)
      Returns aCollector which partitions the input elements according to aPredicate, and organizes them into aMap<Boolean, List<T>>. The returnedMap always contains mappings for bothfalse andtrue keys. There are no guarantees on the type, mutability, serializability, or thread-safety of theMap orList 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:
      aCollector 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 aCollector which partitions the input elements according to aPredicate, reduces the values in each partition according to anotherCollector, and organizes them into aMap<Boolean, D> whose values are the result of the downstream reduction.

      The returnedMap always contains mappings for bothfalse andtrue keys. There are no guarantees on the type, mutability, serializability, or thread-safety of theMap 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 - aCollector implementing the downstream reduction
      Returns:
      aCollector 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 aCollector that accumulates elements into aMap whose keys and values are the result of applying the provided mapping functions to the input elements.

      If the mapped keys contain duplicates (according toObject.equals(Object)), anIllegalStateException is thrown when the collection operation is performed. If the mapped keys might have duplicates, usetoMap(Function, Function, BinaryOperator) instead.

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

      API Note:
      It is common for either the key or the value to be the input elements. In this case, the utility methodFunction.identity() may be helpful. For example, the following produces aMap mapping students to their grade point average:
       Map<Student, Double> studentToGPA   = students.stream().collect(     toMap(Function.identity(),           student -> computeGPA(student)));
      And the following produces aMap mapping a unique identifier to students:
       Map<String, Student> studentIdToStudent   = students.stream().collect(     toMap(Student::getId,           Function.identity()));
      Implementation Note:
      The returnedCollector is not concurrent. For parallel stream pipelines, thecombiner 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 theMap in encounter order, usingtoConcurrentMap(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:
      aCollector which collects elements into aMap 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 aCollector that accumulates the input elements into anunmodifiable 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 toObject.equals(Object)), anIllegalStateException is thrown when the collection operation is performed. If the mapped keys might have duplicates, usetoUnmodifiableMap(Function, Function, BinaryOperator) to handle merging of the values.

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

      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:
      aCollector that accumulates the input elements into anunmodifiable 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:
      10
      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 aCollector that accumulates elements into aMap whose keys and values are the result of applying the provided mapping functions to the input elements.

      If the mapped keys contain duplicates (according toObject.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 theMap returned.

      API Note:
      There are multiple ways to deal with collisions between multiple elements mapping to the same key. The other forms oftoMap simply use a merge function that throws unconditionally, but you can easily write more flexible merge policies. For example, if you have a stream ofPerson, 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 aMap 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 returnedCollector is not concurrent. For parallel stream pipelines, thecombiner 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 theMap in encounter order, usingtoConcurrentMap(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 toMap.merge(Object, Object, BiFunction)
      Returns:
      aCollector which collects elements into aMap 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 aCollector that accumulates the input elements into anunmodifiable 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 toObject.equals(Object)), the value mapping function is applied to each equal element, and the results are merged using the provided merging function.

      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 toMap.merge(Object, Object, BiFunction), must be non-null
      Returns:
      aCollector that accumulates the input elements into anunmodifiable 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:
      10
      See Also:

    • toMap

      public static <T, K, U, M extendsMap<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 aCollector that accumulates elements into aMap whose keys and values are the result of applying the provided mapping functions to the input elements.

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

      Implementation Note:
      The returnedCollector is not concurrent. For parallel stream pipelines, thecombiner 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 theMap in encounter order, usingtoConcurrentMap(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 resultingMap
      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 toMap.merge(Object, Object, BiFunction)
      mapFactory - a supplier providing a new emptyMap into which the results will be inserted
      Returns:
      aCollector which collects elements into aMap whose keys are the result of applying a key mapping function to the input elements, and whose values are the result of applying a value mapping function to all input elements equal to the key and combining them using the merge function
      See Also:

    • toConcurrentMap

      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 concurrentCollector that accumulates elements into aConcurrentMap whose keys and values are the result of applying the provided mapping functions to the input elements.

      If the mapped keys contain duplicates (according toObject.equals(Object)), anIllegalStateException is thrown when the collection operation is performed. If the mapped keys may have duplicates, usetoConcurrentMap(Function, Function, BinaryOperator) instead.

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

      API Note:
      It is common for either the key or the value to be the input elements. In this case, the utility methodFunction.identity() may be helpful. For example, the following produces aConcurrentMap mapping students to their grade point average:
       ConcurrentMap<Student, Double> studentToGPA   = students.stream().collect(     toConcurrentMap(Function.identity(),                     student -> computeGPA(student)));
      And the following produces aConcurrentMap mapping a unique identifier to students:
       ConcurrentMap<String, Student> studentIdToStudent   = students.stream().collect(     toConcurrentMap(Student::getId,                     Function.identity()));

      This is aconcurrent andunordered 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, unorderedCollector which collects elements into aConcurrentMap 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 concurrentCollector that accumulates elements into aConcurrentMap whose keys and values are the result of applying the provided mapping functions to the input elements.

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

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

      API Note:
      There are multiple ways to deal with collisions between multiple elements mapping to the same key. The other forms oftoConcurrentMap simply use a merge function that throws unconditionally, but you can easily write more flexible merge policies. For example, if you have a stream ofPerson, 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 aConcurrentMap 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 aconcurrent andunordered 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 toMap.merge(Object, Object, BiFunction)
      Returns:
      a concurrent, unorderedCollector which collects elements into aConcurrentMap whose keys are the result of applying a key mapping function to the input elements, and whose values are the result of applying a value mapping function to all input elements equal to the key and combining them using the merge function
      See Also:

    • toConcurrentMap

      public static <T, K, U, M extendsConcurrentMap<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 concurrentCollector that accumulates elements into aConcurrentMap whose keys and values are the result of applying the provided mapping functions to the input elements.

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

      This is aconcurrent andunordered 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 resultingConcurrentMap
      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 toMap.merge(Object, Object, BiFunction)
      mapFactory - a supplier providing a new emptyConcurrentMap into which the results will be inserted
      Returns:
      a concurrent, unorderedCollector which collects elements into aConcurrentMap 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:

    • summarizingInt

      public static <T>Collector<T, ?,IntSummaryStatistics> summarizingInt(ToIntFunction<? super T> mapper)
      Returns aCollector which applies anint-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:
      aCollector implementing the summary-statistics reduction
      See Also:

    • summarizingLong

      public static <T>Collector<T, ?,LongSummaryStatistics> summarizingLong(ToLongFunction<? super T> mapper)
      Returns aCollector which applies anlong-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:
      aCollector implementing the summary-statistics reduction
      See Also:

    • summarizingDouble

      public static <T>Collector<T, ?,DoubleSummaryStatistics> summarizingDouble(ToDoubleFunction<? super T> mapper)
      Returns aCollector which applies andouble-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:
      aCollector 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 aCollector 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 isCollector.Characteristics.UNORDERED if both downstream collectors are unordered andCollector.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:
      aCollector which aggregates the results of two supplied collectors.
      Since:
      12