Module java.base
Package java.util.stream

Class Collectors

java.lang.Object
java.util.stream.Collectors
public final class Collectors extends Object
Implementations of Collector that implement various useful reduction operations, such as accumulating elements into collections, summarizing elements according to various criteria, etc.

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


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

 // Accumulate names into a TreeSet
 Set<String> set = people.stream()
   .map(Person::getName)
   .collect(Collectors.toCollection(TreeSet::new));

 // Convert elements to strings and concatenate them, separated by commas
 String joined = things.stream()
   .map(Object::toString)
   .collect(Collectors.joining(", "));

 // Compute sum of salaries of employee
 int total = employees.stream()
   .collect(Collectors.summingInt(Employee::getSalary));

 // Group employees by department
 Map<Department, List<Employee>> byDept = employees.stream()
   .collect(Collectors.groupingBy(Employee::getDepartment));

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

 // Partition students into passing and failing
 Map<Boolean, List<Student>> passingFailing = students.stream()
   .collect(Collectors.partitioningBy(s -> s.getGrade() >= PASS_THRESHOLD));
Since:
1.8

  • Method Summary

    Modifier and Type
    Method
    Description
    static <T> Collector<T,?,Double>
    averagingDouble(ToDoubleFunction<? super T> mapper)
    Returns a Collector that produces the arithmetic mean of a double-valued function applied to the input elements.
    static <T> Collector<T,?,Double>
    averagingInt(ToIntFunction<? super T> mapper)
    Returns a Collector that produces the arithmetic mean of an integer-valued function applied to the input elements.
    static <T> Collector<T,?,Double>
    averagingLong(ToLongFunction<? super T> mapper)
    Returns a Collector that produces the arithmetic mean of a long-valued function applied to the input elements.
    static <T, A, R, RR> Collector<T,A,RR>
    collectingAndThen(Collector<T,A,R> downstream, Function<R,RR> finisher)
    Adapts a Collector to perform an additional finishing transformation.
    static <T> Collector<T,?,Long>
    counting()
    Returns a Collector accepting elements of type T that counts the number of input elements.
    static <T, A, R> Collector<T,?,R>
    filtering(Predicate<? super T> predicate, Collector<? super T,A,R> downstream)
    Adapts a Collector to one accepting elements of the same type T by applying the predicate to each input element and only accumulating if the predicate returns true.
    static <T, U, A, R> Collector<T,?,R>
    flatMapping(Function<? super T,? extends Stream<? extends U>> mapper, Collector<? super U,A,R> downstream)
    Adapts a Collector accepting elements of type U to one accepting elements of type T by applying a flat mapping function to each input element before accumulation.
    static <T, K> Collector<T,?,Map<K,List<T>>>
    groupingBy(Function<? super T,? extends K> classifier)
    Returns a Collector implementing a "group by" operation on input elements of type T, grouping elements according to a classification function, and returning the results in a Map.
    static <T, K, D, A, M extends Map<K, D>>
    Collector<T,?,M>
    groupingBy(Function<? super T,? extends K> classifier, Supplier<M> mapFactory, Collector<? super T,A,D> downstream)
    Returns a Collector implementing a cascaded "group by" operation on input elements of type T, grouping elements according to a classification function, and then performing a reduction operation on the values associated with a given key using the specified downstream Collector.
    static <T, K, A, D> Collector<T,?,Map<K,D>>
    groupingBy(Function<? super T,? extends K> classifier, Collector<? super T,A,D> downstream)
    Returns a Collector implementing a cascaded "group by" operation on input elements of type T, grouping elements according to a classification function, and then performing a reduction operation on the values associated with a given key using the specified downstream Collector.
    static <T, K> Collector<T,?,ConcurrentMap<K,List<T>>>
    groupingByConcurrent(Function<? super T,? extends K> classifier)
    Returns a concurrent Collector implementing a "group by" operation on input elements of type T, grouping elements according to a classification function.
    static <T, K, A, D, M extends ConcurrentMap<K, D>>
    Collector<T,?,M>
    groupingByConcurrent(Function<? super T,? extends K> classifier, Supplier<M> mapFactory, Collector<? super T,A,D> downstream)
    Returns a concurrent Collector implementing a cascaded "group by" operation on input elements of type T, grouping elements according to a classification function, and then performing a reduction operation on the values associated with a given key using the specified downstream Collector.
    static <T, K, A, D> Collector<T,?,ConcurrentMap<K,D>>
    groupingByConcurrent(Function<? super T,? extends K> classifier, Collector<? super T,A,D> downstream)
    Returns a concurrent Collector implementing a cascaded "group by" operation on input elements of type T, grouping elements according to a classification function, and then performing a reduction operation on the values associated with a given key using the specified downstream Collector.
    static Collector<CharSequence,?,String>
    joining()
    Returns a Collector that concatenates the input elements into a String, in encounter order.
    static Collector<CharSequence,?,String>
    joining(CharSequence delimiter)
    Returns a Collector that concatenates the input elements, separated by the specified delimiter, in encounter order.
    static Collector<CharSequence,?,String>
    joining(CharSequence delimiter, CharSequence prefix, CharSequence suffix)
    Returns a Collector that concatenates the input elements, separated by the specified delimiter, with the specified prefix and suffix, in encounter order.
    static <T, U, A, R> Collector<T,?,R>
    mapping(Function<? super T,? extends U> mapper, Collector<? super U,A,R> downstream)
    Adapts a Collector accepting elements of type U to one accepting elements of type T by applying a mapping function to each input element before accumulation.
    static <T> Collector<T,?,Optional<T>>
    maxBy(Comparator<? super T> comparator)
    Returns a Collector that produces the maximal element according to a given Comparator, described as an Optional<T>.
    static <T> Collector<T,?,Optional<T>>
    minBy(Comparator<? super T> comparator)
    Returns a Collector that produces the minimal element according to a given Comparator, described as an Optional<T>.
    static <T> Collector<T,?,Map<Boolean,List<T>>>
    partitioningBy(Predicate<? super T> predicate)
    Returns a Collector which partitions the input elements according to a Predicate, and organizes them into a Map<Boolean, List<T>>.
    static <T, D, A> Collector<T,?,Map<Boolean,D>>
    partitioningBy(Predicate<? super T> predicate, Collector<? super T,A,D> downstream)
    Returns a Collector which partitions the input elements according to a Predicate, reduces the values in each partition according to another Collector, and organizes them into a Map<Boolean, D> whose values are the result of the downstream reduction.
    static <T> Collector<T,?,Optional<T>>
    reducing(BinaryOperator<T> op)
    Returns a Collector which performs a reduction of its input elements under a specified BinaryOperator.
    static <T> Collector<T,?,T>
    reducing(T identity, BinaryOperator<T> op)
    Returns a Collector which performs a reduction of its input elements under a specified BinaryOperator using the provided identity.
    static <T, U> Collector<T,?,U>
    reducing(U identity, Function<? super T,? extends U> mapper, BinaryOperator<U> op)
    Returns a Collector which performs a reduction of its input elements under a specified mapping function and BinaryOperator.
    static <T> Collector<T,?,DoubleSummaryStatistics>
    summarizingDouble(ToDoubleFunction<? super T> mapper)
    Returns a Collector which applies an double-producing mapping function to each input element, and returns summary statistics for the resulting values.
    static <T> Collector<T,?,IntSummaryStatistics>
    summarizingInt(ToIntFunction<? super T> mapper)
    Returns a Collector which applies an int-producing mapping function to each input element, and returns summary statistics for the resulting values.
    static <T> Collector<T,?,LongSummaryStatistics>
    summarizingLong(ToLongFunction<? super T> mapper)
    Returns a Collector which applies an long-producing mapping function to each input element, and returns summary statistics for the resulting values.
    static <T> Collector<T,?,Double>
    summingDouble(ToDoubleFunction<? super T> mapper)
    Returns a Collector that produces the sum of a double-valued function applied to the input elements.
    static <T> Collector<T,?,Integer>
    summingInt(ToIntFunction<? super T> mapper)
    Returns a Collector that produces the sum of 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, R1, R2, R>
    Collector<T,?,R>
    teeing(Collector<? super T,?,R1> downstream1, Collector<? super T,?,R2> downstream2, BiFunction<? super R1,? super R2,R> merger)
    Returns a Collector that is a composite of two downstream collectors.
    static <T, C extends Collection<T>>
    Collector<T,?,C>
    toCollection(Supplier<C> collectionFactory)
    Returns a Collector that accumulates the input elements into a new Collection, in encounter order.
    static <T, K, U> Collector<T,?,ConcurrentMap<K,U>>
    toConcurrentMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper)
    Returns a concurrent Collector that accumulates elements into a ConcurrentMap whose keys and values are the result of applying the provided mapping functions to the input elements.
    static <T, K, U> Collector<T,?,ConcurrentMap<K,U>>
    toConcurrentMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction)
    Returns a concurrent Collector that accumulates elements into a ConcurrentMap whose keys and values are the result of applying the provided mapping functions to the input elements.
    static <T, K, U, M extends ConcurrentMap<K, U>>
    Collector<T,?,M>
    toConcurrentMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction, Supplier<M> mapFactory)
    Returns a concurrent Collector that accumulates elements into a ConcurrentMap whose keys and values are the result of applying the provided mapping functions to the input elements.
    static <T> Collector<T,?,List<T>>
    toList()
    Returns a Collector that accumulates the input elements into a new List.
    static <T, K, U> Collector<T,?,Map<K,U>>
    toMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper)
    Returns a Collector that accumulates elements into a Map whose keys and values are the result of applying the provided mapping functions to the input elements.
    static <T, K, U> Collector<T,?,Map<K,U>>
    toMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction)
    Returns a Collector that accumulates elements into a Map whose keys and values are the result of applying the provided mapping functions to the input elements.
    static <T, K, U, M extends Map<K, U>>
    Collector<T,?,M>
    toMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction, Supplier<M> mapFactory)
    Returns a Collector that accumulates elements into a Map whose keys and values are the result of applying the provided mapping functions to the input elements.
    static <T> Collector<T,?,Set<T>>
    toSet()
    Returns a Collector that accumulates the input elements into a new Set.
    static <T> Collector<T,?,List<T>>
    toUnmodifiableList()
    Returns a Collector that accumulates the input elements into an unmodifiable List in encounter order.
    static <T, K, U> Collector<T,?,Map<K,U>>
    toUnmodifiableMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper)
    Returns a Collector that accumulates the input elements into an unmodifiable Map, whose keys and values are the result of applying the provided mapping functions to the input elements.
    static <T, K, U> Collector<T,?,Map<K,U>>
    toUnmodifiableMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction)
    Returns a Collector that accumulates the input elements into an unmodifiable Map, whose keys and values are the result of applying the provided mapping functions to the input elements.
    static <T> Collector<T,?,Set<T>>
    toUnmodifiableSet()
    Returns a Collector that accumulates the input elements into an unmodifiable Set.

    Methods inherited from class java.lang.Object

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

  • Method Details

    • toCollection

      public static <T, C extends Collection<T>> Collector<T,?,C> toCollection(Supplier<C> collectionFactory)
      Returns a Collector that accumulates the input elements into a new Collection, in encounter order. The Collection is created by the provided factory.
      Type Parameters:
      T - the type of the input elements
      C - the type of the resulting Collection
      Parameters:
      collectionFactory - a supplier providing a new empty Collection into which the results will be inserted
      Returns:
      a Collector which collects all the input elements into a Collection, in encounter order

    • toList

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

    • toUnmodifiableList

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

    • toSet

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

      This is an unordered Collector.

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

    • toUnmodifiableSet

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

      This is an unordered Collector.

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

    • joining

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

    • joining

      public static Collector<CharSequence,?,String> joining(CharSequence delimiter)
      Returns a Collector that concatenates the input elements, separated by the specified delimiter, in encounter order.
      Parameters:
      delimiter - the delimiter to be used between each element
      Returns:
      A Collector which concatenates CharSequence elements, separated by the specified delimiter, in encounter order

    • joining

      public static Collector<CharSequence,?,String> joining(CharSequence delimiter, CharSequence prefix, CharSequence suffix)
      Returns a Collector that concatenates the input elements, separated by the specified delimiter, with the specified prefix and suffix, in encounter order.
      Parameters:
      delimiter - the delimiter to be used between each element
      prefix - the sequence of characters to be used at the beginning of the joined result
      suffix - the sequence of characters to be used at the end of the joined result
      Returns:
      A Collector which concatenates CharSequence elements, separated by the specified delimiter, in encounter order

    • mapping

      public static <T, U, A, R> Collector<T,?,R> mapping(Function<? super T,? extends U> mapper, Collector<? super U,A,R> downstream)
      Adapts a Collector accepting elements of type U to one accepting elements of type T by applying a mapping function to each input element before accumulation.
      Type Parameters:
      T - the type of the input elements
      U - type of elements accepted by downstream collector
      A - intermediate accumulation type of the downstream collector
      R - result type of collector
      Parameters:
      mapper - a function to be applied to the input elements
      downstream - a collector which will accept mapped values
      Returns:
      a collector which applies the mapping function to the input elements and provides the mapped results to the downstream collector

    • flatMapping

      public static <T, U, A, R> Collector<T,?,R> flatMapping(Function<? super T,? extends Stream<? extends U>> mapper, Collector<? super U,A,R> downstream)
      Adapts a Collector accepting elements of type U to one accepting elements of type T by applying a flat mapping function to each input element before accumulation. The flat mapping function maps an input element to a stream covering zero or more output elements that are then accumulated downstream. Each mapped stream is closed after its contents have been placed downstream. (If a mapped stream is null an empty stream is used, instead.)
      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 a Collector to one accepting elements of the same type T by applying the predicate to each input element and only accumulating if the predicate returns true.
      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 a Collector to perform an additional finishing transformation. For example, one could adapt the toList() collector to always produce an immutable list with: 
      
 List<String> list = people.stream().collect(
   collectingAndThen(toList(),
                     Collections::unmodifiableList));
      Type Parameters:
      T - the type of the input elements
      A - intermediate accumulation type of the downstream collector
      R - result type of the downstream collector
      RR - result type of the resulting collector
      Parameters:
      downstream - a collector
      finisher - a function to be applied to the final result of the downstream collector
      Returns:
      a collector which performs the action of the downstream collector, followed by an additional finishing step

    • counting

      public static <T> Collector<T,?,Long> counting()
      Returns a Collector accepting elements of type T that counts the number of input elements. If no elements are present, the result is 0.
      Type Parameters:
      T - the type of the input elements
      Returns:
      a Collector that counts the input elements

    • minBy

      public static <T> Collector<T,?,Optional<T>> minBy(Comparator<? super T> comparator)
      Returns a Collector that produces the minimal element according to a given Comparator, described as an Optional<T>.
      Type Parameters:
      T - the type of the input elements
      Parameters:
      comparator - a Comparator for comparing elements
      Returns:
      a Collector that produces the minimal value

    • maxBy

      public static <T> Collector<T,?,Optional<T>> maxBy(Comparator<? super T> comparator)
      Returns a Collector that produces the maximal element according to a given Comparator, described as an Optional<T>.
      Type Parameters:
      T - the type of the input elements
      Parameters:
      comparator - a Comparator for comparing elements
      Returns:
      a Collector that produces the maximal value

    • summingInt

      public static <T> Collector<T,?,Integer> summingInt(ToIntFunction<? super T> mapper)
      Returns a Collector that produces the sum of a integer-valued function applied to the input elements. If no elements are present, the result is 0.
      Type Parameters:
      T - the type of the input elements
      Parameters:
      mapper - a function extracting the property to be summed
      Returns:
      a Collector that produces the sum of a derived property

    • summingLong

      public static <T> Collector<T,?,Long> summingLong(ToLongFunction<? super T> mapper)
      Returns a Collector that produces the sum of a long-valued function applied to the input elements. If no elements are present, the result is 0.
      Type Parameters:
      T - the type of the input elements
      Parameters:
      mapper - a function extracting the property to be summed
      Returns:
      a Collector that produces the sum of a derived property

    • summingDouble

      public static <T> Collector<T,?,Double> summingDouble(ToDoubleFunction<? super T> mapper)
      Returns a Collector that produces the sum of a double-valued function applied to the input elements. If no elements are present, the result is 0.

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

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

    • averagingInt

      public static <T> Collector<T,?,Double> averagingInt(ToIntFunction<? super T> mapper)
      Returns a Collector that produces the arithmetic mean of an integer-valued function applied to the input elements. If no elements are present, the result is 0.
      Type Parameters:
      T - the type of the input elements
      Parameters:
      mapper - a function extracting the property to be averaged
      Returns:
      a Collector that produces the arithmetic mean of a derived property

    • averagingLong

      public static <T> Collector<T,?,Double> averagingLong(ToLongFunction<? super T> mapper)
      Returns a Collector that produces the arithmetic mean of a long-valued function applied to the input elements. If no elements are present, the result is 0.
      Type Parameters:
      T - the type of the input elements
      Parameters:
      mapper - a function extracting the property to be averaged
      Returns:
      a Collector that produces the arithmetic mean of a derived property

    • averagingDouble

      public static <T> Collector<T,?,Double> averagingDouble(ToDoubleFunction<? super T> mapper)
      Returns a Collector that produces the arithmetic mean of a double-valued function applied to the input elements. If no elements are present, the result is 0.

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

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

    • reducing

      public static <T> Collector<T,?,T> reducing(T identity, BinaryOperator<T> op)
      Returns a Collector which performs a reduction of its input elements under a specified BinaryOperator using the provided identity.
      Type Parameters:
      T - element type for the input and output of the reduction
      Parameters:
      identity - the identity value for the reduction (also, the value that is returned when there are no input elements)
      op - a BinaryOperator<T> used to reduce the input elements
      Returns:
      a Collector which implements the reduction operation
      See Also:

    • reducing

      public static <T> Collector<T,?,Optional<T>> reducing(BinaryOperator<T> op)
      Returns a Collector which performs a reduction of its input elements under a specified BinaryOperator. The result is described as an Optional<T>.
      Type Parameters:
      T - element type for the input and output of the reduction
      Parameters:
      op - a BinaryOperator<T> used to reduce the input elements
      Returns:
      a Collector which implements the reduction operation
      See Also:

    • reducing

      public static <T, U> Collector<T,?,U> reducing(U identity, Function<? super T,? extends U> mapper, BinaryOperator<U> op)
      Returns a Collector which performs a reduction of its input elements under a specified mapping function and BinaryOperator. This is a generalization of reducing(Object, BinaryOperator) which allows a transformation of the elements before reduction.
      Type Parameters:
      T - the type of the input elements
      U - the type of the mapped values
      Parameters:
      identity - the identity value for the reduction (also, the value that is returned when there are no input elements)
      mapper - a mapping function to apply to each input value
      op - a BinaryOperator<U> used to reduce the mapped values
      Returns:
      a Collector implementing the map-reduce operation
      See Also:

    • groupingBy

      public static <T, K> Collector<T,?,Map<K,List<T>>> groupingBy(Function<? super T,? extends K> classifier)
      Returns a Collector implementing a "group by" operation on input elements of type T, grouping elements according to a classification function, and returning the results in a Map.

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

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

      Type Parameters:
      T - the type of the input elements
      K - the type of the keys
      Parameters:
      classifier - the classifier function mapping input elements to keys
      Returns:
      a Collector implementing the group-by operation
      See Also:

    • groupingBy

      public static <T, K, A, D> Collector<T,?,Map<K,D>> groupingBy(Function<? super T,? extends K> classifier, Collector<? super T,A,D> downstream)
      Returns a Collector implementing a cascaded "group by" operation on input elements of type T, grouping elements according to a classification function, and then performing a reduction operation on the values associated with a given key using the specified downstream Collector.

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

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

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

      
 Map<City, Set<String>> namesByCity
   = people.stream().collect(
     groupingBy(Person::getCity,
                mapping(Person::getLastName,
                        toSet())));
      Type Parameters:
      T - the type of the input elements
      K - the type of the keys
      A - the intermediate accumulation type of the downstream collector
      D - the result type of the downstream reduction
      Parameters:
      classifier - a classifier function mapping input elements to keys
      downstream - a Collector implementing the downstream reduction
      Returns:
      a Collector implementing the cascaded group-by operation
      See Also:

    • groupingBy

      public static <T, K, D, A, M extends Map<K, D>> Collector<T,?,M> groupingBy(Function<? super T,? extends K> classifier, Supplier<M> mapFactory, Collector<? super T,A,D> downstream)
      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())));
      Type Parameters:
      T - the type of the input elements
      K - the type of the keys
      A - the intermediate accumulation type of the downstream collector
      D - the result type of the downstream reduction
      M - the type of the resulting Map
      Parameters:
      classifier - a classifier function mapping input elements to keys
      downstream - a Collector implementing the downstream reduction
      mapFactory - a supplier providing a new empty Map into which the results will be inserted
      Returns:
      a Collector implementing the cascaded group-by operation
      See Also:

    • groupingByConcurrent

      public static <T, K> Collector<T,?,ConcurrentMap<K,List<T>>> groupingByConcurrent(Function<? super T,? extends K> classifier)
      Returns a concurrent Collector implementing a "group by" operation on input elements of type T, grouping elements according to a classification function.

      This is a concurrent and unordered Collector.

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

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

      Type Parameters:
      T - the type of the input elements
      K - the type of the keys
      Parameters:
      classifier - a classifier function mapping input elements to keys
      Returns:
      a concurrent, unordered Collector implementing the group-by operation
      See Also:

    • groupingByConcurrent

      public static <T, K, A, D> Collector<T,?,ConcurrentMap<K,D>> groupingByConcurrent(Function<? super T,? extends K> classifier, Collector<? super T,A,D> downstream)
      Returns a concurrent Collector implementing a cascaded "group by" operation on input elements of type T, grouping elements according to a classification function, and then performing a reduction operation on the values associated with a given key using the specified downstream Collector.

      This is a concurrent and unordered Collector.

      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 ConcurrentMap<K, D>.

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

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

      
 ConcurrentMap<City, Set<String>> namesByCity
   = people.stream().collect(
     groupingByConcurrent(Person::getCity,
                          mapping(Person::getLastName,
                                  toSet())));
      Type Parameters:
      T - the type of the input elements
      K - the type of the keys
      A - the intermediate accumulation type of the downstream collector
      D - the result type of the downstream reduction
      Parameters:
      classifier - a classifier function mapping input elements to keys
      downstream - a Collector implementing the downstream reduction
      Returns:
      a concurrent, unordered Collector implementing the cascaded group-by operation
      See Also:

    • groupingByConcurrent

      public static <T, K, A, D, M extends ConcurrentMap<K, D>> Collector<T,?,M> groupingByConcurrent(Function<? super T,? extends K> classifier, Supplier<M> mapFactory, Collector<? super T,A,D> downstream)
      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 ConcurrentMap<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 resulting ConcurrentMap
      Parameters:
      classifier - a classifier function mapping input elements to keys
      downstream - a Collector implementing the downstream reduction
      mapFactory - a supplier providing a new empty ConcurrentMap into which the results will be inserted
      Returns:
      a concurrent, unordered Collector implementing the cascaded group-by operation
      See Also:

    • partitioningBy

      public static <T> Collector<T,?,Map<Boolean,List<T>>> partitioningBy(Predicate<? super T> predicate)
      Returns a Collector which partitions the input elements according to a Predicate, and organizes them into a Map<Boolean, List<T>>. The returned Map always contains mappings for both false and true keys. There are no guarantees on the type, mutability, serializability, or thread-safety of the Map or List returned.
      Type Parameters:
      T - the type of the input elements
      Parameters:
      predicate - a predicate used for classifying input elements
      Returns:
      a Collector implementing the partitioning operation
      See Also:

    • partitioningBy

      public static <T, D, A> Collector<T,?,Map<Boolean,D>> partitioningBy(Predicate<? super T> predicate, Collector<? super T,A,D> downstream)
      Returns a Collector which partitions the input elements according to a Predicate, reduces the values in each partition according to another Collector, and organizes them into a Map<Boolean, D> whose values are the result of the downstream reduction.

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

      Type Parameters:
      T - the type of the input elements
      A - the intermediate accumulation type of the downstream collector
      D - the result type of the downstream reduction
      Parameters:
      predicate - a predicate used for classifying input elements
      downstream - a Collector implementing the downstream reduction
      Returns:
      a Collector implementing the cascaded partitioning operation
      See Also:

    • toMap

      public static <T, K, U> Collector<T,?,Map<K,U>> toMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper)
      Returns a Collector that accumulates elements into a Map whose keys and values are the result of applying the provided mapping functions to the input elements.

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

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

      Type Parameters:
      T - the type of the input elements
      K - the output type of the key mapping function
      U - the output type of the value mapping function
      Parameters:
      keyMapper - a mapping function to produce keys
      valueMapper - a mapping function to produce values
      Returns:
      a Collector which collects elements into a Map whose keys and values are the result of applying mapping functions to the input elements
      See Also:

    • toUnmodifiableMap

      public static <T, K, U> Collector<T,?,Map<K,U>> toUnmodifiableMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper)
      Returns a Collector that accumulates the input elements into an unmodifiable Map, whose keys and values are the result of applying the provided mapping functions to the input elements.

      If the mapped keys contain duplicates (according to Object.equals(Object)), an IllegalStateException is thrown when the collection operation is performed. If the mapped keys might have duplicates, use toUnmodifiableMap(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:
      a Collector that accumulates the input elements into an unmodifiable Map, whose keys and values are the result of applying the provided mapping functions to the input elements
      Throws:
      NullPointerException - if either keyMapper or valueMapper is null
      Since:
      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 a Collector that accumulates elements into a Map whose keys and values are the result of applying the provided mapping functions to the input elements.

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

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

      Type Parameters:
      T - the type of the input elements
      K - the output type of the key mapping function
      U - the output type of the value mapping function
      Parameters:
      keyMapper - a mapping function to produce keys
      valueMapper - a mapping function to produce values
      mergeFunction - a merge function, used to resolve collisions between values associated with the same key, as supplied to Map.merge(Object, Object, BiFunction)
      Returns:
      a Collector which collects elements into a Map whose keys are the result of applying a key mapping function to the input elements, and whose values are the result of applying a value mapping function to all input elements equal to the key and combining them using the merge function
      See Also:

    • toUnmodifiableMap

      public static <T, K, U> Collector<T,?,Map<K,U>> toUnmodifiableMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction)
      Returns a Collector that accumulates the input elements into an unmodifiable Map, whose keys and values are the result of applying the provided mapping functions to the input elements.

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

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

      Type Parameters:
      T - the type of the input elements
      K - the output type of the key mapping function
      U - the output type of the value mapping function
      Parameters:
      keyMapper - a mapping function to produce keys, must be non-null
      valueMapper - a mapping function to produce values, must be non-null
      mergeFunction - a merge function, used to resolve collisions between values associated with the same key, as supplied to Map.merge(Object, Object, BiFunction), must be non-null
      Returns:
      a Collector that accumulates the input elements into an unmodifiable Map, whose keys and values are the result of applying the provided mapping functions to the input elements
      Throws:
      NullPointerException - if the keyMapper, valueMapper, or mergeFunction is null
      Since:
      10
      See Also:

    • toMap

      public static <T, K, U, M extends Map<K, U>> Collector<T,?,M> toMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction, Supplier<M> mapFactory)
      Returns a Collector that accumulates elements into a Map whose keys and values are the result of applying the provided mapping functions to the input elements.

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

      Type Parameters:
      T - the type of the input elements
      K - the output type of the key mapping function
      U - the output type of the value mapping function
      M - the type of the resulting Map
      Parameters:
      keyMapper - a mapping function to produce keys
      valueMapper - a mapping function to produce values
      mergeFunction - a merge function, used to resolve collisions between values associated with the same key, as supplied to Map.merge(Object, Object, BiFunction)
      mapFactory - a supplier providing a new empty Map into which the results will be inserted
      Returns:
      a Collector which collects elements into a Map whose keys are the result of applying a key mapping function to the input elements, and whose values are the result of applying a value mapping function to all input elements equal to the key and combining them using the merge function
      See Also:

    • toConcurrentMap

      public static <T, K, U> Collector<T,?,ConcurrentMap<K,U>> toConcurrentMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper)
      Returns a concurrent Collector that accumulates elements into a ConcurrentMap whose keys and values are the result of applying the provided mapping functions to the input elements.

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

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

      Type Parameters:
      T - the type of the input elements
      K - the output type of the key mapping function
      U - the output type of the value mapping function
      Parameters:
      keyMapper - the mapping function to produce keys
      valueMapper - the mapping function to produce values
      Returns:
      a concurrent, unordered Collector which collects elements into a ConcurrentMap whose keys are the result of applying a key mapping function to the input elements, and whose values are the result of applying a value mapping function to the input elements
      See Also:

    • toConcurrentMap

      public static <T, K, U> Collector<T,?,ConcurrentMap<K,U>> toConcurrentMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction)
      Returns a concurrent Collector that accumulates elements into a ConcurrentMap whose keys and values are the result of applying the provided mapping functions to the input elements.

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

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

      Type Parameters:
      T - the type of the input elements
      K - the output type of the key mapping function
      U - the output type of the value mapping function
      Parameters:
      keyMapper - a mapping function to produce keys
      valueMapper - a mapping function to produce values
      mergeFunction - a merge function, used to resolve collisions between values associated with the same key, as supplied to Map.merge(Object, Object, BiFunction)
      Returns:
      a concurrent, unordered Collector which collects elements into a ConcurrentMap whose keys are the result of applying a key mapping function to the input elements, and whose values are the result of applying a value mapping function to all input elements equal to the key and combining them using the merge function
      See Also:

    • toConcurrentMap

      public static <T, K, U, M extends ConcurrentMap<K, U>> Collector<T,?,M> toConcurrentMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction, Supplier<M> mapFactory)
      Returns a concurrent Collector that accumulates elements into a ConcurrentMap whose keys and values are the result of applying the provided mapping functions to the input elements.

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

      This is a concurrent and unordered Collector.

      Type Parameters:
      T - the type of the input elements
      K - the output type of the key mapping function
      U - the output type of the value mapping function
      M - the type of the resulting ConcurrentMap
      Parameters:
      keyMapper - a mapping function to produce keys
      valueMapper - a mapping function to produce values
      mergeFunction - a merge function, used to resolve collisions between values associated with the same key, as supplied to Map.merge(Object, Object, BiFunction)
      mapFactory - a supplier providing a new empty ConcurrentMap into which the results will be inserted
      Returns:
      a concurrent, unordered Collector which collects elements into a ConcurrentMap whose keys are the result of applying a key mapping function to the input elements, and whose values are the result of applying a value mapping function to all input elements equal to the key and combining them using the merge function
      See Also:

    • summarizingInt

      public static <T> Collector<T,?,IntSummaryStatistics> summarizingInt(ToIntFunction<? super T> mapper)
      Returns a Collector which applies an int-producing mapping function to each input element, and returns summary statistics for the resulting values.
      Type Parameters:
      T - the type of the input elements
      Parameters:
      mapper - a mapping function to apply to each element
      Returns:
      a Collector implementing the summary-statistics reduction
      See Also:

    • summarizingLong

      public static <T> Collector<T,?,LongSummaryStatistics> summarizingLong(ToLongFunction<? super T> mapper)
      Returns a Collector which applies an long-producing mapping function to each input element, and returns summary statistics for the resulting values.
      Type Parameters:
      T - the type of the input elements
      Parameters:
      mapper - the mapping function to apply to each element
      Returns:
      a Collector implementing the summary-statistics reduction
      See Also:

    • summarizingDouble

      public static <T> Collector<T,?,DoubleSummaryStatistics> summarizingDouble(ToDoubleFunction<? super T> mapper)
      Returns a Collector which applies an double-producing mapping function to each input element, and returns summary statistics for the resulting values.
      Type Parameters:
      T - the type of the input elements
      Parameters:
      mapper - a mapping function to apply to each element
      Returns:
      a Collector implementing the summary-statistics reduction
      See Also:

    • teeing

      public static <T, R1, R2, R> Collector<T,?,R> teeing(Collector<? super T,?,R1> downstream1, Collector<? super T,?,R2> downstream2, BiFunction<? super R1,? super R2,R> merger)
      Returns a Collector that is a composite of two downstream collectors. Every element passed to the resulting collector is processed by both downstream collectors, then their results are merged using the specified merge function into the final result.

      The resulting collector functions do the following:

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

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

      Type Parameters:
      T - the type of the input elements
      R1 - the result type of the first collector
      R2 - the result type of the second collector
      R - the final result type
      Parameters:
      downstream1 - the first downstream collector
      downstream2 - the second downstream collector
      merger - the function which merges two results into the single one
      Returns:
      a Collector which aggregates the results of two supplied collectors.
      Since:
      12