Collectors (Java SE 10 & JDK 10 ) (original) (raw)

Method Summary

All Methods Static Methods Concrete Methods

Modifier and Type	Method	Description
static 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 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 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 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 typeT 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>>	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 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 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 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 downstreamCollector.
static <T,K> Collector<T,?,ConcurrentMap<K,List>>	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 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 aString, 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 Collector<T,?,Optional>	maxBy(Comparator<? super T> comparator)	Returns a Collector that produces the maximal element according to a given Comparator, described as an Optional.
static Collector<T,?,Optional>	minBy(Comparator<? super T> comparator)	Returns a Collector that produces the minimal element according to a given Comparator, described as an Optional.
static Collector<T,?,Map<Boolean,List>>	partitioningBy(Predicate<? super T> predicate)	Returns a Collector which partitions the input elements according to a Predicate, and organizes them into aMap<Boolean, List>.
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 aMap<Boolean, D> whose values are the result of the downstream reduction.
static Collector<T,?,Optional>	reducing(BinaryOperator op)	Returns a Collector which performs a reduction of its input elements under a specified BinaryOperator.
static Collector<T,?,T>	reducing(T identity,BinaryOperator 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 op)	Returns a Collector which performs a reduction of its input elements under a specified mapping function andBinaryOperator.
static 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 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 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 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 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 Collector<T,?,Long>	summingLong(ToLongFunction<? super T> mapper)	Returns a Collector that produces the sum of a long-valued function applied to the input elements.
static <T,C extends Collection>Collector<T,?,C>	toCollection(Supplier 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 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 mergeFunction)	Returns a concurrent Collector 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 extends ConcurrentMap<K,U>>Collector<T,?,M>	toConcurrentMap(Function<? super T,? extends K> keyMapper,Function<? super T,? extends U> valueMapper,BinaryOperator mergeFunction,Supplier mapFactory)	Returns a concurrent Collector that accumulates elements into aConcurrentMap whose keys and values are the result of applying the provided mapping functions to the input elements.
static Collector<T,?,List>	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 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 mergeFunction)	Returns a Collector 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 extends Map<K,U>>Collector<T,?,M>	toMap(Function<? super T,? extends K> keyMapper,Function<? super T,? extends U> valueMapper,BinaryOperator mergeFunction,Supplier mapFactory)	Returns a Collector that accumulates elements into aMap whose keys and values are the result of applying the provided mapping functions to the input elements.
static Collector<T,?,Set>	toSet()	Returns a Collector that accumulates the input elements into a new Set.
static Collector<T,?,List>	toUnmodifiableList()	Returns a Collector 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 a Collector 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 mergeFunction)	Returns a Collector 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 Collector<T,?,Set>	toUnmodifiableSet()	Returns a Collector that accumulates the input elements into anunmodifiable Set.

   * ### Methods declared in class java.lang.[Object](../../../java/lang/Object.html "class in java.lang")  
   `[clone](../../../java/lang/Object.html#clone%28%29), [equals](../../../java/lang/Object.html#equals%28java.lang.Object%29), [finalize](../../../java/lang/Object.html#finalize%28%29), [getClass](../../../java/lang/Object.html#getClass%28%29), [hashCode](../../../java/lang/Object.html#hashCode%28%29), [notify](../../../java/lang/Object.html#notify%28%29), [notifyAll](../../../java/lang/Object.html#notifyAll%28%29), [toString](../../../java/lang/Object.html#toString%28%29), [wait](../../../java/lang/Object.html#wait%28%29), [wait](../../../java/lang/Object.html#wait%28long%29), [wait](../../../java/lang/Object.html#wait%28long,int%29)`

Method Detail

 * #### toCollection  
 public static <T,C extends [Collection](../../../java/util/Collection.html "interface in java.util")<T>> [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,C> toCollection([Supplier](../../../java/util/function/Supplier.html "interface in java.util.function")<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](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,[List](../../../java/util/List.html "interface in java.util")<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)](../../../java/util/stream/Collectors.html#toCollection%28java.util.function.Supplier%29).  
 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](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,[List](../../../java/util/List.html "interface in java.util")<T>> toUnmodifiableList()  
 Returns a `Collector` that accumulates the input elements into an[unmodifiable List](../List.html#unmodifiable) 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](../List.html#unmodifiable) in encounter order  
 Since:  
 10  
 * #### toSet  
 public static <T> [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,[Set](../../../java/util/Set.html "interface in java.util")<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)](../../../java/util/stream/Collectors.html#toCollection%28java.util.function.Supplier%29).  
 This is an [unordered](../../../java/util/stream/Collector.Characteristics.html#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](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,[Set](../../../java/util/Set.html "interface in java.util")<T>> toUnmodifiableSet()  
 Returns a `Collector` that accumulates the input elements into an[unmodifiable Set](../Set.html#unmodifiable). 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](../../../java/util/stream/Collector.Characteristics.html#UNORDERED) Collector.  
 Type Parameters:  
 `T` \- the type of the input elements  
 Returns:  
 a `Collector` that accumulates the input elements into an[unmodifiable Set](../Set.html#unmodifiable)  
 Since:  
 10  
 * #### joining  
 public static [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<[CharSequence](../../../java/lang/CharSequence.html "interface in java.lang"),?,[String](../../../java/lang/String.html "class in java.lang")> 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](../../../java/util/stream/Collector.html "interface in java.util.stream")<[CharSequence](../../../java/lang/CharSequence.html "interface in java.lang"),?,[String](../../../java/lang/String.html "class in java.lang")> joining([CharSequence](../../../java/lang/CharSequence.html "interface in java.lang") 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](../../../java/util/stream/Collector.html "interface in java.util.stream")<[CharSequence](../../../java/lang/CharSequence.html "interface in java.lang"),?,[String](../../../java/lang/String.html "class in java.lang")> joining([CharSequence](../../../java/lang/CharSequence.html "interface in java.lang") delimiter,  
                                                        [CharSequence](../../../java/lang/CharSequence.html "interface in java.lang") prefix,  
                                                        [CharSequence](../../../java/lang/CharSequence.html "interface in java.lang") 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](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,R> mapping([Function](../../../java/util/function/Function.html "interface in java.util.function")<? super T,? extends U> mapper,  
                                                  [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<? super U,A,R> downstream)  
 Adapts a `Collector` accepting elements of type `U` to one accepting elements of type `T` by applying a mapping function to each input element before accumulation.  
 API Note:  
 The `mapping()` collectors are most useful when used in a multi-level reduction, such as downstream of a `groupingBy` or`partitioningBy`. For example, given a stream of`Person`, to accumulate the set of last names in each city:  
 ```  
  Map<City, Set<String>> lastNamesByCity  
    = people.stream().collect(  
      groupingBy(Person::getCity,  
                 mapping(Person::getLastName,  
                         toSet())));  
        
 ```  
 Type Parameters:  
 `T` \- the type of the input elements  
 `U` \- type of elements accepted by downstream collector  
 `A` \- intermediate accumulation type of the downstream collector  
 `R` \- result type of collector  
 Parameters:  
 `mapper` \- a function to be applied to the input elements  
 `downstream` \- a collector which will accept mapped values  
 Returns:  
 a collector which applies the mapping function to the input elements and provides the mapped results to the downstream collector  
 * #### flatMapping  
 public static <T,U,A,R> [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,R> flatMapping([Function](../../../java/util/function/Function.html "interface in java.util.function")<? super T,? extends [Stream](../../../java/util/stream/Stream.html "interface in java.util.stream")<? extends U>> mapper,  
                                                      [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<? 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](../../../java/util/stream/Stream.html "interface in java.util.stream") covering zero or more output elements that are then accumulated downstream. Each mapped stream is [closed](../../../java/util/stream/BaseStream.html#close%28%29) after its contents have been placed downstream. (If a mapped stream is `null` an empty stream is used, instead.)  
 API Note:  
 The `flatMapping()` collectors are most useful when used in a multi-level reduction, such as downstream of a `groupingBy` or`partitioningBy`. For example, given a stream of`Order`, to accumulate the set of line items for each customer:  
 ```  
  Map<String, Set<LineItem>> itemsByCustomerName  
    = orders.stream().collect(  
      groupingBy(Order::getCustomerName,  
                 flatMapping(order -> order.getLineItems().stream(),  
                             toSet())));  
        
 ```  
 Type Parameters:  
 `T` \- the type of the input elements  
 `U` \- type of elements accepted by downstream collector  
 `A` \- intermediate accumulation type of the downstream collector  
 `R` \- result type of collector  
 Parameters:  
 `mapper` \- a function to be applied to the input elements, which returns a stream of results  
 `downstream` \- a collector which will receive the elements of the stream returned by mapper  
 Returns:  
 a collector which applies the mapping function to the input elements and provides the flat mapped results to the downstream collector  
 Since:  
 9  
 * #### filtering  
 public static <T,A,R> [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,R> filtering([Predicate](../../../java/util/function/Predicate.html "interface in java.util.function")<? super T> predicate,  
                                                  [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<? super T,A,R> downstream)  
 Adapts a `Collector` to one accepting elements of the same type`T` by applying the predicate to each input element and only accumulating if the predicate returns `true`.  
 API Note:  
 The `filtering()` collectors are most useful when used in a multi-level reduction, such as downstream of a `groupingBy` or`partitioningBy`. For example, given a stream of`Employee`, to accumulate the employees in each department that have a salary above a certain threshold:  
 ```  
  Map<Department, Set<Employee>> wellPaidEmployeesByDepartment  
    = employees.stream().collect(  
      groupingBy(Employee::getDepartment,  
                 filtering(e -> e.getSalary() > 2000,  
                           toSet())));  
        
 ```  
  A filtering collector differs from a stream's `filter()` operation. In this example, suppose there are no employees whose salary is above the threshold in some department. Using a filtering collector as shown above would result in a mapping from that department to an empty `Set`. If a stream `filter()` operation were done instead, there would be no mapping for that department at all.  
 Type Parameters:  
 `T` \- the type of the input elements  
 `A` \- intermediate accumulation type of the downstream collector  
 `R` \- result type of collector  
 Parameters:  
 `predicate` \- a predicate to be applied to the input elements  
 `downstream` \- a collector which will accept values that match the predicate  
 Returns:  
 a collector which applies the predicate to the input elements and provides matching elements to the downstream collector  
 Since:  
 9  
 * #### collectingAndThen  
 public static <T,A,R,RR> [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,A,RR> collectingAndThen([Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,A,R> downstream,  
                                                              [Function](../../../java/util/function/Function.html "interface in java.util.function")<R,RR> finisher)  
 Adapts a `Collector` to perform an additional finishing transformation. For example, one could adapt the [toList()](../../../java/util/stream/Collectors.html#toList%28%29) 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](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,[Long](../../../java/lang/Long.html "class in java.lang")> counting()  
 Returns a `Collector` accepting elements of type `T` that counts the number of input elements. If no elements are present, the result is 0.  
 Implementation Requirements:  
 This produces a result equivalent to:  
 ```  
      reducing(0L, e -> 1L, Long::sum)  
        
 ```  
 Type Parameters:  
 `T` \- the type of the input elements  
 Returns:  
 a `Collector` that counts the input elements  
 * #### minBy  
 public static <T> [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,[Optional](../../../java/util/Optional.html "class in java.util")<T>> minBy([Comparator](../../../java/util/Comparator.html "interface in java.util")<? super T> comparator)  
 Returns a `Collector` that produces the minimal element according to a given `Comparator`, described as an `Optional<T>`.  
 Implementation Requirements:  
 This produces a result equivalent to:  
 ```  
      reducing(BinaryOperator.minBy(comparator))  
        
 ```  
 Type Parameters:  
 `T` \- the type of the input elements  
 Parameters:  
 `comparator` \- a `Comparator` for comparing elements  
 Returns:  
 a `Collector` that produces the minimal value  
 * #### maxBy  
 public static <T> [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,[Optional](../../../java/util/Optional.html "class in java.util")<T>> maxBy([Comparator](../../../java/util/Comparator.html "interface in java.util")<? super T> comparator)  
 Returns a `Collector` that produces the maximal element according to a given `Comparator`, described as an `Optional<T>`.  
 Implementation Requirements:  
 This produces a result equivalent to:  
 ```  
      reducing(BinaryOperator.maxBy(comparator))  
        
 ```  
 Type Parameters:  
 `T` \- the type of the input elements  
 Parameters:  
 `comparator` \- a `Comparator` for comparing elements  
 Returns:  
 a `Collector` that produces the maximal value  
 * #### summingInt  
 public static <T> [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,[Integer](../../../java/lang/Integer.html "class in java.lang")> summingInt([ToIntFunction](../../../java/util/function/ToIntFunction.html "interface in java.util.function")<? 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](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,[Long](../../../java/lang/Long.html "class in java.lang")> summingLong([ToLongFunction](../../../java/util/function/ToLongFunction.html "interface in java.util.function")<? 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](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,[Double](../../../java/lang/Double.html "class in java.lang")> summingDouble([ToDoubleFunction](../../../java/util/function/ToDoubleFunction.html "interface in java.util.function")<? 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](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,[Double](../../../java/lang/Double.html "class in java.lang")> averagingInt([ToIntFunction](../../../java/util/function/ToIntFunction.html "interface in java.util.function")<? 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](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,[Double](../../../java/lang/Double.html "class in java.lang")> averagingLong([ToLongFunction](../../../java/util/function/ToLongFunction.html "interface in java.util.function")<? 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](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,[Double](../../../java/lang/Double.html "class in java.lang")> averagingDouble([ToDoubleFunction](../../../java/util/function/ToDoubleFunction.html "interface in java.util.function")<? super T> mapper)  
 Returns a `Collector` that produces the arithmetic mean of a double-valued function applied to the input elements. If no elements are present, the result is 0.  
 The average returned can vary depending upon the order in which values are recorded, due to accumulated rounding error in addition of values of differing magnitudes. Values sorted by increasing absolute magnitude tend to yield more accurate results. If any recorded value is a `NaN` or the sum is at any point a `NaN` then the average will be `NaN`.  
 Implementation Note:  
 The `double` format can represent all consecutive integers in the range -253 to 253. If the pipeline has more than 253 values, the divisor in the average computation will saturate at 253, leading to additional numerical errors.  
 Type Parameters:  
 `T` \- the type of the input elements  
 Parameters:  
 `mapper` \- a function extracting the property to be averaged  
 Returns:  
 a `Collector` that produces the arithmetic mean of a derived property  
 * #### reducing  
 public static <T> [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,T> reducing(T identity,  
                                             [BinaryOperator](../../../java/util/function/BinaryOperator.html "interface in java.util.function")<T> op)  
 Returns a `Collector` which performs a reduction of its input elements under a specified `BinaryOperator` using the provided identity.  
 API Note:  
 The `reducing()` collectors are most useful when used in a multi-level reduction, downstream of `groupingBy` or`partitioningBy`. To perform a simple reduction on a stream, use [Stream.reduce(Object, BinaryOperator)](../../../java/util/stream/Stream.html#reduce%28T,java.util.function.BinaryOperator%29)} instead.  
 Type Parameters:  
 `T` \- element type for the input and output of the reduction  
 Parameters:  
 `identity` \- the identity value for the reduction (also, the value that is returned when there are no input elements)  
 `op` \- a `BinaryOperator<T>` used to reduce the input elements  
 Returns:  
 a `Collector` which implements the reduction operation  
 See Also:  
 [reducing(BinaryOperator)](../../../java/util/stream/Collectors.html#reducing%28java.util.function.BinaryOperator%29), [reducing(Object, Function, BinaryOperator)](../../../java/util/stream/Collectors.html#reducing%28U,java.util.function.Function,java.util.function.BinaryOperator%29)  
 * #### reducing  
 public static <T> [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,[Optional](../../../java/util/Optional.html "class in java.util")<T>> reducing([BinaryOperator](../../../java/util/function/BinaryOperator.html "interface in java.util.function")<T> op)  
 Returns a `Collector` which performs a reduction of its input elements under a specified `BinaryOperator`. The result is described as an `Optional<T>`.  
 API Note:  
 The `reducing()` collectors are most useful when used in a multi-level reduction, downstream of `groupingBy` or`partitioningBy`. To perform a simple reduction on a stream, use [Stream.reduce(BinaryOperator)](../../../java/util/stream/Stream.html#reduce%28java.util.function.BinaryOperator%29) instead.  
 For example, given a stream of `Person`, to calculate tallest person in each city:  
 ```  
  Comparator<Person> byHeight = Comparator.comparing(Person::getHeight);  
  Map<City, Optional<Person>> tallestByCity  
    = people.stream().collect(  
      groupingBy(Person::getCity,  
                 reducing(BinaryOperator.maxBy(byHeight))));  
        
 ```  
 Type Parameters:  
 `T` \- element type for the input and output of the reduction  
 Parameters:  
 `op` \- a `BinaryOperator<T>` used to reduce the input elements  
 Returns:  
 a `Collector` which implements the reduction operation  
 See Also:  
 [reducing(Object, BinaryOperator)](../../../java/util/stream/Collectors.html#reducing%28T,java.util.function.BinaryOperator%29), [reducing(Object, Function, BinaryOperator)](../../../java/util/stream/Collectors.html#reducing%28U,java.util.function.Function,java.util.function.BinaryOperator%29)  
 * #### reducing  
 public static <T,U> [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,U> reducing(U identity,  
                                               [Function](../../../java/util/function/Function.html "interface in java.util.function")<? super T,? extends U> mapper,  
                                               [BinaryOperator](../../../java/util/function/BinaryOperator.html "interface in java.util.function")<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)](../../../java/util/stream/Collectors.html#reducing%28T,java.util.function.BinaryOperator%29) which allows a transformation of the elements before reduction.  
 API Note:  
 The `reducing()` collectors are most useful when used in a multi-level reduction, downstream of `groupingBy` or`partitioningBy`. To perform a simple map-reduce on a stream, use [Stream.map(Function)](../../../java/util/stream/Stream.html#map%28java.util.function.Function%29) and [Stream.reduce(Object, BinaryOperator)](../../../java/util/stream/Stream.html#reduce%28T,java.util.function.BinaryOperator%29) instead.  
 For example, given a stream of `Person`, to calculate the longest last name of residents in each city:  
 ```  
  Comparator<String> byLength = Comparator.comparing(String::length);  
  Map<City, String> longestLastNameByCity  
    = people.stream().collect(  
      groupingBy(Person::getCity,  
                 reducing("",  
                          Person::getLastName,  
                          BinaryOperator.maxBy(byLength))));  
        
 ```  
 Type Parameters:  
 `T` \- the type of the input elements  
 `U` \- the type of the mapped values  
 Parameters:  
 `identity` \- the identity value for the reduction (also, the value that is returned when there are no input elements)  
 `mapper` \- a mapping function to apply to each input value  
 `op` \- a `BinaryOperator<U>` used to reduce the mapped values  
 Returns:  
 a `Collector` implementing the map-reduce operation  
 See Also:  
 [reducing(Object, BinaryOperator)](../../../java/util/stream/Collectors.html#reducing%28T,java.util.function.BinaryOperator%29), [reducing(BinaryOperator)](../../../java/util/stream/Collectors.html#reducing%28java.util.function.BinaryOperator%29)  
 * #### groupingBy  
 public static <T,K> [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,[Map](../../../java/util/Map.html "interface in java.util")<K,[List](../../../java/util/List.html "interface in java.util")<T>>> groupingBy([Function](../../../java/util/function/Function.html "interface in java.util.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 `List`s containing the input elements which map to the associated key under the classification function.  
 There are no guarantees on the type, mutability, serializability, or thread-safety of the `Map` or `List` objects returned.  
 Implementation Requirements:  
 This produces a result similar to:  
 ```  
      groupingBy(classifier, toList());  
        
 ```  
 Implementation Note:  
 The returned `Collector` is not concurrent. For parallel stream pipelines, the `combiner` function operates by merging the keys from one map into another, which can be an expensive operation. If preservation of the order in which elements appear in the resulting `Map` collector is not required, using [groupingByConcurrent(Function)](../../../java/util/stream/Collectors.html#groupingByConcurrent%28java.util.function.Function%29) may offer better parallel performance.  
 Type Parameters:  
 `T` \- the type of the input elements  
 `K` \- the type of the keys  
 Parameters:  
 `classifier` \- the classifier function mapping input elements to keys  
 Returns:  
 a `Collector` implementing the group-by operation  
 See Also:  
 [groupingBy(Function, Collector)](../../../java/util/stream/Collectors.html#groupingBy%28java.util.function.Function,java.util.stream.Collector%29), [groupingBy(Function, Supplier, Collector)](../../../java/util/stream/Collectors.html#groupingBy%28java.util.function.Function,java.util.function.Supplier,java.util.stream.Collector%29), [groupingByConcurrent(Function)](../../../java/util/stream/Collectors.html#groupingByConcurrent%28java.util.function.Function%29)  
 * #### groupingBy  
 public static <T,K,A,D> [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,[Map](../../../java/util/Map.html "interface in java.util")<K,D>> groupingBy([Function](../../../java/util/function/Function.html "interface in java.util.function")<? super T,? extends K> classifier,  
                                                            [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<? super T,A,D> downstream)  
 Returns a `Collector` implementing a cascaded "group by" operation on input elements of type `T`, grouping elements according to a classification function, and then performing a reduction operation on the values associated with a given key using the specified downstream`Collector`.  
 The classification function maps elements to some key type `K`. The downstream collector operates on elements of type `T` and produces a result of type `D`. The resulting collector produces a`Map<K, D>`.  
 There are no guarantees on the type, mutability, serializability, or thread-safety of the `Map` returned.  
 For example, to compute the set of last names of people in each city:  
 ```  
  Map<City, Set<String>> namesByCity  
    = people.stream().collect(  
      groupingBy(Person::getCity,  
                 mapping(Person::getLastName,  
                         toSet())));  
        
 ```  
 Implementation Note:  
 The returned `Collector` is not concurrent. For parallel stream pipelines, the `combiner` function operates by merging the keys from one map into another, which can be an expensive operation. If preservation of the order in which elements are presented to the downstream collector is not required, using [groupingByConcurrent(Function, Collector)](../../../java/util/stream/Collectors.html#groupingByConcurrent%28java.util.function.Function,java.util.stream.Collector%29) may offer better parallel performance.  
 Type Parameters:  
 `T` \- the type of the input elements  
 `K` \- the type of the keys  
 `A` \- the intermediate accumulation type of the downstream collector  
 `D` \- the result type of the downstream reduction  
 Parameters:  
 `classifier` \- a classifier function mapping input elements to keys  
 `downstream` \- a `Collector` implementing the downstream reduction  
 Returns:  
 a `Collector` implementing the cascaded group-by operation  
 See Also:  
 [groupingBy(Function)](../../../java/util/stream/Collectors.html#groupingBy%28java.util.function.Function%29), [groupingBy(Function, Supplier, Collector)](../../../java/util/stream/Collectors.html#groupingBy%28java.util.function.Function,java.util.function.Supplier,java.util.stream.Collector%29), [groupingByConcurrent(Function, Collector)](../../../java/util/stream/Collectors.html#groupingByConcurrent%28java.util.function.Function,java.util.stream.Collector%29)  
 * #### groupingBy  
 public static <T,K,D,A,M extends [Map](../../../java/util/Map.html "interface in java.util")<K,D>> [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,M> groupingBy([Function](../../../java/util/function/Function.html "interface in java.util.function")<? super T,? extends K> classifier,  
                                                                        [Supplier](../../../java/util/function/Supplier.html "interface in java.util.function")<M> mapFactory,  
                                                                        [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<? super T,A,D> downstream)  
 Returns a `Collector` implementing a cascaded "group by" operation on input elements of type `T`, grouping elements according to a classification function, and then performing a reduction operation on the values associated with a given key using the specified downstream`Collector`. The `Map` produced by the Collector is created with the supplied factory function.  
 The classification function maps elements to some key type `K`. The downstream collector operates on elements of type `T` and produces a result of type `D`. The resulting collector produces a`Map<K, D>`.  
 For example, to compute the set of last names of people in each city, where the city names are sorted:  
 ```  
  Map<City, Set<String>> namesByCity  
    = people.stream().collect(  
      groupingBy(Person::getCity,  
                 TreeMap::new,  
                 mapping(Person::getLastName,  
                         toSet())));  
        
 ```  
 Implementation Note:  
 The returned `Collector` is not concurrent. For parallel stream pipelines, the `combiner` function operates by merging the keys from one map into another, which can be an expensive operation. If preservation of the order in which elements are presented to the downstream collector is not required, using [groupingByConcurrent(Function, Supplier, Collector)](../../../java/util/stream/Collectors.html#groupingByConcurrent%28java.util.function.Function,java.util.function.Supplier,java.util.stream.Collector%29) 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  
 `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:  
 [groupingBy(Function, Collector)](../../../java/util/stream/Collectors.html#groupingBy%28java.util.function.Function,java.util.stream.Collector%29), [groupingBy(Function)](../../../java/util/stream/Collectors.html#groupingBy%28java.util.function.Function%29), [groupingByConcurrent(Function, Supplier, Collector)](../../../java/util/stream/Collectors.html#groupingByConcurrent%28java.util.function.Function,java.util.function.Supplier,java.util.stream.Collector%29)  
 * #### groupingByConcurrent  
 public static <T,K> [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,[ConcurrentMap](../../../java/util/concurrent/ConcurrentMap.html "interface in java.util.concurrent")<K,[List](../../../java/util/List.html "interface in java.util")<T>>> groupingByConcurrent([Function](../../../java/util/function/Function.html "interface in java.util.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](../../../java/util/stream/Collector.Characteristics.html#CONCURRENT) and[unordered](../../../java/util/stream/Collector.Characteristics.html#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 `List`s containing the input elements which map to the associated key under the classification function.  
 There are no guarantees on the type, mutability, or serializability of the `ConcurrentMap` or `List` objects returned, or of the thread-safety of the `List` objects returned.  
 Implementation Requirements:  
 This produces a result similar to:  
 ```  
      groupingByConcurrent(classifier, toList());  
        
 ```  
 Type Parameters:  
 `T` \- the type of the input elements  
 `K` \- the type of the keys  
 Parameters:  
 `classifier` \- a classifier function mapping input elements to keys  
 Returns:  
 a concurrent, unordered `Collector` implementing the group-by operation  
 See Also:  
 [groupingBy(Function)](../../../java/util/stream/Collectors.html#groupingBy%28java.util.function.Function%29), [groupingByConcurrent(Function, Collector)](../../../java/util/stream/Collectors.html#groupingByConcurrent%28java.util.function.Function,java.util.stream.Collector%29), [groupingByConcurrent(Function, Supplier, Collector)](../../../java/util/stream/Collectors.html#groupingByConcurrent%28java.util.function.Function,java.util.function.Supplier,java.util.stream.Collector%29)  
 * #### groupingByConcurrent  
 public static <T,K,A,D> [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,[ConcurrentMap](../../../java/util/concurrent/ConcurrentMap.html "interface in java.util.concurrent")<K,D>> groupingByConcurrent([Function](../../../java/util/function/Function.html "interface in java.util.function")<? super T,? extends K> classifier,  
                                                                                [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<? 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](../../../java/util/stream/Collector.Characteristics.html#CONCURRENT) and[unordered](../../../java/util/stream/Collector.Characteristics.html#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:  
 [groupingBy(Function, Collector)](../../../java/util/stream/Collectors.html#groupingBy%28java.util.function.Function,java.util.stream.Collector%29), [groupingByConcurrent(Function)](../../../java/util/stream/Collectors.html#groupingByConcurrent%28java.util.function.Function%29), [groupingByConcurrent(Function, Supplier, Collector)](../../../java/util/stream/Collectors.html#groupingByConcurrent%28java.util.function.Function,java.util.function.Supplier,java.util.stream.Collector%29)  
 * #### groupingByConcurrent  
 public static <T,K,A,D,M extends [ConcurrentMap](../../../java/util/concurrent/ConcurrentMap.html "interface in java.util.concurrent")<K,D>> [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,M> groupingByConcurrent([Function](../../../java/util/function/Function.html "interface in java.util.function")<? super T,? extends K> classifier,  
                                                                                            [Supplier](../../../java/util/function/Supplier.html "interface in java.util.function")<M> mapFactory,  
                                                                                            [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<? 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](../../../java/util/stream/Collector.Characteristics.html#CONCURRENT) and[unordered](../../../java/util/stream/Collector.Characteristics.html#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:  
 [groupingByConcurrent(Function)](../../../java/util/stream/Collectors.html#groupingByConcurrent%28java.util.function.Function%29), [groupingByConcurrent(Function, Collector)](../../../java/util/stream/Collectors.html#groupingByConcurrent%28java.util.function.Function,java.util.stream.Collector%29), [groupingBy(Function, Supplier, Collector)](../../../java/util/stream/Collectors.html#groupingBy%28java.util.function.Function,java.util.function.Supplier,java.util.stream.Collector%29)  
 * #### partitioningBy  
 public static <T> [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,[Map](../../../java/util/Map.html "interface in java.util")<[Boolean](../../../java/lang/Boolean.html "class in java.lang"),[List](../../../java/util/List.html "interface in java.util")<T>>> partitioningBy([Predicate](../../../java/util/function/Predicate.html "interface in java.util.function")<? super T> predicate)  
 Returns a `Collector` which partitions the input elements according to a `Predicate`, and organizes them into a`Map<Boolean, List<T>>`. The returned `Map` always contains mappings for both`false` and `true` keys. There are no guarantees on the type, mutability, serializability, or thread-safety of the `Map` or `List` returned.  
 API Note:  
 If a partition has no elements, its value in the result Map will be an empty List.  
 Type Parameters:  
 `T` \- the type of the input elements  
 Parameters:  
 `predicate` \- a predicate used for classifying input elements  
 Returns:  
 a `Collector` implementing the partitioning operation  
 See Also:  
 [partitioningBy(Predicate, Collector)](../../../java/util/stream/Collectors.html#partitioningBy%28java.util.function.Predicate,java.util.stream.Collector%29)  
 * #### partitioningBy  
 public static <T,D,A> [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,[Map](../../../java/util/Map.html "interface in java.util")<[Boolean](../../../java/lang/Boolean.html "class in java.lang"),D>> partitioningBy([Predicate](../../../java/util/function/Predicate.html "interface in java.util.function")<? super T> predicate,  
                                                                    [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<? super T,A,D> downstream)  
 Returns a `Collector` which partitions the input elements according to a `Predicate`, reduces the values in each partition according to another `Collector`, and organizes them into a`Map<Boolean, D>` whose values are the result of the downstream reduction.  
  The returned `Map` always contains mappings for both`false` and `true` keys. There are no guarantees on the type, mutability, serializability, or thread-safety of the `Map` returned.  
 API Note:  
 If a partition has no elements, its value in the result Map will be obtained by calling the downstream collector's supplier function and then applying the finisher function.  
 Type Parameters:  
 `T` \- the type of the input elements  
 `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:  
 [partitioningBy(Predicate)](../../../java/util/stream/Collectors.html#partitioningBy%28java.util.function.Predicate%29)  
 * #### toMap  
 public static <T,K,U> [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,[Map](../../../java/util/Map.html "interface in java.util")<K,U>> toMap([Function](../../../java/util/function/Function.html "interface in java.util.function")<? super T,? extends K> keyMapper,  
                                                     [Function](../../../java/util/function/Function.html "interface in java.util.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)](../../../java/lang/Object.html#equals%28java.lang.Object%29)), an `IllegalStateException` is thrown when the collection operation is performed. If the mapped keys might have duplicates, use [toMap(Function, Function, BinaryOperator)](../../../java/util/stream/Collectors.html#toMap%28java.util.function.Function,java.util.function.Function,java.util.function.BinaryOperator%29) instead.  
 There are no guarantees on the type, mutability, serializability, or thread-safety of the `Map` returned.  
 API Note:  
 It is common for either the key or the value to be the input elements. In this case, the utility method[Function.identity()](../../../java/util/function/Function.html#identity%28%29) may be helpful. For example, the following produces a `Map` mapping students to their grade point average:  
 ```  
  Map<Student, Double> studentToGPA  
    = students.stream().collect(  
      toMap(Function.identity(),  
            student -> computeGPA(student)));  
        
 ```  
  And the following produces a `Map` mapping a unique identifier to students:  
 ```  
  Map<String, Student> studentIdToStudent  
    = students.stream().collect(  
      toMap(Student::getId,  
            Function.identity()));  
        
 ```  
 Implementation Note:  
 The returned `Collector` is not concurrent. For parallel stream pipelines, the `combiner` function operates by merging the keys from one map into another, which can be an expensive operation. If it is not required that results are inserted into the `Map` in encounter order, using [toConcurrentMap(Function, Function)](../../../java/util/stream/Collectors.html#toConcurrentMap%28java.util.function.Function,java.util.function.Function%29) may offer better parallel performance.  
 Type Parameters:  
 `T` \- the type of the input elements  
 `K` \- the output type of the key mapping function  
 `U` \- the output type of the value mapping function  
 Parameters:  
 `keyMapper` \- a mapping function to produce keys  
 `valueMapper` \- a mapping function to produce values  
 Returns:  
 a `Collector` which collects elements into a `Map` whose keys and values are the result of applying mapping functions to the input elements  
 See Also:  
 [toMap(Function, Function, BinaryOperator)](../../../java/util/stream/Collectors.html#toMap%28java.util.function.Function,java.util.function.Function,java.util.function.BinaryOperator%29), [toMap(Function, Function, BinaryOperator, Supplier)](../../../java/util/stream/Collectors.html#toMap%28java.util.function.Function,java.util.function.Function,java.util.function.BinaryOperator,java.util.function.Supplier%29), [toConcurrentMap(Function, Function)](../../../java/util/stream/Collectors.html#toConcurrentMap%28java.util.function.Function,java.util.function.Function%29)  
 * #### toUnmodifiableMap  
 public static <T,K,U> [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,[Map](../../../java/util/Map.html "interface in java.util")<K,U>> toUnmodifiableMap([Function](../../../java/util/function/Function.html "interface in java.util.function")<? super T,? extends K> keyMapper,  
                                                                 [Function](../../../java/util/function/Function.html "interface in java.util.function")<? super T,? extends U> valueMapper)  
 Returns a `Collector` that accumulates the input elements into an[unmodifiable Map](../Map.html#unmodifiable), 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)](../../../java/lang/Object.html#equals%28java.lang.Object%29)), an `IllegalStateException` is thrown when the collection operation is performed. If the mapped keys might have duplicates, use [toUnmodifiableMap(Function, Function, BinaryOperator)](../../../java/util/stream/Collectors.html#toUnmodifiableMap%28java.util.function.Function,java.util.function.Function,java.util.function.BinaryOperator%29) 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](../Map.html#unmodifiable), whose keys and values are the result of applying the provided mapping functions to the input elements  
 Throws:  
 `[NullPointerException](../../../java/lang/NullPointerException.html "class in java.lang")` \- if either keyMapper or valueMapper is null  
 Since:  
 10  
 See Also:  
 [toUnmodifiableMap(Function, Function, BinaryOperator)](../../../java/util/stream/Collectors.html#toUnmodifiableMap%28java.util.function.Function,java.util.function.Function,java.util.function.BinaryOperator%29)  
 * #### toMap  
 public static <T,K,U> [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,[Map](../../../java/util/Map.html "interface in java.util")<K,U>> toMap([Function](../../../java/util/function/Function.html "interface in java.util.function")<? super T,? extends K> keyMapper,  
                                                     [Function](../../../java/util/function/Function.html "interface in java.util.function")<? super T,? extends U> valueMapper,  
                                                     [BinaryOperator](../../../java/util/function/BinaryOperator.html "interface in java.util.function")<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)](../../../java/lang/Object.html#equals%28java.lang.Object%29)), the value mapping function is applied to each equal element, and the results are merged using the provided merging function.  
 There are no guarantees on the type, mutability, serializability, or thread-safety of the `Map` returned.  
 API Note:  
 There are multiple ways to deal with collisions between multiple elements mapping to the same key. The other forms of `toMap` simply use a merge function that throws unconditionally, but you can easily write more flexible merge policies. For example, if you have a stream of `Person`, and you want to produce a "phone book" mapping name to address, but it is possible that two persons have the same name, you can do as follows to gracefully deal with these collisions, and produce a`Map` mapping names to a concatenated list of addresses:  
 ```  
  Map<String, String> phoneBook  
    = people.stream().collect(  
      toMap(Person::getName,  
            Person::getAddress,  
            (s, a) -> s + ", " + a));  
        
 ```  
 Implementation Note:  
 The returned `Collector` is not concurrent. For parallel stream pipelines, the `combiner` function operates by merging the keys from one map into another, which can be an expensive operation. If it is not required that results are merged into the `Map` in encounter order, using [toConcurrentMap(Function, Function, BinaryOperator)](../../../java/util/stream/Collectors.html#toConcurrentMap%28java.util.function.Function,java.util.function.Function,java.util.function.BinaryOperator%29) may offer better parallel performance.  
 Type Parameters:  
 `T` \- the type of the input elements  
 `K` \- the output type of the key mapping function  
 `U` \- the output type of the value mapping function  
 Parameters:  
 `keyMapper` \- a mapping function to produce keys  
 `valueMapper` \- a mapping function to produce values  
 `mergeFunction` \- a merge function, used to resolve collisions between values associated with the same key, as supplied to [Map.merge(Object, Object, BiFunction)](../../../java/util/Map.html#merge%28K,V,java.util.function.BiFunction%29)  
 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:  
 [toMap(Function, Function)](../../../java/util/stream/Collectors.html#toMap%28java.util.function.Function,java.util.function.Function%29), [toMap(Function, Function, BinaryOperator, Supplier)](../../../java/util/stream/Collectors.html#toMap%28java.util.function.Function,java.util.function.Function,java.util.function.BinaryOperator,java.util.function.Supplier%29), [toConcurrentMap(Function, Function, BinaryOperator)](../../../java/util/stream/Collectors.html#toConcurrentMap%28java.util.function.Function,java.util.function.Function,java.util.function.BinaryOperator%29)  
 * #### toUnmodifiableMap  
 public static <T,K,U> [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,[Map](../../../java/util/Map.html "interface in java.util")<K,U>> toUnmodifiableMap([Function](../../../java/util/function/Function.html "interface in java.util.function")<? super T,? extends K> keyMapper,  
                                                                 [Function](../../../java/util/function/Function.html "interface in java.util.function")<? super T,? extends U> valueMapper,  
                                                                 [BinaryOperator](../../../java/util/function/BinaryOperator.html "interface in java.util.function")<U> mergeFunction)  
 Returns a `Collector` that accumulates the input elements into an[unmodifiable Map](../Map.html#unmodifiable), 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)](../../../java/lang/Object.html#equals%28java.lang.Object%29)), 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)](../../../java/util/Map.html#merge%28K,V,java.util.function.BiFunction%29), must be non-null  
 Returns:  
 a `Collector` that accumulates the input elements into an[unmodifiable Map](../Map.html#unmodifiable), whose keys and values are the result of applying the provided mapping functions to the input elements  
 Throws:  
 `[NullPointerException](../../../java/lang/NullPointerException.html "class in java.lang")` \- if the keyMapper, valueMapper, or mergeFunction is null  
 Since:  
 10  
 See Also:  
 [toUnmodifiableMap(Function, Function)](../../../java/util/stream/Collectors.html#toUnmodifiableMap%28java.util.function.Function,java.util.function.Function%29)  
 * #### toMap  
 public static <T,K,U,M extends [Map](../../../java/util/Map.html "interface in java.util")<K,U>> [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,M> toMap([Function](../../../java/util/function/Function.html "interface in java.util.function")<? super T,? extends K> keyMapper,  
                                                                 [Function](../../../java/util/function/Function.html "interface in java.util.function")<? super T,? extends U> valueMapper,  
                                                                 [BinaryOperator](../../../java/util/function/BinaryOperator.html "interface in java.util.function")<U> mergeFunction,  
                                                                 [Supplier](../../../java/util/function/Supplier.html "interface in java.util.function")<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)](../../../java/lang/Object.html#equals%28java.lang.Object%29)), the value mapping function is applied to each equal element, and the results are merged using the provided merging function. The `Map` is created by a provided supplier function.  
 Implementation Note:  
 The returned `Collector` is not concurrent. For parallel stream pipelines, the `combiner` function operates by merging the keys from one map into another, which can be an expensive operation. If it is not required that results are merged into the `Map` in encounter order, using [toConcurrentMap(Function, Function, BinaryOperator, Supplier)](../../../java/util/stream/Collectors.html#toConcurrentMap%28java.util.function.Function,java.util.function.Function,java.util.function.BinaryOperator,java.util.function.Supplier%29) may offer better parallel performance.  
 Type Parameters:  
 `T` \- the type of the input elements  
 `K` \- the output type of the key mapping function  
 `U` \- the output type of the value mapping function  
 `M` \- the type of the resulting `Map`  
 Parameters:  
 `keyMapper` \- a mapping function to produce keys  
 `valueMapper` \- a mapping function to produce values  
 `mergeFunction` \- a merge function, used to resolve collisions between values associated with the same key, as supplied to [Map.merge(Object, Object, BiFunction)](../../../java/util/Map.html#merge%28K,V,java.util.function.BiFunction%29)  
 `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:  
 [toMap(Function, Function)](../../../java/util/stream/Collectors.html#toMap%28java.util.function.Function,java.util.function.Function%29), [toMap(Function, Function, BinaryOperator)](../../../java/util/stream/Collectors.html#toMap%28java.util.function.Function,java.util.function.Function,java.util.function.BinaryOperator%29), [toConcurrentMap(Function, Function, BinaryOperator, Supplier)](../../../java/util/stream/Collectors.html#toConcurrentMap%28java.util.function.Function,java.util.function.Function,java.util.function.BinaryOperator,java.util.function.Supplier%29)  
 * #### toConcurrentMap  
 public static <T,K,U> [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,[ConcurrentMap](../../../java/util/concurrent/ConcurrentMap.html "interface in java.util.concurrent")<K,U>> toConcurrentMap([Function](../../../java/util/function/Function.html "interface in java.util.function")<? super T,? extends K> keyMapper,  
                                                                         [Function](../../../java/util/function/Function.html "interface in java.util.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)](../../../java/lang/Object.html#equals%28java.lang.Object%29)), an `IllegalStateException` is thrown when the collection operation is performed. If the mapped keys may have duplicates, use[toConcurrentMap(Function, Function, BinaryOperator)](../../../java/util/stream/Collectors.html#toConcurrentMap%28java.util.function.Function,java.util.function.Function,java.util.function.BinaryOperator%29) instead.  
 There are no guarantees on the type, mutability, or serializability of the `ConcurrentMap` returned.  
 API Note:  
 It is common for either the key or the value to be the input elements. In this case, the utility method[Function.identity()](../../../java/util/function/Function.html#identity%28%29) may be helpful. For example, the following produces a `ConcurrentMap` mapping students to their grade point average:  
 ```  
  ConcurrentMap<Student, Double> studentToGPA  
    = students.stream().collect(  
      toConcurrentMap(Function.identity(),  
                      student -> computeGPA(student)));  
        
 ```  
  And the following produces a `ConcurrentMap` mapping a unique identifier to students:  
 ```  
  ConcurrentMap<String, Student> studentIdToStudent  
    = students.stream().collect(  
      toConcurrentMap(Student::getId,  
                      Function.identity()));  
        
 ```  
 This is a [concurrent](../../../java/util/stream/Collector.Characteristics.html#CONCURRENT) and[unordered](../../../java/util/stream/Collector.Characteristics.html#UNORDERED) Collector.  
 Type Parameters:  
 `T` \- the type of the input elements  
 `K` \- the output type of the key mapping function  
 `U` \- the output type of the value mapping function  
 Parameters:  
 `keyMapper` \- the mapping function to produce keys  
 `valueMapper` \- the mapping function to produce values  
 Returns:  
 a concurrent, unordered `Collector` which collects elements into a`ConcurrentMap` whose keys are the result of applying a key mapping function to the input elements, and whose values are the result of applying a value mapping function to the input elements  
 See Also:  
 [toMap(Function, Function)](../../../java/util/stream/Collectors.html#toMap%28java.util.function.Function,java.util.function.Function%29), [toConcurrentMap(Function, Function, BinaryOperator)](../../../java/util/stream/Collectors.html#toConcurrentMap%28java.util.function.Function,java.util.function.Function,java.util.function.BinaryOperator%29), [toConcurrentMap(Function, Function, BinaryOperator, Supplier)](../../../java/util/stream/Collectors.html#toConcurrentMap%28java.util.function.Function,java.util.function.Function,java.util.function.BinaryOperator,java.util.function.Supplier%29)  
 * #### toConcurrentMap  
 public static <T,K,U> [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,[ConcurrentMap](../../../java/util/concurrent/ConcurrentMap.html "interface in java.util.concurrent")<K,U>> toConcurrentMap([Function](../../../java/util/function/Function.html "interface in java.util.function")<? super T,? extends K> keyMapper,  
                                                                         [Function](../../../java/util/function/Function.html "interface in java.util.function")<? super T,? extends U> valueMapper,  
                                                                         [BinaryOperator](../../../java/util/function/BinaryOperator.html "interface in java.util.function")<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)](../../../java/lang/Object.html#equals%28java.lang.Object%29)), 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.  
 API Note:  
 There are multiple ways to deal with collisions between multiple elements mapping to the same key. The other forms of `toConcurrentMap` simply use a merge function that throws unconditionally, but you can easily write more flexible merge policies. For example, if you have a stream of `Person`, and you want to produce a "phone book" mapping name to address, but it is possible that two persons have the same name, you can do as follows to gracefully deal with these collisions, and produce a`ConcurrentMap` mapping names to a concatenated list of addresses:  
 ```  
  ConcurrentMap<String, String> phoneBook  
    = people.stream().collect(  
      toConcurrentMap(Person::getName,  
                      Person::getAddress,  
                      (s, a) -> s + ", " + a));  
        
 ```  
 This is a [concurrent](../../../java/util/stream/Collector.Characteristics.html#CONCURRENT) and[unordered](../../../java/util/stream/Collector.Characteristics.html#UNORDERED) Collector.  
 Type Parameters:  
 `T` \- the type of the input elements  
 `K` \- the output type of the key mapping function  
 `U` \- the output type of the value mapping function  
 Parameters:  
 `keyMapper` \- a mapping function to produce keys  
 `valueMapper` \- a mapping function to produce values  
 `mergeFunction` \- a merge function, used to resolve collisions between values associated with the same key, as supplied to [Map.merge(Object, Object, BiFunction)](../../../java/util/Map.html#merge%28K,V,java.util.function.BiFunction%29)  
 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(Function, Function)](../../../java/util/stream/Collectors.html#toConcurrentMap%28java.util.function.Function,java.util.function.Function%29), [toConcurrentMap(Function, Function, BinaryOperator, Supplier)](../../../java/util/stream/Collectors.html#toConcurrentMap%28java.util.function.Function,java.util.function.Function,java.util.function.BinaryOperator,java.util.function.Supplier%29), [toMap(Function, Function, BinaryOperator)](../../../java/util/stream/Collectors.html#toMap%28java.util.function.Function,java.util.function.Function,java.util.function.BinaryOperator%29)  
 * #### toConcurrentMap  
 public static <T,K,U,M extends [ConcurrentMap](../../../java/util/concurrent/ConcurrentMap.html "interface in java.util.concurrent")<K,U>> [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,M> toConcurrentMap([Function](../../../java/util/function/Function.html "interface in java.util.function")<? super T,? extends K> keyMapper,  
                                                                                     [Function](../../../java/util/function/Function.html "interface in java.util.function")<? super T,? extends U> valueMapper,  
                                                                                     [BinaryOperator](../../../java/util/function/BinaryOperator.html "interface in java.util.function")<U> mergeFunction,  
                                                                                     [Supplier](../../../java/util/function/Supplier.html "interface in java.util.function")<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)](../../../java/lang/Object.html#equals%28java.lang.Object%29)), 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](../../../java/util/stream/Collector.Characteristics.html#CONCURRENT) and[unordered](../../../java/util/stream/Collector.Characteristics.html#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)](../../../java/util/Map.html#merge%28K,V,java.util.function.BiFunction%29)  
 `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:  
 [toConcurrentMap(Function, Function)](../../../java/util/stream/Collectors.html#toConcurrentMap%28java.util.function.Function,java.util.function.Function%29), [toConcurrentMap(Function, Function, BinaryOperator)](../../../java/util/stream/Collectors.html#toConcurrentMap%28java.util.function.Function,java.util.function.Function,java.util.function.BinaryOperator%29), [toMap(Function, Function, BinaryOperator, Supplier)](../../../java/util/stream/Collectors.html#toMap%28java.util.function.Function,java.util.function.Function,java.util.function.BinaryOperator,java.util.function.Supplier%29)  
 * #### summarizingInt  
 public static <T> [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,[IntSummaryStatistics](../../../java/util/IntSummaryStatistics.html "class in java.util")> summarizingInt([ToIntFunction](../../../java/util/function/ToIntFunction.html "interface in java.util.function")<? 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:  
 [summarizingDouble(ToDoubleFunction)](../../../java/util/stream/Collectors.html#summarizingDouble%28java.util.function.ToDoubleFunction%29), [summarizingLong(ToLongFunction)](../../../java/util/stream/Collectors.html#summarizingLong%28java.util.function.ToLongFunction%29)  
 * #### summarizingLong  
 public static <T> [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,[LongSummaryStatistics](../../../java/util/LongSummaryStatistics.html "class in java.util")> summarizingLong([ToLongFunction](../../../java/util/function/ToLongFunction.html "interface in java.util.function")<? 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(ToDoubleFunction)](../../../java/util/stream/Collectors.html#summarizingDouble%28java.util.function.ToDoubleFunction%29), [summarizingInt(ToIntFunction)](../../../java/util/stream/Collectors.html#summarizingInt%28java.util.function.ToIntFunction%29)  
 * #### summarizingDouble  
 public static <T> [Collector](../../../java/util/stream/Collector.html "interface in java.util.stream")<T,?,[DoubleSummaryStatistics](../../../java/util/DoubleSummaryStatistics.html "class in java.util")> summarizingDouble([ToDoubleFunction](../../../java/util/function/ToDoubleFunction.html "interface in java.util.function")<? 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:  
 [summarizingLong(ToLongFunction)](../../../java/util/stream/Collectors.html#summarizingLong%28java.util.function.ToLongFunction%29), [summarizingInt(ToIntFunction)](../../../java/util/stream/Collectors.html#summarizingInt%28java.util.function.ToIntFunction%29)

Collectors (Java SE 10 & JDK 10 ) (original) (raw)

Method Summary

Method Detail