Collectors (Java Platform SE 8 )

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

All Methods Static Methods Concrete Methods
Modifier and Type	Method and 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,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,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,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> 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> Collector<T,?,ConcurrentMap<K,D>>`	`groupingByConcurrent(Function<? super T,? extends K> classifier, Collector<? super T,A,D> downstream)` Returns a concurrent `Collector` implementing a cascaded "group by" operation on input elements of type `T`, grouping elements according to a classification function, and then performing a reduction operation on the values associated with a given key using the specified downstream `Collector`.
`static <T,K,A,D,M extends ConcurrentMap<K,D>> Collector<T,?,M>`	`groupingByConcurrent(Function<? super T,? extends K> classifier, Supplier<M> mapFactory, Collector<? super T,A,D> downstream)` Returns a concurrent `Collector` implementing a cascaded "group by" operation on input elements of type `T`, grouping elements according to a classification function, and then performing a reduction operation on the values associated with a given key using the specified downstream `Collector`.
`static Collector<CharSequence,?,String>`	`joining()` 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,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> mapSupplier)` Returns a concurrent `Collector` that accumulates elements into a `ConcurrentMap` whose keys and values are the result of applying the provided mapping functions to the input elements.
`static <T> 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> mapSupplier)` Returns a `Collector` that accumulates elements into a `Map` whose keys and values are the result of applying the provided mapping functions to the input elements.
`static <T> Collector<T,?,Set<T>>`	`toSet()` Returns a `Collector` that accumulates the input elements into a new `Set`.

Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

- Method Detail
 - 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 which returns a new, empty Collection of the appropriate type
 
 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
 - 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
 - joining
```
public static Collector<CharSequence,?,String> joining()
```
 Returns a Collector that concatenates the input elements into a String, in encounter order.
 
 Returns:
 
 a Collector that concatenates the input elements into a String, in encounter order
 - joining
```
public static Collector<CharSequence,?,String> joining(CharSequence delimiter)
```
 Returns a Collector that concatenates the input elements, separated by the specified delimiter, in encounter order.
 
 Parameters:
 
 delimiter - the delimiter to be used between each element
 
 Returns:
 
 A Collector which concatenates CharSequence elements, separated by the specified delimiter, in encounter order
 - joining
```
public static Collector<CharSequence,?,String> joining(CharSequence delimiter,
 CharSequence prefix,
 CharSequence suffix)
```
 Returns a Collector that concatenates the input elements, separated by the specified delimiter, with the specified prefix and suffix, in encounter order.
 
 Parameters:
 
 delimiter - the delimiter to be used between each element
 
 prefix - the sequence of characters to be used at the beginning of the joined result
 
 suffix - the sequence of characters to be used at the end of the joined result
 
 Returns:
 
 A Collector which concatenates CharSequence elements, separated by the specified delimiter, in encounter order
 - mapping
```
public static <T,U,A,R> Collector<T,?,R> mapping(Function<? super T,? extends U> mapper,
 Collector<? super U,A,R> downstream)
```
 Adapts a Collector accepting elements of type U to one accepting elements of type T by applying a mapping function to each input element before accumulation.
 API Note:
 The mapping() collectors are most useful when used in a multi-level reduction, such as downstream of a groupingBy or partitioningBy. For example, given a stream of Person, to accumulate the set of last names in each city:
 Map<City, Set<String>> lastNamesByCity = people.stream().collect(groupingBy(Person::getCity, mapping(Person::getLastName, toSet())));
 Type Parameters:
 
 T - the type of the input elements
 
 U - type of elements accepted by downstream collector
 
 A - intermediate accumulation type of the downstream collector
 
 R - result type of collector
 
 Parameters:
 
 mapper - a function to be applied to the input elements
 
 downstream - a collector which will accept mapped values
 
 Returns:
 
 a collector which applies the mapping function to the input elements and provides the mapped results to the downstream collector
 - 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> people
 = people.stream().collect(collectingAndThen(toList(), Collections::unmodifiableList));
 
```
 Type Parameters:
 
 T - the type of the input elements
 
 A - intermediate accumulation type of the downstream collector
 
 R - result type of the downstream collector
 
 RR - result type of the resulting collector
 
 Parameters:
 
 downstream - a collector
 
 finisher - a function to be applied to the final result of the downstream collector
 
 Returns:
 
 a collector which performs the action of the downstream collector, followed by an additional finishing step
 - counting
```
public static <T> Collector<T,?,Long> counting()
```
 Returns a Collector accepting elements of type T that counts the number of input elements. If no elements are present, the result is 0.
 Implementation Requirements:
 This produces a result equivalent to:
 reducing(0L, e -> 1L, Long::sum)
 Type Parameters:
 
 T - the type of the input elements
 
 Returns:
 
 a Collector that counts the input elements
 - minBy
```
public static <T> Collector<T,?,Optional<T>> minBy(Comparator<? super T> comparator)
```
 Returns a Collector that produces the minimal element according to a given Comparator, described as an Optional<T>.
 Implementation Requirements:
 This produces a result equivalent to:
 reducing(BinaryOperator.minBy(comparator))
 Type Parameters:
 
 T - the type of the input elements
 
 Parameters:
 
 comparator - a Comparator for comparing elements
 
 Returns:
 
 a Collector that produces the minimal value
 - maxBy
```
public static <T> Collector<T,?,Optional<T>> maxBy(Comparator<? super T> comparator)
```
 Returns a Collector that produces the maximal element according to a given Comparator, described as an Optional<T>.
 Implementation Requirements:
 This produces a result equivalent to:
 reducing(BinaryOperator.maxBy(comparator))
 Type Parameters:
 
 T - the type of the input elements
 
 Parameters:
 
 comparator - a Comparator for comparing elements
 
 Returns:
 
 a Collector that produces the maximal value
 - summingInt
```
public static <T> Collector<T,?,Integer> summingInt(ToIntFunction<? super T> mapper)
```
 Returns a Collector that produces the sum of 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 summed
 
 Returns:
 
 a Collector that produces the sum 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 summed
 
 Returns:
 
 a Collector that produces the sum of a derived property
 - averagingDouble
```
public static <T> Collector<T,?,Double> averagingDouble(ToDoubleFunction<? super T> mapper)
```
 Returns a Collector that produces the arithmetic mean of a double-valued function applied to the input elements. If no elements are present, the result is 0.
 The average returned can vary depending upon the order in which values are recorded, due to accumulated rounding error in addition of values of differing magnitudes. Values sorted by increasing absolute magnitude tend to yield more accurate results. If any recorded value is a NaN or the sum is at any point a NaN then the average will be NaN.
 
 Implementation Note:
 
 The double format can represent all consecutive integers in the range -2⁵³ to 2⁵³. If the pipeline has more than 2⁵³ values, the divisor in the average computation will saturate at 2⁵³, leading to additional numerical errors.
 
 Type Parameters:
 
 T - the type of the input elements
 
 Parameters:
 
 mapper - a function extracting the property to be summed
 
 Returns:
 
 a Collector that produces the sum of a derived property
 - reducing
```
public static <T> Collector<T,?,T> reducing(T identity,
 BinaryOperator<T> op)
```
 Returns a Collector which performs a reduction of its input elements under a specified BinaryOperator using the provided identity.
 
 API Note:
 
 The reducing() collectors are most useful when used in a multi-level reduction, downstream of groupingBy or partitioningBy. To perform a simple reduction on a stream, use Stream.reduce(Object, BinaryOperator)} instead.
 
 Type Parameters:
 
 T - element type for the input and output of the reduction
 
 Parameters:
 
 identity - the identity value for the reduction (also, the value that is returned when there are no input elements)
 
 op - a BinaryOperator<T> used to reduce the input elements
 
 Returns:
 
 a Collector which implements the reduction operation
 
 See Also:
 
 reducing(BinaryOperator), reducing(Object, Function, BinaryOperator)
 - reducing
```
public static <T> Collector<T,?,Optional<T>> reducing(BinaryOperator<T> op)
```
 Returns a Collector which performs a reduction of its input elements under a specified BinaryOperator. The result is described as an Optional<T>.
 API Note:
 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) instead.
 For example, given a stream of Person, to calculate tallest person in each city:
 Comparator<Person> byHeight = Comparator.comparing(Person::getHeight); Map<City, Person> tallestByCity = people.stream().collect(groupingBy(Person::getCity, reducing(BinaryOperator.maxBy(byHeight))));
 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), reducing(Object, Function, BinaryOperator)
 - reducing
```
public 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 and BinaryOperator. This is a generalization of reducing(Object, BinaryOperator) which allows a transformation of the elements before reduction.
 API Note:
 The reducing() collectors are most useful when used in a multi-level reduction, downstream of groupingBy or partitioningBy. To perform a simple map-reduce on a stream, use Stream.map(Function) and Stream.reduce(Object, BinaryOperator) instead.
 For example, given a stream of Person, to calculate the longest last name of residents in each city:
 Comparator<String> byLength = Comparator.comparing(String::length); Map<City, String> longestLastNameByCity = people.stream().collect(groupingBy(Person::getCity, reducing(Person::getLastName, BinaryOperator.maxBy(byLength))));
 Type Parameters:
 
 T - the type of the input elements
 
 U - the type of the mapped values
 
 Parameters:
 
 identity - the identity value for the reduction (also, the value that is returned when there are no input elements)
 
 mapper - a mapping function to apply to each input value
 
 op - a BinaryOperator used to reduce the mapped values
 
 Returns:
 
 a Collector implementing the map-reduce operation
 
 See Also:
 
 reducing(Object, BinaryOperator), reducing(BinaryOperator)
 - groupingBy
```
public static <T,K> Collector<T,?,Map<K,List<T>>> groupingBy(Function<? super T,? extends K> classifier)
```
 Returns a Collector implementing a "group by" operation on input elements of type T, grouping elements according to a classification function, and returning the results in a Map.
 The classification function maps elements to some key type K. The collector produces a Map<K, List<T>> whose keys are the values resulting from applying the classification function to the input elements, and whose corresponding values are Lists containing the input elements which map to the associated key under the classification function.
 There are no guarantees on the type, mutability, serializability, or thread-safety of the Map or List objects returned.
 Implementation Requirements:
 This produces a result similar to:
 groupingBy(classifier, toList());
 Implementation Note:
 
 The returned Collector is not concurrent. For parallel stream pipelines, the combiner function operates by merging the keys from one map into another, which can be an expensive operation. If preservation of the order in which elements appear in the resulting Map collector is not required, using groupingByConcurrent(Function) may offer better parallel performance.
 
 Type Parameters:
 
 T - the type of the input elements
 
 K - the type of the keys
 
 Parameters:
 
 classifier - the classifier function mapping input elements to keys
 
 Returns:
 
 a Collector implementing the group-by operation
 
 See Also:
 
 groupingBy(Function, Collector), groupingBy(Function, Supplier, Collector), groupingByConcurrent(Function)
 - groupingBy
```
public static <T,K,A,D> Collector<T,?,Map<K,D>> groupingBy(Function<? super T,? extends K> classifier,
 Collector<? super T,A,D> downstream)
```
 Returns a Collector implementing a cascaded "group by" operation on input elements of type T, grouping elements according to a classification function, and then performing a reduction operation on the values associated with a given key using the specified downstream Collector.
 The classification function maps elements to some key type K. The downstream collector operates on elements of type T and produces a result of type D. The resulting collector produces a Map<K, D>.
 There are no guarantees on the type, mutability, serializability, or thread-safety of the Map returned.
 For example, to compute the set of last names of people in each city:
```
 Map<City, Set<String>> namesByCity
 = people.stream().collect(groupingBy(Person::getCity,
 mapping(Person::getLastName, toSet())));
 
```
 Implementation Note:
 
 The returned Collector is not concurrent. For parallel stream pipelines, the combiner function operates by merging the keys from one map into another, which can be an expensive operation. If preservation of the order in which elements are presented to the downstream collector is not required, using groupingByConcurrent(Function, Collector) may offer better parallel performance.
 
 Type Parameters:
 
 T - the type of the input elements
 
 K - the type of the keys
 
 A - the intermediate accumulation type of the downstream collector
 
 D - the result type of the downstream reduction
 
 Parameters:
 
 classifier - a classifier function mapping input elements to keys
 
 downstream - a Collector implementing the downstream reduction
 
 Returns:
 
 a Collector implementing the cascaded group-by operation
 
 See Also:
 
 groupingBy(Function), groupingBy(Function, Supplier, Collector), groupingByConcurrent(Function, Collector)
 - 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())));
 
```
 Implementation Note:
 
 The returned Collector is not concurrent. For parallel stream pipelines, the combiner function operates by merging the keys from one map into another, which can be an expensive operation. If preservation of the order in which elements are presented to the downstream collector is not required, using groupingByConcurrent(Function, Supplier, Collector) may offer better parallel performance.
 
 Type Parameters:
 
 T - the type of the input elements
 
 K - the type of the keys
 
 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 function which, when called, produces a new empty Map of the desired type
 
 Returns:
 
 a Collector implementing the cascaded group-by operation
 
 See Also:
 
 groupingBy(Function, Collector), groupingBy(Function), groupingByConcurrent(Function, Supplier, Collector)
 - 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 Map or List objects returned, or of the thread-safety of the List objects returned.
 Implementation Requirements:
 This produces a result similar to:
 groupingByConcurrent(classifier, toList());
 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), groupingByConcurrent(Function, Collector), groupingByConcurrent(Function, Supplier, Collector)
 - 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 Map<K, D>.
 For example, to compute the set of last names of people in each city, where the city names are sorted:
```
 ConcurrentMap<City, Set<String>> namesByCity
 = people.stream().collect(groupingByConcurrent(Person::getCity,
 mapping(Person::getLastName, toSet())));
 
```
 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), groupingByConcurrent(Function), groupingByConcurrent(Function, Supplier, Collector)
 - 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 Map<K, D>.
 For example, to compute the set of last names of people in each city, where the city names are sorted:
```
 ConcurrentMap<City, Set<String>> namesByCity
 = people.stream().collect(groupingBy(Person::getCity, ConcurrentSkipListMap::new,
 mapping(Person::getLastName, toSet())));
 
```
 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 function which, when called, produces a new empty ConcurrentMap of the desired type
 
 Returns:
 
 a concurrent, unordered Collector implementing the cascaded group-by operation
 
 See Also:
 
 groupingByConcurrent(Function), groupingByConcurrent(Function, Collector), groupingBy(Function, Supplier, Collector)
 - 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>>. 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
 
 Parameters:
 
 predicate - a predicate used for classifying input elements
 
 Returns:
 
 a Collector implementing the partitioning operation
 
 See Also:
 
 partitioningBy(Predicate, Collector)
 - 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.
 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:
 
 partitioningBy(Predicate)
 - 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 contains duplicates (according to Object.equals(Object)), an IllegalStateException is thrown when the collection operation is performed. If the mapped keys may have duplicates, use toMap(Function, Function, BinaryOperator) instead.
 API Note:
 It is common for either the key or the value to be the input elements. In this case, the utility method Function.identity() may be helpful. For example, the following produces a Map mapping students to their grade point average:
 Map<Student, Double> studentToGPA students.stream().collect(toMap(Functions.identity(), student -> computeGPA(student)));
 And the following produces a Map mapping a unique identifier to students:
 Map<String, Student> studentIdToStudent students.stream().collect(toMap(Student::getId, Functions.identity());
 Implementation Note:
 
 The returned Collector is not concurrent. For parallel stream pipelines, the combiner function operates by merging the keys from one map into another, which can be an expensive operation. If it is not required that results are inserted into the Map in encounter order, using toConcurrentMap(Function, Function) may offer better parallel performance.
 
 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), toMap(Function, Function, BinaryOperator, Supplier), toConcurrentMap(Function, Function)
 - 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 mergeFunction)
```
 Returns a Collector that accumulates elements into a Map whose keys and values are the result of applying the provided mapping functions to the input elements.
 If the mapped keys contains duplicates (according to Object.equals(Object)), the value mapping function is applied to each equal element, and the results are merged using the provided merging function.
 API Note:
 There are multiple ways to deal with collisions between multiple elements mapping to the same key. The other forms of toMap simply use a merge function that throws unconditionally, but you can easily write more flexible merge policies. For example, if you have a stream of Person, and you want to produce a "phone book" mapping name to address, but it is possible that two persons have the same name, you can do as follows to gracefully deals with these collisions, and produce a Map mapping names to a concatenated list of addresses:
 Map<String, String> phoneBook people.stream().collect(toMap(Person::getName, Person::getAddress, (s, a) -> s + ", " + a));
 Implementation Note:
 
 The returned Collector is not concurrent. For parallel stream pipelines, the combiner function operates by merging the keys from one map into another, which can be an expensive operation. If it is not required that results are merged into the Map in encounter order, using toConcurrentMap(Function, Function, BinaryOperator) may offer better parallel performance.
 
 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:
 
 toMap(Function, Function), toMap(Function, Function, BinaryOperator, Supplier), toConcurrentMap(Function, Function, BinaryOperator)
 - 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 mergeFunction,
 Supplier<M> mapSupplier)
```
 Returns a Collector that accumulates elements into a Map whose keys and values are the result of applying the provided mapping functions to the input elements.
 If the mapped keys contains duplicates (according to 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.
 
 Implementation Note:
 
 The returned Collector is not concurrent. For parallel stream pipelines, the combiner function operates by merging the keys from one map into another, which can be an expensive operation. If it is not required that results are merged into the Map in encounter order, using toConcurrentMap(Function, Function, BinaryOperator, Supplier) may offer better parallel performance.
 
 Type Parameters:
 
 T - the type of the input elements
 
 K - the output type of the key mapping function
 
 U - the output type of the value mapping function
 
 M - the type of the resulting Map
 
 Parameters:
 
 keyMapper - a mapping function to produce keys
 
 valueMapper - a mapping function to produce values
 
 mergeFunction - a merge function, used to resolve collisions between values associated with the same key, as supplied to Map.merge(Object, Object, BiFunction)
 
 mapSupplier - a function which returns 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), toMap(Function, Function, BinaryOperator), toConcurrentMap(Function, Function, BinaryOperator, Supplier)
 - 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 contains 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.
 API Note:
 It is common for either the key or the value to be the input elements. In this case, the utility method Function.identity() may be helpful. For example, the following produces a Map mapping students to their grade point average:
 Map<Student, Double> studentToGPA students.stream().collect(toMap(Functions.identity(), student -> computeGPA(student)));
 And the following produces a Map mapping a unique identifier to students:
 Map<String, Student> studentIdToStudent students.stream().collect(toConcurrentMap(Student::getId, Functions.identity());
 
 This is a concurrent and unordered Collector.
 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), toConcurrentMap(Function, Function, BinaryOperator), toConcurrentMap(Function, Function, BinaryOperator, Supplier)
 - 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 mergeFunction)
```
 Returns a concurrent Collector that accumulates elements into a ConcurrentMap whose keys and values are the result of applying the provided mapping functions to the input elements.
 If the mapped keys contains duplicates (according to Object.equals(Object)), the value mapping function is applied to each equal element, and the results are merged using the provided merging function.
 API Note:
 There are multiple ways to deal with collisions between multiple elements mapping to the same key. The other forms of toConcurrentMap simply use a merge function that throws unconditionally, but you can easily write more flexible merge policies. For example, if you have a stream of Person, and you want to produce a "phone book" mapping name to address, but it is possible that two persons have the same name, you can do as follows to gracefully deals with these collisions, and produce a Map mapping names to a concatenated list of addresses:
 Map<String, String> phoneBook people.stream().collect(toConcurrentMap(Person::getName, Person::getAddress, (s, a) -> s + ", " + a));
 
 This is a concurrent and unordered Collector.
 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(Function, Function), toConcurrentMap(Function, Function, BinaryOperator, Supplier), toMap(Function, Function, BinaryOperator)
 - 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 mergeFunction,
 Supplier<M> mapSupplier)
```
 Returns a concurrent Collector that accumulates elements into a ConcurrentMap whose keys and values are the result of applying the provided mapping functions to the input elements.
 If the mapped keys contains duplicates (according to 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)
 
 mapSupplier - a function which returns a new, empty Map 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), toConcurrentMap(Function, Function, BinaryOperator), toMap(Function, Function, BinaryOperator, Supplier)
 - 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:
 
 summarizingDouble(ToDoubleFunction), summarizingLong(ToLongFunction)
 - 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(ToDoubleFunction), summarizingInt(ToIntFunction)
 - 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:
 
 summarizingLong(ToLongFunction), summarizingInt(ToIntFunction)

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Class Collectors

Method Summary

Methods inherited from class java.lang.Object

Method Detail

toCollection

toList

toSet

joining

joining

joining

mapping

collectingAndThen

counting

minBy

maxBy

summingInt

summingLong

summingDouble

averagingInt

averagingLong

averagingDouble

reducing

reducing

reducing

groupingBy

groupingBy

groupingBy

groupingByConcurrent

groupingByConcurrent

groupingByConcurrent

partitioningBy

partitioningBy

toMap

toMap

toMap

toConcurrentMap

toConcurrentMap

toConcurrentMap

summarizingInt

summarizingLong

summarizingDouble