Collectors

java.lang.Object
- java.util.stream.Collectors

public final class Collectors
extends Object

Implementations of Collector that implement various useful reduction operations, such as accumulating elements into collections, summarizing elements according to various criteria, etc.

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

 // Accumulate names into a List List<String> list = people.stream().map(Person::getName).collect(Collectors.toList()); // Accumulate names into a TreeSet Set<String> set = people.stream().map(Person::getName).collect(Collectors.toCollection(TreeSet::new)); // Convert elements to strings and concatenate them, separated by commas String joined = things.stream() .map(Object::toString) .collect(Collectors.joining(", ")); // Compute sum of salaries of employee int total = employees.stream() .collect(Collectors.summingInt(Employee::getSalary))); // Group employees by department Map<Department, List<Employee>> byDept = employees.stream() .collect(Collectors.groupingBy(Employee::getDepartment)); // Compute sum of salaries by department Map<Department, Integer> totalByDept = employees.stream() .collect(Collectors.groupingBy(Employee::getDepartment, Collectors.summingInt(Employee::getSalary))); // Partition students into passing and failing Map<Boolean, List<Student>> passingFailing = students.stream() .collect(Collectors.partitioningBy(s -> s.getGrade() >= PASS_THRESHOLD));

Since:: 1.8

Method Summary

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.
 
 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.
 
 Type Parameters:
 
 T - the type of the input elements
 
 Returns:
 
 a Collector that counts the input elements
 - minBy
```
public static <T> Collector<T,?,Optional<T>> minBy(Comparator<? super T> comparator)
```
 Returns a Collector that produces the minimal element according to a given Comparator, described as an Optional<T>.
 
 Type Parameters:
 
 T - the type of the input elements
 
 Parameters:
 
 comparator - a Comparator for comparing elements
 
 Returns:
 
 a Collector that produces the minimal value
 - maxBy
```
public static <T> Collector<T,?,Optional<T>> maxBy(Comparator<? super T> comparator)
```
 Returns a Collector that produces the maximal element according to a given Comparator, described as an Optional<T>.
 
 Type Parameters:
 
 T - the type of the input elements
 
 Parameters:
 
 comparator - a Comparator for comparing elements
 
 Returns:
 
 a Collector that produces the maximal value
 - summingInt
```
public static <T> Collector<T,?,Integer> summingInt(ToIntFunction<? super T> mapper)
```
 Returns a Collector that produces the sum of a integer-valued function applied to the input elements. If no elements are present, the result is 0.
 
 Type Parameters:
 
 T - the type of the input elements
 
 Parameters:
 
 mapper - a function extracting the property to be summed
 
 Returns:
 
 a Collector that produces the sum of a derived property
 - summingLong
```
public static <T> Collector<T,?,Long> summingLong(ToLongFunction<? super T> mapper)
```
 Returns a Collector that produces the sum of a long-valued function applied to the input elements. If no elements are present, the result is 0.
 
 Type Parameters:
 
 T - the type of the input elements
 
 Parameters:
 
 mapper - a function extracting the property to be summed
 
 Returns:
 
 a Collector that produces the sum of a derived property
 - summingDouble
```
public static <T> Collector<T,?,Double> summingDouble(ToDoubleFunction<? super T> mapper)
```
 Returns a Collector that produces the sum of a double-valued function applied to the input elements. If no elements are present, the result is 0.
 The sum returned can vary depending upon the order in which values are recorded, due to accumulated rounding error in addition of values of differing magnitudes. Values sorted by increasing absolute magnitude tend to yield more accurate results. If any recorded value is a NaN or the sum is at any point a NaN then the sum will be NaN.
 
 Type Parameters:
 
 T - the type of the input elements
 
 Parameters:
 
 mapper - a function extracting the property to be summed
 
 Returns:
 
 a Collector that produces the sum of a derived property
 - averagingInt
```
public static <T> Collector<T,?,Double> averagingInt(ToIntFunction<? super T> mapper)
```
 Returns a Collector that produces the arithmetic mean of an integer-valued function applied to the input elements. If no elements are present, the result is 0.
 
 Type Parameters:
 
 T - the type of the input elements
 
 Parameters:
 
 mapper - a function extracting the property to be 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.
 
 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.
 
 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>.
 
 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.
 
 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.
 
 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()))); 
```
 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()))); 
```
 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.
 
 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.
 
 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.
 
 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.
 
 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.
 
 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.
 
 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)

Class Collectors

Method Summary

Methods

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