com.annimon.stream

Class Collectors

java.lang.Object

com.annimon.stream.Collectors

public final class Collectors
extends Object

Common implementations of Collector interface.

See Also:

Collector

Method Summary

Modifier and Type	Method and Description
static <T> Collector<T,?,Double>	averaging(Function<? super T,Double> mapper) Returns a Collector that calculates average of input elements.
static <T,A,IR,OR> Collector<T,A,OR>	collectingAndThen(Collector<T,A,IR> c, Function<IR,OR> finisher) Returns a Collector that performs additional transformation.
static <T> Collector<T,?,Long>	counting() Returns a Collector 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 that performs grouping operation by given classifier.
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 that performs grouping operation by given classifier.
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 that performs grouping operation by given classifier.
static Collector<CharSequence,?,String>	joining() Returns a Collector that concatenates input elements into new string.
static Collector<CharSequence,?,String>	joining(CharSequence delimiter) Returns a Collector that concatenates input elements into new string.
static Collector<CharSequence,?,String>	joining(CharSequence delimiter, CharSequence prefix, CharSequence suffix) Returns a Collector that concatenates input elements into new string.
static Collector<CharSequence,?,String>	joining(CharSequence delimiter, CharSequence prefix, CharSequence suffix, String emptyValue) Returns a Collector that concatenates input elements into new string.
static <T,U,A,R> Collector<T,?,R>	mapping(Function<? super T,? extends U> mapper, Collector<? super U,A,R> downstream) Returns a Collector that performs mapping function before accumulation.
static <T,R> Collector<T,?,R>	reducing(R identity, Function<? super T,? extends R> mapper, BinaryOperator<R> op) Returns a Collector that reduces the input elements.
static <T> Collector<T,?,T>	reducing(T identity, BinaryOperator<T> op) Returns a Collector that reduces the input elements.
static <T,R extends Collection<T>> Collector<T,?,R>	toCollection(Supplier<R> collectionSupplier) Returns a Collector that fills new Collection, provided by collectionSupplier, with input elements.
static <T> Collector<T,?,List<T>>	toList() Returns a Collector that fills new List with input elements.
static <T,K,V> Collector<T,?,Map<K,V>>	toMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends V> valueMapper) Returns a Collector that fills new Map with input elements.
static <T,K,V,M extends Map<K,V>> Collector<T,?,M>	toMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends V> valueMapper, Supplier<M> mapFactory) Returns a Collector that fills new List with input elements.
static <T> Collector<T,?,Set<T>>	toSet() Returns a Collector that fills new Set with input elements.

Methods inherited from class java.lang.Object

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

Method Detail

toCollection

public static <T,R extends Collection<T>> Collector<T,?,R> toCollection(Supplier<R> collectionSupplier)

Returns a Collector that fills new Collection, provided by collectionSupplier, with input elements.

Type Parameters:

T - the type of the input elements

R - the type of the resulting collection

Parameters:

collectionSupplier - a supplier function that provides new collection

Returns:

a Collector

toList

public static <T> Collector<T,?,List<T>> toList()

Returns a Collector that fills new List with input elements.

Type Parameters:

T - the type of the input elements

Returns:

a Collector

toSet

public static <T> Collector<T,?,Set<T>> toSet()

Returns a Collector that fills new Set with input elements.

Type Parameters:

T - the type of the input elements

Returns:

a Collector

toMap

public static <T,K,V> Collector<T,?,Map<K,V>> toMap(Function<? super T,? extends K> keyMapper,
                                                    Function<? super T,? extends V> valueMapper)

Returns a Collector that fills new Map with input elements.

Type Parameters:

T - the type of the input elements

K - the result type of key mapping function

V - the result type of value mapping function

Parameters:

keyMapper - a mapping function to produce keys

valueMapper - a mapping function to produce values

Returns:

a Collector

toMap

public static <T,K,V,M extends Map<K,V>> Collector<T,?,M> toMap(Function<? super T,? extends K> keyMapper,
                                                                Function<? super T,? extends V> valueMapper,
                                                                Supplier<M> mapFactory)

Returns a Collector that fills new List with input elements.

Type Parameters:

T - the type of the input elements

K - the result type of key mapping function

V - the result type of 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

mapFactory - a supplier function that provides new Map

Returns:

a Collector

joining

public static Collector<CharSequence,?,String> joining()

Returns a Collector that concatenates input elements into new string.

Returns:

a Collector

joining

public static Collector<CharSequence,?,String> joining(CharSequence delimiter)

Returns a Collector that concatenates input elements into new string.

Parameters:

delimiter - the delimeter between each element

Returns:

a Collector

joining

public static Collector<CharSequence,?,String> joining(CharSequence delimiter,
                                                       CharSequence prefix,
                                                       CharSequence suffix)

Returns a Collector that concatenates input elements into new string.

Parameters:

delimiter - the delimeter between each element

prefix - the prefix of result

suffix - the suffix of result

Returns:

a Collector

joining

public static Collector<CharSequence,?,String> joining(CharSequence delimiter,
                                                       CharSequence prefix,
                                                       CharSequence suffix,
                                                       String emptyValue)

Returns a Collector that concatenates input elements into new string.

Parameters:

delimiter - the delimeter between each element

prefix - the prefix of result

suffix - the suffix of result

emptyValue - the string which replaces empty element if exists

Returns:

a Collector

averaging

public static <T> Collector<T,?,Double> averaging(Function<? super T,Double> mapper)

Returns a Collector that calculates average of input elements.

Type Parameters:

T - the type of the input elements

Parameters:

mapper - the mapping function which extracts value from element to calculate result

Returns:

a Collector

counting

public static <T> Collector<T,?,Long> counting()

Returns a Collector that counts the number of input elements.

Type Parameters:

T - the type of the input elements

Returns:

a Collector

reducing

public static <T> Collector<T,?,T> reducing(T identity,
                                            BinaryOperator<T> op)

Returns a Collector that reduces the input elements.

Type Parameters:

T - the type of the input elements

Parameters:

identity - the initial value

op - the operator to reduce elements

Returns:

a Collector

See Also:

reducing(java.lang.Object, com.annimon.stream.function.Function, com.annimon.stream.function.BinaryOperator)

reducing

public static <T,R> Collector<T,?,R> reducing(R identity,
                                              Function<? super T,? extends R> mapper,
                                              BinaryOperator<R> op)

Returns a Collector that reduces the input elements.

Type Parameters:

T - the type of the input elements

R - the type of the output elements

Parameters:

identity - the initial value

mapper - the mapping function

op - the operator to reduce elements

Returns:

a Collector

See Also:

reducing(java.lang.Object, com.annimon.stream.function.BinaryOperator)

mapping

public static <T,U,A,R> Collector<T,?,R> mapping(Function<? super T,? extends U> mapper,
                                                 Collector<? super U,A,R> downstream)

Returns a Collector that performs mapping function before accumulation.

Type Parameters:

T - the type of the input elements

U - the result type of mapping function

A - the accumulation type

R - the result type of collector

Parameters:

mapper - a function that performs mapping to input elements

downstream - the collector of mapped elements

Returns:

a Collector

collectingAndThen

public static <T,A,IR,OR> Collector<T,A,OR> collectingAndThen(Collector<T,A,IR> c,
                                                              Function<IR,OR> finisher)

Returns a Collector that performs additional transformation.

Type Parameters:

T - the type of the input elements

A - the accumulation type

IR - the input type of transformation function

OR - the output type of transformation function

Parameters:

c - the input Collector

finisher - the final transformation function

Returns:

a Collector

groupingBy

public static <T,K> Collector<T,?,Map<K,List<T>>> groupingBy(Function<? super T,? extends K> classifier)

Returns a Collector that performs grouping operation by given classifier.

Type Parameters:

T - the type of the input elements

K - the type of the keys

Parameters:

classifier - the classifier function

Returns:

a Collector

See Also:

groupingBy(com.annimon.stream.function.Function, com.annimon.stream.Collector), groupingBy(com.annimon.stream.function.Function, com.annimon.stream.function.Supplier, com.annimon.stream.Collector)

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 that performs grouping operation by given classifier.

Type Parameters:

T - the type of the input elements

K - the type of the keys

A - the accumulation type

D - the result type of downstream reduction

Parameters:

classifier - the classifier function

downstream - the collector of mapped elements

Returns:

a Collector

See Also:

groupingBy(com.annimon.stream.function.Function), groupingBy(com.annimon.stream.function.Function, com.annimon.stream.function.Supplier, com.annimon.stream.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 that performs grouping operation by given classifier.

Type Parameters:

T - the type of the input elements

K - the type of the keys

A - the accumulation type

D - the result type of downstream reduction

M - the type of the resulting Map

Parameters:

classifier - the classifier function

mapFactory - a supplier function that provides new Map

downstream - the collector of mapped elements

Returns:

a Collector

See Also:

groupingBy(com.annimon.stream.function.Function), groupingBy(com.annimon.stream.function.Function, com.annimon.stream.Collector)