public final class Collectors extends Object
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));
| 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>> |
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>> |
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>> |
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>> |
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>> |
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. |
public static <T,C extends Collection<T>> Collector<T,?,C> toCollection(Supplier<C> collectionFactory)
Collector that accumulates the input elements into a
new Collection, in encounter order. The Collection is
created by the provided factory.T - the type of the input elementsC - the type of the resulting CollectioncollectionFactory - a Supplier which returns a new, empty
Collection of the appropriate typeCollector which collects all the input elements into a
Collection, in encounter orderpublic static <T> Collector<T,?,List<T>> toList()
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).T - the type of the input elementsCollector which collects all the input elements into a
List, in encounter orderpublic static <T> Collector<T,?,Set<T>> toSet()
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.
T - the type of the input elementsCollector which collects all the input elements into a
Setpublic static Collector<CharSequence,?,String> joining()
Collector that concatenates the input elements into a
String, in encounter order.Collector that concatenates the input elements into a
String, in encounter orderpublic static Collector<CharSequence,?,String> joining(CharSequence delimiter)
Collector that concatenates the input elements,
separated by the specified delimiter, in encounter order.delimiter - the delimiter to be used between each elementCollector which concatenates CharSequence elements,
separated by the specified delimiter, in encounter orderpublic static Collector<CharSequence,?,String> joining(CharSequence delimiter, CharSequence prefix, CharSequence suffix)
Collector that concatenates the input elements,
separated by the specified delimiter, with the specified prefix and
suffix, in encounter order.delimiter - the delimiter to be used between each elementprefix - the sequence of characters to be used at the beginning
of the joined resultsuffix - the sequence of characters to be used at the end
of the joined resultCollector which concatenates CharSequence elements,
separated by the specified delimiter, in encounter orderpublic static <T,U,A,R> Collector<T,?,R> mapping(Function<? super T,? extends U> mapper, Collector<? super U,A,R> downstream)
Collector accepting elements of type U to one
accepting elements of type T by applying a mapping function to
each input element before accumulation.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())));
T - the type of the input elementsU - type of elements accepted by downstream collectorA - intermediate accumulation type of the downstream collectorR - result type of collectormapper - a function to be applied to the input elementsdownstream - a collector which will accept mapped valuespublic static <T,A,R,RR> Collector<T,A,RR> collectingAndThen(Collector<T,A,R> downstream, Function<R,RR> finisher)
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));
T - the type of the input elementsA - intermediate accumulation type of the downstream collectorR - result type of the downstream collectorRR - result type of the resulting collectordownstream - a collectorfinisher - a function to be applied to the final result of the downstream collectorpublic static <T> Collector<T,?,Long> counting()
Collector accepting elements of type T that
counts the number of input elements. If no elements are present, the
result is 0.
reducing(0L, e -> 1L, Long::sum)
T - the type of the input elementsCollector that counts the input elementspublic static <T> Collector<T,?,Optional<T>> minBy(Comparator<? super T> comparator)
Collector that produces the minimal element according
to a given Comparator, described as an Optional<T>.
reducing(BinaryOperator.minBy(comparator))
T - the type of the input elementscomparator - a Comparator for comparing elementsCollector that produces the minimal valuepublic static <T> Collector<T,?,Optional<T>> maxBy(Comparator<? super T> comparator)
Collector that produces the maximal element according
to a given Comparator, described as an Optional<T>.
reducing(BinaryOperator.maxBy(comparator))
T - the type of the input elementscomparator - a Comparator for comparing elementsCollector that produces the maximal valuepublic static <T> Collector<T,?,Integer> summingInt(ToIntFunction<? super T> mapper)
Collector that produces the sum of a integer-valued
function applied to the input elements. If no elements are present,
the result is 0.T - the type of the input elementsmapper - a function extracting the property to be summedCollector that produces the sum of a derived propertypublic static <T> Collector<T,?,Long> summingLong(ToLongFunction<? super T> mapper)
Collector that produces the sum of a long-valued
function applied to the input elements. If no elements are present,
the result is 0.T - the type of the input elementsmapper - a function extracting the property to be summedCollector that produces the sum of a derived propertypublic static <T> Collector<T,?,Double> summingDouble(ToDoubleFunction<? super T> mapper)
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.
T - the type of the input elementsmapper - a function extracting the property to be summedCollector that produces the sum of a derived propertypublic static <T> Collector<T,?,Double> averagingInt(ToIntFunction<? super T> mapper)
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.T - the type of the input elementsmapper - a function extracting the property to be summedCollector that produces the sum of a derived propertypublic static <T> Collector<T,?,Double> averagingLong(ToLongFunction<? super T> mapper)
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.T - the type of the input elementsmapper - a function extracting the property to be summedCollector that produces the sum of a derived propertypublic static <T> Collector<T,?,Double> averagingDouble(ToDoubleFunction<? super T> mapper)
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.
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.T - the type of the input elementsmapper - a function extracting the property to be summedCollector that produces the sum of a derived propertypublic static <T> Collector<T,?,T> reducing(T identity, BinaryOperator<T> op)
Collector which performs a reduction of its
input elements under a specified BinaryOperator using the
provided identity.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.T - element type for the input and output of the reductionidentity - 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 elementsCollector which implements the reduction operationreducing(BinaryOperator),
reducing(Object, Function, BinaryOperator)public static <T> Collector<T,?,Optional<T>> reducing(BinaryOperator<T> op)
Collector which performs a reduction of its
input elements under a specified BinaryOperator. The result
is described as an Optional<T>.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))));
T - element type for the input and output of the reductionop - a BinaryOperator<T> used to reduce the input elementsCollector which implements the reduction operationreducing(Object, BinaryOperator),
reducing(Object, Function, BinaryOperator)public static <T,U> Collector<T,?,U> reducing(U identity, Function<? super T,? extends U> mapper, BinaryOperator<U> op)
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.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))));
T - the type of the input elementsU - the type of the mapped valuesidentity - 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 valueop - a BinaryOperator<U> used to reduce the mapped valuesCollector implementing the map-reduce operationreducing(Object, BinaryOperator),
reducing(BinaryOperator)public static <T,K> Collector<T,?,Map<K,List<T>>> groupingBy(Function<? super T,? extends K> classifier)
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.
groupingBy(classifier, toList());
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.T - the type of the input elementsK - the type of the keysclassifier - the classifier function mapping input elements to keysCollector implementing the group-by operationgroupingBy(Function, Collector),
groupingBy(Function, Supplier, Collector),
groupingByConcurrent(Function)public static <T,K,A,D> Collector<T,?,Map<K,D>> groupingBy(Function<? super T,? extends K> classifier, Collector<? super T,A,D> downstream)
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())));
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.T - the type of the input elementsK - the type of the keysA - the intermediate accumulation type of the downstream collectorD - the result type of the downstream reductionclassifier - a classifier function mapping input elements to keysdownstream - a Collector implementing the downstream reductionCollector implementing the cascaded group-by operationgroupingBy(Function),
groupingBy(Function, Supplier, Collector),
groupingByConcurrent(Function, Collector)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)
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())));
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.T - the type of the input elementsK - the type of the keysA - the intermediate accumulation type of the downstream collectorD - the result type of the downstream reductionM - the type of the resulting Mapclassifier - a classifier function mapping input elements to keysdownstream - a Collector implementing the downstream reductionmapFactory - a function which, when called, produces a new empty
Map of the desired typeCollector implementing the cascaded group-by operationgroupingBy(Function, Collector),
groupingBy(Function),
groupingByConcurrent(Function, Supplier, Collector)public static <T,K> Collector<T,?,ConcurrentMap<K,List<T>>> groupingByConcurrent(Function<? super T,? extends K> classifier)
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.
groupingByConcurrent(classifier, toList());
T - the type of the input elementsK - the type of the keysclassifier - a classifier function mapping input elements to keysCollector implementing the group-by operationgroupingBy(Function),
groupingByConcurrent(Function, Collector),
groupingByConcurrent(Function, Supplier, Collector)public static <T,K,A,D> Collector<T,?,ConcurrentMap<K,D>> groupingByConcurrent(Function<? super T,? extends K> classifier, Collector<? super T,A,D> downstream)
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())));
T - the type of the input elementsK - the type of the keysA - the intermediate accumulation type of the downstream collectorD - the result type of the downstream reductionclassifier - a classifier function mapping input elements to keysdownstream - a Collector implementing the downstream reductionCollector implementing the cascaded group-by operationgroupingBy(Function, Collector),
groupingByConcurrent(Function),
groupingByConcurrent(Function, Supplier, Collector)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)
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())));
T - the type of the input elementsK - the type of the keysA - the intermediate accumulation type of the downstream collectorD - the result type of the downstream reductionM - the type of the resulting ConcurrentMapclassifier - a classifier function mapping input elements to keysdownstream - a Collector implementing the downstream reductionmapFactory - a function which, when called, produces a new empty
ConcurrentMap of the desired typeCollector implementing the cascaded group-by operationgroupingByConcurrent(Function),
groupingByConcurrent(Function, Collector),
groupingBy(Function, Supplier, Collector)public static <T> Collector<T,?,Map<Boolean,List<T>>> partitioningBy(Predicate<? super T> predicate)
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.T - the type of the input elementspredicate - a predicate used for classifying input elementsCollector implementing the partitioning operationpartitioningBy(Predicate, Collector)public static <T,D,A> Collector<T,?,Map<Boolean,D>> partitioningBy(Predicate<? super T> predicate, Collector<? super T,A,D> downstream)
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.
T - the type of the input elementsA - the intermediate accumulation type of the downstream collectorD - the result type of the downstream reductionpredicate - a predicate used for classifying input elementsdownstream - a Collector implementing the downstream
reductionCollector implementing the cascaded partitioning
operationpartitioningBy(Predicate)public static <T,K,U> Collector<T,?,Map<K,U>> toMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper)
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.
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)));
Map mapping a unique identifier to
students:
Map<String, Student> studentIdToStudent
students.stream().collect(toMap(Student::getId,
Functions.identity());
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.T - the type of the input elementsK - the output type of the key mapping functionU - the output type of the value mapping functionkeyMapper - a mapping function to produce keysvalueMapper - a mapping function to produce valuesCollector which collects elements into a Map
whose keys and values are the result of applying mapping functions to
the input elementstoMap(Function, Function, BinaryOperator),
toMap(Function, Function, BinaryOperator, Supplier),
toConcurrentMap(Function, Function)public static <T,K,U> Collector<T,?,Map<K,U>> toMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction)
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.
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));
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.T - the type of the input elementsK - the output type of the key mapping functionU - the output type of the value mapping functionkeyMapper - a mapping function to produce keysvalueMapper - a mapping function to produce valuesmergeFunction - a merge function, used to resolve collisions between
values associated with the same key, as supplied
to Map.merge(Object, Object, BiFunction)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 functiontoMap(Function, Function),
toMap(Function, Function, BinaryOperator, Supplier),
toConcurrentMap(Function, Function, BinaryOperator)public static <T,K,U,M extends Map<K,U>> Collector<T,?,M> toMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction, Supplier<M> mapSupplier)
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.
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.T - the type of the input elementsK - the output type of the key mapping functionU - the output type of the value mapping functionM - the type of the resulting MapkeyMapper - a mapping function to produce keysvalueMapper - a mapping function to produce valuesmergeFunction - 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 insertedCollector 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 functiontoMap(Function, Function),
toMap(Function, Function, BinaryOperator),
toConcurrentMap(Function, Function, BinaryOperator, Supplier)public static <T,K,U> Collector<T,?,ConcurrentMap<K,U>> toConcurrentMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper)
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.
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)));
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.
T - the type of the input elementsK - the output type of the key mapping functionU - the output type of the value mapping functionkeyMapper - the mapping function to produce keysvalueMapper - the mapping function to produce valuesCollector 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 elementstoMap(Function, Function),
toConcurrentMap(Function, Function, BinaryOperator),
toConcurrentMap(Function, Function, BinaryOperator, Supplier)public static <T,K,U> Collector<T,?,ConcurrentMap<K,U>> toConcurrentMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction)
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.
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.
T - the type of the input elementsK - the output type of the key mapping functionU - the output type of the value mapping functionkeyMapper - a mapping function to produce keysvalueMapper - a mapping function to produce valuesmergeFunction - a merge function, used to resolve collisions between
values associated with the same key, as supplied
to Map.merge(Object, Object, BiFunction)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 functiontoConcurrentMap(Function, Function),
toConcurrentMap(Function, Function, BinaryOperator, Supplier),
toMap(Function, Function, BinaryOperator)public static <T,K,U,M extends ConcurrentMap<K,U>> Collector<T,?,M> toConcurrentMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction, Supplier<M> mapSupplier)
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.
T - the type of the input elementsK - the output type of the key mapping functionU - the output type of the value mapping functionM - the type of the resulting ConcurrentMapkeyMapper - a mapping function to produce keysvalueMapper - a mapping function to produce valuesmergeFunction - 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 insertedCollector 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 functiontoConcurrentMap(Function, Function),
toConcurrentMap(Function, Function, BinaryOperator),
toMap(Function, Function, BinaryOperator, Supplier)public static <T> Collector<T,?,IntSummaryStatistics> summarizingInt(ToIntFunction<? super T> mapper)
Collector which applies an int-producing
mapping function to each input element, and returns summary statistics
for the resulting values.T - the type of the input elementsmapper - a mapping function to apply to each elementCollector implementing the summary-statistics reductionsummarizingDouble(ToDoubleFunction),
summarizingLong(ToLongFunction)public static <T> Collector<T,?,LongSummaryStatistics> summarizingLong(ToLongFunction<? super T> mapper)
Collector which applies an long-producing
mapping function to each input element, and returns summary statistics
for the resulting values.T - the type of the input elementsmapper - the mapping function to apply to each elementCollector implementing the summary-statistics reductionsummarizingDouble(ToDoubleFunction),
summarizingInt(ToIntFunction)public static <T> Collector<T,?,DoubleSummaryStatistics> summarizingDouble(ToDoubleFunction<? super T> mapper)
Collector which applies an double-producing
mapping function to each input element, and returns summary statistics
for the resulting values.T - the type of the input elementsmapper - a mapping function to apply to each elementCollector implementing the summary-statistics reductionsummarizingLong(ToLongFunction),
summarizingInt(ToIntFunction) Submit a bug or feature
For further API reference and developer documentation, see Java SE Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.
Copyright © 1993, 2024, Oracle and/or its affiliates. All rights reserved. Use is subject to license terms. Also see the documentation redistribution policy.