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