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));
Public Method Summary
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, D, A, M extends Map<K, D>> Collector<T, ?, M> |
groupingBy(Function<? super T, ? extends K> classifier, Supplier<M> mapFactory, Collector<? super T, A, D> downstream)
Returns a
Collector implementing a cascaded "group by" operation
on input elements of type T , grouping elements according to a
classification function, and then performing a reduction operation on
the values associated with a given key using the specified downstream
Collector . |
static <T, K, 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, 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> 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, M extends ConcurrentMap<K, D>> Collector<T, ?, M> |
groupingByConcurrent(Function<? super T, ? extends K> classifier, Supplier<M> mapFactory, Collector<? super T, A, D> downstream)
Returns a concurrent
Collector implementing a cascaded "group by"
operation on input elements of type T , grouping elements
according to a classification function, and then performing a reduction
operation on the values associated with a given key using the specified
downstream Collector . |
static Collector<CharSequence, ?, String> |
joining(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 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()
Returns a
Collector that concatenates the input elements into a
String , in encounter order. |
static <T, U, A, R> Collector<T, ?, R> | |
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, 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, ?, 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> Collector<T, ?, DoubleSummaryStatistics> |
summarizingDouble(ToDoubleFunction<? super T> mapper)
Returns a
Collector which applies an double -producing
mapping function to each input element, and returns summary statistics
for the resulting values. |
static <T> Collector<T, ?, IntSummaryStatistics> |
summarizingInt(ToIntFunction<? super T> mapper)
Returns a
Collector which applies an int -producing
mapping function to each input element, and returns summary statistics
for the resulting values. |
static <T> Collector<T, ?, LongSummaryStatistics> |
summarizingLong(ToLongFunction<? super T> mapper)
Returns a
Collector which applies an long -producing
mapping function to each input element, and returns summary statistics
for the resulting values. |
static <T> Collector<T, ?, Double> |
summingDouble(ToDoubleFunction<? super T> mapper)
Returns a
Collector that produces the sum of a double-valued
function applied to the input elements. |
static <T> Collector<T, ?, Integer> |
summingInt(ToIntFunction<? super T> mapper)
Returns a
Collector that produces the sum of a integer-valued
function applied to the input elements. |
static <T> Collector<T, ?, Long> |
summingLong(ToLongFunction<? super T> mapper)
Returns a
Collector that produces the sum of a long-valued
function applied to the input elements. |
static <T, C extends Collection<T>> Collector<T, ?, C> |
toCollection(Supplier<C> collectionFactory)
Returns a
Collector that accumulates the input elements into a
new Collection , in encounter order. |
static <T, K, U, M extends ConcurrentMap<K, U>> Collector<T, ?, M> |
toConcurrentMap(Function<? super T, ? extends K> keyMapper, Function<? super T, ? extends U> valueMapper, BinaryOperator<U> mergeFunction, Supplier<M> mapSupplier)
Returns a concurrent
Collector that accumulates elements into a
ConcurrentMap whose keys and values are the result of applying
the provided mapping functions to the input elements. |
static <T, 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> 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> 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, BinaryOperator<U> mergeFunction)
Returns a
Collector that accumulates elements into a
Map whose keys and values are the result of applying the provided
mapping functions to the input elements. |
static <T, K, U, M extends Map<K, U>> Collector<T, ?, M> |
toMap(Function<? super T, ? extends K> keyMapper, Function<? super T, ? extends U> valueMapper, BinaryOperator<U> mergeFunction, Supplier<M> mapSupplier)
Returns a
Collector that accumulates elements into a
Map whose keys and values are the result of applying the provided
mapping functions to the input elements. |
static <T, K, U> Collector<T, ?, Map<K, U>> | |
static <T> Collector<T, ?, Set<T>> |
toSet()
Returns a
Collector that accumulates the input elements into a
new Set . |
Inherited Method Summary
Public Methods
public static 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
.
Parameters
mapper | a function extracting the property to be summed |
---|
Returns
- a
Collector
that produces the sum of a derived property
public static 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.
Parameters
mapper | a function extracting the property to be summed |
---|
Returns
- a
Collector
that produces the sum of a derived property
public static 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.
Parameters
mapper | a function extracting the property to be summed |
---|
Returns
- a
Collector
that produces the sum of a derived property
public static Collector<T, A, RR> collectingAndThen (Collector<T, A, R> downstream, Function<R, RR> finisher)
Adapts a Collector
to perform an additional finishing
transformation. For example, one could adapt the toList()
collector to always produce an immutable list with:
List<String> people
= people.stream().collect(collectingAndThen(toList(), Collections::unmodifiableList));
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
public static 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.
Returns
- a
Collector
that counts the input elements
public static 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.
Parameters
classifier | the classifier function mapping input elements to keys |
---|
Returns
- a
Collector
implementing the group-by operation
public static 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())));
Parameters
classifier | a classifier function mapping input elements to keys |
---|---|
mapFactory | a function which, when called, produces a new empty
Map of the desired type |
downstream | a Collector implementing the downstream reduction |
Returns
- a
Collector
implementing the cascaded group-by operation
public static 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())));
Parameters
classifier | a classifier function mapping input elements to keys |
---|---|
downstream | a Collector implementing the downstream reduction |
Returns
- a
Collector
implementing the cascaded group-by operation
public static Collector<T, ?, ConcurrentMap<K, D>> groupingByConcurrent (Function<? super T, ? extends K> classifier, Collector<? super T, A, D> downstream)
Returns a concurrent Collector
implementing a cascaded "group by"
operation on input elements of type T
, grouping elements
according to a classification function, and then performing a reduction
operation on the values associated with a given key using the specified
downstream Collector
.
This is a concurrent
and
unordered
Collector.
The classification function maps elements to some key type K
.
The downstream collector operates on elements of type T
and
produces a result of type D
. The resulting collector produces a
Map<K, D>
.
For example, to compute the set of last names of people in each city, where the city names are sorted:
ConcurrentMap<City, Set<String>> namesByCity
= people.stream().collect(groupingByConcurrent(Person::getCity,
mapping(Person::getLastName, toSet())));
Parameters
classifier | a classifier function mapping input elements to keys |
---|---|
downstream | a Collector implementing the downstream reduction |
Returns
- a concurrent, unordered
Collector
implementing the cascaded group-by operation
public static 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 Map
or List
objects returned, or of the
thread-safety of the List
objects returned.
Parameters
classifier | a classifier function mapping input elements to keys |
---|
Returns
- a concurrent, unordered
Collector
implementing the group-by operation
public static Collector<T, ?, M> groupingByConcurrent (Function<? super T, ? extends K> classifier, Supplier<M> mapFactory, Collector<? super T, A, D> downstream)
Returns a concurrent Collector
implementing a cascaded "group by"
operation on input elements of type T
, grouping elements
according to a classification function, and then performing a reduction
operation on the values associated with a given key using the specified
downstream Collector
. The ConcurrentMap
produced by the
Collector is created with the supplied factory function.
This is a concurrent
and
unordered
Collector.
The classification function maps elements to some key type K
.
The downstream collector operates on elements of type T
and
produces a result of type D
. The resulting collector produces a
Map<K, D>
.
For example, to compute the set of last names of people in each city, where the city names are sorted:
ConcurrentMap<City, Set<String>> namesByCity
= people.stream().collect(groupingBy(Person::getCity, ConcurrentSkipListMap::new,
mapping(Person::getLastName, toSet())));
Parameters
classifier | a classifier function mapping input elements to keys |
---|---|
mapFactory | a function which, when called, produces a new empty
ConcurrentMap of the desired type |
downstream | a Collector implementing the downstream reduction |
Returns
- a concurrent, unordered
Collector
implementing the cascaded group-by operation
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
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
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
public static 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.
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
public static 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>
.
Parameters
comparator | a Comparator for comparing elements |
---|
Returns
- a
Collector
that produces the maximal value
public static 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>
.
Parameters
comparator | a Comparator for comparing elements |
---|
Returns
- a
Collector
that produces the minimal value
public static Collector<T, ?, Map<Boolean, List<T>>> partitioningBy (Predicate<? super T> predicate)
Returns a Collector
which partitions the input elements according
to a Predicate
, and organizes them into a
Map<Boolean, List<T>>
.
There are no guarantees on the type, mutability,
serializability, or thread-safety of the Map
returned.
Parameters
predicate | a predicate used for classifying input elements |
---|
Returns
- a
Collector
implementing the partitioning operation
See Also
public static Collector<T, ?, Map<Boolean, D>> partitioningBy (Predicate<? super T> predicate, Collector<? super T, A, D> downstream)
Returns a Collector
which partitions the input elements according
to a Predicate
, reduces the values in each partition according to
another Collector
, and organizes them into a
Map<Boolean, D>
whose values are the result of the downstream
reduction.
There are no guarantees on the type, mutability,
serializability, or thread-safety of the Map
returned.
Parameters
predicate | a predicate used for classifying input elements |
---|---|
downstream | a Collector implementing the downstream
reduction |
Returns
- a
Collector
implementing the cascaded partitioning operation
See Also
public static 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>
.
Parameters
op | a BinaryOperator<T> used to reduce the input elements |
---|
Returns
- a
Collector
which implements the reduction operation
public static 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.
Parameters
identity | the identity value for the reduction (also, the value that is returned when there are no input elements) |
---|---|
mapper | a mapping function to apply to each input value |
op | a BinaryOperator<U> used to reduce the mapped values |
Returns
- a
Collector
implementing the map-reduce operation
public static 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.
Parameters
identity | the identity value for the reduction (also, the value that is returned when there are no input elements) |
---|---|
op | a BinaryOperator<T> used to reduce the input elements |
Returns
- a
Collector
which implements the reduction operation
public static 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.
Parameters
mapper | a mapping function to apply to each element |
---|
Returns
- a
Collector
implementing the summary-statistics reduction
public static 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.
Parameters
mapper | a mapping function to apply to each element |
---|
Returns
- a
Collector
implementing the summary-statistics reduction
public static 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.
Parameters
mapper | the mapping function to apply to each element |
---|
Returns
- a
Collector
implementing the summary-statistics reduction
public static 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
.
Parameters
mapper | a function extracting the property to be summed |
---|
Returns
- a
Collector
that produces the sum of a derived property
public static 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.
Parameters
mapper | a function extracting the property to be summed |
---|
Returns
- a
Collector
that produces the sum of a derived property
public static 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.
Parameters
mapper | a function extracting the property to be summed |
---|
Returns
- a
Collector
that produces the sum of a derived property
public static 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.
Parameters
collectionFactory | a Supplier which returns a new, empty
Collection of the appropriate type |
---|
Returns
- a
Collector
which collects all the input elements into aCollection
, in encounter order
public static Collector<T, ?, M> toConcurrentMap (Function<? super T, ? extends K> keyMapper, Function<? super T, ? extends U> valueMapper, BinaryOperator<U> mergeFunction, Supplier<M> mapSupplier)
Returns a concurrent Collector
that accumulates elements into a
ConcurrentMap
whose keys and values are the result of applying
the provided mapping functions to the input elements.
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.
Parameters
keyMapper | a mapping function to produce keys |
---|---|
valueMapper | a mapping function to produce values |
mergeFunction | a merge function, used to resolve collisions between
values associated with the same key, as supplied
to Map.merge(Object, Object, BiFunction) |
mapSupplier | a function which returns a new, empty Map into
which the results will be inserted |
Returns
- a concurrent, unordered
Collector
which collects elements into 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
public static 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 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.
Parameters
keyMapper | a mapping function to produce keys |
---|---|
valueMapper | a mapping function to produce values |
mergeFunction | a merge function, used to resolve collisions between
values associated with the same key, as supplied
to Map.merge(Object, Object, BiFunction) |
Returns
- a concurrent, unordered
Collector
which collects elements into 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
public static Collector<T, ?, ConcurrentMap<K, U>> toConcurrentMap (Function<? super T, ? extends K> keyMapper, Function<? super T, ? extends U> valueMapper)
Returns a concurrent Collector
that accumulates elements into a
ConcurrentMap
whose keys and values are the result of applying
the provided mapping functions to the input elements.
If the mapped keys contains duplicates (according to
Object.equals(Object)
), an IllegalStateException
is
thrown when the collection operation is performed. If the mapped keys
may have duplicates, use
toConcurrentMap(Function, Function, BinaryOperator)
instead.
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
public static 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)
.
Returns
- a
Collector
which collects all the input elements into aList
, in encounter order
public static 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 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.
Parameters
keyMapper | a mapping function to produce keys |
---|---|
valueMapper | a mapping function to produce values |
mergeFunction | a merge function, used to resolve collisions between
values associated with the same key, as supplied
to Map.merge(Object, Object, BiFunction) |
Returns
- a
Collector
which collects elements into 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
public static Collector<T, ?, M> toMap (Function<? super T, ? extends K> keyMapper, Function<? super T, ? extends U> valueMapper, BinaryOperator<U> mergeFunction, Supplier<M> mapSupplier)
Returns a Collector
that accumulates elements into a
Map
whose keys and values are the result of applying the provided
mapping functions to the input elements.
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.
Parameters
keyMapper | a mapping function to produce keys |
---|---|
valueMapper | a mapping function to produce values |
mergeFunction | a merge function, used to resolve collisions between
values associated with the same key, as supplied
to Map.merge(Object, Object, BiFunction) |
mapSupplier | a function which returns a new, empty Map into
which the results will be inserted |
Returns
- a
Collector
which collects elements into 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
public static Collector<T, ?, Map<K, U>> toMap (Function<? super T, ? extends K> keyMapper, Function<? super T, ? extends U> valueMapper)
Returns a Collector
that accumulates elements into a
Map
whose keys and values are the result of applying the provided
mapping functions to the input elements.
If the mapped keys contains duplicates (according to
Object.equals(Object)
), an IllegalStateException
is
thrown when the collection operation is performed. If the mapped keys
may have duplicates, use toMap(Function, Function, BinaryOperator)
instead.
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
public static 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.
Returns
- a
Collector
which collects all the input elements into aSet