Collectors

AI-generated Key Takeaways

• The Collectors class provides pre-built reduction operations for Java Streams to simplify data processing.

• It offers methods for accumulating elements into collections like Lists and Sets, as well as Maps based on grouping or partitioning.

• It supports calculating summary statistics such as average, sum, count, min, and max for numeric stream elements.

• Collectors can be used to concatenate stream elements into strings using joining() with optional delimiters.

• Methods like mapping() and collectingAndThen() allow for customizing and chaining collector operations for complex data manipulations.

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));

 

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>	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, 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>>	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> Collector<T, ?, Set<T>>	toSet() Returns a Collector that accumulates the input elements into a new Set.

Inherited Method Summary

From class java.lang.Object

Object	clone() Creates and returns a copy of this Object.
boolean	equals(Object obj) Compares this instance with the specified object and indicates if they are equal.
void	finalize() Invoked when the garbage collector has detected that this instance is no longer reachable.
final Class<?>	getClass() Returns the unique instance of Class that represents this object's class.
int	hashCode() Returns an integer hash code for this object.
final void	notify() Causes a thread which is waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
final void	notifyAll() Causes all threads which are waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
String	toString() Returns a string containing a concise, human-readable description of this object.
final void	wait(long timeout, int nanos) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait(long timeout) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait() Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object.

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 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.

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)

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

See Also

• groupingBy(Function, Collector)
• groupingBy(Function)
• groupingByConcurrent(Function, Supplier, Collector)

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

See Also

• groupingBy(Function)
• groupingBy(Function, Supplier, Collector)
• groupingByConcurrent(Function, Collector)

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

See Also

• groupingBy(Function, Collector)
• groupingByConcurrent(Function)
• groupingByConcurrent(Function, Supplier, Collector)

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 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.

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)

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

See Also

• groupingByConcurrent(Function)
• groupingByConcurrent(Function, Collector)
• groupingBy(Function, Supplier, Collector)

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 a String, 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

• partitioningBy(Predicate, Collector)

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

• partitioningBy(Predicate)

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

See Also

• reducing(Object, BinaryOperator)
• reducing(Object, Function, BinaryOperator)

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

See Also

• reducing(Object, BinaryOperator)
• reducing(BinaryOperator)

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

See Also

• reducing(BinaryOperator)
• reducing(Object, Function, BinaryOperator)

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

See Also

• summarizingLong(ToLongFunction)
• summarizingInt(ToIntFunction)

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

See Also

• summarizingDouble(ToDoubleFunction)
• summarizingLong(ToLongFunction)

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

See Also

• summarizingDouble(ToDoubleFunction)
• summarizingInt(ToIntFunction)

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 a Collection, 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 a ConcurrentMap whose keys are the result of applying a key mapping function to the input elements, and whose values are the result of applying a value mapping function to all input elements equal to the key and combining them using the merge function

See Also

• toConcurrentMap(Function, Function)
• toConcurrentMap(Function, Function, BinaryOperator)
• toMap(Function, Function, BinaryOperator, Supplier)

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 a ConcurrentMap whose keys are the result of applying a key mapping function to the input elements, and whose values are the result of applying a value mapping function to all input elements equal to the key and combining them using the merge function

See Also

• toConcurrentMap(Function, Function)
• toConcurrentMap(Function, Function, BinaryOperator, Supplier)
• toMap(Function, Function, BinaryOperator)

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 a ConcurrentMap whose keys are the result of applying a key mapping function to the input elements, and whose values are the result of applying a value mapping function to the input elements

See Also

• toMap(Function, Function)
• toConcurrentMap(Function, Function, BinaryOperator)
• toConcurrentMap(Function, Function, BinaryOperator, Supplier)

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 a List, 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 a Map whose keys are the result of applying a key mapping function to the input elements, and whose values are the result of applying a value mapping function to all input elements equal to the key and combining them using the merge function

See Also

• toMap(Function, Function)
• toMap(Function, Function, BinaryOperator, Supplier)
• toConcurrentMap(Function, Function, BinaryOperator)

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 a Map whose keys are the result of applying a key mapping function to the input elements, and whose values are the result of applying a value mapping function to all input elements equal to the key and combining them using the merge function

See Also

• toMap(Function, Function)
• toMap(Function, Function, BinaryOperator)
• toConcurrentMap(Function, Function, BinaryOperator, Supplier)

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 a Map whose keys and values are the result of applying mapping functions to the input elements

See Also

• toMap(Function, Function, BinaryOperator)
• toMap(Function, Function, BinaryOperator, Supplier)
• toConcurrentMap(Function, Function)

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 a Set