jdk.util.stream.Collectors — clojure-interop/java.util.stream 1.0.5

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));

*averaging-double clj

(*averaging-double 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.

mapper - a function extracting the property to be summed - java.util.function.ToDoubleFunction

returns: a Collector that produces the sum of a derived property - <T> java.util.stream.Collector<T,?,java.lang.Double>

*averaging-int clj

(*averaging-int 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.

mapper - a function extracting the property to be summed - java.util.function.ToIntFunction

returns: a Collector that produces the sum of a derived property - <T> java.util.stream.Collector<T,?,java.lang.Double>

*averaging-long clj

(*averaging-long 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.

mapper - a function extracting the property to be summed - java.util.function.ToLongFunction

returns: a Collector that produces the sum of a derived property - <T> java.util.stream.Collector<T,?,java.lang.Double>

*collecting-and-then clj

(*collecting-and-then downstream 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));

downstream - a collector - java.util.stream.Collector finisher - a function to be applied to the final result of the downstream collector - java.util.function.Function

returns: a collector which performs the action of the downstream collector, followed by an additional finishing step - <T,A,R,RR> java.util.stream.Collector<T,A,RR>

*counting clj

(*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 - <T> java.util.stream.Collector<T,?,java.lang.Long>

*grouping-by clj

(*grouping-by classifier)

(*grouping-by classifier downstream)

(*grouping-by classifier map-factory 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())));

classifier - a classifier function mapping input elements to keys - java.util.function.Function map-factory - a function which, when called, produces a new empty Map of the desired type - java.util.function.Supplier downstream - a Collector implementing the downstream reduction - java.util.stream.Collector

returns: a Collector implementing the cascaded group-by operation - <T,K,D,A,M extends java.util.Map<K,D>> java.util.stream.Collector<T,?,M>

*grouping-by-concurrent clj

(*grouping-by-concurrent classifier)

(*grouping-by-concurrent classifier downstream)

(*grouping-by-concurrent classifier map-factory 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())));

classifier - a classifier function mapping input elements to keys - java.util.function.Function map-factory - a function which, when called, produces a new empty ConcurrentMap of the desired type - java.util.function.Supplier downstream - a Collector implementing the downstream reduction - java.util.stream.Collector

returns: a concurrent, unordered Collector implementing the cascaded group-by operation - <T,K,A,D,M extends java.util.concurrent.ConcurrentMap<K,D>> java.util.stream.Collector<T,?,M>

*joining clj

(*joining)

(*joining delimiter)

(*joining delimiter prefix suffix)

Returns a Collector that concatenates the input elements, separated by the specified delimiter, with the specified prefix and suffix, in encounter order.

delimiter - the delimiter to be used between each element - java.lang.CharSequence prefix - the sequence of characters to be used at the beginning of the joined result - java.lang.CharSequence suffix - the sequence of characters to be used at the end of the joined result - java.lang.CharSequence

returns: A Collector which concatenates CharSequence elements, separated by the specified delimiter, in encounter order - java.util.stream.Collector<java.lang.CharSequence,?,java.lang.String>

*mapping clj

(*mapping mapper 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.

mapper - a function to be applied to the input elements - java.util.function.Function downstream - a collector which will accept mapped values - java.util.stream.Collector

returns: a collector which applies the mapping function to the input elements and provides the mapped results to the downstream collector - <T,U,A,R> java.util.stream.Collector<T,?,R>

*max-by clj

(*max-by comparator)

Returns a Collector that produces the maximal element according to a given Comparator, described as an Optional<T>.

comparator - a Comparator for comparing elements - java.util.Comparator

returns: a Collector that produces the maximal value - <T> java.util.stream.Collector<T,?,java.util.Optional<T>>

*min-by clj

(*min-by comparator)

Returns a Collector that produces the minimal element according to a given Comparator, described as an Optional<T>.

comparator - a Comparator for comparing elements - java.util.Comparator

returns: a Collector that produces the minimal value - <T> java.util.stream.Collector<T,?,java.util.Optional<T>>

*partitioning-by clj

(*partitioning-by predicate)

(*partitioning-by predicate 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.

predicate - a predicate used for classifying input elements - java.util.function.Predicate downstream - a Collector implementing the downstream reduction - java.util.stream.Collector

returns: a Collector implementing the cascaded partitioning operation - <T,D,A> java.util.stream.Collector<T,?,java.util.Map<java.lang.Boolean,D>>

*reducing clj

(*reducing op)

(*reducing identity op)

(*reducing identity mapper 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.

identity - the identity value for the reduction (also, the value that is returned when there are no input elements) - U mapper - a mapping function to apply to each input value - java.util.function.Function op - a BinaryOperator<U> used to reduce the mapped values - java.util.function.BinaryOperator

returns: a Collector implementing the map-reduce operation - <T,U> java.util.stream.Collector<T,?,U>

*summarizing-double clj

(*summarizing-double mapper)

Returns a Collector which applies an double-producing mapping function to each input element, and returns summary statistics for the resulting values.

mapper - a mapping function to apply to each element - java.util.function.ToDoubleFunction

returns: a Collector implementing the summary-statistics reduction - <T> java.util.stream.Collector<T,?,java.util.DoubleSummaryStatistics>

*summarizing-int clj

(*summarizing-int mapper)

Returns a Collector which applies an int-producing mapping function to each input element, and returns summary statistics for the resulting values.

mapper - a mapping function to apply to each element - java.util.function.ToIntFunction

returns: a Collector implementing the summary-statistics reduction - <T> java.util.stream.Collector<T,?,java.util.IntSummaryStatistics>

*summarizing-long clj

(*summarizing-long mapper)

Returns a Collector which applies an long-producing mapping function to each input element, and returns summary statistics for the resulting values.

mapper - the mapping function to apply to each element - java.util.function.ToLongFunction

returns: a Collector implementing the summary-statistics reduction - <T> java.util.stream.Collector<T,?,java.util.LongSummaryStatistics>

*summing-double clj

(*summing-double 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.

mapper - a function extracting the property to be summed - java.util.function.ToDoubleFunction

returns: a Collector that produces the sum of a derived property - <T> java.util.stream.Collector<T,?,java.lang.Double>

*summing-int clj

(*summing-int 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.

mapper - a function extracting the property to be summed - java.util.function.ToIntFunction

returns: a Collector that produces the sum of a derived property - <T> java.util.stream.Collector<T,?,java.lang.Integer>

*summing-long clj

(*summing-long 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.

mapper - a function extracting the property to be summed - java.util.function.ToLongFunction

returns: a Collector that produces the sum of a derived property - <T> java.util.stream.Collector<T,?,java.lang.Long>

*to-collection clj

(*to-collection collection-factory)

Returns a Collector that accumulates the input elements into a new Collection, in encounter order. The Collection is created by the provided factory.

collection-factory - a Supplier which returns a new, empty Collection of the appropriate type - java.util.function.Supplier

returns: a Collector which collects all the input elements into a Collection, in encounter order - <T,C extends java.util.Collection<T>> java.util.stream.Collector<T,?,C>

*to-concurrent-map clj

(*to-concurrent-map key-mapper value-mapper)

(*to-concurrent-map key-mapper value-mapper merge-function)

(*to-concurrent-map key-mapper value-mapper merge-function map-supplier)

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.

key-mapper - a mapping function to produce keys - java.util.function.Function value-mapper - a mapping function to produce values - java.util.function.Function merge-function - a merge function, used to resolve collisions between values associated with the same key, as supplied to Map.merge(Object, Object, BiFunction) - java.util.function.BinaryOperator map-supplier - a function which returns a new, empty Map into which the results will be inserted - java.util.function.Supplier

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 - <T,K,U,M extends java.util.concurrent.ConcurrentMap<K,U>> java.util.stream.Collector<T,?,M>

*to-list clj

(*to-list)

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 - <T> java.util.stream.Collector<T,?,java.util.List<T>>

*to-map clj

(*to-map key-mapper value-mapper)

(*to-map key-mapper value-mapper merge-function)

(*to-map key-mapper value-mapper merge-function map-supplier)

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.

key-mapper - a mapping function to produce keys - java.util.function.Function value-mapper - a mapping function to produce values - java.util.function.Function merge-function - a merge function, used to resolve collisions between values associated with the same key, as supplied to Map.merge(Object, Object, BiFunction) - java.util.function.BinaryOperator map-supplier - a function which returns a new, empty Map into which the results will be inserted - java.util.function.Supplier

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 - <T,K,U,M extends java.util.Map<K,U>> java.util.stream.Collector<T,?,M>

*to-set clj

(*to-set)

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 - <T> java.util.stream.Collector<T,?,java.util.Set<T>>