The collector produces a Map> whose keys are the The classification function maps elements to some key type K. Input elements of type T, grouping elements according to aĬlassification function, and returning the results in a Map. Returns a Collector implementing a 'group by' operation on

groupingBy public static Collector> groupingBy( Function classifier).

That is returned when there are no input elements) mapper - a mapping function to apply to each input value op - a BinaryOperator used to reduce the mapped values Returns: a Collector implementing the map-reduce operation See Also: reducing(Object, BinaryOperator), Type Parameters: T - the type of the input elements U - the type of the mapped values Parameters: identity - the identity value for the reduction (also, the value Reducing(Person::getLastName, BinaryOperator.maxBy(byLength)))) Use Stream.map(Function) and Stream.reduce(Object, BinaryOperator)įor example, given a stream of Person, to calculate the longestĬomparator byLength = paring(String::length) To perform a simple map-reduce on a stream, API Note: The reducing() collectors are most useful when used in a Reducing(Object, BinaryOperator) which allows a transformation Input elements under a specified mapping function andīinaryOperator.

reducing public static Collector reducing(U identity,.

Reducing(Object, Function, BinaryOperator) Type Parameters: T - element type for the input and output of the reduction Parameters: op - a BinaryOperator used to reduce the input elements Returns: a Collector which implements the reduction operation See Also: reducing(Object, BinaryOperator), = people.stream().collect(groupingBy(Person::getCity, reducing(BinaryOperator.maxBy(byHeight)))) Use Stream.reduce(BinaryOperator) instead.įor example, given a stream of Person, to calculate tallestĬomparator byHeight = paring(Person::getHeight) To perform a simple reduction on a stream, Multi-level reduction, downstream of groupingBy or Input elements under a specified BinaryOperator. Returns a Collector which performs a reduction of its

reducing public static Collector reducing(T identity,.Type Parameters: T - the type of the input elements Parameters: mapper - a function extracting the property to be summed Returns: a Collector that produces the sum of a derived property Values, the divisor in the average computation will saturate atĢ 53, leading to additional numerical errors. Implementation Note: The double format can represent allĬonsecutive integers in the range -2 53 to Value is a NaN or the sum is at any point a NaN then theĪverage will be NaN. Values sorted by increasingĪbsolute magnitude tend to yield more accurate results. Values are recorded, due to accumulated rounding error inĪddition of values of differing magnitudes. The average returned can vary depending upon the order in which Returns a Collector that produces the arithmetic mean of a double-valuedįunction applied to the input elements. collectingAndThen public static Collector collectingAndThen( Collector downstream,.

Type Parameters: T - the type of the input elements U - type of elements accepted by downstream collector A - intermediate accumulation type of the downstream collector R - result type of collector Parameters: mapper - a function to be applied to the input elements downstream - a collector which will accept mapped values Returns: a collector which applies the mapping function to the inputĮlements and provides the mapped results to the downstream collector = people.stream().collect(groupingBy(Person::getCity, Person, to accumulate the set of last names in each city: Multi-level reduction, such as downstream of a groupingBy or API Note: The mapping() collectors are most useful when used in a collect(Collectors.partitioningBy(s -> s.getGrade() >= PASS_THRESHOLD)) Īdapts a Collector accepting elements of type U to oneĪccepting elements of type T by applying a mapping function toĮach input element before accumulation. Partition students into passing and failing

collect(oupingBy(Employee::getDepartment,Ĭollectors.summingInt(Employee::getSalary))) collect(oupingBy(Employee::getDepartment)) collect(Collectors.summingInt(Employee::getSalary))) Convert elements to strings and concatenate them, separated by commas Set set = people.stream().map(Person::getName).collect(Collectors.toCollection(TreeSet::new)) List list = people.stream().map(Person::getName).collect(Collectors.toList()) The following are examples of using the predefined collectors to perform Operations, such as accumulating elements into collections, summarizingĮlements according to various criteria, etc. Implementations of Collector that implement various useful reduction

Code collector