- java.lang.Object
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- java.util.stream.Collectors
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public final class Collectors extends Object
Implementations ofCollectorthat implement various useful reduction operations, such as accumulating elements into collections, summarizing elements according to various criteria, etc.The following are examples of using the predefined collectors to perform common mutable reduction tasks:
// Accumulate names into a List List<String> list = people.stream() .map(Person::getName) .collect(Collectors.toList()); // Accumulate names into a TreeSet Set<String> set = people.stream() .map(Person::getName) .collect(Collectors.toCollection(TreeSet::new)); // Convert elements to strings and concatenate them, separated by commas String joined = things.stream() .map(Object::toString) .collect(Collectors.joining(", ")); // Compute sum of salaries of employee int total = employees.stream() .collect(Collectors.summingInt(Employee::getSalary)); // Group employees by department Map<Department, List<Employee>> byDept = employees.stream() .collect(Collectors.groupingBy(Employee::getDepartment)); // Compute sum of salaries by department Map<Department, Integer> totalByDept = employees.stream() .collect(Collectors.groupingBy(Employee::getDepartment, Collectors.summingInt(Employee::getSalary))); // Partition students into passing and failing Map<Boolean, List<Student>> passingFailing = students.stream() .collect(Collectors.partitioningBy(s -> s.getGrade() >= PASS_THRESHOLD));- Since:
- 1.8
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Method Summary
All Methods Static Methods Concrete Methods Modifier and Type Method Description static <T> Collector<T,?,Double>averagingDouble(ToDoubleFunction<? super T> mapper)Returns aCollectorthat 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 aCollectorthat 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 aCollectorthat 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 aCollectorto perform an additional finishing transformation.static <T> Collector<T,?,Long>counting()Returns aCollectoraccepting elements of typeTthat counts the number of input elements.static <T,A,R>
Collector<T,?,R>filtering(Predicate<? super T> predicate, Collector<? super T,A,R> downstream)Adapts aCollectorto one accepting elements of the same typeTby applying the predicate to each input element and only accumulating if the predicate returnstrue.static <T,U,A,R>
Collector<T,?,R>flatMapping(Function<? super T,? extends Stream<? extends U>> mapper, Collector<? super U,A,R> downstream)Adapts aCollectoraccepting elements of typeUto one accepting elements of typeTby applying a flat mapping function to each input element before accumulation.static <T,K>
Collector<T,?,Map<K,List<T>>>groupingBy(Function<? super T,? extends K> classifier)Returns aCollectorimplementing a "group by" operation on input elements of typeT, grouping elements according to a classification function, and returning the results in aMap.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 aCollectorimplementing a cascaded "group by" operation on input elements of typeT, grouping elements according to a classification function, and then performing a reduction operation on the values associated with a given key using the specified downstreamCollector.static <T,K,A,D>
Collector<T,?,Map<K,D>>groupingBy(Function<? super T,? extends K> classifier, Collector<? super T,A,D> downstream)Returns aCollectorimplementing a cascaded "group by" operation on input elements of typeT, grouping elements according to a classification function, and then performing a reduction operation on the values associated with a given key using the specified downstreamCollector.static <T,K>
Collector<T,?,ConcurrentMap<K,List<T>>>groupingByConcurrent(Function<? super T,? extends K> classifier)Returns a concurrentCollectorimplementing a "group by" operation on input elements of typeT, 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 concurrentCollectorimplementing a cascaded "group by" operation on input elements of typeT, grouping elements according to a classification function, and then performing a reduction operation on the values associated with a given key using the specified downstreamCollector.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 concurrentCollectorimplementing a cascaded "group by" operation on input elements of typeT, grouping elements according to a classification function, and then performing a reduction operation on the values associated with a given key using the specified downstreamCollector.static Collector<CharSequence,?,String>joining()Returns aCollectorthat concatenates the input elements into aString, in encounter order.static Collector<CharSequence,?,String>joining(CharSequence delimiter)Returns aCollectorthat concatenates the input elements, separated by the specified delimiter, in encounter order.static Collector<CharSequence,?,String>joining(CharSequence delimiter, CharSequence prefix, CharSequence suffix)Returns aCollectorthat concatenates the input elements, separated by the specified delimiter, with the specified prefix and suffix, 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 aCollectoraccepting elements of typeUto one accepting elements of typeTby applying a mapping function to each input element before accumulation.static <T> Collector<T,?,Optional<T>>maxBy(Comparator<? super T> comparator)Returns aCollectorthat produces the maximal element according to a givenComparator, described as anOptional<T>.static <T> Collector<T,?,Optional<T>>minBy(Comparator<? super T> comparator)Returns aCollectorthat produces the minimal element according to a givenComparator, described as anOptional<T>.static <T> Collector<T,?,Map<Boolean,List<T>>>partitioningBy(Predicate<? super T> predicate)Returns aCollectorwhich partitions the input elements according to aPredicate, and organizes them into aMap<Boolean, List<T>>.static <T,D,A>
Collector<T,?,Map<Boolean,D>>partitioningBy(Predicate<? super T> predicate, Collector<? super T,A,D> downstream)Returns aCollectorwhich partitions the input elements according to aPredicate, reduces the values in each partition according to anotherCollector, and organizes them into aMap<Boolean, D>whose values are the result of the downstream reduction.static <T> Collector<T,?,Optional<T>>reducing(BinaryOperator<T> op)Returns aCollectorwhich performs a reduction of its input elements under a specifiedBinaryOperator.static <T> Collector<T,?,T>reducing(T identity, BinaryOperator<T> op)Returns aCollectorwhich performs a reduction of its input elements under a specifiedBinaryOperatorusing the provided identity.static <T,U>
Collector<T,?,U>reducing(U identity, Function<? super T,? extends U> mapper, BinaryOperator<U> op)Returns aCollectorwhich performs a reduction of its input elements under a specified mapping function andBinaryOperator.static <T> Collector<T,?,DoubleSummaryStatistics>summarizingDouble(ToDoubleFunction<? super T> mapper)Returns aCollectorwhich applies andouble-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 aCollectorwhich applies anint-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 aCollectorwhich applies anlong-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 aCollectorthat produces the sum of a double-valued function applied to the input elements.static <T> Collector<T,?,Integer>summingInt(ToIntFunction<? super T> mapper)Returns aCollectorthat produces the sum of a integer-valued function applied to the input elements.static <T> Collector<T,?,Long>summingLong(ToLongFunction<? super T> mapper)Returns aCollectorthat 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 aCollectorthat accumulates the input elements into a newCollection, in encounter order.static <T,K,U>
Collector<T,?,ConcurrentMap<K,U>>toConcurrentMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper)Returns a concurrentCollectorthat accumulates elements into aConcurrentMapwhose 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 concurrentCollectorthat accumulates elements into aConcurrentMapwhose keys and values are the result of applying the provided mapping functions to the input elements.static <T,K,U,M extends ConcurrentMap<K,U>>
Collector<T,?,M>toConcurrentMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction, Supplier<M> mapFactory)Returns a concurrentCollectorthat accumulates elements into aConcurrentMapwhose keys and values are the result of applying the provided mapping functions to the input elements.static <T> Collector<T,?,List<T>>toList()Returns aCollectorthat accumulates the input elements into a newList.static <T,K,U>
Collector<T,?,Map<K,U>>toMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper)Returns aCollectorthat accumulates elements into aMapwhose 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, BinaryOperator<U> mergeFunction)Returns aCollectorthat accumulates elements into aMapwhose 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> mapFactory)Returns aCollectorthat accumulates elements into aMapwhose keys and values are the result of applying the provided mapping functions to the input elements.static <T> Collector<T,?,Set<T>>toSet()Returns aCollectorthat accumulates the input elements into a newSet.static <T> Collector<T,?,List<T>>toUnmodifiableList()Returns aCollectorthat accumulates the input elements into an unmodifiable List in encounter order.static <T,K,U>
Collector<T,?,Map<K,U>>toUnmodifiableMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper)Returns aCollectorthat accumulates the input elements into an unmodifiable Map, whose keys and values are the result of applying the provided mapping functions to the input elements.static <T,K,U>
Collector<T,?,Map<K,U>>toUnmodifiableMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction)Returns aCollectorthat accumulates the input elements into an unmodifiable Map, whose keys and values are the result of applying the provided mapping functions to the input elements.static <T> Collector<T,?,Set<T>>toUnmodifiableSet()Returns aCollectorthat accumulates the input elements into an unmodifiable Set.
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Method Detail
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toCollection
public static <T,C extends Collection<T>> Collector<T,?,C> toCollection(Supplier<C> collectionFactory)
Returns aCollectorthat accumulates the input elements into a newCollection, in encounter order. TheCollectionis created by the provided factory.- Type Parameters:
T- the type of the input elementsC- the type of the resultingCollection- Parameters:
collectionFactory- a supplier providing a new emptyCollectioninto which the results will be inserted- Returns:
- a
Collectorwhich collects all the input elements into aCollection, in encounter order
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toList
public static <T> Collector<T,?,List<T>> toList()
Returns aCollectorthat accumulates the input elements into a newList. There are no guarantees on the type, mutability, serializability, or thread-safety of theListreturned; if more control over the returnedListis required, usetoCollection(Supplier).- Type Parameters:
T- the type of the input elements- Returns:
- a
Collectorwhich collects all the input elements into aList, in encounter order
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toUnmodifiableList
public static <T> Collector<T,?,List<T>> toUnmodifiableList()
Returns aCollectorthat accumulates the input elements into an unmodifiable List in encounter order. The returned Collector disallows null values and will throwNullPointerExceptionif it is presented with a null value.- Type Parameters:
T- the type of the input elements- Returns:
- a
Collectorthat accumulates the input elements into an unmodifiable List in encounter order - Since:
- 10
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toSet
public static <T> Collector<T,?,Set<T>> toSet()
Returns aCollectorthat accumulates the input elements into a newSet. There are no guarantees on the type, mutability, serializability, or thread-safety of theSetreturned; if more control over the returnedSetis required, usetoCollection(Supplier).This is an
unorderedCollector.- Type Parameters:
T- the type of the input elements- Returns:
- a
Collectorwhich collects all the input elements into aSet
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toUnmodifiableSet
public static <T> Collector<T,?,Set<T>> toUnmodifiableSet()
Returns aCollectorthat accumulates the input elements into an unmodifiable Set. The returned Collector disallows null values and will throwNullPointerExceptionif it is presented with a null value. If the input contains duplicate elements, an arbitrary element of the duplicates is preserved.This is an
unorderedCollector.- Type Parameters:
T- the type of the input elements- Returns:
- a
Collectorthat accumulates the input elements into an unmodifiable Set - Since:
- 10
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joining
public static Collector<CharSequence,?,String> joining()
Returns aCollectorthat concatenates the input elements into aString, in encounter order.- Returns:
- a
Collectorthat concatenates the input elements into aString, in encounter order
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joining
public static Collector<CharSequence,?,String> joining(CharSequence delimiter)
Returns aCollectorthat concatenates the input elements, separated by the specified delimiter, in encounter order.- Parameters:
delimiter- the delimiter to be used between each element- Returns:
- A
Collectorwhich concatenates CharSequence elements, separated by the specified delimiter, in encounter order
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joining
public static Collector<CharSequence,?,String> joining(CharSequence delimiter, CharSequence prefix, CharSequence suffix)
Returns aCollectorthat 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 elementprefix- the sequence of characters to be used at the beginning of the joined resultsuffix- the sequence of characters to be used at the end of the joined result- Returns:
- A
Collectorwhich concatenates CharSequence elements, separated by the specified delimiter, in encounter order
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mapping
public static <T,U,A,R> Collector<T,?,R> mapping(Function<? super T,? extends U> mapper, Collector<? super U,A,R> downstream)
Adapts aCollectoraccepting elements of typeUto one accepting elements of typeTby applying a mapping function to each input element before accumulation.- API Note:
- The
mapping()collectors are most useful when used in a multi-level reduction, such as downstream of agroupingByorpartitioningBy. For example, given a stream ofPerson, to accumulate the set of last names in each city:Map<City, Set<String>> lastNamesByCity = people.stream().collect( groupingBy(Person::getCity, mapping(Person::getLastName, toSet()))); - Type Parameters:
T- the type of the input elementsU- type of elements accepted by downstream collectorA- intermediate accumulation type of the downstream collectorR- result type of collector- Parameters:
mapper- a function to be applied to the input elementsdownstream- 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
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flatMapping
public static <T,U,A,R> Collector<T,?,R> flatMapping(Function<? super T,? extends Stream<? extends U>> mapper, Collector<? super U,A,R> downstream)
Adapts aCollectoraccepting elements of typeUto one accepting elements of typeTby applying a flat mapping function to each input element before accumulation. The flat mapping function maps an input element to astreamcovering zero or more output elements that are then accumulated downstream. Each mapped stream isclosedafter its contents have been placed downstream. (If a mapped stream isnullan empty stream is used, instead.)- API Note:
- The
flatMapping()collectors are most useful when used in a multi-level reduction, such as downstream of agroupingByorpartitioningBy. For example, given a stream ofOrder, to accumulate the set of line items for each customer:Map<String, Set<LineItem>> itemsByCustomerName = orders.stream().collect( groupingBy(Order::getCustomerName, flatMapping(order -> order.getLineItems().stream(), toSet()))); - Type Parameters:
T- the type of the input elementsU- type of elements accepted by downstream collectorA- intermediate accumulation type of the downstream collectorR- result type of collector- Parameters:
mapper- a function to be applied to the input elements, which returns a stream of resultsdownstream- a collector which will receive the elements of the stream returned by mapper- Returns:
- a collector which applies the mapping function to the input elements and provides the flat mapped results to the downstream collector
- Since:
- 9
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filtering
public static <T,A,R> Collector<T,?,R> filtering(Predicate<? super T> predicate, Collector<? super T,A,R> downstream)
Adapts aCollectorto one accepting elements of the same typeTby applying the predicate to each input element and only accumulating if the predicate returnstrue.- API Note:
- The
filtering()collectors are most useful when used in a multi-level reduction, such as downstream of agroupingByorpartitioningBy. For example, given a stream ofEmployee, to accumulate the employees in each department that have a salary above a certain threshold:
A filtering collector differs from a stream'sMap<Department, Set<Employee>> wellPaidEmployeesByDepartment = employees.stream().collect( groupingBy(Employee::getDepartment, filtering(e -> e.getSalary() > 2000, toSet())));filter()operation. In this example, suppose there are no employees whose salary is above the threshold in some department. Using a filtering collector as shown above would result in a mapping from that department to an emptySet. If a streamfilter()operation were done instead, there would be no mapping for that department at all. - Type Parameters:
T- the type of the input elementsA- intermediate accumulation type of the downstream collectorR- result type of collector- Parameters:
predicate- a predicate to be applied to the input elementsdownstream- a collector which will accept values that match the predicate- Returns:
- a collector which applies the predicate to the input elements and provides matching elements to the downstream collector
- Since:
- 9
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collectingAndThen
public static <T,A,R,RR> Collector<T,A,RR> collectingAndThen(Collector<T,A,R> downstream, Function<R,RR> finisher)
Adapts aCollectorto perform an additional finishing transformation. For example, one could adapt thetoList()collector to always produce an immutable list with:List<String> list = people.stream().collect( collectingAndThen(toList(), Collections::unmodifiableList));- Type Parameters:
T- the type of the input elementsA- intermediate accumulation type of the downstream collectorR- result type of the downstream collectorRR- result type of the resulting collector- Parameters:
downstream- a collectorfinisher- 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
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counting
public static <T> Collector<T,?,Long> counting()
Returns aCollectoraccepting elements of typeTthat counts the number of input elements. If no elements are present, the result is 0.- Implementation Requirements:
- This produces a result equivalent to:
reducing(0L, e -> 1L, Long::sum) - Type Parameters:
T- the type of the input elements- Returns:
- a
Collectorthat counts the input elements
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minBy
public static <T> Collector<T,?,Optional<T>> minBy(Comparator<? super T> comparator)
Returns aCollectorthat produces the minimal element according to a givenComparator, described as anOptional<T>.- Implementation Requirements:
- This produces a result equivalent to:
reducing(BinaryOperator.minBy(comparator)) - Type Parameters:
T- the type of the input elements- Parameters:
comparator- aComparatorfor comparing elements- Returns:
- a
Collectorthat produces the minimal value
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maxBy
public static <T> Collector<T,?,Optional<T>> maxBy(Comparator<? super T> comparator)
Returns aCollectorthat produces the maximal element according to a givenComparator, described as anOptional<T>.- Implementation Requirements:
- This produces a result equivalent to:
reducing(BinaryOperator.maxBy(comparator)) - Type Parameters:
T- the type of the input elements- Parameters:
comparator- aComparatorfor comparing elements- Returns:
- a
Collectorthat produces the maximal value
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summingInt
public static <T> Collector<T,?,Integer> summingInt(ToIntFunction<? super T> mapper)
Returns aCollectorthat produces the sum of a integer-valued function applied to the input elements. If no elements are present, the result is 0.- Type Parameters:
T- the type of the input elements- Parameters:
mapper- a function extracting the property to be summed- Returns:
- a
Collectorthat produces the sum of a derived property
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summingLong
public static <T> Collector<T,?,Long> summingLong(ToLongFunction<? super T> mapper)
Returns aCollectorthat produces the sum of a long-valued function applied to the input elements. If no elements are present, the result is 0.- Type Parameters:
T- the type of the input elements- Parameters:
mapper- a function extracting the property to be summed- Returns:
- a
Collectorthat produces the sum of a derived property
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summingDouble
public static <T> Collector<T,?,Double> summingDouble(ToDoubleFunction<? super T> mapper)
Returns aCollectorthat 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
NaNor the sum is at any point aNaNthen the sum will beNaN.- Type Parameters:
T- the type of the input elements- Parameters:
mapper- a function extracting the property to be summed- Returns:
- a
Collectorthat produces the sum of a derived property
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averagingInt
public static <T> Collector<T,?,Double> averagingInt(ToIntFunction<? super T> mapper)
Returns aCollectorthat produces the arithmetic mean of an integer-valued function applied to the input elements. If no elements are present, the result is 0.- Type Parameters:
T- the type of the input elements- Parameters:
mapper- a function extracting the property to be averaged- Returns:
- a
Collectorthat produces the arithmetic mean of a derived property
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averagingLong
public static <T> Collector<T,?,Double> averagingLong(ToLongFunction<? super T> mapper)
Returns aCollectorthat produces the arithmetic mean of a long-valued function applied to the input elements. If no elements are present, the result is 0.- Type Parameters:
T- the type of the input elements- Parameters:
mapper- a function extracting the property to be averaged- Returns:
- a
Collectorthat produces the arithmetic mean of a derived property
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averagingDouble
public static <T> Collector<T,?,Double> averagingDouble(ToDoubleFunction<? super T> mapper)
Returns aCollectorthat 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
NaNor the sum is at any point aNaNthen the average will beNaN.- Implementation Note:
- The
doubleformat can represent all consecutive integers in the range -253 to 253. If the pipeline has more than 253 values, the divisor in the average computation will saturate at 253, leading to additional numerical errors. - Type Parameters:
T- the type of the input elements- Parameters:
mapper- a function extracting the property to be averaged- Returns:
- a
Collectorthat produces the arithmetic mean of a derived property
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reducing
public static <T> Collector<T,?,T> reducing(T identity, BinaryOperator<T> op)
Returns aCollectorwhich performs a reduction of its input elements under a specifiedBinaryOperatorusing the provided identity.- API Note:
- The
reducing()collectors are most useful when used in a multi-level reduction, downstream ofgroupingByorpartitioningBy. To perform a simple reduction on a stream, useStream.reduce(Object, BinaryOperator)} instead. - Type Parameters:
T- element type for the input and output of the reduction- Parameters:
identity- the identity value for the reduction (also, the value that is returned when there are no input elements)op- aBinaryOperator<T>used to reduce the input elements- Returns:
- a
Collectorwhich implements the reduction operation - See Also:
reducing(BinaryOperator),reducing(Object, Function, BinaryOperator)
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reducing
public static <T> Collector<T,?,Optional<T>> reducing(BinaryOperator<T> op)
Returns aCollectorwhich performs a reduction of its input elements under a specifiedBinaryOperator. The result is described as anOptional<T>.- API Note:
- The
reducing()collectors are most useful when used in a multi-level reduction, downstream ofgroupingByorpartitioningBy. To perform a simple reduction on a stream, useStream.reduce(BinaryOperator)instead.For example, given a stream of
Person, to calculate tallest person in each city:Comparator<Person> byHeight = Comparator.comparing(Person::getHeight); Map<City, Optional<Person>> tallestByCity = people.stream().collect( groupingBy(Person::getCity, reducing(BinaryOperator.maxBy(byHeight)))); - Type Parameters:
T- element type for the input and output of the reduction- Parameters:
op- aBinaryOperator<T>used to reduce the input elements- Returns:
- a
Collectorwhich implements the reduction operation - See Also:
reducing(Object, BinaryOperator),reducing(Object, Function, BinaryOperator)
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reducing
public static <T,U> Collector<T,?,U> reducing(U identity, Function<? super T,? extends U> mapper, BinaryOperator<U> op)
Returns aCollectorwhich performs a reduction of its input elements under a specified mapping function andBinaryOperator. This is a generalization ofreducing(Object, BinaryOperator)which allows a transformation of the elements before reduction.- API Note:
- The
reducing()collectors are most useful when used in a multi-level reduction, downstream ofgroupingByorpartitioningBy. To perform a simple map-reduce on a stream, useStream.map(Function)andStream.reduce(Object, BinaryOperator)instead.For example, given a stream of
Person, to calculate the longest last name of residents in each city:Comparator<String> byLength = Comparator.comparing(String::length); Map<City, String> longestLastNameByCity = people.stream().collect( groupingBy(Person::getCity, reducing("", Person::getLastName, BinaryOperator.maxBy(byLength)))); - Type Parameters:
T- the type of the input elementsU- the type of the mapped values- Parameters:
identity- the identity value for the reduction (also, the value that is returned when there are no input elements)mapper- a mapping function to apply to each input valueop- aBinaryOperator<U>used to reduce the mapped values- Returns:
- a
Collectorimplementing the map-reduce operation - See Also:
reducing(Object, BinaryOperator),reducing(BinaryOperator)
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groupingBy
public static <T,K> Collector<T,?,Map<K,List<T>>> groupingBy(Function<? super T,? extends K> classifier)
Returns aCollectorimplementing a "group by" operation on input elements of typeT, grouping elements according to a classification function, and returning the results in aMap.The classification function maps elements to some key type
K. The collector produces aMap<K, List<T>>whose keys are the values resulting from applying the classification function to the input elements, and whose corresponding values areLists 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
MaporListobjects returned.- Implementation Requirements:
- This produces a result similar to:
groupingBy(classifier, toList()); - Implementation Note:
- The returned
Collectoris not concurrent. For parallel stream pipelines, thecombinerfunction operates by merging the keys from one map into another, which can be an expensive operation. If preservation of the order in which elements appear in the resultingMapcollector is not required, usinggroupingByConcurrent(Function)may offer better parallel performance. - Type Parameters:
T- the type of the input elementsK- the type of the keys- Parameters:
classifier- the classifier function mapping input elements to keys- Returns:
- a
Collectorimplementing the group-by operation - See Also:
groupingBy(Function, Collector),groupingBy(Function, Supplier, Collector),groupingByConcurrent(Function)
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groupingBy
public static <T,K,A,D> Collector<T,?,Map<K,D>> groupingBy(Function<? super T,? extends K> classifier, Collector<? super T,A,D> downstream)
Returns aCollectorimplementing a cascaded "group by" operation on input elements of typeT, grouping elements according to a classification function, and then performing a reduction operation on the values associated with a given key using the specified downstreamCollector.The classification function maps elements to some key type
K. The downstream collector operates on elements of typeTand produces a result of typeD. The resulting collector produces aMap<K, D>.There are no guarantees on the type, mutability, serializability, or thread-safety of the
Mapreturned.For example, to compute the set of last names of people in each city:
Map<City, Set<String>> namesByCity = people.stream().collect( groupingBy(Person::getCity, mapping(Person::getLastName, toSet())));- Implementation Note:
- The returned
Collectoris not concurrent. For parallel stream pipelines, thecombinerfunction operates by merging the keys from one map into another, which can be an expensive operation. If preservation of the order in which elements are presented to the downstream collector is not required, usinggroupingByConcurrent(Function, Collector)may offer better parallel performance. - Type Parameters:
T- the type of the input elementsK- the type of the keysA- the intermediate accumulation type of the downstream collectorD- the result type of the downstream reduction- Parameters:
classifier- a classifier function mapping input elements to keysdownstream- aCollectorimplementing the downstream reduction- Returns:
- a
Collectorimplementing the cascaded group-by operation - See Also:
groupingBy(Function),groupingBy(Function, Supplier, Collector),groupingByConcurrent(Function, Collector)
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groupingBy
public static <T,K,D,A,M extends Map<K,D>> Collector<T,?,M> groupingBy(Function<? super T,? extends K> classifier, Supplier<M> mapFactory, Collector<? super T,A,D> downstream)
Returns aCollectorimplementing a cascaded "group by" operation on input elements of typeT, grouping elements according to a classification function, and then performing a reduction operation on the values associated with a given key using the specified downstreamCollector. TheMapproduced 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 typeTand produces a result of typeD. The resulting collector produces aMap<K, D>.For example, to compute the set of last names of people in each city, where the city names are sorted:
Map<City, Set<String>> namesByCity = people.stream().collect( groupingBy(Person::getCity, TreeMap::new, mapping(Person::getLastName, toSet())));- Implementation Note:
- The returned
Collectoris not concurrent. For parallel stream pipelines, thecombinerfunction operates by merging the keys from one map into another, which can be an expensive operation. If preservation of the order in which elements are presented to the downstream collector is not required, usinggroupingByConcurrent(Function, Supplier, Collector)may offer better parallel performance. - Type Parameters:
T- the type of the input elementsK- the type of the keysA- the intermediate accumulation type of the downstream collectorD- the result type of the downstream reductionM- the type of the resultingMap- Parameters:
classifier- a classifier function mapping input elements to keysdownstream- aCollectorimplementing the downstream reductionmapFactory- a supplier providing a new emptyMapinto which the results will be inserted- Returns:
- a
Collectorimplementing the cascaded group-by operation - See Also:
groupingBy(Function, Collector),groupingBy(Function),groupingByConcurrent(Function, Supplier, Collector)
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groupingByConcurrent
public static <T,K> Collector<T,?,ConcurrentMap<K,List<T>>> groupingByConcurrent(Function<? super T,? extends K> classifier)
Returns a concurrentCollectorimplementing a "group by" operation on input elements of typeT, grouping elements according to a classification function.This is a
concurrentandunorderedCollector.The classification function maps elements to some key type
K. The collector produces aConcurrentMap<K, List<T>>whose keys are the values resulting from applying the classification function to the input elements, and whose corresponding values areLists 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
ConcurrentMaporListobjects returned, or of the thread-safety of theListobjects returned.- Implementation Requirements:
- This produces a result similar to:
groupingByConcurrent(classifier, toList()); - Type Parameters:
T- the type of the input elementsK- the type of the keys- Parameters:
classifier- a classifier function mapping input elements to keys- Returns:
- a concurrent, unordered
Collectorimplementing the group-by operation - See Also:
groupingBy(Function),groupingByConcurrent(Function, Collector),groupingByConcurrent(Function, Supplier, Collector)
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groupingByConcurrent
public static <T,K,A,D> Collector<T,?,ConcurrentMap<K,D>> groupingByConcurrent(Function<? super T,? extends K> classifier, Collector<? super T,A,D> downstream)
Returns a concurrentCollectorimplementing a cascaded "group by" operation on input elements of typeT, grouping elements according to a classification function, and then performing a reduction operation on the values associated with a given key using the specified downstreamCollector.This is a
concurrentandunorderedCollector.The classification function maps elements to some key type
K. The downstream collector operates on elements of typeTand produces a result of typeD. The resulting collector produces aConcurrentMap<K, D>.There are no guarantees on the type, mutability, or serializability of the
ConcurrentMapreturned.For example, to compute the set of last names of people in each city, where the city names are sorted:
ConcurrentMap<City, Set<String>> namesByCity = people.stream().collect( groupingByConcurrent(Person::getCity, mapping(Person::getLastName, toSet())));- Type Parameters:
T- the type of the input elementsK- the type of the keysA- the intermediate accumulation type of the downstream collectorD- the result type of the downstream reduction- Parameters:
classifier- a classifier function mapping input elements to keysdownstream- aCollectorimplementing the downstream reduction- Returns:
- a concurrent, unordered
Collectorimplementing the cascaded group-by operation - See Also:
groupingBy(Function, Collector),groupingByConcurrent(Function),groupingByConcurrent(Function, Supplier, Collector)
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groupingByConcurrent
public static <T,K,A,D,M extends ConcurrentMap<K,D>> Collector<T,?,M> groupingByConcurrent(Function<? super T,? extends K> classifier, Supplier<M> mapFactory, Collector<? super T,A,D> downstream)
Returns a concurrentCollectorimplementing a cascaded "group by" operation on input elements of typeT, grouping elements according to a classification function, and then performing a reduction operation on the values associated with a given key using the specified downstreamCollector. TheConcurrentMapproduced by the Collector is created with the supplied factory function.This is a
concurrentandunorderedCollector.The classification function maps elements to some key type
K. The downstream collector operates on elements of typeTand produces a result of typeD. The resulting collector produces aConcurrentMap<K, D>.For example, to compute the set of last names of people in each city, where the city names are sorted:
ConcurrentMap<City, Set<String>> namesByCity = people.stream().collect( groupingByConcurrent(Person::getCity, ConcurrentSkipListMap::new, mapping(Person::getLastName, toSet())));- Type Parameters:
T- the type of the input elementsK- the type of the keysA- the intermediate accumulation type of the downstream collectorD- the result type of the downstream reductionM- the type of the resultingConcurrentMap- Parameters:
classifier- a classifier function mapping input elements to keysdownstream- aCollectorimplementing the downstream reductionmapFactory- a supplier providing a new emptyConcurrentMapinto which the results will be inserted- Returns:
- a concurrent, unordered
Collectorimplementing the cascaded group-by operation - See Also:
groupingByConcurrent(Function),groupingByConcurrent(Function, Collector),groupingBy(Function, Supplier, Collector)
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partitioningBy
public static <T> Collector<T,?,Map<Boolean,List<T>>> partitioningBy(Predicate<? super T> predicate)
Returns aCollectorwhich partitions the input elements according to aPredicate, and organizes them into aMap<Boolean, List<T>>. The returnedMapalways contains mappings for bothfalseandtruekeys. There are no guarantees on the type, mutability, serializability, or thread-safety of theMaporListreturned.- API Note:
- If a partition has no elements, its value in the result Map will be an empty List.
- Type Parameters:
T- the type of the input elements- Parameters:
predicate- a predicate used for classifying input elements- Returns:
- a
Collectorimplementing the partitioning operation - See Also:
partitioningBy(Predicate, Collector)
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partitioningBy
public static <T,D,A> Collector<T,?,Map<Boolean,D>> partitioningBy(Predicate<? super T> predicate, Collector<? super T,A,D> downstream)
Returns aCollectorwhich partitions the input elements according to aPredicate, reduces the values in each partition according to anotherCollector, and organizes them into aMap<Boolean, D>whose values are the result of the downstream reduction.The returned
Mapalways contains mappings for bothfalseandtruekeys. There are no guarantees on the type, mutability, serializability, or thread-safety of theMapreturned.- API Note:
- If a partition has no elements, its value in the result Map will be obtained by calling the downstream collector's supplier function and then applying the finisher function.
- Type Parameters:
T- the type of the input elementsA- the intermediate accumulation type of the downstream collectorD- the result type of the downstream reduction- Parameters:
predicate- a predicate used for classifying input elementsdownstream- aCollectorimplementing the downstream reduction- Returns:
- a
Collectorimplementing the cascaded partitioning operation - See Also:
partitioningBy(Predicate)
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toMap
public static <T,K,U> Collector<T,?,Map<K,U>> toMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper)
Returns aCollectorthat accumulates elements into aMapwhose keys and values are the result of applying the provided mapping functions to the input elements.If the mapped keys contain duplicates (according to
Object.equals(Object)), anIllegalStateExceptionis thrown when the collection operation is performed. If the mapped keys might have duplicates, usetoMap(Function, Function, BinaryOperator)instead.There are no guarantees on the type, mutability, serializability, or thread-safety of the
Mapreturned.- API Note:
- It is common for either the key or the value to be the input elements.
In this case, the utility method
Function.identity()may be helpful. For example, the following produces aMapmapping students to their grade point average:
And the following produces aMap<Student, Double> studentToGPA = students.stream().collect( toMap(Function.identity(), student -> computeGPA(student)));Mapmapping a unique identifier to students:Map<String, Student> studentIdToStudent = students.stream().collect( toMap(Student::getId, Function.identity())); - Implementation Note:
- The returned
Collectoris not concurrent. For parallel stream pipelines, thecombinerfunction operates by merging the keys from one map into another, which can be an expensive operation. If it is not required that results are inserted into theMapin encounter order, usingtoConcurrentMap(Function, Function)may offer better parallel performance. - Type Parameters:
T- the type of the input elementsK- the output type of the key mapping functionU- the output type of the value mapping function- Parameters:
keyMapper- a mapping function to produce keysvalueMapper- a mapping function to produce values- Returns:
- a
Collectorwhich collects elements into aMapwhose 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)
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toUnmodifiableMap
public static <T,K,U> Collector<T,?,Map<K,U>> toUnmodifiableMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper)
Returns aCollectorthat accumulates the input elements into an unmodifiable Map, whose keys and values are the result of applying the provided mapping functions to the input elements.If the mapped keys contain duplicates (according to
Object.equals(Object)), anIllegalStateExceptionis thrown when the collection operation is performed. If the mapped keys might have duplicates, usetoUnmodifiableMap(Function, Function, BinaryOperator)to handle merging of the values.The returned Collector disallows null keys and values. If either mapping function returns null,
NullPointerExceptionwill be thrown.- Type Parameters:
T- the type of the input elementsK- the output type of the key mapping functionU- the output type of the value mapping function- Parameters:
keyMapper- a mapping function to produce keys, must be non-nullvalueMapper- a mapping function to produce values, must be non-null- Returns:
- a
Collectorthat accumulates the input elements into an unmodifiable Map, whose keys and values are the result of applying the provided mapping functions to the input elements - Throws:
NullPointerException- if either keyMapper or valueMapper is null- Since:
- 10
- See Also:
toUnmodifiableMap(Function, Function, BinaryOperator)
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toMap
public static <T,K,U> Collector<T,?,Map<K,U>> toMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction)
Returns aCollectorthat accumulates elements into aMapwhose keys and values are the result of applying the provided mapping functions to the input elements.If the mapped keys contain duplicates (according to
Object.equals(Object)), the value mapping function is applied to each equal element, and the results are merged using the provided merging function.There are no guarantees on the type, mutability, serializability, or thread-safety of the
Mapreturned.- API Note:
- There are multiple ways to deal with collisions between multiple elements
mapping to the same key. The other forms of
toMapsimply use a merge function that throws unconditionally, but you can easily write more flexible merge policies. For example, if you have a stream ofPerson, and you want to produce a "phone book" mapping name to address, but it is possible that two persons have the same name, you can do as follows to gracefully deal with these collisions, and produce aMapmapping names to a concatenated list of addresses:Map<String, String> phoneBook = people.stream().collect( toMap(Person::getName, Person::getAddress, (s, a) -> s + ", " + a)); - Implementation Note:
- The returned
Collectoris not concurrent. For parallel stream pipelines, thecombinerfunction operates by merging the keys from one map into another, which can be an expensive operation. If it is not required that results are merged into theMapin encounter order, usingtoConcurrentMap(Function, Function, BinaryOperator)may offer better parallel performance. - Type Parameters:
T- the type of the input elementsK- the output type of the key mapping functionU- the output type of the value mapping function- Parameters:
keyMapper- a mapping function to produce keysvalueMapper- a mapping function to produce valuesmergeFunction- a merge function, used to resolve collisions between values associated with the same key, as supplied toMap.merge(Object, Object, BiFunction)- Returns:
- a
Collectorwhich collects elements into aMapwhose 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)
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toUnmodifiableMap
public static <T,K,U> Collector<T,?,Map<K,U>> toUnmodifiableMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction)
Returns aCollectorthat accumulates the input elements into an unmodifiable Map, whose keys and values are the result of applying the provided mapping functions to the input elements.If the mapped keys contain duplicates (according to
Object.equals(Object)), the value mapping function is applied to each equal element, and the results are merged using the provided merging function.The returned Collector disallows null keys and values. If either mapping function returns null,
NullPointerExceptionwill be thrown.- Type Parameters:
T- the type of the input elementsK- the output type of the key mapping functionU- the output type of the value mapping function- Parameters:
keyMapper- a mapping function to produce keys, must be non-nullvalueMapper- a mapping function to produce values, must be non-nullmergeFunction- a merge function, used to resolve collisions between values associated with the same key, as supplied toMap.merge(Object, Object, BiFunction), must be non-null- Returns:
- a
Collectorthat accumulates the input elements into an unmodifiable Map, whose keys and values are the result of applying the provided mapping functions to the input elements - Throws:
NullPointerException- if the keyMapper, valueMapper, or mergeFunction is null- Since:
- 10
- See Also:
toUnmodifiableMap(Function, Function)
-
toMap
public static <T,K,U,M extends Map<K,U>> Collector<T,?,M> toMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction, Supplier<M> mapFactory)
Returns aCollectorthat accumulates elements into aMapwhose keys and values are the result of applying the provided mapping functions to the input elements.If the mapped keys contain duplicates (according to
Object.equals(Object)), the value mapping function is applied to each equal element, and the results are merged using the provided merging function. TheMapis created by a provided supplier function.- Implementation Note:
- The returned
Collectoris not concurrent. For parallel stream pipelines, thecombinerfunction operates by merging the keys from one map into another, which can be an expensive operation. If it is not required that results are merged into theMapin encounter order, usingtoConcurrentMap(Function, Function, BinaryOperator, Supplier)may offer better parallel performance. - Type Parameters:
T- the type of the input elementsK- the output type of the key mapping functionU- the output type of the value mapping functionM- the type of the resultingMap- Parameters:
keyMapper- a mapping function to produce keysvalueMapper- a mapping function to produce valuesmergeFunction- a merge function, used to resolve collisions between values associated with the same key, as supplied toMap.merge(Object, Object, BiFunction)mapFactory- a supplier providing a new emptyMapinto which the results will be inserted- Returns:
- a
Collectorwhich collects elements into aMapwhose 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)
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toConcurrentMap
public static <T,K,U> Collector<T,?,ConcurrentMap<K,U>> toConcurrentMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper)
Returns a concurrentCollectorthat accumulates elements into aConcurrentMapwhose keys and values are the result of applying the provided mapping functions to the input elements.If the mapped keys contain duplicates (according to
Object.equals(Object)), anIllegalStateExceptionis thrown when the collection operation is performed. If the mapped keys may have duplicates, usetoConcurrentMap(Function, Function, BinaryOperator)instead.There are no guarantees on the type, mutability, or serializability of the
ConcurrentMapreturned.- API Note:
- It is common for either the key or the value to be the input elements.
In this case, the utility method
Function.identity()may be helpful. For example, the following produces aConcurrentMapmapping students to their grade point average:
And the following produces aConcurrentMap<Student, Double> studentToGPA = students.stream().collect( toConcurrentMap(Function.identity(), student -> computeGPA(student)));ConcurrentMapmapping a unique identifier to students:ConcurrentMap<String, Student> studentIdToStudent = students.stream().collect( toConcurrentMap(Student::getId, Function.identity()));This is a
concurrentandunorderedCollector. - Type Parameters:
T- the type of the input elementsK- the output type of the key mapping functionU- the output type of the value mapping function- Parameters:
keyMapper- the mapping function to produce keysvalueMapper- the mapping function to produce values- Returns:
- a concurrent, unordered
Collectorwhich collects elements into aConcurrentMapwhose 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)
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toConcurrentMap
public static <T,K,U> Collector<T,?,ConcurrentMap<K,U>> toConcurrentMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction)
Returns a concurrentCollectorthat accumulates elements into aConcurrentMapwhose keys and values are the result of applying the provided mapping functions to the input elements.If the mapped keys contain duplicates (according to
Object.equals(Object)), the value mapping function is applied to each equal element, and the results are merged using the provided merging function.There are no guarantees on the type, mutability, or serializability of the
ConcurrentMapreturned.- API Note:
- There are multiple ways to deal with collisions between multiple elements
mapping to the same key. The other forms of
toConcurrentMapsimply use a merge function that throws unconditionally, but you can easily write more flexible merge policies. For example, if you have a stream ofPerson, and you want to produce a "phone book" mapping name to address, but it is possible that two persons have the same name, you can do as follows to gracefully deal with these collisions, and produce aConcurrentMapmapping names to a concatenated list of addresses:ConcurrentMap<String, String> phoneBook = people.stream().collect( toConcurrentMap(Person::getName, Person::getAddress, (s, a) -> s + ", " + a));This is a
concurrentandunorderedCollector. - Type Parameters:
T- the type of the input elementsK- the output type of the key mapping functionU- the output type of the value mapping function- Parameters:
keyMapper- a mapping function to produce keysvalueMapper- a mapping function to produce valuesmergeFunction- a merge function, used to resolve collisions between values associated with the same key, as supplied toMap.merge(Object, Object, BiFunction)- Returns:
- a concurrent, unordered
Collectorwhich collects elements into aConcurrentMapwhose 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)
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toConcurrentMap
public static <T,K,U,M extends ConcurrentMap<K,U>> Collector<T,?,M> toConcurrentMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction, Supplier<M> mapFactory)
Returns a concurrentCollectorthat accumulates elements into aConcurrentMapwhose keys and values are the result of applying the provided mapping functions to the input elements.If the mapped keys contain duplicates (according to
Object.equals(Object)), the value mapping function is applied to each equal element, and the results are merged using the provided merging function. TheConcurrentMapis created by a provided supplier function.This is a
concurrentandunorderedCollector.- Type Parameters:
T- the type of the input elementsK- the output type of the key mapping functionU- the output type of the value mapping functionM- the type of the resultingConcurrentMap- Parameters:
keyMapper- a mapping function to produce keysvalueMapper- a mapping function to produce valuesmergeFunction- a merge function, used to resolve collisions between values associated with the same key, as supplied toMap.merge(Object, Object, BiFunction)mapFactory- a supplier providing a new emptyConcurrentMapinto which the results will be inserted- Returns:
- a concurrent, unordered
Collectorwhich collects elements into aConcurrentMapwhose 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)
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summarizingInt
public static <T> Collector<T,?,IntSummaryStatistics> summarizingInt(ToIntFunction<? super T> mapper)
Returns aCollectorwhich applies anint-producing mapping function to each input element, and returns summary statistics for the resulting values.- Type Parameters:
T- the type of the input elements- Parameters:
mapper- a mapping function to apply to each element- Returns:
- a
Collectorimplementing the summary-statistics reduction - See Also:
summarizingDouble(ToDoubleFunction),summarizingLong(ToLongFunction)
-
summarizingLong
public static <T> Collector<T,?,LongSummaryStatistics> summarizingLong(ToLongFunction<? super T> mapper)
Returns aCollectorwhich applies anlong-producing mapping function to each input element, and returns summary statistics for the resulting values.- Type Parameters:
T- the type of the input elements- Parameters:
mapper- the mapping function to apply to each element- Returns:
- a
Collectorimplementing the summary-statistics reduction - See Also:
summarizingDouble(ToDoubleFunction),summarizingInt(ToIntFunction)
-
summarizingDouble
public static <T> Collector<T,?,DoubleSummaryStatistics> summarizingDouble(ToDoubleFunction<? super T> mapper)
Returns aCollectorwhich applies andouble-producing mapping function to each input element, and returns summary statistics for the resulting values.- Type Parameters:
T- the type of the input elements- Parameters:
mapper- a mapping function to apply to each element- Returns:
- a
Collectorimplementing the summary-statistics reduction - See Also:
summarizingLong(ToLongFunction),summarizingInt(ToIntFunction)
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