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 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. The Collection
is created by the provided factory.
T
- the type of the input elementsC
- the type of the resulting Collection
collectionFactory
- a Supplier
which returns a new, empty Collection
of the appropriate typeCollector
which collects all the input elements into a Collection
, in encounter orderpublic static <T> 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)
.
T
- the type of the input elementsCollector
which collects all the input elements into a List
, in encounter orderpublic static <T> 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.
T
- the type of the input elementsCollector
which collects all the input elements into a Set
public static Collector<CharSequence,?,String> joining()
Returns a Collector
that concatenates the input elements into a String
, in encounter order.
Collector
that concatenates the input elements into a String
, in encounter orderpublic static Collector<CharSequence,?,String> joining(CharSequence delimiter)
Returns a Collector
that concatenates the input elements, separated by the specified delimiter, in encounter order.
delimiter
- the delimiter to be used between each elementCollector
which concatenates CharSequence elements, separated by the specified delimiter, in encounter orderpublic 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.
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 resultCollector
which concatenates CharSequence elements, separated by the specified delimiter, in encounter orderpublic 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.
mapping()
collectors are most useful when used in a multi-level reduction, such as downstream of a groupingBy
or partitioningBy
. For example, given a stream of Person
, 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())));
T
- the type of the input elementsU
- type of elements accepted by downstream collectorA
- intermediate accumulation type of the downstream collectorR
- result type of collectormapper
- a function to be applied to the input elementsdownstream
- a collector which will accept mapped valuespublic 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. 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));
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 collectordownstream
- a collectorfinisher
- a function to be applied to the final result of the downstream collectorpublic static <T> 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.
reducing(0L, e -> 1L, Long::sum)
T
- the type of the input elementsCollector
that counts the input elementspublic 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>
.
reducing(BinaryOperator.minBy(comparator))
T
- the type of the input elementscomparator
- a Comparator
for comparing elementsCollector
that produces the minimal valuepublic 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>
.
reducing(BinaryOperator.maxBy(comparator))
T
- the type of the input elementscomparator
- a Comparator
for comparing elementsCollector
that produces the maximal valuepublic 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. If no elements are present, the result is 0.
T
- the type of the input elementsmapper
- a function extracting the property to be summedCollector
that produces the sum of a derived propertypublic 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. If no elements are present, the result is 0.
T
- the type of the input elementsmapper
- a function extracting the property to be summedCollector
that produces the sum of a derived propertypublic 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. 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
.
T
- the type of the input elementsmapper
- a function extracting the property to be summedCollector
that produces the sum of a derived propertypublic 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. If no elements are present, the result is 0.
T
- the type of the input elementsmapper
- a function extracting the property to be summedCollector
that produces the sum of a derived propertypublic 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. If no elements are present, the result is 0.
T
- the type of the input elementsmapper
- a function extracting the property to be summedCollector
that produces the sum of a derived propertypublic 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. 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
.
double
format 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.T
- the type of the input elementsmapper
- a function extracting the property to be summedCollector
that produces the sum of a derived propertypublic 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.
reducing()
collectors are most useful when used in a multi-level reduction, downstream of groupingBy
or partitioningBy
. To perform a simple reduction on a stream, use Stream.reduce(Object, BinaryOperator)
} instead.T
- element type for the input and output of the reductionidentity
- 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 elementsCollector
which implements the reduction operationreducing(BinaryOperator)
, reducing(Object, Function, BinaryOperator)
public 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
. The result is described as an Optional<T>
.
reducing()
collectors are most useful when used in a multi-level reduction, downstream of groupingBy
or partitioningBy
. To perform a simple reduction on a stream, use Stream.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, Person> tallestByCity = people.stream().collect(groupingBy(Person::getCity, reducing(BinaryOperator.maxBy(byHeight))));
T
- element type for the input and output of the reductionop
- a BinaryOperator<T>
used to reduce the input elementsCollector
which implements the reduction operationreducing(Object, BinaryOperator)
, reducing(Object, Function, BinaryOperator)
public 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
. This is a generalization of reducing(Object, BinaryOperator)
which allows a transformation of the elements before reduction.
reducing()
collectors are most useful when used in a multi-level reduction, downstream of groupingBy
or partitioningBy
. To perform a simple map-reduce on a stream, use Stream.map(Function)
and Stream.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))));
T
- the type of the input elementsU
- the type of the mapped valuesidentity
- 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
- a BinaryOperator<U>
used to reduce the mapped valuesCollector
implementing the map-reduce operationreducing(Object, BinaryOperator)
, reducing(BinaryOperator)
public 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
.
The classification function maps elements to some key type K
. The collector produces a Map<K, List<T>>
whose keys are the values resulting from applying the classification function to the input elements, and whose corresponding values are List
s containing the input elements which map to the associated key under the classification function.
There are no guarantees on the type, mutability, serializability, or thread-safety of the Map
or List
objects returned.
groupingBy(classifier, toList());
Collector
is not concurrent. For parallel stream pipelines, the combiner
function 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 resulting Map
collector is not required, using groupingByConcurrent(Function)
may offer better parallel performance.T
- the type of the input elementsK
- the type of the keysclassifier
- the classifier function mapping input elements to keysCollector
implementing the group-by operationgroupingBy(Function, Collector)
, groupingBy(Function, Supplier, Collector)
, groupingByConcurrent(Function)
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 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())));
Collector
is not concurrent. For parallel stream pipelines, the combiner
function 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, using groupingByConcurrent(Function, Collector)
may offer better parallel performance.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 reductionclassifier
- a classifier function mapping input elements to keysdownstream
- a Collector
implementing the downstream reductionCollector
implementing the cascaded group-by operationgroupingBy(Function)
, groupingBy(Function, Supplier, Collector)
, groupingByConcurrent(Function, Collector)
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 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())));
Collector
is not concurrent. For parallel stream pipelines, the combiner
function 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, using groupingByConcurrent(Function, Supplier, Collector)
may offer better parallel performance.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 resulting Map
classifier
- a classifier function mapping input elements to keysdownstream
- a Collector
implementing the downstream reductionmapFactory
- a function which, when called, produces a new empty Map
of the desired typeCollector
implementing the cascaded group-by operationgroupingBy(Function, Collector)
, groupingBy(Function)
, groupingByConcurrent(Function, Supplier, Collector)
public 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.
This is a concurrent
and unordered
Collector.
The classification function maps elements to some key type K
. The collector produces a ConcurrentMap<K, List<T>>
whose keys are the values resulting from applying the classification function to the input elements, and whose corresponding values are List
s containing the input elements which map to the associated key under the classification function.
There are no guarantees on the type, mutability, or serializability of the Map
or List
objects returned, or of the thread-safety of the List
objects returned.
groupingByConcurrent(classifier, toList());
T
- the type of the input elementsK
- the type of the keysclassifier
- a classifier function mapping input elements to keysCollector
implementing the group-by operationgroupingBy(Function)
, groupingByConcurrent(Function, Collector)
, groupingByConcurrent(Function, Supplier, Collector)
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 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())));
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 reductionclassifier
- a classifier function mapping input elements to keysdownstream
- a Collector
implementing the downstream reductionCollector
implementing the cascaded group-by operationgroupingBy(Function, Collector)
, groupingByConcurrent(Function)
, groupingByConcurrent(Function, Supplier, Collector)
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 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())));
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 resulting ConcurrentMap
classifier
- a classifier function mapping input elements to keysdownstream
- a Collector
implementing the downstream reductionmapFactory
- a function which, when called, produces a new empty ConcurrentMap
of the desired typeCollector
implementing the cascaded group-by operationgroupingByConcurrent(Function)
, groupingByConcurrent(Function, Collector)
, groupingBy(Function, Supplier, Collector)
public 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>>
. There are no guarantees on the type, mutability, serializability, or thread-safety of the Map
returned.
T
- the type of the input elementspredicate
- a predicate used for classifying input elementsCollector
implementing the partitioning operationpartitioningBy(Predicate, Collector)
public 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.
There are no guarantees on the type, mutability, serializability, or thread-safety of the Map
returned.
T
- the type of the input elementsA
- the intermediate accumulation type of the downstream collectorD
- the result type of the downstream reductionpredicate
- a predicate used for classifying input elementsdownstream
- a Collector
implementing the downstream reductionCollector
implementing the cascaded partitioning operationpartitioningBy(Predicate)
public 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.
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.
Function.identity()
may be helpful. For example, the following produces a Map
mapping students to their grade point average: Map<Student, Double> studentToGPA students.stream().collect(toMap(Functions.identity(), student -> computeGPA(student)));And the following produces a
Map
mapping a unique identifier to students: Map<String, Student> studentIdToStudent students.stream().collect(toMap(Student::getId, Functions.identity());
Collector
is not concurrent. For parallel stream pipelines, the combiner
function 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 the Map
in encounter order, using toConcurrentMap(Function, Function)
may offer better parallel performance.T
- the type of the input elementsK
- the output type of the key mapping functionU
- the output type of the value mapping functionkeyMapper
- a mapping function to produce keysvalueMapper
- a mapping function to produce valuesCollector
which collects elements into a Map
whose keys and values are the result of applying mapping functions to the input elementstoMap(Function, Function, BinaryOperator)
, toMap(Function, Function, BinaryOperator, Supplier)
, toConcurrentMap(Function, Function)
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 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.
toMap
simply use a merge function that throws unconditionally, but you can easily write more flexible merge policies. For example, if you have a stream of Person
, 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 deals with these collisions, and produce a Map
mapping names to a concatenated list of addresses: Map<String, String> phoneBook people.stream().collect(toMap(Person::getName, Person::getAddress, (s, a) -> s + ", " + a));
Collector
is not concurrent. For parallel stream pipelines, the combiner
function 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 the Map
in encounter order, using toConcurrentMap(Function, Function, BinaryOperator)
may offer better parallel performance.T
- the type of the input elementsK
- the output type of the key mapping functionU
- the output type of the value mapping functionkeyMapper
- 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 to Map.merge(Object, Object, BiFunction)
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 functiontoMap(Function, Function)
, toMap(Function, Function, BinaryOperator, Supplier)
, toConcurrentMap(Function, Function, BinaryOperator)
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> 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.
Collector
is not concurrent. For parallel stream pipelines, the combiner
function 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 the Map
in encounter order, using toConcurrentMap(Function, Function, BinaryOperator, Supplier)
may offer better parallel performance.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 resulting Map
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 to Map.merge(Object, Object, BiFunction)
mapSupplier
- a function which returns a new, empty Map
into which the results will be insertedCollector
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 functiontoMap(Function, Function)
, toMap(Function, Function, BinaryOperator)
, toConcurrentMap(Function, Function, BinaryOperator, Supplier)
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 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.
Function.identity()
may be helpful. For example, the following produces a Map
mapping students to their grade point average: Map<Student, Double> studentToGPA students.stream().collect(toMap(Functions.identity(), student -> computeGPA(student)));And the following produces a
Map
mapping a unique identifier to students: Map<String, Student> studentIdToStudent students.stream().collect(toConcurrentMap(Student::getId, Functions.identity());
This is a concurrent
and unordered
Collector.
T
- the type of the input elementsK
- the output type of the key mapping functionU
- the output type of the value mapping functionkeyMapper
- the mapping function to produce keysvalueMapper
- the mapping function to produce valuesCollector
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 elementstoMap(Function, Function)
, toConcurrentMap(Function, Function, BinaryOperator)
, toConcurrentMap(Function, Function, BinaryOperator, Supplier)
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 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.
toConcurrentMap
simply use a merge function that throws unconditionally, but you can easily write more flexible merge policies. For example, if you have a stream of Person
, 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 deals with these collisions, and produce a Map
mapping names to a concatenated list of addresses: Map<String, String> phoneBook people.stream().collect(toConcurrentMap(Person::getName, Person::getAddress, (s, a) -> s + ", " + a));
This is a concurrent
and unordered
Collector.
T
- the type of the input elementsK
- the output type of the key mapping functionU
- the output type of the value mapping functionkeyMapper
- 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 to Map.merge(Object, Object, BiFunction)
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 functiontoConcurrentMap(Function, Function)
, toConcurrentMap(Function, Function, BinaryOperator, Supplier)
, toMap(Function, Function, BinaryOperator)
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> 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.
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 resulting ConcurrentMap
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 to Map.merge(Object, Object, BiFunction)
mapSupplier
- a function which returns a new, empty Map
into which the results will be insertedCollector
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 functiontoConcurrentMap(Function, Function)
, toConcurrentMap(Function, Function, BinaryOperator)
, toMap(Function, Function, BinaryOperator, Supplier)
public 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.
T
- the type of the input elementsmapper
- a mapping function to apply to each elementCollector
implementing the summary-statistics reductionsummarizingDouble(ToDoubleFunction)
, summarizingLong(ToLongFunction)
public 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.
T
- the type of the input elementsmapper
- the mapping function to apply to each elementCollector
implementing the summary-statistics reductionsummarizingDouble(ToDoubleFunction)
, summarizingInt(ToIntFunction)
public 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.
T
- the type of the input elementsmapper
- a mapping function to apply to each elementCollector
implementing the summary-statistics reductionsummarizingLong(ToLongFunction)
, summarizingInt(ToIntFunction)
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