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));
Returns a Collector that produces the arithmetic mean of a double-valued function applied to the input elements.
Returns a Collector that produces the arithmetic mean of an integer-valued function applied to the input elements.
Returns a Collector that produces the arithmetic mean of a long-valued function applied to the input elements.
Adapts a Collector to perform an additional finishing transformation.
Returns a Collector accepting elements of type T that counts the number of input elements.
Adapts a Collector to one accepting elements of the same type T by applying the predicate to each input element and only accumulating if the predicate returns true.
Adapts a Collector accepting elements of type U to one accepting elements of type T by applying a flat mapping function to each input element before accumulation.
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.
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.
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.
Returns a concurrent Collector implementing a "group by" operation on input elements of type T, grouping elements according to a classification function.
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.
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.
Returns a Collector that concatenates the input elements into a String, in encounter order.
Returns a Collector that concatenates the input elements, separated by the specified delimiter, in encounter order.
Returns a Collector that concatenates the input elements, separated by the specified delimiter, with the specified prefix and suffix, in encounter order.
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.
Returns a Collector that produces the maximal element according to a given Comparator, described as an Optional<T>.
Returns a Collector that produces the minimal element according to a given Comparator, described as an Optional<T>.
Returns a Collector which partitions the input elements according to a Predicate, and organizes them into a Map<Boolean, List<T>>.
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.
Returns a Collector which performs a reduction of its input elements under a specified BinaryOperator.
Returns a Collector which performs a reduction of its input elements under a specified BinaryOperator using the provided identity.
Returns a Collector which performs a reduction of its input elements under a specified mapping function and BinaryOperator.
Returns a Collector which applies an double-producing mapping function to each input element, and returns summary statistics for the resulting values.
Returns a Collector which applies an int-producing mapping function to each input element, and returns summary statistics for the resulting values.
Returns a Collector which applies an long-producing mapping function to each input element, and returns summary statistics for the resulting values.
Returns a Collector that produces the sum of a double-valued function applied to the input elements.
Returns a Collector that produces the sum of a integer-valued function applied to the input elements.
Returns a Collector that produces the sum of a long-valued function applied to the input elements.
Returns a Collector that is a composite of two downstream collectors.
Returns a Collector that accumulates the input elements into a new Collection, in encounter order.
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.
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.
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.
Returns a Collector that accumulates the input elements into a new List.
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.
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.
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.
Returns a Collector that accumulates the input elements into a new Set.
Returns a
Collector
that accumulates the input elements into an
unmodifiable Listin encounter order.
Returns a
Collector
that 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.
Returns a
Collector
that 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.
Returns a
Collector
that accumulates the input elements into an
unmodifiable Set.
Methods declared in class java.lang.Objectclone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, waitReturns 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 elements
C - the type of the resulting Collection
collectionFactory - a supplier providing a new empty Collection into which the results will be inserted
Collector which collects all the input elements into a Collection, in encounter order
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 elements
Collector which collects all the input elements into a List, in encounter order
Returns a
Collector
that accumulates the input elements into an
unmodifiable Listin encounter order. The returned Collector disallows null values and will throw
NullPointerException
if it is presented with a null value.
T - the type of the input elements
Collector that accumulates the input elements into an unmodifiable List in encounter order
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 elements
Collector which collects all the input elements into a Set
()
Returns a
Collector
that accumulates the input elements into an
unmodifiable Set. The returned Collector disallows null values and will throw
NullPointerException
if it is presented with a null value. If the input contains duplicate elements, an arbitrary element of the duplicates is preserved.
This is an unordered Collector.
T - the type of the input elements
Collector that accumulates the input elements into an unmodifiable Set
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 order
Returns a Collector that concatenates the input elements, separated by the specified delimiter, in encounter order.
delimiter - the delimiter to be used between each element
Collector which concatenates CharSequence elements, separated by the specified delimiter, in encounter order
Returns a Collector that concatenates the input elements, separated by the specified delimiter, with the specified prefix and suffix, in encounter order.
delimiter - the delimiter to be used between each element
prefix - the sequence of characters to be used at the beginning of the joined result
suffix - the sequence of characters to be used at the end of the joined result
Collector which concatenates CharSequence elements, separated by the specified delimiter, in encounter order
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 elements
U - type of elements accepted by downstream collector
A - intermediate accumulation type of the downstream collector
R - result type of collector
mapper - a function to be applied to the input elements
downstream - a collector which will accept mapped values
Adapts a
Collector
accepting elements of type
U
to one accepting elements of type
T
by applying a flat mapping function to each input element before accumulation. The flat mapping function maps an input element to a
stream
covering zero or more output elements that are then accumulated downstream. Each mapped stream is
closed
after its contents have been placed downstream. (If a mapped stream is
null
an empty stream is used, instead.)
flatMapping() 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 Order, 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())));
T - the type of the input elements
U - type of elements accepted by downstream collector
A - intermediate accumulation type of the downstream collector
R - result type of collector
mapper - a function to be applied to the input elements, which returns a stream of results
downstream - a collector which will receive the elements of the stream returned by mapper
Adapts a Collector to one accepting elements of the same type T by applying the predicate to each input element and only accumulating if the predicate returns true.
filtering() 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 Employee, to accumulate the employees in each department that have a salary above a certain threshold:
Map<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 empty Set. If a stream filter() operation were done instead, there would be no mapping for that department at all.
T - the type of the input elements
A - intermediate accumulation type of the downstream collector
R - result type of collector
predicate - a predicate to be applied to the input elements
downstream - a collector which will accept values that match the predicate
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> list = people.stream().collect(
collectingAndThen(toList(),
Collections::unmodifiableList));
T - the type of the input elements
A - intermediate accumulation type of the downstream collector
R - result type of the downstream collector
RR - result type of the resulting collector
downstream - a collector
finisher - a function to be applied to the final result of the downstream collector
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 elements
Collector that counts the input elements
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 elements
comparator - a Comparator for comparing elements
Collector that produces the minimal value
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 elements
comparator - a Comparator for comparing elements
Collector that produces the maximal value
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 elements
mapper - a function extracting the property to be summed
Collector that produces the sum of a derived property
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 elements
mapper - a function extracting the property to be summed
Collector that produces the sum of a derived property
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 elements
mapper - a function extracting the property to be summed
Collector that produces the sum of a derived property
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 elements
mapper - a function extracting the property to be averaged
Collector that produces the arithmetic mean of a derived property
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 elements
mapper - a function extracting the property to be averaged
Collector that produces the arithmetic mean of a derived property
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 elements
mapper - a function extracting the property to be averaged
Collector that produces the arithmetic mean of a derived property
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 reduction
identity - the identity value for the reduction (also, the value that is returned when there are no input elements)
op - a BinaryOperator<T> used to reduce the input elements
Collector which implements the reduction operation
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, Optional<Person>> tallestByCity
= people.stream().collect(
groupingBy(Person::getCity,
reducing(BinaryOperator.maxBy(byHeight))));
T - element type for the input and output of the reduction
op - a BinaryOperator<T> used to reduce the input elements
Collector which implements the reduction operation
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 elements
U - the type of the mapped values
identity - the identity value for the reduction (also, the value that is returned when there are no input elements)
mapper - a mapping function to apply to each input value
op - a BinaryOperator<U> used to reduce the mapped values
Collector implementing the map-reduce operation
Returns a
Collector
implementing a "group by" operation on input elements of type
T
, grouping elements according to a classification function, and returning the results in a
Map
.
The classification function maps elements to some key type K. The collector produces a Map<K, List<T>> whose keys are the values resulting from applying the classification function to the input elements, and whose corresponding values are Lists containing the input elements which map to the associated key under the classification function.
There are no guarantees on the type, mutability, serializability, or thread-safety of the Map or List objects returned.
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 elements
K - the type of the keys
classifier - the classifier function mapping input elements to keys
Collector implementing the group-by operation
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 elements
K - the type of the keys
A - the intermediate accumulation type of the downstream collector
D - the result type of the downstream reduction
classifier - a classifier function mapping input elements to keys
downstream - a Collector implementing the downstream reduction
Collector implementing the cascaded group-by operation
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 elements
K - the type of the keys
D - the result type of the downstream reduction
A - the intermediate accumulation type of the downstream collector
M - the type of the resulting Map
classifier - a classifier function mapping input elements to keys
mapFactory - a supplier providing a new empty Map into which the results will be inserted
downstream - a Collector implementing the downstream reduction
Collector implementing the cascaded group-by operation
Returns a concurrent
Collector
implementing a "group by" operation on input elements of type
T
, grouping elements according to a classification function.
This is a concurrent and unordered Collector.
The classification function maps elements to some key type K. The collector produces a ConcurrentMap<K, List<T>> whose keys are the values resulting from applying the classification function to the input elements, and whose corresponding values are Lists containing the input elements which map to the associated key under the classification function.
There are no guarantees on the type, mutability, or serializability of the ConcurrentMap or List objects returned, or of the thread-safety of the List objects returned.
groupingByConcurrent(classifier, toList());
T - the type of the input elements
K - the type of the keys
classifier - a classifier function mapping input elements to keys
Collector implementing the group-by operation
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 ConcurrentMap<K, D>.
There are no guarantees on the type, mutability, or serializability of the ConcurrentMap returned.
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 elements
K - the type of the keys
A - the intermediate accumulation type of the downstream collector
D - the result type of the downstream reduction
classifier - a classifier function mapping input elements to keys
downstream - a Collector implementing the downstream reduction
Collector implementing the cascaded group-by operation
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 ConcurrentMap<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())));
T - the type of the input elements
K - the type of the keys
A - the intermediate accumulation type of the downstream collector
D - the result type of the downstream reduction
M - the type of the resulting ConcurrentMap
classifier - a classifier function mapping input elements to keys
mapFactory - a supplier providing a new empty ConcurrentMap into which the results will be inserted
downstream - a Collector implementing the downstream reduction
Collector implementing the cascaded group-by operation
Returns a Collector which partitions the input elements according to a Predicate, and organizes them into a Map<Boolean, List<T>>. The returned Map always contains mappings for both false and true keys. There are no guarantees on the type, mutability, serializability, or thread-safety of the Map or List returned.
T - the type of the input elements
predicate - a predicate used for classifying input elements
Collector implementing the partitioning operation
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.
The returned Map always contains mappings for both false and true keys. There are no guarantees on the type, mutability, serializability, or thread-safety of the Map returned.
T - the type of the input elements
D - the result type of the downstream reduction
A - the intermediate accumulation type of the downstream collector
predicate - a predicate used for classifying input elements
downstream - a Collector implementing the downstream reduction
Collector implementing the cascaded partitioning operation
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 contain duplicates (according to Object.equals(Object)), an IllegalStateException is thrown when the collection operation is performed. If the mapped keys might have duplicates, use toMap(Function, Function, BinaryOperator) instead.
There are no guarantees on the type, mutability, serializability, or thread-safety of the Map returned.
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(Function.identity(),
student -> computeGPA(student)));
Map mapping a unique identifier to students:
Map<String, Student> studentIdToStudent
= students.stream().collect(
toMap(Student::getId,
Function.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 elements
K - the output type of the key mapping function
U - the output type of the value mapping function
keyMapper - a mapping function to produce keys
valueMapper - a mapping function to produce values
Collector which collects elements into a Map whose keys and values are the result of applying mapping functions to the input elements
Returns a
Collector
that 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)), an IllegalStateException is thrown when the collection operation is performed. If the mapped keys might have duplicates, use toUnmodifiableMap(Function, Function, BinaryOperator) to handle merging of the values.
The returned Collector disallows null keys and values. If either mapping function returns null, NullPointerException will be thrown.
T - the type of the input elements
K - the output type of the key mapping function
U - the output type of the value mapping function
keyMapper - a mapping function to produce keys, must be non-null
valueMapper - a mapping function to produce values, must be non-null
Collector that 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
NullPointerException - if either keyMapper or valueMapper is null
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 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 Map returned.
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 deal 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 elements
K - the output type of the key mapping function
U - the output type of the value mapping function
keyMapper - a mapping function to produce keys
valueMapper - a mapping function to produce values
mergeFunction - a merge function, used to resolve collisions between values associated with the same key, as supplied to Map.merge(Object, Object, BiFunction)
Collector which collects elements into a Map whose keys are the result of applying a key mapping function to the input elements, and whose values are the result of applying a value mapping function to all input elements equal to the key and combining them using the merge function
Returns a
Collector
that 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, NullPointerException will be thrown.
T - the type of the input elements
K - the output type of the key mapping function
U - the output type of the value mapping function
keyMapper - a mapping function to produce keys, must be non-null
valueMapper - a mapping function to produce values, must be non-null
mergeFunction - a merge function, used to resolve collisions between values associated with the same key, as supplied to Map.merge(Object, Object, BiFunction), must be non-null
Collector that 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
NullPointerException - if the keyMapper, valueMapper, or mergeFunction is null
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 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 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 elements
K - the output type of the key mapping function
U - the output type of the value mapping function
M - the type of the resulting Map
keyMapper - a mapping function to produce keys
valueMapper - a mapping function to produce values
mergeFunction - a merge function, used to resolve collisions between values associated with the same key, as supplied to Map.merge(Object, Object, BiFunction)
mapFactory - a supplier providing a new empty Map into which the results will be inserted
Collector which collects elements into a Map whose keys are the result of applying a key mapping function to the input elements, and whose values are the result of applying a value mapping function to all input elements equal to the key and combining them using the merge function
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 contain 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.
There are no guarantees on the type, mutability, or serializability of the ConcurrentMap returned.
Function.identity() may be helpful. For example, the following produces a ConcurrentMap mapping students to their grade point average:
ConcurrentMap<Student, Double> studentToGPA
= students.stream().collect(
toConcurrentMap(Function.identity(),
student -> computeGPA(student)));
ConcurrentMap mapping a unique identifier to students:
ConcurrentMap<String, Student> studentIdToStudent
= students.stream().collect(
toConcurrentMap(Student::getId,
Function.identity()));
This is a concurrent and unordered Collector.
T - the type of the input elements
K - the output type of the key mapping function
U - the output type of the value mapping function
keyMapper - the mapping function to produce keys
valueMapper - the mapping function to produce values
Collector which collects elements into a ConcurrentMap whose keys are the result of applying a key mapping function to the input elements, and whose values are the result of applying a value mapping function to the input elements
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 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 ConcurrentMap returned.
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 deal with these collisions, and produce a ConcurrentMap mapping 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 concurrent and unordered Collector.
T - the type of the input elements
K - the output type of the key mapping function
U - the output type of the value mapping function
keyMapper - a mapping function to produce keys
valueMapper - a mapping function to produce values
mergeFunction - a merge function, used to resolve collisions between values associated with the same key, as supplied to Map.merge(Object, Object, BiFunction)
Collector which collects elements into a ConcurrentMap whose keys are the result of applying a key mapping function to the input elements, and whose values are the result of applying a value mapping function to all input elements equal to the key and combining them using the merge function
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 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 ConcurrentMap is created by a provided supplier function.
This is a concurrent and unordered Collector.
T - the type of the input elements
K - the output type of the key mapping function
U - the output type of the value mapping function
M - the type of the resulting ConcurrentMap
keyMapper - a mapping function to produce keys
valueMapper - a mapping function to produce values
mergeFunction - a merge function, used to resolve collisions between values associated with the same key, as supplied to Map.merge(Object, Object, BiFunction)
mapFactory - a supplier providing a new empty ConcurrentMap into which the results will be inserted
Collector which collects elements into a ConcurrentMap whose keys are the result of applying a key mapping function to the input elements, and whose values are the result of applying a value mapping function to all input elements equal to the key and combining them using the merge function
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 elements
mapper - a mapping function to apply to each element
Collector implementing the summary-statistics reduction
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 elements
mapper - the mapping function to apply to each element
Collector implementing the summary-statistics reduction
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 elements
mapper - a mapping function to apply to each element
Collector implementing the summary-statistics reduction
Returns a
Collector
that is a composite of two downstream collectors. Every element passed to the resulting collector is processed by both downstream collectors, then their results are merged using the specified merge function into the final result.
The resulting collector functions do the following:
The resulting collector is Collector.Characteristics.UNORDERED if both downstream collectors are unordered and Collector.Characteristics.CONCURRENT if both downstream collectors are concurrent.
T - the type of the input elements
R1 - the result type of the first collector
R2 - the result type of the second collector
R - the final result type
downstream1 - the first downstream collector
downstream2 - the second downstream collector
merger - the function which merges two results into the single one
Collector which aggregates the results of two supplied collectors.
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