Added in API level 24

Summary: Methods | Inherited Methods

Collectors

public final class Collectors
extends Object

java.lang.Object
↳	java.util.stream.Collectors

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));

Summary

Public methods
`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.
`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.
`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.
`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.
`static <T> Collector<T, ?, Long>`	`counting()` Returns a `Collector` accepting elements of type `T` that 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 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`.
`static <T, U, A, R> Collector<T, ?, R>`	`flatMapping(Function<? super T, ? extends Stream<? 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 flat mapping function to each input element before accumulation.
`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`.
`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`.
`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`.
`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`.
`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`.
`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.
`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.
`static Collector<CharSequence, ?, String>`	`joining(CharSequence delimiter)` Returns a `Collector` that concatenates the input elements, separated by the specified delimiter, in encounter order.
`static Collector<CharSequence, ?, String>`	`joining()` Returns a `Collector` that concatenates the input elements into a `String`, 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 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.
`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>`.
`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>`.
`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.
`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>>`.
`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`.
`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`.
`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.
`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.
`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.
`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.
`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.
`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.
`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.
`static <T, R1, R2, R> Collector<T, ?, R>`	`teeing(Collector<? super T, ?, R1> downstream1, Collector<? super T, ?, R2> downstream2, BiFunction<? super R1, ? super R2, R> merger)` Returns a `Collector` that is a composite of two downstream collectors.
`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.
`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.
`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 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.
`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.
`static <T> Collector<T, ?, List<T>>`	`toList()` Returns a `Collector` that accumulates the input elements into a new `List`.
`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 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.
`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.
`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.
`static <T> Collector<T, ?, Set<T>>`	`toSet()` Returns a `Collector` that accumulates the input elements into a new `Set`.
`static <T> Collector<T, ?, List<T>>`	`toUnmodifiableList()` Returns a `Collector` that 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, BinaryOperator<U> mergeFunction)` 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.
`static <T, K, U> Collector<T, ?, Map<K, U>>`	`toUnmodifiableMap(Function<? super T, ? extends K> keyMapper, Function<? super T, ? extends U> valueMapper)` 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.
`static <T> Collector<T, ?, Set<T>>`	`toUnmodifiableSet()` Returns a `Collector` that accumulates the input elements into an unmodifiable Set.

Inherited methods

From class


        
          java.lang.Object

`Object`	`clone()` Creates and returns a copy of this object.
`boolean`	`equals(Object obj)` Indicates whether some other object is "equal to" this one.
`void`	`finalize()` Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.
`final Class<?>`	`getClass()` Returns the runtime class of this `Object`.
`int`	`hashCode()` Returns a hash code value for the object.
`final void`	`notify()` Wakes up a single thread that is waiting on this object's monitor.
`final void`	`notifyAll()` Wakes up all threads that are waiting on this object's monitor.
`String`	`toString()` Returns a string representation of the object.
`final void`	`wait(long timeoutMillis, int nanos)` Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.
`final void`	`wait(long timeoutMillis)` Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.
`final void`	`wait()` Causes the current thread to wait until it is awakened, typically by being notified or interrupted.

Public methods

averagingDouble

Added in API level 24

public static 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.

Implementation Note:

The 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.

Parameters
`mapper`	`ToDoubleFunction`: a function extracting the property to be averaged

Returns
`Collector<T, ?, Double>`	a `Collector` that produces the arithmetic mean of a derived property

averagingInt

Added in API level 24

public static 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.

Parameters
`mapper`	`ToIntFunction`: a function extracting the property to be averaged

Returns
`Collector<T, ?, Double>`	a `Collector` that produces the arithmetic mean of a derived property

averagingLong

Added in API level 24

public static 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.

Parameters
`mapper`	`ToLongFunction`: a function extracting the property to be averaged

Returns
`Collector<T, ?, Double>`	a `Collector` that produces the arithmetic mean of a derived property

collectingAndThen

Added in API level 24

public static 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> list = people.stream().collect(
   collectingAndThen(toList(),
                     Collections::unmodifiableList));

Parameters
`downstream`	`Collector`: a collector
`finisher`	`Function`: a function to be applied to the final result of the downstream collector

Returns
`Collector<T, A, RR>`	a collector which performs the action of the downstream collector, followed by an additional finishing step

counting

Added in API level 24

public static 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.

Implementation Requirements:

This produces a result equivalent to:
```
reducing(0L, e -> 1L, Long::sum)
 
```

Returns
`Collector<T, ?, Long>`	a `Collector` that counts the input elements

filtering

Added in API level 33

public static Collector<T, ?, R> filtering (Predicate<? super T> predicate, 
                Collector<? super T, A, R> downstream)

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.

API Note:

The 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())));
 
```
A filtering collector differs from a stream's 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.

Parameters
`predicate`	`Predicate`: a predicate to be applied to the input elements
`downstream`	`Collector`: a collector which will accept values that match the predicate

Returns
`Collector<T, ?, R>`	a collector which applies the predicate to the input elements and provides matching elements to the downstream collector

flatMapping

Added in API level 33

public static Collector<T, ?, R> flatMapping (Function<? super T, ? extends Stream<? 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 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.)

API Note:

The 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())));

Parameters
`mapper`	`Function`: a function to be applied to the input elements, which returns a stream of results
`downstream`	`Collector`: a collector which will receive the elements of the stream returned by mapper

Returns
`Collector<T, ?, R>`	a collector which applies the mapping function to the input elements and provides the flat mapped results to the downstream collector

groupingBy

Added in API level 24

public static 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())));

Implementation Note:

The returned 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(java.util.function.Function, java.util.function.Supplier, java.util.stream.Collector) may offer better parallel performance.

Parameters
`classifier`	`Function`: a classifier function mapping input elements to keys
`mapFactory`	`Supplier`: a supplier providing a new empty `Map` into which the results will be inserted
`downstream`	`Collector`: a `Collector` implementing the downstream reduction

Returns
`Collector<T, ?, M>`	a `Collector` implementing the cascaded group-by operation

See also:

groupingBy

Added in API level 24

public static 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 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.

Implementation Requirements:

This produces a result similar to:
```
groupingBy(classifier, toList());
 
```

Implementation Note:

The returned 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(java.util.function.Function) may offer better parallel performance.

Parameters
`classifier`	`Function`: the classifier function mapping input elements to keys

Returns
`Collector<T, ?, Map<K, List<T>>>`	a `Collector` implementing the group-by operation

See also:

groupingBy

Added in API level 24

public static Collector<T, ?, Map<K, D>> groupingBy (Function<? super T, ? extends K> classifier, 
                Collector<? super T, A, D> downstream)

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())));

Implementation Note:

The returned 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(java.util.function.Function, java.util.stream.Collector) may offer better parallel performance.

Parameters
`classifier`	`Function`: a classifier function mapping input elements to keys
`downstream`	`Collector`: a `Collector` implementing the downstream reduction

Returns
`Collector<T, ?, Map<K, D>>`	a `Collector` implementing the cascaded group-by operation

See also:

groupingByConcurrent

Added in API level 24

public static 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.

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())));

Parameters
`classifier`	`Function`: a classifier function mapping input elements to keys
`mapFactory`	`Supplier`: a supplier providing a new empty `ConcurrentMap` into which the results will be inserted
`downstream`	`Collector`: a `Collector` implementing the downstream reduction

Returns
`Collector<T, ?, M>`	a concurrent, unordered `Collector` implementing the cascaded group-by operation

See also:

groupingByConcurrent

Added in API level 24

public static Collector<T, ?, ConcurrentMap<K, D>> groupingByConcurrent (Function<? super T, ? extends K> classifier, 
                Collector<? super T, A, D> downstream)

This is a concurrent and unordered Collector.

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())));

Parameters
`classifier`	`Function`: a classifier function mapping input elements to keys
`downstream`	`Collector`: a `Collector` implementing the downstream reduction

Returns
`Collector<T, ?, ConcurrentMap<K, D>>`	a concurrent, unordered `Collector` implementing the cascaded group-by operation

See also:

groupingByConcurrent

Added in API level 24

public static 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 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.

Implementation Requirements:

This produces a result similar to:

groupingByConcurrent(classifier, toList());

Parameters
`classifier`	`Function`: a classifier function mapping input elements to keys

Returns
`Collector<T, ?, ConcurrentMap<K, List<T>>>`	a concurrent, unordered `Collector` implementing the group-by operation

See also:

joining

Added in API level 24

public 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.

Parameters
`delimiter`	`CharSequence`: the delimiter to be used between each element
`prefix`	`CharSequence`: the sequence of characters to be used at the beginning of the joined result
`suffix`	`CharSequence`: the sequence of characters to be used at the end of the joined result

Returns
`Collector<CharSequence, ?, String>`	A `Collector` which concatenates CharSequence elements, separated by the specified delimiter, in encounter order

joining

Added in API level 24

public static Collector<CharSequence, ?, String> joining (CharSequence delimiter)

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

Parameters
`delimiter`	`CharSequence`: the delimiter to be used between each element

Returns
`Collector<CharSequence, ?, String>`	A `Collector` which concatenates CharSequence elements, separated by the specified delimiter, in encounter order

joining

Added in API level 24

public static Collector<CharSequence, ?, String> joining ()

Returns a Collector that concatenates the input elements into a String, in encounter order.

Returns
`Collector<CharSequence, ?, String>`	a `Collector` that concatenates the input elements into a `String`, in encounter order

mapping

Added in API level 24

public static 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.

API Note:

The 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())));
 
```

Parameters
`mapper`	`Function`: a function to be applied to the input elements
`downstream`	`Collector`: a collector which will accept mapped values

Returns
`Collector<T, ?, R>`	a collector which applies the mapping function to the input elements and provides the mapped results to the downstream collector

maxBy

Added in API level 24

public static 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>.

Implementation Requirements:

This produces a result equivalent to:

reducing(BinaryOperator.maxBy(comparator))

Parameters
`comparator`	`Comparator`: a `Comparator` for comparing elements

Returns
`Collector<T, ?, Optional<T>>`	a `Collector` that produces the maximal value

minBy

Added in API level 24

public static 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>.

Implementation Requirements:

This produces a result equivalent to:

reducing(BinaryOperator.minBy(comparator))

Parameters
`comparator`	`Comparator`: a `Comparator` for comparing elements

Returns
`Collector<T, ?, Optional<T>>`	a `Collector` that produces the minimal value

partitioningBy

Added in API level 24

public static 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.

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.

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.

Parameters
`predicate`	`Predicate`: a predicate used for classifying input elements
`downstream`	`Collector`: a `Collector` implementing the downstream reduction

Returns
`Collector<T, ?, Map<Boolean, D>>`	a `Collector` implementing the cascaded partitioning operation

See also:

partitioningBy(Predicate)

partitioningBy

Added in API level 24

public static 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>>. 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.

API Note:

If a partition has no elements, its value in the result Map will be an empty List.

Parameters
`predicate`	`Predicate`: a predicate used for classifying input elements

Returns
`Collector<T, ?, Map<Boolean, List<T>>>`	a `Collector` implementing the partitioning operation

See also:

partitioningBy(Predicate, Collector)

reducing

Added in API level 24

public static 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>.

API Note:

The 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))));
 
```

Parameters
`op`	`BinaryOperator`: a `BinaryOperator<T>` used to reduce the input elements

Returns
`Collector<T, ?, Optional<T>>`	a `Collector` which implements the reduction operation

See also:

reducing

Added in API level 24

public static 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(java.lang.Object, java.util.function.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 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))));
 
```

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

Returns
`Collector<T, ?, U>`	a `Collector` implementing the map-reduce operation

See also:

reducing

Added in API level 24

public static 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.

API Note:

The 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.

Parameters
`identity`	`T`: the identity value for the reduction (also, the value that is returned when there are no input elements)
`op`	`BinaryOperator`: a `BinaryOperator<T>` used to reduce the input elements

Returns
`Collector<T, ?, T>`	a `Collector` which implements the reduction operation

See also:

summarizingDouble

Added in API level 24

public static 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.

Parameters
`mapper`	`ToDoubleFunction`: a mapping function to apply to each element

Returns
`Collector<T, ?, DoubleSummaryStatistics>`	a `Collector` implementing the summary-statistics reduction

See also:

summarizingInt

Added in API level 24

public static 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.

Parameters
`mapper`	`ToIntFunction`: a mapping function to apply to each element

Returns
`Collector<T, ?, IntSummaryStatistics>`	a `Collector` implementing the summary-statistics reduction

See also:

summarizingLong

Added in API level 24

public static 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.

Parameters
`mapper`	`ToLongFunction`: the mapping function to apply to each element

Returns
`Collector<T, ?, LongSummaryStatistics>`	a `Collector` implementing the summary-statistics reduction

See also:

summingDouble

Added in API level 24

public static 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.

Parameters
`mapper`	`ToDoubleFunction`: a function extracting the property to be summed

Returns
`Collector<T, ?, Double>`	a `Collector` that produces the sum of a derived property

summingInt

Added in API level 24

public static 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.

Parameters
`mapper`	`ToIntFunction`: a function extracting the property to be summed

Returns
`Collector<T, ?, Integer>`	a `Collector` that produces the sum of a derived property

summingLong

Added in API level 24

public static 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.

Parameters
`mapper`	`ToLongFunction`: a function extracting the property to be summed

Returns
`Collector<T, ?, Long>`	a `Collector` that produces the sum of a derived property

teeing

Added in API level 35

public static Collector<T, ?, R> teeing (Collector<? super T, ?, R1> downstream1, 
                Collector<? super T, ?, R2> downstream2, 
                BiFunction<? super R1, ? super R2, R> merger)

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:

supplier: creates a result container that contains result containers obtained by calling each collector's supplier
accumulator: calls each collector's accumulator with its result container and the input element
combiner: calls each collector's combiner with two result containers
finisher: calls each collector's finisher with its result container, then calls the supplied merger and returns its result.

The resulting collector is Collector.Characteristics#UNORDERED if both downstream collectors are unordered and Collector.Characteristics#CONCURRENT if both downstream collectors are concurrent.

Parameters
`downstream1`	`Collector`: the first downstream collector
`downstream2`	`Collector`: the second downstream collector
`merger`	`BiFunction`: the function which merges two results into the single one

Returns
`Collector<T, ?, R>`	a `Collector` which aggregates the results of two supplied collectors.

toCollection

Added in API level 24

public static 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.

Parameters
`collectionFactory`	`Supplier`: a supplier providing a new empty `Collection` into which the results will be inserted

Returns
`Collector<T, ?, C>`	a `Collector` which collects all the input elements into a `Collection`, in encounter order

toConcurrentMap

Added in API level 24

public static 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 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(java.util.function.Function, java.util.function.Function, java.util.function.BinaryOperator) instead.

There are no guarantees on the type, mutability, or serializability of the ConcurrentMap returned.

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 a ConcurrentMap mapping students to their grade point average:
```
ConcurrentMap<Student, Double> studentToGPA
   = students.stream().collect(
     toConcurrentMap(Function.identity(),
                     student -> computeGPA(student)));
 
```
And the following produces a 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.

Parameters
`keyMapper`	`Function`: the mapping function to produce keys
`valueMapper`	`Function`: the mapping function to produce values

Returns
`Collector<T, ?, ConcurrentMap<K, U>>`	a concurrent, unordered `Collector` which collects elements into a `ConcurrentMap` whose keys are the result of applying a key mapping function to the input elements, and whose values are the result of applying a value mapping function to the input elements

See also:

toConcurrentMap

Added in API level 24

public static Collector<T, ?, M> toConcurrentMap (Function<? super T, ? extends K> keyMapper, 
                Function<? super T, ? extends U> valueMapper, 
                BinaryOperator<U> mergeFunction, 
                Supplier<M> mapFactory)

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.

Parameters
`keyMapper`	`Function`: a mapping function to produce keys
`valueMapper`	`Function`: a mapping function to produce values
`mergeFunction`	`BinaryOperator`: a merge function, used to resolve collisions between values associated with the same key, as supplied to `Map#merge(Object, Object, BiFunction)`
`mapFactory`	`Supplier`: a supplier providing a new empty `ConcurrentMap` into which the results will be inserted

Returns
`Collector<T, ?, M>`	a concurrent, unordered `Collector` which collects elements into a `ConcurrentMap` whose keys are the result of applying a key mapping function to the input elements, and whose values are the result of applying a value mapping function to all input elements equal to the key and combining them using the merge function

See also:

toConcurrentMap

Added in API level 24

public static 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.

There are no guarantees on the type, mutability, or serializability of the ConcurrentMap returned.

API Note:

There are multiple ways to deal with collisions between multiple elements mapping to the same key. The other forms of 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.

Parameters
`keyMapper`	`Function`: a mapping function to produce keys
`valueMapper`	`Function`: a mapping function to produce values
`mergeFunction`	`BinaryOperator`: a merge function, used to resolve collisions between values associated with the same key, as supplied to `Map#merge(Object, Object, BiFunction)`

Returns
`Collector<T, ?, ConcurrentMap<K, U>>`	a concurrent, unordered `Collector` which collects elements into a `ConcurrentMap` whose keys are the result of applying a key mapping function to the input elements, and whose values are the result of applying a value mapping function to all input elements equal to the key and combining them using the merge function

See also:

toList

Added in API level 24

public static 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(java.util.function.Supplier).

Returns
`Collector<T, ?, List<T>>`	a `Collector` which collects all the input elements into a `List`, in encounter order

toMap

Added in API level 24

public static Collector<T, ?, M> toMap (Function<? super T, ? extends K> keyMapper, 
                Function<? super T, ? extends U> valueMapper, 
                BinaryOperator<U> mergeFunction, 
                Supplier<M> mapFactory)

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.

Implementation Note:

The returned 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(java.util.function.Function, java.util.function.Function, java.util.function.BinaryOperator, java.util.function.Supplier) may offer better parallel performance.

Parameters
`keyMapper`	`Function`: a mapping function to produce keys
`valueMapper`	`Function`: a mapping function to produce values
`mergeFunction`	`BinaryOperator`: a merge function, used to resolve collisions between values associated with the same key, as supplied to `Map#merge(Object, Object, BiFunction)`
`mapFactory`	`Supplier`: a supplier providing a new empty `Map` into which the results will be inserted

Returns
`Collector<T, ?, M>`	a `Collector` which collects elements into a `Map` whose keys are the result of applying a key mapping function to the input elements, and whose values are the result of applying a value mapping function to all input elements equal to the key and combining them using the merge function

See also:

toMap

Added in API level 24

public static 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.

There are no guarantees on the type, mutability, serializability, or thread-safety of the Map returned.

API Note:

There are multiple ways to deal with collisions between multiple elements mapping to the same key. The other forms of 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));
 
```

Implementation Note:

The returned 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(java.util.function.Function, java.util.function.Function, java.util.function.BinaryOperator) may offer better parallel performance.

Parameters
`keyMapper`	`Function`: a mapping function to produce keys
`valueMapper`	`Function`: a mapping function to produce values
`mergeFunction`	`BinaryOperator`: a merge function, used to resolve collisions between values associated with the same key, as supplied to `Map#merge(Object, Object, BiFunction)`

Returns
`Collector<T, ?, Map<K, U>>`	a `Collector` which collects elements into a `Map` whose keys are the result of applying a key mapping function to the input elements, and whose values are the result of applying a value mapping function to all input elements equal to the key and combining them using the merge function

See also:

toMap

Added in API level 24

public static 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 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(java.util.function.Function, java.util.function.Function, java.util.function.BinaryOperator) instead.

There are no guarantees on the type, mutability, serializability, or thread-safety of the Map returned.

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 a Map mapping students to their grade point average:
```
Map<Student, Double> studentToGPA
   = students.stream().collect(
     toMap(Function.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,
           Function.identity()));
 
```

Implementation Note:

The returned 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(java.util.function.Function, java.util.function.Function) may offer better parallel performance.

Parameters
`keyMapper`	`Function`: a mapping function to produce keys
`valueMapper`	`Function`: a mapping function to produce values

Returns
`Collector<T, ?, Map<K, U>>`	a `Collector` which collects elements into a `Map` whose keys and values are the result of applying mapping functions to the input elements

See also:

toSet

Added in API level 24

public static 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(java.util.function.Supplier).

This is an unordered Collector.

Returns
`Collector<T, ?, Set<T>>`	a `Collector` which collects all the input elements into a `Set`

toUnmodifiableList

Added in API level 33

public static Collector<T, ?, List<T>> toUnmodifiableList ()

Returns a Collector that accumulates the input elements into an unmodifiable List in encounter order. The returned Collector disallows null values and will throw NullPointerException if it is presented with a null value.

Returns
`Collector<T, ?, List<T>>`	a `Collector` that accumulates the input elements into an unmodifiable List in encounter order

toUnmodifiableMap

Added in API level 33

public static Collector<T, ?, Map<K, U>> toUnmodifiableMap (Function<? super T, ? extends K> keyMapper, 
                Function<? super T, ? extends U> valueMapper, 
                BinaryOperator<U> mergeFunction)

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.

The returned Collector disallows null keys and values. If either mapping function returns null, NullPointerException will be thrown.

Parameters
`keyMapper`	`Function`: a mapping function to produce keys, must be non-null
`valueMapper`	`Function`: a mapping function to produce values, must be non-null
`mergeFunction`	`BinaryOperator`: 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

Returns
`Collector<T, ?, Map<K, U>>`	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

Throws
`NullPointerException`	if the keyMapper, valueMapper, or mergeFunction is null

See also:

toUnmodifiableMap(Function, Function)

toUnmodifiableMap

Added in API level 33

public static Collector<T, ?, Map<K, U>> toUnmodifiableMap (Function<? super T, ? extends K> keyMapper, 
                Function<? super T, ? extends U> valueMapper)

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(java.util.function.Function, java.util.function.Function, java.util.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.

Parameters
`keyMapper`	`Function`: a mapping function to produce keys, must be non-null
`valueMapper`	`Function`: a mapping function to produce values, must be non-null

Returns
`Collector<T, ?, Map<K, U>>`	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

Throws
`NullPointerException`	if either keyMapper or valueMapper is null

See also:

toUnmodifiableMap(Function, Function, BinaryOperator)

toUnmodifiableSet

Added in API level 33

public static Collector<T, ?, Set<T>> toUnmodifiableSet ()

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.

Returns
`Collector<T, ?, Set<T>>`	a `Collector` that accumulates the input elements into an unmodifiable Set