 Type Parameters:
T
 the type of input elements to the reduction operationA
 the mutable accumulation type of the reduction operation (often hidden as an implementation detail)R
 the result type of the reduction operation
Examples of mutable reduction operations include:
accumulating elements into a Collection
; concatenating
strings using a StringBuilder
; computing summary information about
elements such as sum, min, max, or average; computing "pivot table" summaries
such as "maximum valued transaction by seller", etc. The class Collectors
provides implementations of many common mutable reductions.
A Collector
is specified by four functions that work together to
accumulate entries into a mutable result container, and optionally perform
a final transform on the result. They are:
 creation of a new result container (
supplier()
)  incorporating a new data element into a result container (
accumulator()
)  combining two result containers into one (
combiner()
)  performing an optional final transform on the container (
finisher()
)
Collectors also have a set of characteristics, such as
Collector.Characteristics.CONCURRENT
, that provide hints that can be used by a
reduction implementation to provide better performance.
A sequential implementation of a reduction using a collector would create a single result container using the supplier function, and invoke the accumulator function once for each input element. A parallel implementation would partition the input, create a result container for each partition, accumulate the contents of each partition into a subresult for that partition, and then use the combiner function to merge the subresults into a combined result.
To ensure that sequential and parallel executions produce equivalent results, the collector functions must satisfy an identity and an associativity constraints.
The identity constraint says that for any partially accumulated result,
combining it with an empty result container must produce an equivalent
result. That is, for a partially accumulated result a
that is the
result of any series of accumulator and combiner invocations, a
must
be equivalent to combiner.apply(a, supplier.get())
.
The associativity constraint says that splitting the computation must
produce an equivalent result. That is, for any input elements t1
and t2
, the results r1
and r2
in the computation
below must be equivalent:
A a1 = supplier.get();
accumulator.accept(a1, t1);
accumulator.accept(a1, t2);
R r1 = finisher.apply(a1); // result without splitting
A a2 = supplier.get();
accumulator.accept(a2, t1);
A a3 = supplier.get();
accumulator.accept(a3, t2);
R r2 = finisher.apply(combiner.apply(a2, a3)); // result with splitting
For collectors that do not have the UNORDERED
characteristic,
two accumulated results a1
and a2
are equivalent if
finisher.apply(a1).equals(finisher.apply(a2))
. For unordered
collectors, equivalence is relaxed to allow for nonequality related to
differences in order. (For example, an unordered collector that accumulated
elements to a List
would consider two lists equivalent if they
contained the same elements, ignoring order.)
Libraries that implement reduction based on Collector
, such as
Stream.collect(Collector)
, must adhere to the following constraints:
 The first argument passed to the accumulator function, both arguments passed to the combiner function, and the argument passed to the finisher function must be the result of a previous invocation of the result supplier, accumulator, or combiner functions.
 The implementation should not do anything with the result of any of the result supplier, accumulator, or combiner functions other than to pass them again to the accumulator, combiner, or finisher functions, or return them to the caller of the reduction operation.
 If a result is passed to the combiner or finisher function, and the same object is not returned from that function, it is never used again.
 Once a result is passed to the combiner or finisher function, it is never passed to the accumulator function again.
 For nonconcurrent collectors, any result returned from the result
supplier, accumulator, or combiner functions must be serially
threadconfined. This enables collection to occur in parallel without
the
Collector
needing to implement any additional synchronization. The reduction implementation must manage that the input is properly partitioned, that partitions are processed in isolation, and combining happens only after accumulation is complete.  For concurrent collectors, an implementation is free to (but not
required to) implement reduction concurrently. A concurrent reduction
is one where the accumulator function is called concurrently from
multiple threads, using the same concurrentlymodifiable result container,
rather than keeping the result isolated during accumulation.
A concurrent reduction should only be applied if the collector has the
Collector.Characteristics.UNORDERED
characteristics or if the originating data is unordered.
In addition to the predefined implementations in Collectors
, the
static factory methods of(Supplier, BiConsumer, BinaryOperator, Characteristics...)
can be used to construct collectors. For example, you could create a collector
that accumulates widgets into a TreeSet
with:
Collector<Widget, ?, TreeSet<Widget>> intoSet =
Collector.of(TreeSet::new, TreeSet::add,
(left, right) > { left.addAll(right); return left; });
(This behavior is also implemented by the predefined collector
Collectors.toCollection(Supplier)
). API Note:
 Performing a reduction operation with a
Collector
should produce a result equivalent to:A container = collector.supplier().get(); for (T t : data) collector.accumulator().accept(container, t); return collector.finisher().apply(container);
However, the library is free to partition the input, perform the reduction on the partitions, and then use the combiner function to combine the partial results to achieve a parallel reduction. (Depending on the specific reduction operation, this may perform better or worse, depending on the relative cost of the accumulator and combiner functions.)
Collectors are designed to be composed; many of the methods in
Collectors
are functions that take a collector and produce a new collector. For example, given the following collector that computes the sum of the salaries of a stream of employees:
If we wanted to create a collector to tabulate the sum of salaries by department, we could reuse the "sum of salaries" logic usingCollector<Employee, ?, Integer> summingSalaries = Collectors.summingInt(Employee::getSalary))
Collectors.groupingBy(Function, Collector)
:Collector<Employee, ?, Map<Department, Integer>> summingSalariesByDept = Collectors.groupingBy(Employee::getDepartment, summingSalaries);
 Since:
 1.8
 See Also:

Nested Class Summary
Modifier and TypeInterfaceDescriptionstatic enum
Characteristics indicating properties of aCollector
, which can be used to optimize reduction implementations. 
Method Summary
Modifier and TypeMethodDescriptionBiConsumer
<A, T> A function that folds a value into a mutable result container.Returns aSet
ofCollector.Characteristics
indicating the characteristics of this Collector.combiner()
A function that accepts two partial results and merges them.finisher()
Perform the final transformation from the intermediate accumulation typeA
to the final result typeR
.static <T,
A, R> Collector <T, A, R> of
(Supplier<A> supplier, BiConsumer<A, T> accumulator, BinaryOperator<A> combiner, Function<A, R> finisher, Collector.Characteristics... characteristics) Returns a newCollector
described by the givensupplier
,accumulator
,combiner
, andfinisher
functions.static <T,
R> Collector <T, R, R> of
(Supplier<R> supplier, BiConsumer<R, T> accumulator, BinaryOperator<R> combiner, Collector.Characteristics... characteristics) Returns a newCollector
described by the givensupplier
,accumulator
, andcombiner
functions.supplier()
A function that creates and returns a new mutable result container.

Method Details

supplier
A function that creates and returns a new mutable result container. Returns:
 a function which returns a new, mutable result container

accumulator
BiConsumer<A,T> accumulator()A function that folds a value into a mutable result container. Returns:
 a function which folds a value into a mutable result container

combiner
BinaryOperator<A> combiner()A function that accepts two partial results and merges them. The combiner function may fold state from one argument into the other and return that, or may return a new result container. Returns:
 a function which combines two partial results into a combined result

finisher
Perform the final transformation from the intermediate accumulation typeA
to the final result typeR
.If the characteristic
IDENTITY_FINISH
is set, this function may be presumed to be an identity transform with an unchecked cast fromA
toR
. Returns:
 a function which transforms the intermediate result to the final result

characteristics
Set<Collector.Characteristics> characteristics()Returns aSet
ofCollector.Characteristics
indicating the characteristics of this Collector. This set should be immutable. Returns:
 an immutable set of collector characteristics

of
static <T,R> Collector<T,R, ofR> (Supplier<R> supplier, BiConsumer<R, T> accumulator, BinaryOperator<R> combiner, Collector.Characteristics... characteristics) Returns a newCollector
described by the givensupplier
,accumulator
, andcombiner
functions. The resultingCollector
has theCollector.Characteristics.IDENTITY_FINISH
characteristic. Type Parameters:
T
 The type of input elements for the new collectorR
 The type of intermediate accumulation result, and final result, for the new collector Parameters:
supplier
 The supplier function for the new collectoraccumulator
 The accumulator function for the new collectorcombiner
 The combiner function for the new collectorcharacteristics
 The collector characteristics for the new collector Returns:
 the new
Collector
 Throws:
NullPointerException
 if any argument is null

of
static <T,A, Collector<T,R> A, ofR> (Supplier<A> supplier, BiConsumer<A, T> accumulator, BinaryOperator<A> combiner, Function<A, R> finisher, Collector.Characteristics... characteristics) Returns a newCollector
described by the givensupplier
,accumulator
,combiner
, andfinisher
functions. Type Parameters:
T
 The type of input elements for the new collectorA
 The intermediate accumulation type of the new collectorR
 The final result type of the new collector Parameters:
supplier
 The supplier function for the new collectoraccumulator
 The accumulator function for the new collectorcombiner
 The combiner function for the new collectorfinisher
 The finisher function for the new collectorcharacteristics
 The collector characteristics for the new collector Returns:
 the new
Collector
 Throws:
NullPointerException
 if any argument is null
