Collector
public
interface
Collector
java.util.stream.Collector<T, A, R> 
A mutable reduction operation that accumulates input elements into a mutable result container, optionally transforming the accumulated result into a final representation after all input elements have been processed. Reduction operations can be performed either sequentially or in parallel.
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
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
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
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
toCollection(Supplier)
).
See also:
Summary
Nested classes  

enum 
Collector.Characteristics
Characteristics indicating properties of a 
Public methods  

abstract
BiConsumer<A, T>

accumulator()
A function that folds a value into a mutable result container. 
abstract
Set<Collector.Characteristics>

characteristics()
Returns a 
abstract
BinaryOperator<A>

combiner()
A function that accepts two partial results and merges them. 
abstract
Function<A, R>

finisher()
Perform the final transformation from the intermediate accumulation type

static
<T, A, R>
Collector<T, A, R>

of(Supplier<A> supplier, BiConsumer<A, T> accumulator, BinaryOperator<A> combiner, Function<A, R> finisher, Characteristics... characteristics)
Returns a new 
static
<T, R>
Collector<T, R, R>

of(Supplier<R> supplier, BiConsumer<R, T> accumulator, BinaryOperator<R> combiner, Characteristics... characteristics)
Returns a new 
abstract
Supplier<A>

supplier()
A function that creates and returns a new mutable result container. 
Public methods
accumulator
BiConsumer<A, T> accumulator ()
A function that folds a value into a mutable result container.
Returns  

BiConsumer<A, T> 
a function which folds a value into a mutable result container 
characteristics
Set<Collector.Characteristics> characteristics ()
Returns a Set
of Collector.Characteristics
indicating
the characteristics of this Collector. This set should be immutable.
Returns  

Set<Collector.Characteristics> 
an immutable set of collector characteristics 
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  

BinaryOperator<A> 
a function which combines two partial results into a combined result 
finisher
Function<A, R> finisher ()
Perform the final transformation from the intermediate accumulation type
A
to the final result type R
.
If the characteristic IDENTITY_TRANSFORM
is
set, this function may be presumed to be an identity transform with an
unchecked cast from A
to R
.
Returns  

Function<A, R> 
a function which transforms the intermediate result to the final result 
of
Collector<T, A, R> of (Supplier<A> supplier, BiConsumer<A, T> accumulator, BinaryOperator<A> combiner, Function<A, R> finisher, Characteristics... characteristics)
Returns a new Collector
described by the given supplier
,
accumulator
, combiner
, and finisher
functions.
Parameters  

supplier 
Supplier : The supplier function for the new collector 
accumulator 
BiConsumer : The accumulator function for the new collector 
combiner 
BinaryOperator : The combiner function for the new collector 
finisher 
Function : The finisher function for the new collector 
characteristics 
Characteristics : The collector characteristics for the new
collector 
Returns  

Collector<T, A, R> 
the new Collector

Throws  

NullPointerException 
if any argument is null 
of
Collector<T, R, R> of (Supplier<R> supplier, BiConsumer<R, T> accumulator, BinaryOperator<R> combiner, Characteristics... characteristics)
Returns a new Collector
described by the given supplier
,
accumulator
, and combiner
functions. The resulting
Collector
has the Collector.Characteristics.IDENTITY_FINISH
characteristic.
Parameters  

supplier 
Supplier : The supplier function for the new collector 
accumulator 
BiConsumer : The accumulator function for the new collector 
combiner 
BinaryOperator : The combiner function for the new collector 
characteristics 
Characteristics : The collector characteristics for the new
collector 
Returns  

Collector<T, R, R> 
the new Collector

Throws  

NullPointerException 
if any argument is null 
supplier
Supplier<A> supplier ()
A function that creates and returns a new mutable result container.
Returns  

Supplier<A> 
a function which returns a new, mutable result container 