- Type Parameters:
T
- the type of elements returned by this Spliterator
- All Known Subinterfaces:
Spliterator.OfDouble
,Spliterator.OfInt
,Spliterator.OfLong
,Spliterator.OfPrimitive<T,
T_CONS, T_SPLITR>
- All Known Implementing Classes:
Spliterators.AbstractDoubleSpliterator
,Spliterators.AbstractIntSpliterator
,Spliterators.AbstractLongSpliterator
,Spliterators.AbstractSpliterator
Collection
, an IO channel, or a generator function.
A Spliterator may traverse elements individually (tryAdvance()
) or sequentially in bulk
(forEachRemaining()
).
A Spliterator may also partition off some of its elements (using
trySplit()
) as another Spliterator, to be used in
possibly-parallel operations. Operations using a Spliterator that
cannot split, or does so in a highly imbalanced or inefficient
manner, are unlikely to benefit from parallelism. Traversal
and splitting exhaust elements; each Spliterator is useful for only a single
bulk computation.
A Spliterator also reports a set of characteristics()
of its
structure, source, and elements from among ORDERED
,
DISTINCT
, SORTED
, SIZED
, NONNULL
,
IMMUTABLE
, CONCURRENT
, and SUBSIZED
. These may
be employed by Spliterator clients to control, specialize or simplify
computation. For example, a Spliterator for a Collection
would
report SIZED
, a Spliterator for a Set
would report
DISTINCT
, and a Spliterator for a SortedSet
would also
report SORTED
. Characteristics are reported as a simple unioned bit
set.
Some characteristics additionally constrain method behavior; for example if
ORDERED
, traversal methods must conform to their documented ordering.
New characteristics may be defined in the future, so implementors should not
assign meanings to unlisted values.
A Spliterator that does not report IMMUTABLE
or
CONCURRENT
is expected to have a documented policy concerning:
when the spliterator binds to the element source; and detection of
structural interference of the element source detected after binding. A
late-binding Spliterator binds to the source of elements at the
point of first traversal, first split, or first query for estimated size,
rather than at the time the Spliterator is created. A Spliterator that is
not late-binding binds to the source of elements at the point of
construction or first invocation of any method. Modifications made to the
source prior to binding are reflected when the Spliterator is traversed.
After binding a Spliterator should, on a best-effort basis, throw
ConcurrentModificationException
if structural interference is
detected. Spliterators that do this are called fail-fast. The
bulk traversal method (forEachRemaining()
) of a
Spliterator may optimize traversal and check for structural interference
after all elements have been traversed, rather than checking per-element and
failing immediately.
Spliterators can provide an estimate of the number of remaining elements
via the estimateSize()
method. Ideally, as reflected in characteristic
SIZED
, this value corresponds exactly to the number of elements
that would be encountered in a successful traversal. However, even when not
exactly known, an estimated value may still be useful to operations
being performed on the source, such as helping to determine whether it is
preferable to split further or traverse the remaining elements sequentially.
Despite their obvious utility in parallel algorithms, spliterators are not
expected to be thread-safe; instead, implementations of parallel algorithms
using spliterators should ensure that the spliterator is only used by one
thread at a time. This is generally easy to attain via serial
thread-confinement, which often is a natural consequence of typical
parallel algorithms that work by recursive decomposition. A thread calling
trySplit()
may hand over the returned Spliterator to another thread,
which in turn may traverse or further split that Spliterator. The behaviour
of splitting and traversal is undefined if two or more threads operate
concurrently on the same spliterator. If the original thread hands a
spliterator off to another thread for processing, it is best if that handoff
occurs before any elements are consumed with tryAdvance()
, as certain guarantees (such as the accuracy of
estimateSize()
for SIZED
spliterators) are only valid before
traversal has begun.
Primitive subtype specializations of Spliterator
are provided for
int
, long
, and double
values.
The subtype default implementations of
tryAdvance(java.util.function.Consumer)
and forEachRemaining(java.util.function.Consumer)
box
primitive values to instances of their corresponding wrapper class. Such
boxing may undermine any performance advantages gained by using the primitive
specializations. To avoid boxing, the corresponding primitive-based methods
should be used. For example,
Spliterator.OfPrimitive.tryAdvance(java.util.function.IntConsumer)
and Spliterator.OfPrimitive.forEachRemaining(java.util.function.IntConsumer)
should be used in preference to
Spliterator.OfInt.tryAdvance(java.util.function.Consumer)
and
Spliterator.OfInt.forEachRemaining(java.util.function.Consumer)
.
Traversal of primitive values using boxing-based methods
tryAdvance()
and
forEachRemaining()
does not affect the order in which the values, transformed to boxed values,
are encountered.
- API Note:
Spliterators, like
Iterator
s, are for traversing the elements of a source. TheSpliterator
API was designed to support efficient parallel traversal in addition to sequential traversal, by supporting decomposition as well as single-element iteration. In addition, the protocol for accessing elements via a Spliterator is designed to impose smaller per-element overhead thanIterator
, and to avoid the inherent race involved in having separate methods forhasNext()
andnext()
.For mutable sources, arbitrary and non-deterministic behavior may occur if the source is structurally interfered with (elements added, replaced, or removed) between the time that the Spliterator binds to its data source and the end of traversal. For example, such interference will produce arbitrary, non-deterministic results when using the
java.util.stream
framework.Structural interference of a source can be managed in the following ways (in approximate order of decreasing desirability):
- The source cannot be structurally interfered with.
For example, an instance ofCopyOnWriteArrayList
is an immutable source. A Spliterator created from the source reports a characteristic ofIMMUTABLE
. - The source manages concurrent modifications.
For example, a key set of aConcurrentHashMap
is a concurrent source. A Spliterator created from the source reports a characteristic ofCONCURRENT
. - The mutable source provides a late-binding and fail-fast Spliterator.
Late binding narrows the window during which interference can affect the calculation; fail-fast detects, on a best-effort basis, that structural interference has occurred after traversal has commenced and throwsConcurrentModificationException
. For example,ArrayList
, and many other non-concurrentCollection
classes in the JDK, provide a late-binding, fail-fast spliterator. - The mutable source provides a non-late-binding but fail-fast Spliterator.
The source increases the likelihood of throwingConcurrentModificationException
since the window of potential interference is larger. - The mutable source provides a late-binding and non-fail-fast Spliterator.
The source risks arbitrary, non-deterministic behavior after traversal has commenced since interference is not detected. - The mutable source provides a non-late-binding and non-fail-fast
Spliterator.
The source increases the risk of arbitrary, non-deterministic behavior since non-detected interference may occur after construction.
Example. Here is a class (not a very useful one, except for illustration) that maintains an array in which the actual data are held in even locations, and unrelated tag data are held in odd locations. Its Spliterator ignores the tags.
class TaggedArray<T> { private final Object[] elements; // immutable after construction TaggedArray(T[] data, Object[] tags) { int size = data.length; if (tags.length != size) throw new IllegalArgumentException(); this.elements = new Object[2 * size]; for (int i = 0, j = 0; i < size; ++i) { elements[j++] = data[i]; elements[j++] = tags[i]; } } public Spliterator<T> spliterator() { return new TaggedArraySpliterator<>(elements, 0, elements.length); } static class TaggedArraySpliterator<T> implements Spliterator<T> { private final Object[] array; private int origin; // current index, advanced on split or traversal private final int fence; // one past the greatest index TaggedArraySpliterator(Object[] array, int origin, int fence) { this.array = array; this.origin = origin; this.fence = fence; } public void forEachRemaining(Consumer<? super T> action) { for (; origin < fence; origin += 2) action.accept((T) array[origin]); } public boolean tryAdvance(Consumer<? super T> action) { if (origin < fence) { action.accept((T) array[origin]); origin += 2; return true; } else // cannot advance return false; } public Spliterator<T> trySplit() { int lo = origin; // divide range in half int mid = ((lo + fence) >>> 1) & ~1; // force midpoint to be even if (lo < mid) { // split out left half origin = mid; // reset this Spliterator's origin return new TaggedArraySpliterator<>(array, lo, mid); } else // too small to split return null; } public long estimateSize() { return (long)((fence - origin) / 2); } public int characteristics() { return ORDERED | SIZED | IMMUTABLE | SUBSIZED; } } }
As an example how a parallel computation framework, such as the
java.util.stream
package, would use Spliterator in a parallel computation, here is one way to implement an associated parallel forEach, that illustrates the primary usage idiom of splitting off subtasks until the estimated amount of work is small enough to perform sequentially. Here we assume that the order of processing across subtasks doesn't matter; different (forked) tasks may further split and process elements concurrently in undetermined order. This example uses aCountedCompleter
; similar usages apply to other parallel task constructions.static <T> void parEach(TaggedArray<T> a, Consumer<T> action) { Spliterator<T> s = a.spliterator(); long targetBatchSize = s.estimateSize() / (ForkJoinPool.getCommonPoolParallelism() * 8); new ParEach(null, s, action, targetBatchSize).invoke(); } static class ParEach<T> extends CountedCompleter<Void> { final Spliterator<T> spliterator; final Consumer<T> action; final long targetBatchSize; ParEach(ParEach<T> parent, Spliterator<T> spliterator, Consumer<T> action, long targetBatchSize) { super(parent); this.spliterator = spliterator; this.action = action; this.targetBatchSize = targetBatchSize; } public void compute() { Spliterator<T> sub; while (spliterator.estimateSize() > targetBatchSize && (sub = spliterator.trySplit()) != null) { addToPendingCount(1); new ParEach<>(this, sub, action, targetBatchSize).fork(); } spliterator.forEachRemaining(action); propagateCompletion(); } }
- The source cannot be structurally interfered with.
- Implementation Note:
- If the boolean system property
org.openjdk.java.util.stream.tripwire
is set totrue
then diagnostic warnings are reported if boxing of primitive values occur when operating on primitive subtype specializations. - Since:
- 1.8
- See Also:
-
Nested Class Summary
Modifier and TypeInterfaceDescriptionstatic interface
A Spliterator specialized fordouble
values.static interface
A Spliterator specialized forint
values.static interface
A Spliterator specialized forlong
values.static interface
A Spliterator specialized for primitive values. -
Field Summary
Modifier and TypeFieldDescriptionstatic final int
Characteristic value signifying that the element source may be safely concurrently modified (allowing additions, replacements, and/or removals) by multiple threads without external synchronization.static final int
Characteristic value signifying that, for each pair of encountered elementsx, y
,!x.equals(y)
.static final int
Characteristic value signifying that the element source cannot be structurally modified; that is, elements cannot be added, replaced, or removed, so such changes cannot occur during traversal.static final int
Characteristic value signifying that the source guarantees that encountered elements will not benull
.static final int
Characteristic value signifying that an encounter order is defined for elements.static final int
Characteristic value signifying that the value returned fromestimateSize()
prior to traversal or splitting represents a finite size that, in the absence of structural source modification, represents an exact count of the number of elements that would be encountered by a complete traversal.static final int
Characteristic value signifying that encounter order follows a defined sort order.static final int
-
Method Summary
Modifier and TypeMethodDescriptionint
Returns a set of characteristics of this Spliterator and its elements.long
Returns an estimate of the number of elements that would be encountered by aforEachRemaining(java.util.function.Consumer<? super T>)
traversal, or returnsLong.MAX_VALUE
if infinite, unknown, or too expensive to compute.default void
forEachRemaining
(Consumer<? super T> action) Performs the given action for each remaining element, sequentially in the current thread, until all elements have been processed or the action throws an exception.default Comparator
<? super T> default long
default boolean
hasCharacteristics
(int characteristics) Returnstrue
if this Spliterator'scharacteristics()
contain all of the given characteristics.boolean
tryAdvance
(Consumer<? super T> action) If a remaining element exists, performs the given action on it, returningtrue
; else returnsfalse
.trySplit()
If this spliterator can be partitioned, returns a Spliterator covering elements, that will, upon return from this method, not be covered by this Spliterator.
-
Field Details
-
ORDERED
static final int ORDEREDCharacteristic value signifying that an encounter order is defined for elements. If so, this Spliterator guarantees that methodtrySplit()
splits a strict prefix of elements, that methodtryAdvance(java.util.function.Consumer<? super T>)
steps by one element in prefix order, and thatforEachRemaining(java.util.function.Consumer<? super T>)
performs actions in encounter order.A
Collection
has an encounter order if the correspondingCollection.iterator()
documents an order. If so, the encounter order is the same as the documented order. Otherwise, a collection does not have an encounter order. -
DISTINCT
static final int DISTINCTCharacteristic value signifying that, for each pair of encountered elementsx, y
,!x.equals(y)
. This applies for example, to a Spliterator based on aSet
.- See Also:
-
SORTED
static final int SORTEDCharacteristic value signifying that encounter order follows a defined sort order. If so, methodgetComparator()
returns the associated Comparator, ornull
if all elements areComparable
and are sorted by their natural ordering.A Spliterator that reports
SORTED
must also reportORDERED
.- API Note:
- The spliterators for
Collection
classes in the JDK that implementNavigableSet
orSortedSet
reportSORTED
. - See Also:
-
SIZED
static final int SIZEDCharacteristic value signifying that the value returned fromestimateSize()
prior to traversal or splitting represents a finite size that, in the absence of structural source modification, represents an exact count of the number of elements that would be encountered by a complete traversal.- API Note:
- Most Spliterators for Collections, that cover all elements of a
Collection
report this characteristic. Sub-spliterators, such as those forHashSet
, that cover a sub-set of elements and approximate their reported size do not. - See Also:
-
NONNULL
static final int NONNULLCharacteristic value signifying that the source guarantees that encountered elements will not benull
. (This applies, for example, to most concurrent collections, queues, and maps.)- See Also:
-
IMMUTABLE
static final int IMMUTABLECharacteristic value signifying that the element source cannot be structurally modified; that is, elements cannot be added, replaced, or removed, so such changes cannot occur during traversal. A Spliterator that does not reportIMMUTABLE
orCONCURRENT
is expected to have a documented policy (for example throwingConcurrentModificationException
) concerning structural interference detected during traversal.- See Also:
-
CONCURRENT
static final int CONCURRENTCharacteristic value signifying that the element source may be safely concurrently modified (allowing additions, replacements, and/or removals) by multiple threads without external synchronization. If so, the Spliterator is expected to have a documented policy concerning the impact of modifications during traversal.A top-level Spliterator should not report both
CONCURRENT
andSIZED
, since the finite size, if known, may change if the source is concurrently modified during traversal. Such a Spliterator is inconsistent and no guarantees can be made about any computation using that Spliterator. Sub-spliterators may reportSIZED
if the sub-split size is known and additions or removals to the source are not reflected when traversing.A top-level Spliterator should not report both
CONCURRENT
andIMMUTABLE
, since they are mutually exclusive. Such a Spliterator is inconsistent and no guarantees can be made about any computation using that Spliterator. Sub-spliterators may reportIMMUTABLE
if additions or removals to the source are not reflected when traversing.- API Note:
- Most concurrent collections maintain a consistency policy guaranteeing accuracy with respect to elements present at the point of Spliterator construction, but possibly not reflecting subsequent additions or removals.
- See Also:
-
SUBSIZED
static final int SUBSIZEDCharacteristic value signifying that all Spliterators resulting fromtrySplit()
will be bothSIZED
andSUBSIZED
. (This means that all child Spliterators, whether direct or indirect, will beSIZED
.)A Spliterator that does not report
SIZED
as required bySUBSIZED
is inconsistent and no guarantees can be made about any computation using that Spliterator.- API Note:
- Some spliterators, such as the top-level spliterator for an
approximately balanced binary tree, will report
SIZED
but notSUBSIZED
, since it is common to know the size of the entire tree but not the exact sizes of subtrees. - See Also:
-
-
Method Details
-
tryAdvance
If a remaining element exists, performs the given action on it, returningtrue
; else returnsfalse
. If this Spliterator isORDERED
the action is performed on the next element in encounter order. Exceptions thrown by the action are relayed to the caller.Subsequent behavior of a spliterator is unspecified if the action throws an exception.
- Parameters:
action
- The action- Returns:
false
if no remaining elements existed upon entry to this method, elsetrue
.- Throws:
NullPointerException
- if the specified action is null
-
forEachRemaining
Performs the given action for each remaining element, sequentially in the current thread, until all elements have been processed or the action throws an exception. If this Spliterator isORDERED
, actions are performed in encounter order. Exceptions thrown by the action are relayed to the caller.Subsequent behavior of a spliterator is unspecified if the action throws an exception.
- Implementation Requirements:
- The default implementation repeatedly invokes
tryAdvance(java.util.function.Consumer<? super T>)
until it returnsfalse
. It should be overridden whenever possible. - Parameters:
action
- The action- Throws:
NullPointerException
- if the specified action is null
-
trySplit
Spliterator<T> trySplit()If this spliterator can be partitioned, returns a Spliterator covering elements, that will, upon return from this method, not be covered by this Spliterator.If this Spliterator is
ORDERED
, the returned Spliterator must cover a strict prefix of the elements.Unless this Spliterator covers an infinite number of elements, repeated calls to
trySplit()
must eventually returnnull
. Upon non-null return:- the value reported for
estimateSize()
before splitting, must, after splitting, be greater than or equal toestimateSize()
for this and the returned Spliterator; and - if this Spliterator is
SUBSIZED
, thenestimateSize()
for this spliterator before splitting must be equal to the sum ofestimateSize()
for this and the returned Spliterator after splitting.
This method may return
null
for any reason, including emptiness, inability to split after traversal has commenced, data structure constraints, and efficiency considerations.- API Note:
- An ideal
trySplit
method efficiently (without traversal) divides its elements exactly in half, allowing balanced parallel computation. Many departures from this ideal remain highly effective; for example, only approximately splitting an approximately balanced tree, or for a tree in which leaf nodes may contain either one or two elements, failing to further split these nodes. However, large deviations in balance and/or overly inefficienttrySplit
mechanics typically result in poor parallel performance. - Returns:
- a
Spliterator
covering some portion of the elements, ornull
if this spliterator cannot be split
- the value reported for
-
estimateSize
long estimateSize()Returns an estimate of the number of elements that would be encountered by aforEachRemaining(java.util.function.Consumer<? super T>)
traversal, or returnsLong.MAX_VALUE
if infinite, unknown, or too expensive to compute.If this Spliterator is
SIZED
and has not yet been partially traversed or split, or this Spliterator isSUBSIZED
and has not yet been partially traversed, this estimate must be an accurate count of elements that would be encountered by a complete traversal. Otherwise, this estimate may be arbitrarily inaccurate, but must decrease as specified across invocations oftrySplit()
.- API Note:
- Even an inexact estimate is often useful and inexpensive to compute. For example, a sub-spliterator of an approximately balanced binary tree may return a value that estimates the number of elements to be half of that of its parent; if the root Spliterator does not maintain an accurate count, it could estimate size to be the power of two corresponding to its maximum depth.
- Returns:
- the estimated size, or
Long.MAX_VALUE
if infinite, unknown, or too expensive to compute.
-
getExactSizeIfKnown
default long getExactSizeIfKnown()- Implementation Requirements:
- The default implementation returns the result of
estimateSize()
if the Spliterator reports a characteristic ofSIZED
, and-1
otherwise. - Returns:
- the exact size, if known, else
-1
.
-
characteristics
int characteristics()Returns a set of characteristics of this Spliterator and its elements. The result is represented as ORed values fromORDERED
,DISTINCT
,SORTED
,SIZED
,NONNULL
,IMMUTABLE
,CONCURRENT
,SUBSIZED
. Repeated calls tocharacteristics()
on a given spliterator, prior to or in-between calls totrySplit
, should always return the same result.If a Spliterator reports an inconsistent set of characteristics (either those returned from a single invocation or across multiple invocations), no guarantees can be made about any computation using this Spliterator.
- API Note:
- The characteristics of a given spliterator before splitting
may differ from the characteristics after splitting. For specific
examples see the characteristic values
SIZED
,SUBSIZED
andCONCURRENT
. - Returns:
- a representation of characteristics
-
hasCharacteristics
default boolean hasCharacteristics(int characteristics) Returnstrue
if this Spliterator'scharacteristics()
contain all of the given characteristics.- Implementation Requirements:
- The default implementation returns true if the corresponding bits of the given characteristics are set.
- Parameters:
characteristics
- the characteristics to check for- Returns:
true
if all the specified characteristics are present, elsefalse
-
getComparator
If this Spliterator's source isSORTED
by aComparator
, returns thatComparator
. If the source isSORTED
in natural order, returnsnull
. Otherwise, if the source is notSORTED
, throwsIllegalStateException
.- Implementation Requirements:
- The default implementation always throws
IllegalStateException
. - Returns:
- a Comparator, or
null
if the elements are sorted in the natural order. - Throws:
IllegalStateException
- if the spliterator does not report a characteristic ofSORTED
.
-