Spliterator

public interface Spliterator
Known Indirect Subclasses

An object for traversing and partitioning elements of a source. The source of elements covered by a Spliterator could be, for example, an array, a 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 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, tryAdvance(java.util.function.IntConsumer) and forEachRemaining(java.util.function.IntConsumer) should be used in preference to tryAdvance(java.util.function.Consumer) and 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.

See Also

Nested Class Summary

interface Spliterator.OfDouble A Spliterator specialized for double values. 
interface Spliterator.OfInt A Spliterator specialized for int values. 
interface Spliterator.OfLong A Spliterator specialized for long values. 
interface Spliterator.OfPrimitive<T, T_CONS, T_SPLITR extends OfPrimitive<T, T_CONS, T_SPLITR>> A Spliterator specialized for primitive values. 

Constant Summary

int CONCURRENT Characteristic value signifying that the element source may be safely concurrently modified (allowing additions, replacements, and/or removals) by multiple threads without external synchronization.
int DISTINCT Characteristic value signifying that, for each pair of encountered elements x, y, !x.equals(y).
int IMMUTABLE 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.
int NONNULL Characteristic value signifying that the source guarantees that encountered elements will not be null.
int ORDERED Characteristic value signifying that an encounter order is defined for elements.
int SIZED Characteristic value signifying that the value returned from estimateSize() 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.
int SORTED Characteristic value signifying that encounter order follows a defined sort order.
int SUBSIZED Characteristic value signifying that all Spliterators resulting from trySplit() will be both SIZED and SUBSIZED.

Public Method Summary

abstract int
characteristics()
Returns a set of characteristics of this Spliterator and its elements.
abstract long
estimateSize()
Returns an estimate of the number of elements that would be encountered by a forEachRemaining(Consumer) traversal, or returns MAX_VALUE if infinite, unknown, or too expensive to compute.
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.
Comparator<? super T>
getComparator()
If this Spliterator's source is SORTED by a Comparator, returns that Comparator.
long
getExactSizeIfKnown()
Convenience method that returns estimateSize() if this Spliterator is SIZED, else -1.
boolean
hasCharacteristics(int characteristics)
Returns true if this Spliterator's characteristics() contain all of the given characteristics.
abstract boolean
tryAdvance(Consumer<? super T> action)
If a remaining element exists, performs the given action on it, returning true; else returns false.
abstract 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.

Constants

public static final int CONCURRENT

Characteristic 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 and SIZED, 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 report SIZED if the sub-split size is known and additions or removals to the source are not reflected when traversing.

Constant Value: