Module java.base
Package java.util

Interface Spliterator<T>

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
public interface Spliterator<T>
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 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.OfInt.tryAdvance(java.util.function.IntConsumer) and Spliterator.OfInt.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.

Since:
1.8
See Also:

  • Nested Class Summary

    Nested Classes
    Modifier and Type
    Interface
    Description
    static interface 
    Spliterator.OfDouble
    A Spliterator specialized for double values.
    static interface 
    Spliterator.OfInt
    A Spliterator specialized for int values.
    static interface 
    Spliterator.OfLong
    A Spliterator specialized for long values.
    static interface 
    Spliterator.OfPrimitive<T,T_CONS,T_SPLITR extends Spliterator.OfPrimitive<T,T_CONS,T_SPLITR>>
    A Spliterator specialized for primitive values.

  • Field Summary

    Fields
    Modifier and Type
    Field
    Description
    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.
    static final int
    DISTINCT
    Characteristic value signifying that, for each pair of encountered elements x, y, !x.equals(y).
    static final 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.
    static final int
    NONNULL
    Characteristic value signifying that the source guarantees that encountered elements will not be null.
    static final int
    ORDERED
    Characteristic value signifying that an encounter order is defined for elements.
    static final 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.
    static final int
    SORTED
    Characteristic value signifying that encounter order follows a defined sort order.
    static final int
    SUBSIZED
    Characteristic value signifying that all Spliterators resulting from trySplit() will be both SIZED and SUBSIZED.

  • Method Summary

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

  • Field Details

    • ORDERED

      static final int ORDERED
      Characteristic value signifying that an encounter order is defined for elements. If so, this Spliterator guarantees that method trySplit() splits a strict prefix of elements, that method tryAdvance(java.util.function.Consumer<? super T>) steps by one element in prefix order, and that forEachRemaining(java.util.function.Consumer<? super T>) performs actions in encounter order.

      A Collection has an encounter order if the corresponding Collection.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.

      See Also:

    • DISTINCT

      static final int DISTINCT
      Characteristic value signifying that, for each pair of encountered elements x, y, !x.equals(y). This applies for example, to a Spliterator based on a Set.
      See Also:

    • SORTED

      static final int SORTED
      Characteristic value signifying that encounter order follows a defined sort order. If so, method getComparator() returns the associated Comparator, or null if all elements are Comparable and are sorted by their natural ordering.

      A Spliterator that reports SORTED must also report ORDERED.

      See Also:

    • SIZED

      static final 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.
      See Also:

    • NONNULL

      static final int NONNULL
      Characteristic value signifying that the source guarantees that encountered elements will not be null. (This applies, for example, to most concurrent collections, queues, and maps.)
      See Also:

    • IMMUTABLE

      static final 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. A Spliterator that does not report IMMUTABLE or CONCURRENT is expected to have a documented policy (for example throwing ConcurrentModificationException) concerning structural interference detected during traversal.
      See Also:

    • CONCURRENT

      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.

      A top-level Spliterator should not report both CONCURRENT and IMMUTABLE, 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 report IMMUTABLE if additions or removals to the source are not reflected when traversing.

      See Also:

    • SUBSIZED

      static final int SUBSIZED
      Characteristic value signifying that all Spliterators resulting from trySplit() will be both SIZED and SUBSIZED. (This means that all child Spliterators, whether direct or indirect, will be SIZED.)

      A Spliterator that does not report SIZED as required by SUBSIZED is inconsistent and no guarantees can be made about any computation using that Spliterator.

      See Also:

  • Method Details

    • tryAdvance

      boolean tryAdvance(Consumer<? super T> action)
      If a remaining element exists, performs the given action on it, returning true; else returns false. If this Spliterator is ORDERED 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, else true.
      Throws:
      NullPointerException - if the specified action is null

    • forEachRemaining

      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. If this Spliterator is ORDERED, 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.

      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 return null. Upon non-null return:

      • the value reported for estimateSize() before splitting, must, after splitting, be greater than or equal to estimateSize() for this and the returned Spliterator; and
      • if this Spliterator is SUBSIZED, then estimateSize() for this spliterator before splitting must be equal to the sum of estimateSize() 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.

      Returns:
      a Spliterator covering some portion of the elements, or null if this spliterator cannot be split

    • estimateSize

      long estimateSize()
      Returns an estimate of the number of elements that would be encountered by a forEachRemaining(java.util.function.Consumer<? super T>) traversal, or returns Long.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 is SUBSIZED 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 of trySplit().

      Returns:
      the estimated size, or Long.MAX_VALUE if infinite, unknown, or too expensive to compute.

    • getExactSizeIfKnown

      default long getExactSizeIfKnown()
      Convenience method that returns estimateSize() if this Spliterator is SIZED, else -1.
      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 from ORDERED, DISTINCT, SORTED, SIZED, NONNULL, IMMUTABLE, CONCURRENT, SUBSIZED. Repeated calls to characteristics() on a given spliterator, prior to or in-between calls to trySplit, 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.

      Returns:
      a representation of characteristics

    • hasCharacteristics

      default boolean hasCharacteristics(int characteristics)
      Returns true if this Spliterator's characteristics() contain all of the given characteristics.
      Parameters:
      characteristics - the characteristics to check for
      Returns:
      true if all the specified characteristics are present, else false

    • getComparator

      default Comparator<? super T> getComparator()
      If this Spliterator's source is SORTED by a Comparator, returns that Comparator. If the source is SORTED in natural order, returns null. Otherwise, if the source is not SORTED, 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 of SORTED.