java11-spliterator

Overview of spliterator inferface.

Reference: https://docs.oracle.com/javase/8/docs/api/java/util/Spliterator.html
Reference: https://docs.oracle.com/javase/8/docs/api/java/util/stream/StreamSupport.html
Reference: https://www.baeldung.com/java-spliterator

preface

To see exemplary implementation of Spliterator please refer my other github project: https://github.com/mtumilowicz/java11-spliterator-forkjoin

Spliterator

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.

methods

boolean tryAdvance(Consumer<? super T> action); - If a remaining element exists, performs the given action on it, returning true; else returns false.
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.
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.

returns a Spliterator covering some portion of the elements, or null if this spliterator cannot be split
long estimateSize(); - Returns an estimate of the number of elements that would be encountered by a forEachRemaining 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.

default long getExactSizeIfKnown() - default implementation is self explanatory

default long getExactSizeIfKnown() {
        return (characteristics() & SIZED) == 0 ? -1L : estimateSize();
}

int characteristics(); - returns a set of characteristics of this Spliterator and its elements. The result is represented as ORed values (https://github.com/mtumilowicz/java11-ORed-container) from:
- 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 steps by one element in prefix order, and that forEachRemaining performs actions in encounter order.
- DISTINCT
  
  Characteristic value signifying that, for each pair of encountered elements !x.equals(y).
- 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.
- 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.
- NONNULL
  
  Characteristic value signifying that the source guarantees that encountered elements will not be null.
- 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.
- 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.
- 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.)
  
  Some spliterators, such as the top-level spliterator for an approximately balanced binary tree, will report SIZED but not SUBSIZED, since it is common to know the size of the entire tree but not the exact sizes of subtrees.
default boolean hasCharacteristics(int characteristics)
default Comparator<? super T> getComparator() - If this Spliterator's source is SORTED by a Comparator, returns that Comparator. If the source is SORTED in Comparable natural order, returns null. Otherwise, if the source is not SORTED, throws IllegalStateException.

Spliterator.OfPrimitive

A Spliterator specialized for primitive values.

Note that this interface extends Spliterator<T>.

We have three main primitive spliterators:

OfInt
OfLong
OfDouble

StreamSupport

Low-level utility methods for creating and manipulating streams.

This class is mostly for library writers presenting stream views of data structures; most static stream methods intended for end users are in the various Stream classes.

public static <T> Stream<T> stream(Spliterator<T> spliterator, boolean parallel)
public static <T> Stream<T> stream(Supplier<? extends Spliterator<T>> supplier, int characteristics, boolean parallel)
public static IntStream intStream(Spliterator.OfInt spliterator, boolean parallel)
public static IntStream intStream(Supplier<? extends Spliterator.OfInt> supplier, int characteristics, boolean parallel)
similar methods for long and double

Spliterators

Static classes and methods for operating on or creating instances of Spliterator and its primitive specializations like Spliterator.OfInt.

This method is provided as an implementation convenience for Spliterators which store portions of their elements in arrays, and need fine control over Spliterator characteristics. Most other situations in which a Spliterator for an array is needed should use Arrays#spliterator(T/int/double/long[]).

The returned spliterator always reports the characteristics SIZED and SUBSIZED. The caller may provide additional characteristics for the spliterator to report; it is common to additionally specify IMMUTABLE and ORDERED.

Spliterators.spliterator(new int[]{1, 2, 3}, IMMUTABLE | ORDERED);

// SIZED, SUBSIZED, ORDERED, IMMUTABLE
Spliterator.OfInt spliterator = Arrays.spliterator(new int[]{1, 2, 3});

project description

We provide tests for:

SpliteratorOfIntTest - example of methods of SpliteratorOfInt
SpliteratorTest - example of methods of SpliteratorTest
SpliteratorsTest - example of obtaining OfInt spliterator with dedicated characteristics.

Note that StreamSupport examples are in: https://github.com/mtumilowicz/java11-spliterator-forkjoin.

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