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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

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 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

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 * accompanied this code).

 *

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package java.util.stream;

import java.util.Arrays;

import java.util.Comparator;

import java.util.Iterator;

import java.util.Objects;

import java.util.Optional;

import java.util.Spliterator;

import java.util.Spliterators;

import java.util.function.BiConsumer;

import java.util.function.BiFunction;

import java.util.function.BinaryOperator;

import java.util.function.Consumer;

import java.util.function.Function;

import java.util.function.IntFunction;

import java.util.function.Predicate;

import java.util.function.Supplier;

import java.util.function.ToDoubleFunction;

import java.util.function.ToIntFunction;

import java.util.function.ToLongFunction;

import java.util.function.UnaryOperator;

// @@@ Specification to-do list @@@

// - Describe the difference between sequential and parallel streams

// - More general information about reduce, better definitions for associativity, more description of

//   how reduce employs parallelism, more examples

// - Role of stream flags in various operations, specifically ordering

//   - Whether each op preserves encounter order

// @@@ Specification to-do list @@@

/**

 * A sequence of elements supporting sequential and parallel bulk operations.

 * Streams support lazy intermediate operations (transforming a stream to

 * another stream) such as {@code filter} and {@code map}, and terminal

 * operations (consuming the contents of a stream to produce a result or

 * side-effect), such as {@code forEach}, {@code findFirst}, and {@code

 * iterator}.  Once an operation has been performed on a stream, it

 * is considered <em>consumed</em> and no longer usable for other operations.

 *

 * <p>For sequential stream pipelines, all operations are performed in the

 * <a href="package-summary.html#Ordering">encounter order</a> of the pipeline

 * source, if the pipeline source has a defined encounter order.

 *

 * <p>For parallel stream pipelines, unless otherwise specified, intermediate

 * stream operations preserve the <a href="package-summary.html#Ordering">

 * encounter order</a> of their source, and terminal operations

 * respect the encounter order of their source, if the source

 * has an encounter order.  Provided that and parameters to stream operations

 * satisfy the <a href="package-summary.html#NonInterference">non-interference

 * requirements</a>, and excepting differences arising from the absence of

 * a defined encounter order, the result of a stream pipeline should be the

 * stable across multiple executions of the same operations on the same source.

 * However, the timing and thread in which side-effects occur (for those

 * operations which are allowed to produce side-effects, such as

 * {@link #forEach(Consumer)}), are explicitly nondeterministic for parallel

 * execution of stream pipelines.

 *

 * <p>Unless otherwise noted, passing a {@code null} argument to any stream

 * method may result in a {@link NullPointerException}.

 *

 * @apiNote

 * Streams are not data structures; they do not manage the storage for their

 * elements, nor do they support access to individual elements.  However,

 * you can use the {@link #iterator()} or {@link #spliterator()} operations to

 * perform a controlled traversal.

 *

 * @param <T> type of elements

 * @since 1.8

 * @see <a href="package-summary.html">java.util.stream</a>

 */

public interface Stream<T> extends BaseStream<T, Stream<T>> {

    /**

     * Returns a stream consisting of the elements of this stream that match

     * the given predicate.

     *

     * <p>This is an <a href="package-summary.html#StreamOps">intermediate

     * operation</a>.

     *

     * @param predicate a <a href="package-summary.html#NonInterference">

     *                  non-interfering, stateless</a> predicate to apply to

     *                  each element to determine if it should be included

     * @return the new stream

     */

    Stream<T> filter(Predicate<? super T> predicate);

    /**

     * Returns a stream consisting of the results of applying the given

     * function to the elements of this stream.

     *

     * <p>This is an <a href="package-summary.html#StreamOps">intermediate

     * operation</a>.

     *

     * @param <R> The element type of the new stream

     * @param mapper a <a href="package-summary.html#NonInterference">

     *               non-interfering, stateless</a> function to apply to each

     *               element

     * @return the new stream

     */

    <R> Stream<R> map(Function<? super T, ? extends R> mapper);

    /**

     * Returns an {@code IntStream} consisting of the results of applying the

     * given function to the elements of this stream.

     *

     * <p>This is an <a href="package-summary.html#StreamOps">

     *     intermediate operation</a>.

     *

     * @param mapper a <a href="package-summary.html#NonInterference">

     *               non-interfering, stateless</a> function to apply to each

     *               element

     * @return the new stream

     */

    IntStream mapToInt(ToIntFunction<? super T> mapper);

    /**

     * Returns a {@code LongStream} consisting of the results of applying the

     * given function to the elements of this stream.

     *

     * <p>This is an <a href="package-summary.html#StreamOps">intermediate

     * operation</a>.

     *

     * @param mapper a <a href="package-summary.html#NonInterference">

     *               non-interfering, stateless</a> function to apply to each

     *               element

     * @return the new stream

     */

    LongStream mapToLong(ToLongFunction<? super T> mapper);

    /**

     * Returns a {@code DoubleStream} consisting of the results of applying the

     * given function to the elements of this stream.

     *

     * <p>This is an <a href="package-summary.html#StreamOps">intermediate

     * operation</a>.

     *

     * @param mapper a <a href="package-summary.html#NonInterference">

     *               non-interfering, stateless</a> function to apply to each

     *               element

     * @return the new stream

     */

    DoubleStream mapToDouble(ToDoubleFunction<? super T> mapper);

    /**

     * Returns a stream consisting of the results of replacing each element of

     * this stream with the contents of the stream produced by applying the

     * provided mapping function to each element.  If the result of the mapping

     * function is {@code null}, this is treated as if the result is an empty

     * stream.

     *

     * <p>This is an <a href="package-summary.html#StreamOps">intermediate

     * operation</a>.

     *

     * @apiNote

     * The {@code flatMap()} operation has the effect of applying a one-to-many

     * tranformation to the elements of the stream, and then flattening the

     * resulting elements into a new stream. For example, if {@code orders}

     * is a stream of purchase orders, and each purchase order contains a

     * collection of line items, then the following produces a stream of line

     * items:

     * <pre>{@code

     *     orderStream.flatMap(order -> order.getLineItems().stream())...

     * }</pre>

     *

     * @param <R> The element type of the new stream

     * @param mapper a <a href="package-summary.html#NonInterference">

     *               non-interfering, stateless</a> function to apply to each

     *               element which produces a stream of new values

     * @return the new stream

     */

    <R> Stream<R> flatMap(Function<? super T, ? extends Stream<? extends R>> mapper);

    /**

     * Returns an {@code IntStream} consisting of the results of replacing each

     * element of this stream with the contents of the stream produced by

     * applying the provided mapping function to each element.  If the result of

     * the mapping function is {@code null}, this is treated as if the result is

     * an empty stream.

     *

     * <p>This is an <a href="package-summary.html#StreamOps">intermediate

     * operation</a>.

     *

     * @param mapper a <a href="package-summary.html#NonInterference">

     *               non-interfering, stateless</a> function to apply to each

     *               element which produces a stream of new values

     * @return the new stream

     */

    IntStream flatMapToInt(Function<? super T, ? extends IntStream> mapper);

    /**

     * Returns a {@code LongStream} consisting of the results of replacing each

     * element of this stream with the contents of the stream produced

     * by applying the provided mapping function to each element.  If the result

     * of the mapping function is {@code null}, this is treated as if the

     * result is an empty stream.

     *

     * <p>This is an <a href="package-summary.html#StreamOps">intermediate

     * operation</a>.

     *

     * @param mapper a <a href="package-summary.html#NonInterference">

     *               non-interfering, stateless</a> function to apply to

     *               each element which produces a stream of new values

     * @return the new stream

     */

    LongStream flatMapToLong(Function<? super T, ? extends LongStream> mapper);

    /**

     * Returns a {@code DoubleStream} consisting of the results of replacing each

     * element of this stream with the contents of the stream produced

     * by applying the provided mapping function to each element.  If the result

     * of the mapping function is {@code null}, this is treated as if the result

     * is an empty stream.

     *

     * <p>This is an <a href="package-summary.html#StreamOps">intermediate

     * operation</a>.

     *

     * @param mapper a <a href="package-summary.html#NonInterference">

     *               non-interfering, stateless</a> function to apply to each

     *               element which produces a stream of new values

     * @return the new stream

     */

    DoubleStream flatMapToDouble(Function<? super T, ? extends DoubleStream> mapper);

    /**

     * Returns a stream consisting of the distinct elements (according to

     * {@link Object#equals(Object)}) of this stream.

     *

     * <p>This is a <a href="package-summary.html#StreamOps">stateful

     * intermediate operation</a>.

     *

     * @return the new stream

     */

    Stream<T> distinct();

    /**

     * Returns a stream consisting of the elements of this stream, sorted

     * according to natural order.  If the elements of this stream are not

     * {@code Comparable}, a {@code java.lang.ClassCastException} may be thrown

     * when the stream pipeline is executed.

     *

     * <p>This is a <a href="package-summary.html#StreamOps">stateful

     * intermediate operation</a>.

     *

     * @return the new stream

     */

    Stream<T> sorted();

    /**

     * Returns a stream consisting of the elements of this stream, sorted

     * according to the provided {@code Comparator}.

     *

     * <p>This is a <a href="package-summary.html#StreamOps">stateful

     * intermediate operation</a>.

     *

     * @param comparator a <a href="package-summary.html#NonInterference">

     *                   non-interfering, stateless</a> {@code Comparator} to

     *                   be used to compare stream elements

     * @return the new stream

     */

    Stream<T> sorted(Comparator<? super T> comparator);

    /**

     * Returns a stream consisting of the elements of this stream, additionally

     * performing the provided action on each element as elements are consumed

     * from the resulting stream.

     *

     * <p>This is an <a href="package-summary.html#StreamOps">intermediate

     * operation</a>.

     *

     * <p>For parallel stream pipelines, the action may be called at

     * whatever time and in whatever thread the element is made available by the

     * upstream operation.  If the action modifies shared state,

     * it is responsible for providing the required synchronization.

     *

     * @apiNote This method exists mainly to support debugging, where you want

     * to see the elements as they flow past a certain point in a pipeline:

     * <pre>{@code

     *     list.stream()

     *         .filter(filteringFunction)

     *         .peek(e -> {System.out.println("Filtered value: " + e); });

     *         .map(mappingFunction)

     *         .peek(e -> {System.out.println("Mapped value: " + e); });

     *         .collect(Collectors.intoList());

     * }</pre>

     *

     * @param consumer a <a href="package-summary.html#NonInterference">

     *                 non-interfering</a> action to perform on the elements as

     *                 they are consumed from the stream

     * @return the new stream

     */

    Stream<T> peek(Consumer<? super T> consumer);

    /**

     * Returns a stream consisting of the elements of this stream, truncated

     * to be no longer than {@code maxSize} in length.

     *

     * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting

     * stateful intermediate operation</a>.

     *

     * @param maxSize the number of elements the stream should be limited to

     * @return the new stream

     * @throws IllegalArgumentException if {@code maxSize} is negative

     */

    Stream<T> limit(long maxSize);

    /**

     * Returns a stream consisting of the remaining elements of this stream

     * after indexing {@code startInclusive} elements into the stream. If the

     * {@code startInclusive} index lies past the end of this stream then an

     * empty stream will be returned.

     *

     * <p>This is a <a href="package-summary.html#StreamOps">stateful

     * intermediate operation</a>.

     *

     * @param startInclusive the number of leading elements to skip

     * @return the new stream

     * @throws IllegalArgumentException if {@code startInclusive} is negative

     */

    Stream<T> substream(long startInclusive);

    /**

     * Returns a stream consisting of the remaining elements of this stream

     * after indexing {@code startInclusive} elements into the stream and

     * truncated to contain no more than {@code endExclusive - startInclusive}

     * elements. If the {@code startInclusive} index lies past the end

     * of this stream then an empty stream will be returned.

     *

     * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting

     * stateful intermediate operation</a>.

     *

     * @param startInclusive the starting position of the substream, inclusive

     * @param endExclusive the ending position of the substream, exclusive

     * @return the new stream

     * @throws IllegalArgumentException if {@code startInclusive} or

     * {@code endExclusive} is negative or {@code startInclusive} is greater

     * than {@code endExclusive}

     */

    Stream<T> substream(long startInclusive, long endExclusive);

    /**

     * Performs an action for each element of this stream.

     *

     * <p>This is a <a href="package-summary.html#StreamOps">terminal

     * operation</a>.

     *

     * <p>For parallel stream pipelines, this operation does <em>not</em>

     * guarantee to respect the encounter order of the stream, as doing so

     * would sacrifice the benefit of parallelism.  For any given element, the

     * action may be performed at whatever time and in whatever thread the

     * library chooses.  If the action accesses shared state, it is

     * responsible for providing the required synchronization.

     *

     * @param action a <a href="package-summary.html#NonInterference">

     *               non-interfering</a> action to perform on the elements

     */

    void forEach(Consumer<? super T> action);

    /**

     * Performs an action for each element of this stream, guaranteeing that

     * each element is processed in encounter order for streams that have a

     * defined encounter order.

     *

     * <p>This is a <a href="package-summary.html#StreamOps">terminal

     * operation</a>.

     *

     * @param action a <a href="package-summary.html#NonInterference">

     *               non-interfering</a> action to perform on the elements

     * @see #forEach(Consumer)

     */

    void forEachOrdered(Consumer<? super T> action);

    /**

     * Returns an array containing the elements of this stream.

     *

     * <p>This is a <a href="package-summary.html#StreamOps">terminal

     * operation</a>.

     *

     * @return an array containing the elements of this stream

     */

    Object[] toArray();

    /**

     * Returns an array containing the elements of this stream, using the

     * provided {@code generator} function to allocate the returned array.

     *

     * <p>This is a <a href="package-summary.html#StreamOps">terminal

     * operation</a>.

     *

     * @param <A> the element type of the resulting array

     * @param generator a function which produces a new array of the desired

     *                  type and the provided length

     * @return an array containing the elements in this stream

     * @throws ArrayStoreException if the runtime type of the array returned

     * from the array generator is not a supertype of the runtime type of every

     * element in this stream

     */

    <A> A[] toArray(IntFunction<A[]> generator);

    /**

     * Performs a <a href="package-summary.html#Reduction">reduction</a> on the

     * elements of this stream, using the provided identity value and an

     * <a href="package-summary.html#Associativity">associative</a>

     * accumulation function, and returns the reduced value.  This is equivalent

     * to:

     * <pre>{@code

     *     T result = identity;

     *     for (T element : this stream)

     *         result = accumulator.apply(result, element)

     *     return result;

     * }</pre>

     *

     * but is not constrained to execute sequentially.

     *

     * <p>The {@code identity} value must be an identity for the accumulator

     * function. This means that for all {@code t},

     * {@code accumulator.apply(identity, t)} is equal to {@code t}.

     * The {@code accumulator} function must be an

     * <a href="package-summary.html#Associativity">associative</a> function.

     *

     * <p>This is a <a href="package-summary.html#StreamOps">terminal

     * operation</a>.

     *

     * @apiNote Sum, min, max, average, and string concatenation are all special

     * cases of reduction. Summing a stream of numbers can be expressed as:

     *

     * <pre>{@code

     *     Integer sum = integers.reduce(0, (a, b) -> a+b);

     * }</pre>

     *

     * or more compactly:

     *

     * <pre>{@code

     *     Integer sum = integers.reduce(0, Integer::sum);

     * }</pre>

     *

     * <p>While this may seem a more roundabout way to perform an aggregation

     * compared to simply mutating a running total in a loop, reduction

     * operations parallelize more gracefully, without needing additional

     * synchronization and with greatly reduced risk of data races.

     *

     * @param identity the identity value for the accumulating function

     * @param accumulator an <a href="package-summary.html#Associativity">associative</a>

     *                    <a href="package-summary.html#NonInterference">non-interfering,

     *                    stateless</a> function for combining two values

     * @return the result of the reduction

     */

    T reduce(T identity, BinaryOperator<T> accumulator);

    /**

     * Performs a <a href="package-summary.html#Reduction">reduction</a> on the

     * elements of this stream, using an

     * <a href="package-summary.html#Associativity">associative</a> accumulation

     * function, and returns an {@code Optional} describing the reduced value,

     * if any. This is equivalent to:

     * <pre>{@code

     *     boolean foundAny = false;

     *     T result = null;

     *     for (T element : this stream) {

     *         if (!foundAny) {

     *             foundAny = true;

     *             result = element;

     *         }

     *         else

     *             result = accumulator.apply(result, element);

     *     }

     *     return foundAny ? Optional.of(result) : Optional.empty();

     * }</pre>

     *

     * but is not constrained to execute sequentially.

     *

     * <p>The {@code accumulator} function must be an

     * <a href="package-summary.html#Associativity">associative</a> function.

     *

     * <p>This is a <a href="package-summary.html#StreamOps">terminal

     * operation</a>.

     *

     * @param accumulator an <a href="package-summary.html#Associativity">associative</a>

     *                    <a href="package-summary.html#NonInterference">non-interfering,

     *                    stateless</a> function for combining two values

     * @return the result of the reduction

     * @see #reduce(Object, BinaryOperator)

     * @see #min(java.util.Comparator)

     * @see #max(java.util.Comparator)

     */

    Optional<T> reduce(BinaryOperator<T> accumulator);

    /**

     * Performs a <a href="package-summary.html#Reduction">reduction</a> on the

     * elements of this stream, using the provided identity, accumulation

     * function, and a combining functions.  This is equivalent to:

     * <pre>{@code

     *     U result = identity;

     *     for (T element : this stream)

     *         result = accumulator.apply(result, element)

     *     return result;

     * }</pre>

     *

     * but is not constrained to execute sequentially.

     *

     * <p>The {@code identity} value must be an identity for the combiner

     * function.  This means that for all {@code u}, {@code combiner(identity, u)}

     * is equal to {@code u}.  Additionally, the {@code combiner} function

     * must be compatible with the {@code accumulator} function; for all

     * {@code u} and {@code t}, the following must hold: