/*
 
* Copyright (c) 2012, 2013, Oracle and/or its affiliates. All rights reserved.
 
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 
*
 
* 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
 
* version 2 for more details (a copy is included in the LICENSE file that
 
* accompanied this code).
 
*
 
* You should have received a copy of the GNU General Public License version
 
* 2 along with this work; if not, write to the Free Software Foundation,
 
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 
*
 
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 
* or visit www.oracle.com if you need additional information or have any
 
* questions.
 
*/
package java.util.stream;

import java.nio.charset.Charset;
import java.nio.file.Files;
import java.nio.file.Path;
import java.util.Arrays;
import java.util.Collection;
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.concurrent.ConcurrentHashMap;
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;

/**
 
* A sequence of elements supporting sequential and parallel aggregate
 
* operations.
  
The following example illustrates an aggregate operation using
 
* {@link Stream} and {@link IntStream}:
 
*
 
* <pre>{@code
 
*
     
int sum = widgets.stream()
 
*
                      
.filter(w -> w.getColor() == RED)
 
*
                      
.mapToInt(w -> w.getWeight())
 
*
                      
.sum();
 
* }</pre>
 
*
 
* In this example, {@code widgets} is a {@code Collection<Widget>}.
  
We create
 
* a stream of {@code Widget} objects via {@link Collection#stream Collection.stream()},
 
* filter it to produce a stream containing only the red widgets, and then
 
* transform it into a stream of {@code int} values representing the weight of
 
* each red widget. Then this stream is summed to produce a total weight.
 
*
 
* <p>In addition to {@code Stream}, which is a stream of object references,
 
* there are primitive specializations for {@link IntStream}, {@link LongStream},
 
* and {@link DoubleStream}, all of which are referred to as "streams" and
 
* conform to the characteristics and restrictions described here.
 
*
 
* <p>To perform a computation, stream
 
*<a href="package-summary.html#StreamOps">operations</a>
 
are composed into a
 
* <em>stream pipeline</em>.
  
A stream pipeline consists of a source (which
 
* might be an array, a collection, a generator function, an I/O channel,
 
* etc), zero or more <em>intermediate operations</em> (which transform a
 
* stream into another stream, such as {@link Stream#filter(Predicate)}), and a
 
* <em>terminal operation</em> (which produces a result or side-effect, such
 
* as {@link Stream#count()} or {@link Stream#forEach(Consumer)}).
 
* Streams are lazy; computation on the source data is only performed when the
 
* terminal operation is initiated, and source elements are consumed only
 
* as needed.
 
*
 
* <p>Collections and streams, while bearing some superficial similarities,
 
* have different goals.
  
Collections are primarily concerned with the efficient
 
* management of, and access to, their elements.
  
By contrast, streams do not
 
* provide a means to directly access or manipulate their elements, and are
 
* instead concerned with declaratively describing their source and the
 
* computational operations which will be performed in aggregate on that source.
 
* However, if the provided stream operations do not offer the desired
 
* functionality, the
 
and {@link #spliterator()} operations
 
* can be used to perform a controlled traversal.
 
*
 
* <p>A stream pipeline, like the "widgets" example above, can be viewed as
 
* a <em>query</em> on the stream source.
  
Unless the source was explicitly
 
* designed for concurrent modification (such as a {@link ConcurrentHashMap}),
 
* unpredictable or erroneous behavior may result from modifying the stream
 
* source while it is being queried.
 
*
 
* <p>Most stream operations accept parameters that describe user-specified
 
* behavior, such as the lambda expression {@code w -> w.getWeight()} passed to
 
* {@code mapToInt} in the example above.
  
To preserve correct behavior,
 
* these <em>behavioral parameters</em>:
 
* <ul>
 
* <li>must be<a href="package-summary.html#NonInterference">non-interfering</a>
 
* (they do not modify the stream source); and</li>
 
* <li>in most cases must be<a href="package-summary.html#Statelessness">stateless</a>
 
* (their result should not depend on any state that might change during execution
 
* of the stream pipeline).</li>
 
* </ul>
 
*
 
* <p>Such parameters are always instances of a
 
*<a href="../function/package-summary.html">functional interface</a>
 
such
 
* as , and are often lambda expressions or
 
* method references.
  
Unless otherwise specified these parameters must be
 
* <em>non-null</em>.
 
*
 
* <p>A stream should be operated on (invoking an intermediate or terminal stream
 
* operation) only once.
  
This rules out, for example, "forked" streams, where
 
* the same source feeds two or more pipelines, or multiple traversals of the
 
* same stream.
  
A stream implementation may throw {@link IllegalStateException}
 
* if it detects that the stream is being reused. However, since some stream
 
* operations may return their receiver rather than a new stream object, it may
 
* not be possible to detect reuse in all cases.
 
*
 
* <p>Streams have a
 
method and implement {@link AutoCloseable},
 
* but nearly all stream instances do not actually need to be closed after use.
 
* Generally, only streams whose source is an IO channel (such as those returned
 
* by {@link Files#lines(Path, Charset)}) will require closing.
  
Most streams
 
* are backed by collections, arrays, or generating functions, which require no
 
* special resource management.
  
(If a stream does require closing, it can be
 
* declared as a resource in a {@code try}-with-resources statement.)
 
*
 
* <p>Stream pipelines may execute either sequentially or in
 
*<a href="package-summary.html#Parallelism">parallel</a>.
  
This
 
* execution mode is a property of the stream.
  
Streams are created
 
* with an initial choice of sequential or parallel execution.
  
(For example,
 
* {@link Collection#stream() Collection.stream()} creates a sequential stream,
 
* and {@link Collection#parallelStream() Collection.parallelStream()} creates
 
* a parallel one.)
  
This choice of execution mode may be modified by the
 
*
 
or {@link #parallel()} methods, and may be queried with
 
* the
 
method.
 
*
 
* @param <T> the type of the stream elements
 
* @since 1.8
 
* @see IntStream
 
* @see LongStream
 
* @see DoubleStream
 
* @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</a>,
     
*
                  
<a href="package-summary.html#Statelessness">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</a>,
     
*
               
<a href="package-summary.html#Statelessness">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</a>,
     
*
               
<a href="package-summary.html#Statelessness">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</a>,
     
*
               
<a href="package-summary.html#Statelessness">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</a>,
     
*
               
<a href="package-summary.html#Statelessness">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 a mapped stream produced by applying
     
* the provided mapping function to each element.
  
Each mapped stream is
     
* {@link java.util.stream.BaseStream#close() closed} after its contents
     
* have been placed into this stream.
  
(If a mapped stream is {@code null}
     
* an empty stream is used, instead.)
     
*
     
* <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
     
* transformation to the elements of the stream, and then flattening the
     
* resulting elements into a new stream.
     
*
     
* <p><b>Examples.</b>
     
*
     
* <p>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 containing all the line items in all the orders:
     
* <pre>{@code
     
*orders.flatMap(order -> order.getLineItems().stream())...
     
* }</pre>
     
*
     
* <p>If {@code path} is the path to a file, then the following produces a
     
* stream of the {@code words} contained in that file:
     
* <pre>{@code
     
*Stream<String> lines = Files.lines(path, StandardCharsets.UTF_8);
     
*Stream<String> words = lines.flatMap(line -> Stream.of(line.split(" +")));
     
* }</pre>
     
* The {@code mapper} function passed to {@code flatMap} splits a line,
     
* using a simple regular expression, into an array of words, and then
     
* creates a stream of words from that array.
     
*
     
* @param <R> The element type of the new stream
     
* @param mapper a
 
<a href="package-summary.html#NonInterference">non-interfering</a>,
     
*
               
<a href="package-summary.html#Statelessness">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 a mapped stream produced by
     
* applying the provided mapping function to each element.
  
Each mapped
     
* stream is {@link java.util.stream.BaseStream#close() closed} after its
     
* contents have been placed into this stream.
  
(If a mapped stream is
     
* {@code null} an empty stream is used, instead.)
     
*
     
* <p>This is an <a href="package-summary.html#StreamOps">intermediate
     
* operation</a>.
     
*
     
* @param mapper a
 
<a href="package-summary.html#NonInterference">non-interfering</a>,
     
*
               
<a href="package-summary.html#Statelessness">stateless</a>
     
*
               
function to apply to each element which produces a stream
     
*
               
of new values
     
* @return the new stream
     
* @see #flatMap(Function)
     
*/

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

    
/**
     
* Returns an {@code LongStream} consisting of the results of replacing each
     
* element of this stream with the contents of a mapped stream produced by
     
* applying the provided mapping function to each element.
  
Each mapped
     
* stream is {@link java.util.stream.BaseStream#close() closed} after its
     
* contents have been placed into this stream.
  
(If a mapped stream is
     
* {@code null} an empty stream is used, instead.)
     
*
     
* <p>This is an <a href="package-summary.html#StreamOps">intermediate
     
* operation</a>.
     
*
     
* @param mapper a
 
<a href="package-summary.html#NonInterference">non-interfering</a>,
     
*
               
<a href="package-summary.html#Statelessness">stateless</a>
     
*
               
function to apply to each element which produces a stream
     
*
               
of new values
     
* @return the new stream
     
* @see #flatMap(Function)
     
*/

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

    
/**
     
* Returns an {@code DoubleStream} consisting of the results of replacing
     
* each element of this stream with the contents of a mapped stream produced
     
* by applying the provided mapping function to each element.
  
Each mapped
     
* stream is {@link java.util.stream.BaseStream#close() closed} after its
     
* contents have placed been into this stream.
  
(If a mapped stream is
     
* {@code null} an empty stream is used, instead.)
     
*
     
* <p>This is an <a href="package-summary.html#StreamOps">intermediate
     
* operation</a>.
     
*
     
* @param mapper a
 
<a href="package-summary.html#NonInterference">non-interfering</a>,
     
*
               
<a href="package-summary.html#Statelessness">stateless</a>
     
*
               
function to apply to each element which produces a stream
     
*
               
of new values
     
* @return the new stream
     
* @see #flatMap(Function)
     
*/

    
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>For ordered streams, the selection of distinct elements is stable
     
* (for duplicated elements, the element appearing first in the encounter
     
* order is preserved.)
  
For unordered streams, no stability guarantees
     
* are made.
     
*
     
* <p>This is a <a href="package-summary.html#StreamOps">stateful
     
* intermediate operation</a>.
     
*
     
* @apiNote
     
* Preserving stability for {@code distinct()} in parallel pipelines is
     
* relatively expensive (requires that the operation act as a full barrier,
     
* with substantial buffering overhead), and stability is often not needed.
     
* Using an unordered stream source (such as {@link #generate(Supplier)})
     
* or removing the ordering constraint with
 
 
may result
     
* in significantly more efficient execution for {@code distinct()} in parallel
     
* pipelines, if the semantics of your situation permit.
  
If consistency
     
* with encounter order is required, and you are experiencing poor performance
     
* or memory utilization with {@code distinct()} in parallel pipelines,
     
* switching to sequential execution with
 
 
may improve
     
* performance.
     
*
     
* @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 terminal operation is executed.
     
*
     
* <p>For ordered streams, the sort is stable.
  
For unordered streams, no
     
* stability guarantees are made.
     
*
     
* <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>For ordered streams, the sort is stable.
  
For unordered streams, no
     
* stability guarantees are made.
     
*
     
* <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</a>,
     
*
                   
<a href="package-summary.html#Statelessness">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
     
*Stream.of("one", "two", "three", "four")
     
*
         
.filter(e -> e.length() > 3)
     
*
         
.peek(e -> System.out.println("Filtered value: " + e))
     
*
         
.map(String::toUpperCase)
     
*
         
.peek(e -> System.out.println("Mapped value: " + e))
     
*
         
.collect(Collectors.toList());
     
* }</pre>
     
*
     
* @param action 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> action);

    
/**
     
* 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>.
     
*
     
* @apiNote
     
* While {@code limit()} is generally a cheap operation on sequential
     
* stream pipelines, it can be quite expensive on ordered parallel pipelines,
     
* especially for large values of {@code maxSize}, since {@code limit(n)}
     
* is constrained to return not just any <em>n</em> elements, but the
     
* <em>first n</em> elements in the encounter order.
  
Using an unordered
     
* stream source (such as {@link #generate(Supplier)}) or removing the
     
* ordering constraint with
 
 
may result in significant
     
* speedups of {@code limit()} in parallel pipelines, if the semantics of
     
* your situation permit.
  
If consistency with encounter order is required,
     
* and you are experiencing poor performance or memory utilization with
     
* {@code limit()} in parallel pipelines, switching to sequential execution
     
* with
 
 
may improve performance.
     
*
     
* @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 discarding the first {@code n} elements of the stream.
     
* If this stream contains fewer than {@code n} elements then an
     
* empty stream will be returned.
     
*
     
* <p>This is a <a href="package-summary.html#StreamOps">stateful
     
* intermediate operation</a>.
     
*
     
* @apiNote
     
* While {@code skip()} is generally a cheap operation on sequential
     
* stream pipelines, it can be quite expensive on ordered parallel pipelines,
     
* especially for large values of {@code n}, since {@code skip(n)}
     
* is constrained to skip not just any <em>n</em> elements, but the
     
* <em>first n</em> elements in the encounter order.
  
Using an unordered
     
* stream source (such as {@link #generate(Supplier)}) or removing the
     
* ordering constraint with
 
 
may result in significant
     
* speedups of {@code skip()} in parallel pipelines, if the semantics of
     
* your situation permit.
  
If consistency with encounter order is required,
     
* and you are experiencing poor performance or memory utilization with
     
* {@code skip()} in parallel pipelines, switching to sequential execution
     
* with
 
 
may improve performance.
     
*
     
* @param n the number of leading elements to skip
     
* @return the new stream
     
* @throws IllegalArgumentException if {@code n} is negative
     
*/

    
Stream<T> skip(long n);

    
/**
     
* Performs an action for each element of this stream.
     
*
     
* <p>This is a <a href="package-summary.html#StreamOps">terminal
     
* operation</a>.
     
*
     
* <p>The behavior of this operation is explicitly nondeterministic.
     
* 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, in the encounter
     
* order of the stream if the stream has a defined encounter order.
     
*
     
* <p>This is a <a href="package-summary.html#StreamOps">terminal
     
* operation</a>.
     
*
     
* <p>This operation processes the elements one at a time, in encounter
     
* order if one exists.
  
Performing the action for one element
     
*
 
<a href="../concurrent/package-summary.html#MemoryVisibility"><i>happens-before</i></a>
     
* performing the action for subsequent elements, but for any given element,
     
* the action may be performed in whatever thread the library chooses.
     
*
     
* @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, as
     
* well as any additional arrays that might be required for a partitioned
     
* execution or for resizing.
     
*
     
* <p>This is a <a href="package-summary.html#StreamOps">terminal
     
* operation</a>.
     
*
     
* @apiNote
     
* The generator function takes an integer, which is the size of the
     
* desired array, and produces an array of the desired size.
  
This can be
     
* concisely expressed with an array constructor reference:
     
* <pre>{@code
     
*Person[] men = people.stream()
     
*
                          
.filter(p -> p.getGender() == MALE)
     
*
                          
.toArray(Person[]::new);
     
* }</pre>
     
*
     
* @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:
     
*
     
* <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</a>,
     
*
                    
<a href="package-summary.html#Statelessness">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</a>,
     
*
                    
<a href="package-summary.html#Statelessness">stateless</a>
     
*
                    
function for combining two values
     
* @return an {@link Optional} describing the result of the reduction
     
* @throws NullPointerException if the result of the reduction is null
     
* @see #reduce(Object, BinaryOperator)
     
* @see #min(Comparator)
     
* @see #max(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 and
     
* 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:
     
* <pre>{@code
     
*combiner.apply(u, accumulator.apply(identity, t)) == accumulator.apply(u, t)
     
* }</pre>
     
*
     
* <p>This is a <a href="package-summary.html#StreamOps">terminal
     
* operation</a>.
     
*
     
* @apiNote Many reductions using this form can be represented more simply
     
* by an explicit combination of {@code map} and {@code reduce} operations.
     
* The {@code accumulator} function acts as a fused mapper and accumulator,
     
* which can sometimes be more efficient than separate mapping and reduction,
     
* such as when knowing the previously reduced value allows you to avoid
     
* some computation.
     
*
     
* @param <U> The type of the result
     
* @param identity the identity value for the combiner function
     
* @param accumulator an
 
<a href="package-summary.html#Associativity">associative</a>,
     
*
                    
<a href="package-summary.html#NonInterference">non-interfering</a>,
     
*
                    
<a href="package-summary.html#Statelessness">stateless</a>
     
*
                    
function for incorporating an additional element into a result
     
* @param combiner an
 
<a href="package-summary.html#Associativity">associative</a>,
     
*
                    
<a href="package-summary.html#NonInterference">non-interfering</a>,
     
*
                    
<a href="package-summary.html#Statelessness">stateless</a>
     
*
                    
function for combining two values, which must be
     
*
                    
compatible with the accumulator function
     
* @return the result of the reduction
     
* @see #reduce(BinaryOperator)
     
* @see #reduce(Object, BinaryOperator)
     
*/

    
<U> U reduce(U identity,
                 
BiFunction<U, ? super T, U> accumulator,
                 
BinaryOperator<U> combiner);

    
/**
     
* Performs a <a href="package-summary.html#MutableReduction">mutable
     
* reduction</a> operation on the elements of this stream.
  
A mutable
     
* reduction is one in which the reduced value is a mutable result container,
     
* such as an {@code ArrayList}, and elements are incorporated by updating
     
* the state of the result rather than by replacing the result.
  
This
     
* produces a result equivalent to:
     
* <pre>{@code
     
*R result = supplier.get();
     
*for (T element : this stream)
     
*
         
accumulator.accept(result, element);
     
*return result;
     
* }</pre>
     
*
     
* <p>Like {@link #reduce(Object, BinaryOperator)}, {@code collect} operations
     
* can be parallelized without requiring additional synchronization.
     
*
     
* <p>This is a <a href="package-summary.html#StreamOps">terminal
     
* operation</a>.
     
*
     
* @apiNote There are many existing classes in the JDK whose signatures are
     
* well-suited for use with method references as arguments to {@code collect()}.
     
* For example, the following will accumulate strings into an {@code ArrayList}:
     
* <pre>{@code
     
*List<String> asList = stringStream.collect(ArrayList::new, ArrayList::add,
     
*
                                                
ArrayList::addAll);
     
* }</pre>
     
*
     
* <p>The following will take a stream of strings and concatenates them into a
     
* single string:
     
* <pre>{@code
     
*String concat = stringStream.collect(StringBuilder::new, StringBuilder::append,
     
*
                                          
StringBuilder::append)
     
*
                                 
.toString();
     
* }</pre>
     
*
     
* @param <R> type of the result
     
* @param supplier a function that creates a new result container. For a
     
*
                 
parallel execution, this function may be called
     
*
                 
multiple times and must return a fresh value each time.
     
* @param accumulator an
 
<a href="package-summary.html#Associativity">associative</a>,
     
*
                    
<a href="package-summary.html#NonInterference">non-interfering</a>,
     
*
                    
<a href="package-summary.html#Statelessness">stateless</a>
     
*
                    
function for incorporating an additional element into a result
     
* @param combiner an
 
<a href="package-summary.html#Associativity">associative</a>,
     
*
                    
<a href="package-summary.html#NonInterference">non-interfering</a>,
     
*
                    
<a href="package-summary.html#Statelessness">stateless</a>
     
*
                    
function for combining two values, which must be
     
*
                    
compatible with the accumulator function
     
* @return the result of the reduction
     
*/

    
<R> R collect(Supplier<R> supplier,
                  
BiConsumer<R, ? super T> accumulator,
                  
BiConsumer<R, R> combiner);

    
/**
     
* Performs a <a href="package-summary.html#MutableReduction">mutable
     
* reduction</a> operation on the elements of this stream using a
     
* {@code Collector}.
  
A {@code Collector}
     
* encapsulates the functions used as arguments to
     
* {@link #collect(Supplier, BiConsumer, BiConsumer)}, allowing for reuse of
     
* collection strategies and composition of collect operations such as
     
* multiple-level grouping or partitioning.
     
*
     
* <p>If the stream is parallel, and the {@code Collector}
     
* is {@link Collector.Characteristics#CONCURRENT concurrent}, and
     
* either the stream is unordered or the collector is
     
* {@link Collector.Characteristics#UNORDERED unordered},
     
* then a concurrent reduction will be performed (see {@link Collector} for
     
* details on concurrent reduction.)
     
*
     
* <p>This is a <a href="package-summary.html#StreamOps">terminal
     
* operation</a>.
     
*
     
* <p>When executed in parallel, multiple intermediate results may be
     
* instantiated, populated, and merged so as to maintain isolation of
     
* mutable data structures.
  
Therefore, even when executed in parallel
     
* with non-thread-safe data structures (such as {@code ArrayList}), no
     
* additional synchronization is needed for a parallel reduction.
     
*
     
* @apiNote
     
* The following will accumulate strings into an ArrayList:
     
* <pre>{@code
     
*List<String> asList = stringStream.collect(Collectors.toList());
     
* }</pre>
     
*
     
* <p>The following will classify {@code Person} objects by city:
     
* <pre>{@code
     
*Map<String, List<Person>> peopleByCity
     
*
         
= personStream.collect(Collectors.groupingBy(Person::getCity));
     
* }</pre>
     
*
     
* <p>The following will classify {@code Person} objects by state and city,
     
* cascading two {@code Collector}s together:
     
* <pre>{@code
     
*Map<String, Map<String, List<Person>>> peopleByStateAndCity
     
*
         
= personStream.collect(Collectors.groupingBy(Person::getState,
     
*
                                                      
Collectors.groupingBy(Person::getCity)));
     
* }</pre>
     
*
     
* @param <R> the type of the result
     
* @param <A> the intermediate accumulation type of the {@code Collector}
     
* @param collector the {@code Collector} describing the reduction
     
* @return the result of the reduction
     
* @see #collect(Supplier, BiConsumer, BiConsumer)
     
* @see Collectors
     
*/

    
<R, A> R collect(Collector<? super T, A, R> collector);

    
/**
     
* Returns the minimum element of this stream according to the provided
     
* {@code Comparator}.
  
This is a special case of a
     
*
 
<a href="package-summary.html#Reduction">reduction</a>.
     
*
     
* <p>This is a
 
<a href="package-summary.html#StreamOps">terminal operation</a>.
     
*
     
* @param comparator a
 
<a href="package-summary.html#NonInterference">non-interfering</a>,
     
*
                   
<a href="package-summary.html#Statelessness">stateless</a>
     
*
                   
{@code Comparator} to compare elements of this stream
     
* @return an {@code Optional} describing the minimum element of this stream,
     
* or an empty {@code Optional} if the stream is empty
     
* @throws NullPointerException if the minimum element is null
     
*/

    
Optional<T> min(Comparator<? super T> comparator);

    
/**
     
* Returns the maximum element of this stream according to the provided
     
* {@code Comparator}.
  
This is a special case of a
     
*
 
<a href="package-summary.html#Reduction">reduction</a>.
     
*
     
* <p>This is a <a href="package-summary.html#StreamOps">terminal
     
* operation</a>.
     
*
     
* @param comparator a
 
<a href="package-summary.html#NonInterference">non-interfering</a>,
     
*
                   
<a href="package-summary.html#Statelessness">stateless</a>
     
*
                   
{@code Comparator} to compare elements of this stream
     
* @return an {@code Optional} describing the maximum element of this stream,
     
* or an empty {@code Optional} if the stream is empty
     
* @throws NullPointerException if the maximum element is null
     
*/

    
Optional<T> max(Comparator<? super T> comparator);

    
/**
     
* Returns the count of elements in this stream.
  
This is a special case of
     
* a
 
<a href="package-summary.html#Reduction">reduction</a>
 
and is
     
* equivalent to:
     
* <pre>{@code
     
*return mapToLong(e -> 1L).sum();
     
* }</pre>
     
*
     
* <p>This is a
 
<a href="package-summary.html#StreamOps">terminal operation</a>.
     
*
     
* @return the count of elements in this stream
     
*/

    
long count();

    
/**
     
* Returns whether any elements of this stream match the provided
     
* predicate.
  
May not evaluate the predicate on all elements if not
     
* necessary for determining the result.
  
If the stream is empty then
     
* {@code false} is returned and the predicate is not evaluated.
     
*
     
* <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
     
* terminal operation</a>.
     
*
     
* @apiNote
     
* This method evaluates the <em>existential quantification</em> of the
     
* predicate over the elements of the stream (for some x P(x)).
     
*
     
* @param predicate a
 
<a href="package-summary.html#NonInterference">non-interfering</a>,
     
*
                  
<a href="package-summary.html#Statelessness">stateless</a>
     
*
                  
predicate to apply to elements of this stream
     
* @return {@code true} if any elements of the stream match the provided
     
* predicate, otherwise {@code false}
     
*/

    
boolean anyMatch(Predicate<? super T> predicate);

    
/**
     
* Returns whether all elements of this stream match the provided predicate.
     
* May not evaluate the predicate on all elements if not necessary for
     
* determining the result.
  
If the stream is empty then {@code true} is
     
* returned and the predicate is not evaluated.
     
*
     
* <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
     
* terminal operation</a>.
     
*
     
* @apiNote
     
* This method evaluates the <em>universal quantification</em> of the
     
* predicate over the elements of the stream (for all x P(x)).
  
If the
     
* stream is empty, the quantification is said to be <em>vacuously
     
* satisfied</em> and is always {@code true} (regardless of P(x)).
     
*
     
* @param predicate a
 
<a href="package-summary.html#NonInterference">non-interfering</a>,
     
*
                  
<a href="package-summary.html#Statelessness">stateless</a>
     
*
                  
predicate to apply to elements of this stream
     
* @return {@code true} if either all elements of the stream match the
     
* provided predicate or the stream is empty, otherwise {@code false}
     
*/

    
boolean allMatch(Predicate<? super T> predicate);

    
/**
     
* Returns whether no elements of this stream match the provided predicate.
     
* May not evaluate the predicate on all elements if not necessary for
     
* determining the result.
  
If the stream is empty then {@code true} is
     
* returned and the predicate is not evaluated.
     
*
     
* <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
     
* terminal operation</a>.
     
*
     
* @apiNote
     
* This method evaluates the <em>universal quantification</em> of the
     
* negated predicate over the elements of the stream (for all x ~P(x)).
  
If
     
* the stream is empty, the quantification is said to be vacuously satisfied
     
* and is always {@code true}, regardless of P(x).
     
*
     
* @param predicate a
 
<a href="package-summary.html#NonInterference">non-interfering</a>,
     
*
                  
<a href="package-summary.html#Statelessness">stateless</a>
     
*
                  
predicate to apply to elements of this stream
     
* @return {@code true} if either no elements of the stream match the
     
* provided predicate or the stream is empty, otherwise {@code false}
     
*/

    
boolean noneMatch(Predicate<? super T> predicate);

    
/**
     
* Returns an {@link Optional} describing the first element of this stream,
     
* or an empty {@code Optional} if the stream is empty.
  
If the stream has
     
* no encounter order, then any element may be returned.
     
*
     
* <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
     
* terminal operation</a>.
     
*
     
* @return an {@code Optional} describing the first element of this stream,
     
* or an empty {@code Optional} if the stream is empty
     
* @throws NullPointerException if the element selected is null
     
*/

    
Optional<T> findFirst();

    
/**
     
* Returns an {@link Optional} describing some element of the stream, or an
     
* empty {@code Optional} if the stream is empty.
     
*
     
* <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
     
* terminal operation</a>.
     
*
     
* <p>The behavior of this operation is explicitly nondeterministic; it is
     
* free to select any element in the stream.
  
This is to allow for maximal
     
* performance in parallel operations; the cost is that multiple invocations
     
* on the same source may not return the same result.
  
(If a stable result
     
* is desired, use
 
 
instead.)
     
*
     
* @return an {@code Optional} describing some element of this stream, or an
     
* empty {@code Optional} if the stream is empty
     
* @throws NullPointerException if the element selected is null
     
*
 

     
*/

    
Optional<T> findAny();

    
// Static factories

    
/**
     
* Returns a builder for a {@code Stream}.
     
*
     
* @param <T> type of elements
     
* @return a stream builder
     
*/

    
public static<T> Builder<T> builder() {
        
return new Streams.StreamBuilderImpl<>();
    
}

    
/**
     
* Returns an empty sequential {@code Stream}.
     
*
     
* @param <T> the type of stream elements
     
* @return an empty sequential stream
     
*/

    
public static<T> Stream<T> empty() {
        
return StreamSupport.stream(Spliterators.<T>emptySpliterator(), false);
    
}

    
/**
     
* Returns a sequential {@code Stream} containing a single element.
     
*
     
* @param t the single element
     
* @param <T> the type of stream elements
     
* @return a singleton sequential stream
     
*/

    
public static<T> Stream<T> of(T t) {
        
return StreamSupport.stream(new Streams.StreamBuilderImpl<>(t), false);
    
}

    
/**
     
* Returns a sequential ordered stream whose elements are the specified values.
     
*
     
* @param <T> the type of stream elements
     
* @param values the elements of the new stream
     
* @return the new stream
     
*/

    
@SafeVarargs
    
@SuppressWarnings("varargs") // Creating a stream from an array is safe
    
public static<T> Stream<T> of(T... values) {
        
return Arrays.stream(values);
    
}

    
/**
     
* Returns an infinite sequential ordered {@code Stream} produced by iterative
     
* application of a function {@code f} to an initial element {@code seed},
     
* producing a {@code Stream} consisting of {@code seed}, {@code f(seed)},
     
* {@code f(f(seed))}, etc.
     
*
     
* <p>The first element (position {@code 0}) in the {@code Stream} will be
     
* the provided {@code seed}.
  
For {@code n > 0}, the element at position
     
* {@code n}, will be the result of applying the function {@code f} to the
     
* element at position {@code n - 1}.
     
*
     
* @param <T> the type of stream elements
     
* @param seed the initial element
     
* @param f a function to be applied to the previous element to produce
     
*
          
a new element
     
* @return a new sequential {@code Stream}
     
*/

    
public static<T> Stream<T> iterate(final T seed, final UnaryOperator<T> f) {
        
Objects.requireNonNull(f);
        
final Iterator<T> iterator = new Iterator<T>() {
            
@SuppressWarnings("unchecked")
            
T t = (T) Streams.NONE;

            
@Override
            
public boolean hasNext() {
                
return true;
            
}

            
@Override
            
public T next() {
                
return t = (t == Streams.NONE) ? seed : f.apply(t);
            
}
        
};
        
return StreamSupport.stream(Spliterators.spliteratorUnknownSize(
                
iterator,
                
Spliterator.ORDERED | Spliterator.IMMUTABLE), false);
    
}

    
/**
     
* Returns an infinite sequential unordered stream where each element is
     
* generated by the provided {@code Supplier}.
  
This is suitable for
     
* generating constant streams, streams of random elements, etc.
     
*
     
* @param <T> the type of stream elements
     
* @param s the {@code Supplier} of generated elements
     
* @return a new infinite sequential unordered {@code Stream}
     
*/

    
public static<T> Stream<T> generate(Supplier<T> s) {
        
Objects.requireNonNull(s);
        
return StreamSupport.stream(
                
new StreamSpliterators.InfiniteSupplyingSpliterator.OfRef<>(Long.MAX_VALUE, s), false);
    
}

    
/**
     
* Creates a lazily concatenated stream whose elements are all the
     
* elements of the first stream followed by all the elements of the
     
* second stream.
  
The resulting stream is ordered if both
     
* of the input streams are ordered, and parallel if either of the input
     
* streams is parallel.
  
When the resulting stream is closed, the close
     
* handlers for both input streams are invoked.
     
*
     
* @implNote
     
* Use caution when constructing streams from repeated concatenation.
     
* Accessing an element of a deeply concatenated stream can result in deep
     
* call chains, or even {@code StackOverflowException}.
     
*
     
* @param <T> The type of stream elements
     
* @param a the first stream
     
* @param b the second stream
     
* @return the concatenation of the two input streams
     
*/

    
public static <T> Stream<T> concat(Stream<? extends T> a, Stream<? extends T> b) {
        
Objects.requireNonNull(a);
        
Objects.requireNonNull(b);

        
@SuppressWarnings("unchecked")
        
Spliterator<T> split = new Streams.ConcatSpliterator.OfRef<>(
                
(Spliterator<T>) a.spliterator(), (Spliterator<T>) b.spliterator());
        
Stream<T> stream = StreamSupport.stream(split, a.isParallel() || b.isParallel());
        
return stream.onClose(Streams.composedClose(a, b));
    
}

    
/**
     
* A mutable builder for a {@code Stream}.
  
This allows the creation of a
     
* {@code Stream} by generating elements individually and adding them to the
     
* {@code Builder} (without the copying overhead that comes from using
     
* an {@code ArrayList} as a temporary buffer.)
     
*
     
* <p>A stream builder has a lifecycle, which starts in a building
     
* phase, during which elements can be added, and then transitions to a built
     
* phase, after which elements may not be added.
  
The built phase begins
     
* when the {@link #build()} method is called, which creates an ordered
     
* {@code Stream} whose elements are the elements that were added to the stream
     
* builder, in the order they were added.
     
*
     
* @param <T> the type of stream elements
     
* @see Stream#builder()
     
* @since 1.8
     
*/

    
public interface Builder<T> extends Consumer<T> {

        
/**
         
* Adds an element to the stream being built.
         
*
         
* @throws IllegalStateException if the builder has already transitioned to
         
* the built state
         
*/

        
@Override
        
void accept(T t);

        
/**
         
* Adds an element to the stream being built.
         
*
         
* @implSpec
         
* The default implementation behaves as if:
         
* <pre>{@code
         
*
     
accept(t)
         
*
     
return this;
         
* }</pre>
         
*
         
* @param t the element to add
         
* @return {@code this} builder
         
* @throws IllegalStateException if the builder has already transitioned to
         
* the built state
         
*/

        
default Builder<T> add(T t) {
            
accept(t);
            
return this;
        
}

        
/**
         
* Builds the stream, transitioning this builder to the built state.
         
* An {@code IllegalStateException} is thrown if there are further attempts
         
* to operate on the builder after it has entered the built state.
         
*
         
* @return the built stream
         
* @throws IllegalStateException if the builder has already transitioned to
         
* the built state
         
*/

        
Stream<T> build();

    
}
}