1 /*
   2  * Copyright (c) 2012, 2016, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
  15  * accompanied this code).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 package java.util.stream;
  26 
  27 import java.nio.file.Files;
  28 import java.nio.file.Path;
  29 import java.util.Arrays;
  30 import java.util.Collection;
  31 import java.util.Comparator;
  32 import java.util.Objects;
  33 import java.util.Optional;
  34 import java.util.Spliterator;
  35 import java.util.Spliterators;
  36 import java.util.concurrent.ConcurrentHashMap;
  37 import java.util.function.BiConsumer;
  38 import java.util.function.BiFunction;
  39 import java.util.function.BinaryOperator;
  40 import java.util.function.Consumer;
  41 import java.util.function.Function;
  42 import java.util.function.IntFunction;
  43 import java.util.function.Predicate;
  44 import java.util.function.Supplier;
  45 import java.util.function.ToDoubleFunction;
  46 import java.util.function.ToIntFunction;
  47 import java.util.function.ToLongFunction;
  48 import java.util.function.UnaryOperator;
  49 
  50 /**
  51  * A sequence of elements supporting sequential and parallel aggregate
  52  * operations.  The following example illustrates an aggregate operation using
  53  * {@link Stream} and {@link IntStream}:
  54  *
  55  * <pre>{@code
  56  *     int sum = widgets.stream()
  57  *                      .filter(w -> w.getColor() == RED)
  58  *                      .mapToInt(w -> w.getWeight())
  59  *                      .sum();
  60  * }</pre>
  61  *
  62  * In this example, {@code widgets} is a {@code Collection<Widget>}.  We create
  63  * a stream of {@code Widget} objects via {@link Collection#stream Collection.stream()},
  64  * filter it to produce a stream containing only the red widgets, and then
  65  * transform it into a stream of {@code int} values representing the weight of
  66  * each red widget. Then this stream is summed to produce a total weight.
  67  *
  68  * <p>In addition to {@code Stream}, which is a stream of object references,
  69  * there are primitive specializations for {@link IntStream}, {@link LongStream},
  70  * and {@link DoubleStream}, all of which are referred to as "streams" and
  71  * conform to the characteristics and restrictions described here.
  72  *
  73  * <p>To perform a computation, stream
  74  * <a href="package-summary.html#StreamOps">operations</a> are composed into a
  75  * <em>stream pipeline</em>.  A stream pipeline consists of a source (which
  76  * might be an array, a collection, a generator function, an I/O channel,
  77  * etc), zero or more <em>intermediate operations</em> (which transform a
  78  * stream into another stream, such as {@link Stream#filter(Predicate)}), and a
  79  * <em>terminal operation</em> (which produces a result or side-effect, such
  80  * as {@link Stream#count()} or {@link Stream#forEach(Consumer)}).
  81  * Streams are lazy; computation on the source data is only performed when the
  82  * terminal operation is initiated, and source elements are consumed only
  83  * as needed.
  84  *
  85  * <p>A stream implementation is permitted significant latitude in optimizing
  86  * the computation of the result.  For example, a stream implementation is free
  87  * to elide operations (or entire stages) from a stream pipeline -- and
  88  * therefore elide invocation of behavioral parameters -- if it can prove that
  89  * it would not affect the result of the computation.  This means that
  90  * side-effects of behavioral parameters may not always be executed and should
  91  * not be relied upon, unless otherwise specified (such as by the terminal
  92  * operations {@code forEach} and {@code forEachOrdered}). (For a specific
  93  * example of such an optimization, see the API note documented on the
  94  * {@link #count} operation.  For more detail, see the
  95  * <a href="package-summary.html#SideEffects">side-effects</a> section of the
  96  * stream package documentation.)
  97  *
  98  * <p>Collections and streams, while bearing some superficial similarities,
  99  * have different goals.  Collections are primarily concerned with the efficient
 100  * management of, and access to, their elements.  By contrast, streams do not
 101  * provide a means to directly access or manipulate their elements, and are
 102  * instead concerned with declaratively describing their source and the
 103  * computational operations which will be performed in aggregate on that source.
 104  * However, if the provided stream operations do not offer the desired
 105  * functionality, the {@link #iterator()} and {@link #spliterator()} operations
 106  * can be used to perform a controlled traversal.
 107  *
 108  * <p>A stream pipeline, like the "widgets" example above, can be viewed as
 109  * a <em>query</em> on the stream source.  Unless the source was explicitly
 110  * designed for concurrent modification (such as a {@link ConcurrentHashMap}),
 111  * unpredictable or erroneous behavior may result from modifying the stream
 112  * source while it is being queried.
 113  *
 114  * <p>Most stream operations accept parameters that describe user-specified
 115  * behavior, such as the lambda expression {@code w -> w.getWeight()} passed to
 116  * {@code mapToInt} in the example above.  To preserve correct behavior,
 117  * these <em>behavioral parameters</em>:
 118  * <ul>
 119  * <li>must be <a href="package-summary.html#NonInterference">non-interfering</a>
 120  * (they do not modify the stream source); and</li>
 121  * <li>in most cases must be <a href="package-summary.html#Statelessness">stateless</a>
 122  * (their result should not depend on any state that might change during execution
 123  * of the stream pipeline).</li>
 124  * </ul>
 125  *
 126  * <p>Such parameters are always instances of a
 127  * <a href="../function/package-summary.html">functional interface</a> such
 128  * as {@link java.util.function.Function}, and are often lambda expressions or
 129  * method references.  Unless otherwise specified these parameters must be
 130  * <em>non-null</em>.
 131  *
 132  * <p>A stream should be operated on (invoking an intermediate or terminal stream
 133  * operation) only once.  This rules out, for example, "forked" streams, where
 134  * the same source feeds two or more pipelines, or multiple traversals of the
 135  * same stream.  A stream implementation may throw {@link IllegalStateException}
 136  * if it detects that the stream is being reused. However, since some stream
 137  * operations may return their receiver rather than a new stream object, it may
 138  * not be possible to detect reuse in all cases.
 139  *
 140  * <p>Streams have a {@link #close()} method and implement {@link AutoCloseable}.
 141  * Operating on a stream after it has been closed will throw {@link IllegalStateException}.
 142  * Most stream instances do not actually need to be closed after use, as they
 143  * are backed by collections, arrays, or generating functions, which require no
 144  * special resource management. Generally, only streams whose source is an IO channel,
 145  * such as those returned by {@link Files#lines(Path)}, will require closing. If a
 146  * stream does require closing, it must be opened as a resource within a try-with-resources
 147  * statement or similar control structure to ensure that it is closed promptly after its
 148  * operations have completed.
 149  *
 150  * <p>Stream pipelines may execute either sequentially or in
 151  * <a href="package-summary.html#Parallelism">parallel</a>.  This
 152  * execution mode is a property of the stream.  Streams are created
 153  * with an initial choice of sequential or parallel execution.  (For example,
 154  * {@link Collection#stream() Collection.stream()} creates a sequential stream,
 155  * and {@link Collection#parallelStream() Collection.parallelStream()} creates
 156  * a parallel one.)  This choice of execution mode may be modified by the
 157  * {@link #sequential()} or {@link #parallel()} methods, and may be queried with
 158  * the {@link #isParallel()} method.
 159  *
 160  * @param <T> the type of the stream elements
 161  * @since 1.8
 162  * @see IntStream
 163  * @see LongStream
 164  * @see DoubleStream
 165  * @see <a href="package-summary.html">java.util.stream</a>
 166  */
 167 public interface Stream<T> extends BaseStream<T, Stream<T>> {
 168 
 169     /**
 170      * Returns a stream consisting of the elements of this stream that match
 171      * the given predicate.
 172      *
 173      * <p>This is an <a href="package-summary.html#StreamOps">intermediate
 174      * operation</a>.
 175      *
 176      * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>,
 177      *                  <a href="package-summary.html#Statelessness">stateless</a>
 178      *                  predicate to apply to each element to determine if it
 179      *                  should be included
 180      * @return the new stream
 181      */
 182     Stream<T> filter(Predicate<? super T> predicate);
 183 
 184     /**
 185      * Returns a stream consisting of the results of applying the given
 186      * function to the elements of this stream.
 187      *
 188      * <p>This is an <a href="package-summary.html#StreamOps">intermediate
 189      * operation</a>.
 190      *
 191      * @param <R> The element type of the new stream
 192      * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
 193      *               <a href="package-summary.html#Statelessness">stateless</a>
 194      *               function to apply to each element
 195      * @return the new stream
 196      */
 197     <R> Stream<R> map(Function<? super T, ? extends R> mapper);
 198 
 199     /**
 200      * Returns an {@code IntStream} consisting of the results of applying the
 201      * given function to the elements of this stream.
 202      *
 203      * <p>This is an <a href="package-summary.html#StreamOps">
 204      *     intermediate operation</a>.
 205      *
 206      * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
 207      *               <a href="package-summary.html#Statelessness">stateless</a>
 208      *               function to apply to each element
 209      * @return the new stream
 210      */
 211     IntStream mapToInt(ToIntFunction<? super T> mapper);
 212 
 213     /**
 214      * Returns a {@code LongStream} consisting of the results of applying the
 215      * given function to the elements of this stream.
 216      *
 217      * <p>This is an <a href="package-summary.html#StreamOps">intermediate
 218      * operation</a>.
 219      *
 220      * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
 221      *               <a href="package-summary.html#Statelessness">stateless</a>
 222      *               function to apply to each element
 223      * @return the new stream
 224      */
 225     LongStream mapToLong(ToLongFunction<? super T> mapper);
 226 
 227     /**
 228      * Returns a {@code DoubleStream} consisting of the results of applying the
 229      * given function to the elements of this stream.
 230      *
 231      * <p>This is an <a href="package-summary.html#StreamOps">intermediate
 232      * operation</a>.
 233      *
 234      * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
 235      *               <a href="package-summary.html#Statelessness">stateless</a>
 236      *               function to apply to each element
 237      * @return the new stream
 238      */
 239     DoubleStream mapToDouble(ToDoubleFunction<? super T> mapper);
 240 
 241     /**
 242      * Returns a stream consisting of the results of replacing each element of
 243      * this stream with the contents of a mapped stream produced by applying
 244      * the provided mapping function to each element.  Each mapped stream is
 245      * {@link java.util.stream.BaseStream#close() closed} after its contents
 246      * have been placed into this stream.  (If a mapped stream is {@code null}
 247      * an empty stream is used, instead.)
 248      *
 249      * <p>This is an <a href="package-summary.html#StreamOps">intermediate
 250      * operation</a>.
 251      *
 252      * @apiNote
 253      * The {@code flatMap()} operation has the effect of applying a one-to-many
 254      * transformation to the elements of the stream, and then flattening the
 255      * resulting elements into a new stream.
 256      *
 257      * <p><b>Examples.</b>
 258      *
 259      * <p>If {@code orders} is a stream of purchase orders, and each purchase
 260      * order contains a collection of line items, then the following produces a
 261      * stream containing all the line items in all the orders:
 262      * <pre>{@code
 263      *     orders.flatMap(order -> order.getLineItems().stream())...
 264      * }</pre>
 265      *
 266      * <p>If {@code path} is the path to a file, then the following produces a
 267      * stream of the {@code words} contained in that file:
 268      * <pre>{@code
 269      *     Stream<String> lines = Files.lines(path, StandardCharsets.UTF_8);
 270      *     Stream<String> words = lines.flatMap(line -> Stream.of(line.split(" +")));
 271      * }</pre>
 272      * The {@code mapper} function passed to {@code flatMap} splits a line,
 273      * using a simple regular expression, into an array of words, and then
 274      * creates a stream of words from that array.
 275      *
 276      * @param <R> The element type of the new stream
 277      * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
 278      *               <a href="package-summary.html#Statelessness">stateless</a>
 279      *               function to apply to each element which produces a stream
 280      *               of new values
 281      * @return the new stream
 282      */
 283     <R> Stream<R> flatMap(Function<? super T, ? extends Stream<? extends R>> mapper);
 284 
 285     /**
 286      * Returns an {@code IntStream} consisting of the results of replacing each
 287      * element of this stream with the contents of a mapped stream produced by
 288      * applying the provided mapping function to each element.  Each mapped
 289      * stream is {@link java.util.stream.BaseStream#close() closed} after its
 290      * contents have been placed into this stream.  (If a mapped stream is
 291      * {@code null} an empty stream is used, instead.)
 292      *
 293      * <p>This is an <a href="package-summary.html#StreamOps">intermediate
 294      * operation</a>.
 295      *
 296      * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
 297      *               <a href="package-summary.html#Statelessness">stateless</a>
 298      *               function to apply to each element which produces a stream
 299      *               of new values
 300      * @return the new stream
 301      * @see #flatMap(Function)
 302      */
 303     IntStream flatMapToInt(Function<? super T, ? extends IntStream> mapper);
 304 
 305     /**
 306      * Returns an {@code LongStream} consisting of the results of replacing each
 307      * element of this stream with the contents of a mapped stream produced by
 308      * applying the provided mapping function to each element.  Each mapped
 309      * stream is {@link java.util.stream.BaseStream#close() closed} after its
 310      * contents have been placed into this stream.  (If a mapped stream is
 311      * {@code null} an empty stream is used, instead.)
 312      *
 313      * <p>This is an <a href="package-summary.html#StreamOps">intermediate
 314      * operation</a>.
 315      *
 316      * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
 317      *               <a href="package-summary.html#Statelessness">stateless</a>
 318      *               function to apply to each element which produces a stream
 319      *               of new values
 320      * @return the new stream
 321      * @see #flatMap(Function)
 322      */
 323     LongStream flatMapToLong(Function<? super T, ? extends LongStream> mapper);
 324 
 325     /**
 326      * Returns an {@code DoubleStream} consisting of the results of replacing
 327      * each element of this stream with the contents of a mapped stream produced
 328      * by applying the provided mapping function to each element.  Each mapped
 329      * stream is {@link java.util.stream.BaseStream#close() closed} after its
 330      * contents have placed been into this stream.  (If a mapped stream is
 331      * {@code null} an empty stream is used, instead.)
 332      *
 333      * <p>This is an <a href="package-summary.html#StreamOps">intermediate
 334      * operation</a>.
 335      *
 336      * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
 337      *               <a href="package-summary.html#Statelessness">stateless</a>
 338      *               function to apply to each element which produces a stream
 339      *               of new values
 340      * @return the new stream
 341      * @see #flatMap(Function)
 342      */
 343     DoubleStream flatMapToDouble(Function<? super T, ? extends DoubleStream> mapper);
 344 
 345     /**
 346      * Returns a stream consisting of the distinct elements (according to
 347      * {@link Object#equals(Object)}) of this stream.
 348      *
 349      * <p>For ordered streams, the selection of distinct elements is stable
 350      * (for duplicated elements, the element appearing first in the encounter
 351      * order is preserved.)  For unordered streams, no stability guarantees
 352      * are made.
 353      *
 354      * <p>This is a <a href="package-summary.html#StreamOps">stateful
 355      * intermediate operation</a>.
 356      *
 357      * @apiNote
 358      * Preserving stability for {@code distinct()} in parallel pipelines is
 359      * relatively expensive (requires that the operation act as a full barrier,
 360      * with substantial buffering overhead), and stability is often not needed.
 361      * Using an unordered stream source (such as {@link #generate(Supplier)})
 362      * or removing the ordering constraint with {@link #unordered()} may result
 363      * in significantly more efficient execution for {@code distinct()} in parallel
 364      * pipelines, if the semantics of your situation permit.  If consistency
 365      * with encounter order is required, and you are experiencing poor performance
 366      * or memory utilization with {@code distinct()} in parallel pipelines,
 367      * switching to sequential execution with {@link #sequential()} may improve
 368      * performance.
 369      *
 370      * @return the new stream
 371      */
 372     Stream<T> distinct();
 373 
 374     /**
 375      * Returns a stream consisting of the elements of this stream, sorted
 376      * according to natural order.  If the elements of this stream are not
 377      * {@code Comparable}, a {@code java.lang.ClassCastException} may be thrown
 378      * when the terminal operation is executed.
 379      *
 380      * <p>For ordered streams, the sort is stable.  For unordered streams, no
 381      * stability guarantees are made.
 382      *
 383      * <p>This is a <a href="package-summary.html#StreamOps">stateful
 384      * intermediate operation</a>.
 385      *
 386      * @return the new stream
 387      */
 388     Stream<T> sorted();
 389 
 390     /**
 391      * Returns a stream consisting of the elements of this stream, sorted
 392      * according to the provided {@code Comparator}.
 393      *
 394      * <p>For ordered streams, the sort is stable.  For unordered streams, no
 395      * stability guarantees are made.
 396      *
 397      * <p>This is a <a href="package-summary.html#StreamOps">stateful
 398      * intermediate operation</a>.
 399      *
 400      * @param comparator a <a href="package-summary.html#NonInterference">non-interfering</a>,
 401      *                   <a href="package-summary.html#Statelessness">stateless</a>
 402      *                   {@code Comparator} to be used to compare stream elements
 403      * @return the new stream
 404      */
 405     Stream<T> sorted(Comparator<? super T> comparator);
 406 
 407     /**
 408      * Returns a stream consisting of the elements of this stream, additionally
 409      * performing the provided action on each element as elements are consumed
 410      * from the resulting stream.
 411      *
 412      * <p>This is an <a href="package-summary.html#StreamOps">intermediate
 413      * operation</a>.
 414      *
 415      * <p>For parallel stream pipelines, the action may be called at
 416      * whatever time and in whatever thread the element is made available by the
 417      * upstream operation.  If the action modifies shared state,
 418      * it is responsible for providing the required synchronization.
 419      *
 420      * @apiNote This method exists mainly to support debugging, where you want
 421      * to see the elements as they flow past a certain point in a pipeline:
 422      * <pre>{@code
 423      *     Stream.of("one", "two", "three", "four")
 424      *         .filter(e -> e.length() > 3)
 425      *         .peek(e -> System.out.println("Filtered value: " + e))
 426      *         .map(String::toUpperCase)
 427      *         .peek(e -> System.out.println("Mapped value: " + e))
 428      *         .collect(Collectors.toList());
 429      * }</pre>
 430      *
 431      * <p>In cases where the stream implementation is able to optimize away the
 432      * production of some or all the elements (such as with short-circuiting
 433      * operations like {@code findFirst}, or in the example described in
 434      * {@link #count}), the action will not be invoked for those elements.
 435      *
 436      * @param action a <a href="package-summary.html#NonInterference">
 437      *                 non-interfering</a> action to perform on the elements as
 438      *                 they are consumed from the stream
 439      * @return the new stream
 440      */
 441     Stream<T> peek(Consumer<? super T> action);
 442 
 443     /**
 444      * Returns a stream consisting of the elements of this stream, truncated
 445      * to be no longer than {@code maxSize} in length.
 446      *
 447      * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
 448      * stateful intermediate operation</a>.
 449      *
 450      * @apiNote
 451      * While {@code limit()} is generally a cheap operation on sequential
 452      * stream pipelines, it can be quite expensive on ordered parallel pipelines,
 453      * especially for large values of {@code maxSize}, since {@code limit(n)}
 454      * is constrained to return not just any <em>n</em> elements, but the
 455      * <em>first n</em> elements in the encounter order.  Using an unordered
 456      * stream source (such as {@link #generate(Supplier)}) or removing the
 457      * ordering constraint with {@link #unordered()} may result in significant
 458      * speedups of {@code limit()} in parallel pipelines, if the semantics of
 459      * your situation permit.  If consistency with encounter order is required,
 460      * and you are experiencing poor performance or memory utilization with
 461      * {@code limit()} in parallel pipelines, switching to sequential execution
 462      * with {@link #sequential()} may improve performance.
 463      *
 464      * @param maxSize the number of elements the stream should be limited to
 465      * @return the new stream
 466      * @throws IllegalArgumentException if {@code maxSize} is negative
 467      */
 468     Stream<T> limit(long maxSize);
 469 
 470     /**
 471      * Returns a stream consisting of the remaining elements of this stream
 472      * after discarding the first {@code n} elements of the stream.
 473      * If this stream contains fewer than {@code n} elements then an
 474      * empty stream will be returned.
 475      *
 476      * <p>This is a <a href="package-summary.html#StreamOps">stateful
 477      * intermediate operation</a>.
 478      *
 479      * @apiNote
 480      * While {@code skip()} is generally a cheap operation on sequential
 481      * stream pipelines, it can be quite expensive on ordered parallel pipelines,
 482      * especially for large values of {@code n}, since {@code skip(n)}
 483      * is constrained to skip not just any <em>n</em> elements, but the
 484      * <em>first n</em> elements in the encounter order.  Using an unordered
 485      * stream source (such as {@link #generate(Supplier)}) or removing the
 486      * ordering constraint with {@link #unordered()} may result in significant
 487      * speedups of {@code skip()} in parallel pipelines, if the semantics of
 488      * your situation permit.  If consistency with encounter order is required,
 489      * and you are experiencing poor performance or memory utilization with
 490      * {@code skip()} in parallel pipelines, switching to sequential execution
 491      * with {@link #sequential()} may improve performance.
 492      *
 493      * @param n the number of leading elements to skip
 494      * @return the new stream
 495      * @throws IllegalArgumentException if {@code n} is negative
 496      */
 497     Stream<T> skip(long n);
 498 
 499     /**
 500      * Returns, if this stream is ordered, a stream consisting of the longest
 501      * prefix of elements taken from this stream that match the given predicate.
 502      * Otherwise returns, if this stream is unordered, a stream consisting of a
 503      * subset of elements taken from this stream that match the given predicate.
 504      *
 505      * <p>If this stream is ordered then the longest prefix is a contiguous
 506      * sequence of elements of this stream that match the given predicate.  The
 507      * first element of the sequence is the first element of this stream, and
 508      * the element immediately following the last element of the sequence does
 509      * not match the given predicate.
 510      *
 511      * <p>If this stream is unordered, and some (but not all) elements of this
 512      * stream match the given predicate, then the behavior of this operation is
 513      * nondeterministic; it is free to take any subset of matching elements
 514      * (which includes the empty set).
 515      *
 516      * <p>Independent of whether this stream is ordered or unordered if all
 517      * elements of this stream match the given predicate then this operation
 518      * takes all elements (the result is the same as the input), or if no
 519      * elements of the stream match the given predicate then no elements are
 520      * taken (the result is an empty stream).
 521      *
 522      * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
 523      * stateful intermediate operation</a>.
 524      *
 525      * @implSpec
 526      * The default implementation obtains the {@link #spliterator() spliterator}
 527      * of this stream, wraps that spliterator so as to support the semantics
 528      * of this operation on traversal, and returns a new stream associated with
 529      * the wrapped spliterator.  The returned stream preserves the execution
 530      * characteristics of this stream (namely parallel or sequential execution
 531      * as per {@link #isParallel()}) but the wrapped spliterator may choose to
 532      * not support splitting.  When the returned stream is closed, the close
 533      * handlers for both the returned and this stream are invoked.
 534      *
 535      * @apiNote
 536      * While {@code takeWhile()} is generally a cheap operation on sequential
 537      * stream pipelines, it can be quite expensive on ordered parallel
 538      * pipelines, since the operation is constrained to return not just any
 539      * valid prefix, but the longest prefix of elements in the encounter order.
 540      * Using an unordered stream source (such as {@link #generate(Supplier)}) or
 541      * removing the ordering constraint with {@link #unordered()} may result in
 542      * significant speedups of {@code takeWhile()} in parallel pipelines, if the
 543      * semantics of your situation permit.  If consistency with encounter order
 544      * is required, and you are experiencing poor performance or memory
 545      * utilization with {@code takeWhile()} in parallel pipelines, switching to
 546      * sequential execution with {@link #sequential()} may improve performance.
 547      *
 548      * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>,
 549      *                  <a href="package-summary.html#Statelessness">stateless</a>
 550      *                  predicate to apply to elements to determine the longest
 551      *                  prefix of elements.
 552      * @return the new stream
 553      * @since 9
 554      */
 555     default Stream<T> takeWhile(Predicate<? super T> predicate) {
 556         Objects.requireNonNull(predicate);
 557         // Reuses the unordered spliterator, which, when encounter is present,
 558         // is safe to use as long as it configured not to split
 559         return StreamSupport.stream(
 560                 new WhileOps.UnorderedWhileSpliterator.OfRef.Taking<>(spliterator(), true, predicate),
 561                 isParallel()).onClose(this::close);
 562     }
 563 
 564     /**
 565      * Returns, if this stream is ordered, a stream consisting of the remaining
 566      * elements of this stream after dropping the longest prefix of elements
 567      * that match the given predicate.  Otherwise returns, if this stream is
 568      * unordered, a stream consisting of the remaining elements of this stream
 569      * after dropping a subset of elements that match the given predicate.
 570      *
 571      * <p>If this stream is ordered then the longest prefix is a contiguous
 572      * sequence of elements of this stream that match the given predicate.  The
 573      * first element of the sequence is the first element of this stream, and
 574      * the element immediately following the last element of the sequence does
 575      * not match the given predicate.
 576      *
 577      * <p>If this stream is unordered, and some (but not all) elements of this
 578      * stream match the given predicate, then the behavior of this operation is
 579      * nondeterministic; it is free to drop any subset of matching elements
 580      * (which includes the empty set).
 581      *
 582      * <p>Independent of whether this stream is ordered or unordered if all
 583      * elements of this stream match the given predicate then this operation
 584      * drops all elements (the result is an empty stream), or if no elements of
 585      * the stream match the given predicate then no elements are dropped (the
 586      * result is the same as the input).
 587      *
 588      * <p>This is a <a href="package-summary.html#StreamOps">stateful
 589      * intermediate operation</a>.
 590      *
 591      * @implSpec
 592      * The default implementation obtains the {@link #spliterator() spliterator}
 593      * of this stream, wraps that spliterator so as to support the semantics
 594      * of this operation on traversal, and returns a new stream associated with
 595      * the wrapped spliterator.  The returned stream preserves the execution
 596      * characteristics of this stream (namely parallel or sequential execution
 597      * as per {@link #isParallel()}) but the wrapped spliterator may choose to
 598      * not support splitting.  When the returned stream is closed, the close
 599      * handlers for both the returned and this stream are invoked.
 600      *
 601      * @apiNote
 602      * While {@code dropWhile()} is generally a cheap operation on sequential
 603      * stream pipelines, it can be quite expensive on ordered parallel
 604      * pipelines, since the operation is constrained to return not just any
 605      * valid prefix, but the longest prefix of elements in the encounter order.
 606      * Using an unordered stream source (such as {@link #generate(Supplier)}) or
 607      * removing the ordering constraint with {@link #unordered()} may result in
 608      * significant speedups of {@code dropWhile()} in parallel pipelines, if the
 609      * semantics of your situation permit.  If consistency with encounter order
 610      * is required, and you are experiencing poor performance or memory
 611      * utilization with {@code dropWhile()} in parallel pipelines, switching to
 612      * sequential execution with {@link #sequential()} may improve performance.
 613      *
 614      * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>,
 615      *                  <a href="package-summary.html#Statelessness">stateless</a>
 616      *                  predicate to apply to elements to determine the longest
 617      *                  prefix of elements.
 618      * @return the new stream
 619      * @since 9
 620      */
 621     default Stream<T> dropWhile(Predicate<? super T> predicate) {
 622         Objects.requireNonNull(predicate);
 623         // Reuses the unordered spliterator, which, when encounter is present,
 624         // is safe to use as long as it configured not to split
 625         return StreamSupport.stream(
 626                 new WhileOps.UnorderedWhileSpliterator.OfRef.Dropping<>(spliterator(), true, predicate),
 627                 isParallel()).onClose(this::close);
 628     }
 629 
 630     /**
 631      * Performs an action for each element of this stream.
 632      *
 633      * <p>This is a <a href="package-summary.html#StreamOps">terminal
 634      * operation</a>.
 635      *
 636      * <p>The behavior of this operation is explicitly nondeterministic.
 637      * For parallel stream pipelines, this operation does <em>not</em>
 638      * guarantee to respect the encounter order of the stream, as doing so
 639      * would sacrifice the benefit of parallelism.  For any given element, the
 640      * action may be performed at whatever time and in whatever thread the
 641      * library chooses.  If the action accesses shared state, it is
 642      * responsible for providing the required synchronization.
 643      *
 644      * @param action a <a href="package-summary.html#NonInterference">
 645      *               non-interfering</a> action to perform on the elements
 646      */
 647     void forEach(Consumer<? super T> action);
 648 
 649     /**
 650      * Performs an action for each element of this stream, in the encounter
 651      * order of the stream if the stream has a defined encounter order.
 652      *
 653      * <p>This is a <a href="package-summary.html#StreamOps">terminal
 654      * operation</a>.
 655      *
 656      * <p>This operation processes the elements one at a time, in encounter
 657      * order if one exists.  Performing the action for one element
 658      * <a href="../concurrent/package-summary.html#MemoryVisibility"><i>happens-before</i></a>
 659      * performing the action for subsequent elements, but for any given element,
 660      * the action may be performed in whatever thread the library chooses.
 661      *
 662      * @param action a <a href="package-summary.html#NonInterference">
 663      *               non-interfering</a> action to perform on the elements
 664      * @see #forEach(Consumer)
 665      */
 666     void forEachOrdered(Consumer<? super T> action);
 667 
 668     /**
 669      * Returns an array containing the elements of this stream.
 670      *
 671      * <p>This is a <a href="package-summary.html#StreamOps">terminal
 672      * operation</a>.
 673      *
 674      * @return an array containing the elements of this stream
 675      */
 676     Object[] toArray();
 677 
 678     /**
 679      * Returns an array containing the elements of this stream, using the
 680      * provided {@code generator} function to allocate the returned array, as
 681      * well as any additional arrays that might be required for a partitioned
 682      * execution or for resizing.
 683      *
 684      * <p>This is a <a href="package-summary.html#StreamOps">terminal
 685      * operation</a>.
 686      *
 687      * @apiNote
 688      * The generator function takes an integer, which is the size of the
 689      * desired array, and produces an array of the desired size.  This can be
 690      * concisely expressed with an array constructor reference:
 691      * <pre>{@code
 692      *     Person[] men = people.stream()
 693      *                          .filter(p -> p.getGender() == MALE)
 694      *                          .toArray(Person[]::new);
 695      * }</pre>
 696      *
 697      * @param <A> the element type of the resulting array
 698      * @param generator a function which produces a new array of the desired
 699      *                  type and the provided length
 700      * @return an array containing the elements in this stream
 701      * @throws ArrayStoreException if the runtime type of the array returned
 702      * from the array generator is not a supertype of the runtime type of every
 703      * element in this stream
 704      */
 705     <A> A[] toArray(IntFunction<A[]> generator);
 706 
 707     /**
 708      * Performs a <a href="package-summary.html#Reduction">reduction</a> on the
 709      * elements of this stream, using the provided identity value and an
 710      * <a href="package-summary.html#Associativity">associative</a>
 711      * accumulation function, and returns the reduced value.  This is equivalent
 712      * to:
 713      * <pre>{@code
 714      *     T result = identity;
 715      *     for (T element : this stream)
 716      *         result = accumulator.apply(result, element)
 717      *     return result;
 718      * }</pre>
 719      *
 720      * but is not constrained to execute sequentially.
 721      *
 722      * <p>The {@code identity} value must be an identity for the accumulator
 723      * function. This means that for all {@code t},
 724      * {@code accumulator.apply(identity, t)} is equal to {@code t}.
 725      * The {@code accumulator} function must be an
 726      * <a href="package-summary.html#Associativity">associative</a> function.
 727      *
 728      * <p>This is a <a href="package-summary.html#StreamOps">terminal
 729      * operation</a>.
 730      *
 731      * @apiNote Sum, min, max, average, and string concatenation are all special
 732      * cases of reduction. Summing a stream of numbers can be expressed as:
 733      *
 734      * <pre>{@code
 735      *     Integer sum = integers.reduce(0, (a, b) -> a+b);
 736      * }</pre>
 737      *
 738      * or:
 739      *
 740      * <pre>{@code
 741      *     Integer sum = integers.reduce(0, Integer::sum);
 742      * }</pre>
 743      *
 744      * <p>While this may seem a more roundabout way to perform an aggregation
 745      * compared to simply mutating a running total in a loop, reduction
 746      * operations parallelize more gracefully, without needing additional
 747      * synchronization and with greatly reduced risk of data races.
 748      *
 749      * @param identity the identity value for the accumulating function
 750      * @param accumulator an <a href="package-summary.html#Associativity">associative</a>,
 751      *                    <a href="package-summary.html#NonInterference">non-interfering</a>,
 752      *                    <a href="package-summary.html#Statelessness">stateless</a>
 753      *                    function for combining two values
 754      * @return the result of the reduction
 755      */
 756     T reduce(T identity, BinaryOperator<T> accumulator);
 757 
 758     /**
 759      * Performs a <a href="package-summary.html#Reduction">reduction</a> on the
 760      * elements of this stream, using an
 761      * <a href="package-summary.html#Associativity">associative</a> accumulation
 762      * function, and returns an {@code Optional} describing the reduced value,
 763      * if any. This is equivalent to:
 764      * <pre>{@code
 765      *     boolean foundAny = false;
 766      *     T result = null;
 767      *     for (T element : this stream) {
 768      *         if (!foundAny) {
 769      *             foundAny = true;
 770      *             result = element;
 771      *         }
 772      *         else
 773      *             result = accumulator.apply(result, element);
 774      *     }
 775      *     return foundAny ? Optional.of(result) : Optional.empty();
 776      * }</pre>
 777      *
 778      * but is not constrained to execute sequentially.
 779      *
 780      * <p>The {@code accumulator} function must be an
 781      * <a href="package-summary.html#Associativity">associative</a> function.
 782      *
 783      * <p>This is a <a href="package-summary.html#StreamOps">terminal
 784      * operation</a>.
 785      *
 786      * @param accumulator an <a href="package-summary.html#Associativity">associative</a>,
 787      *                    <a href="package-summary.html#NonInterference">non-interfering</a>,
 788      *                    <a href="package-summary.html#Statelessness">stateless</a>
 789      *                    function for combining two values
 790      * @return an {@link Optional} describing the result of the reduction
 791      * @throws NullPointerException if the result of the reduction is null
 792      * @see #reduce(Object, BinaryOperator)
 793      * @see #min(Comparator)
 794      * @see #max(Comparator)
 795      */
 796     Optional<T> reduce(BinaryOperator<T> accumulator);
 797 
 798     /**
 799      * Performs a <a href="package-summary.html#Reduction">reduction</a> on the
 800      * elements of this stream, using the provided identity, accumulation and
 801      * combining functions.  This is equivalent to:
 802      * <pre>{@code
 803      *     U result = identity;
 804      *     for (T element : this stream)
 805      *         result = accumulator.apply(result, element)
 806      *     return result;
 807      * }</pre>
 808      *
 809      * but is not constrained to execute sequentially.
 810      *
 811      * <p>The {@code identity} value must be an identity for the combiner
 812      * function.  This means that for all {@code u}, {@code combiner(identity, u)}
 813      * is equal to {@code u}.  Additionally, the {@code combiner} function
 814      * must be compatible with the {@code accumulator} function; for all
 815      * {@code u} and {@code t}, the following must hold:
 816      * <pre>{@code
 817      *     combiner.apply(u, accumulator.apply(identity, t)) == accumulator.apply(u, t)
 818      * }</pre>
 819      *
 820      * <p>This is a <a href="package-summary.html#StreamOps">terminal
 821      * operation</a>.
 822      *
 823      * @apiNote Many reductions using this form can be represented more simply
 824      * by an explicit combination of {@code map} and {@code reduce} operations.
 825      * The {@code accumulator} function acts as a fused mapper and accumulator,
 826      * which can sometimes be more efficient than separate mapping and reduction,
 827      * such as when knowing the previously reduced value allows you to avoid
 828      * some computation.
 829      *
 830      * @param <U> The type of the result
 831      * @param identity the identity value for the combiner function
 832      * @param accumulator an <a href="package-summary.html#Associativity">associative</a>,
 833      *                    <a href="package-summary.html#NonInterference">non-interfering</a>,
 834      *                    <a href="package-summary.html#Statelessness">stateless</a>
 835      *                    function for incorporating an additional element into a result
 836      * @param combiner an <a href="package-summary.html#Associativity">associative</a>,
 837      *                    <a href="package-summary.html#NonInterference">non-interfering</a>,
 838      *                    <a href="package-summary.html#Statelessness">stateless</a>
 839      *                    function for combining two values, which must be
 840      *                    compatible with the accumulator function
 841      * @return the result of the reduction
 842      * @see #reduce(BinaryOperator)
 843      * @see #reduce(Object, BinaryOperator)
 844      */
 845     <U> U reduce(U identity,
 846                  BiFunction<U, ? super T, U> accumulator,
 847                  BinaryOperator<U> combiner);
 848 
 849     /**
 850      * Performs a <a href="package-summary.html#MutableReduction">mutable
 851      * reduction</a> operation on the elements of this stream.  A mutable
 852      * reduction is one in which the reduced value is a mutable result container,
 853      * such as an {@code ArrayList}, and elements are incorporated by updating
 854      * the state of the result rather than by replacing the result.  This
 855      * produces a result equivalent to:
 856      * <pre>{@code
 857      *     R result = supplier.get();
 858      *     for (T element : this stream)
 859      *         accumulator.accept(result, element);
 860      *     return result;
 861      * }</pre>
 862      *
 863      * <p>Like {@link #reduce(Object, BinaryOperator)}, {@code collect} operations
 864      * can be parallelized without requiring additional synchronization.
 865      *
 866      * <p>This is a <a href="package-summary.html#StreamOps">terminal
 867      * operation</a>.
 868      *
 869      * @apiNote There are many existing classes in the JDK whose signatures are
 870      * well-suited for use with method references as arguments to {@code collect()}.
 871      * For example, the following will accumulate strings into an {@code ArrayList}:
 872      * <pre>{@code
 873      *     List<String> asList = stringStream.collect(ArrayList::new, ArrayList::add,
 874      *                                                ArrayList::addAll);
 875      * }</pre>
 876      *
 877      * <p>The following will take a stream of strings and concatenates them into a
 878      * single string:
 879      * <pre>{@code
 880      *     String concat = stringStream.collect(StringBuilder::new, StringBuilder::append,
 881      *                                          StringBuilder::append)
 882      *                                 .toString();
 883      * }</pre>
 884      *
 885      * @param <R> the type of the mutable result container
 886      * @param supplier a function that creates a new mutable result container.
 887      *                 For a parallel execution, this function may be called
 888      *                 multiple times and must return a fresh value each time.
 889      * @param accumulator an <a href="package-summary.html#Associativity">associative</a>,
 890      *                    <a href="package-summary.html#NonInterference">non-interfering</a>,
 891      *                    <a href="package-summary.html#Statelessness">stateless</a>
 892      *                    function that must fold an element into a result
 893      *                    container.
 894      * @param combiner an <a href="package-summary.html#Associativity">associative</a>,
 895      *                    <a href="package-summary.html#NonInterference">non-interfering</a>,
 896      *                    <a href="package-summary.html#Statelessness">stateless</a>
 897      *                    function that accepts two partial result containers
 898      *                    and merges them, which must be compatible with the
 899      *                    accumulator function.  The combiner function must fold
 900      *                    the elements from the second result container into the
 901      *                    first result container.
 902      * @return the result of the reduction
 903      */
 904     <R> R collect(Supplier<R> supplier,
 905                   BiConsumer<R, ? super T> accumulator,
 906                   BiConsumer<R, R> combiner);
 907 
 908     /**
 909      * Performs a <a href="package-summary.html#MutableReduction">mutable
 910      * reduction</a> operation on the elements of this stream using a
 911      * {@code Collector}.  A {@code Collector}
 912      * encapsulates the functions used as arguments to
 913      * {@link #collect(Supplier, BiConsumer, BiConsumer)}, allowing for reuse of
 914      * collection strategies and composition of collect operations such as
 915      * multiple-level grouping or partitioning.
 916      *
 917      * <p>If the stream is parallel, and the {@code Collector}
 918      * is {@link Collector.Characteristics#CONCURRENT concurrent}, and
 919      * either the stream is unordered or the collector is
 920      * {@link Collector.Characteristics#UNORDERED unordered},
 921      * then a concurrent reduction will be performed (see {@link Collector} for
 922      * details on concurrent reduction.)
 923      *
 924      * <p>This is a <a href="package-summary.html#StreamOps">terminal
 925      * operation</a>.
 926      *
 927      * <p>When executed in parallel, multiple intermediate results may be
 928      * instantiated, populated, and merged so as to maintain isolation of
 929      * mutable data structures.  Therefore, even when executed in parallel
 930      * with non-thread-safe data structures (such as {@code ArrayList}), no
 931      * additional synchronization is needed for a parallel reduction.
 932      *
 933      * @apiNote
 934      * The following will accumulate strings into an ArrayList:
 935      * <pre>{@code
 936      *     List<String> asList = stringStream.collect(Collectors.toList());
 937      * }</pre>
 938      *
 939      * <p>The following will classify {@code Person} objects by city:
 940      * <pre>{@code
 941      *     Map<String, List<Person>> peopleByCity
 942      *         = personStream.collect(Collectors.groupingBy(Person::getCity));
 943      * }</pre>
 944      *
 945      * <p>The following will classify {@code Person} objects by state and city,
 946      * cascading two {@code Collector}s together:
 947      * <pre>{@code
 948      *     Map<String, Map<String, List<Person>>> peopleByStateAndCity
 949      *         = personStream.collect(Collectors.groupingBy(Person::getState,
 950      *                                                      Collectors.groupingBy(Person::getCity)));
 951      * }</pre>
 952      *
 953      * @param <R> the type of the result
 954      * @param <A> the intermediate accumulation type of the {@code Collector}
 955      * @param collector the {@code Collector} describing the reduction
 956      * @return the result of the reduction
 957      * @see #collect(Supplier, BiConsumer, BiConsumer)
 958      * @see Collectors
 959      */
 960     <R, A> R collect(Collector<? super T, A, R> collector);
 961 
 962     /**
 963      * Returns the minimum element of this stream according to the provided
 964      * {@code Comparator}.  This is a special case of a
 965      * <a href="package-summary.html#Reduction">reduction</a>.
 966      *
 967      * <p>This is a <a href="package-summary.html#StreamOps">terminal operation</a>.
 968      *
 969      * @param comparator a <a href="package-summary.html#NonInterference">non-interfering</a>,
 970      *                   <a href="package-summary.html#Statelessness">stateless</a>
 971      *                   {@code Comparator} to compare elements of this stream
 972      * @return an {@code Optional} describing the minimum element of this stream,
 973      * or an empty {@code Optional} if the stream is empty
 974      * @throws NullPointerException if the minimum element is null
 975      */
 976     Optional<T> min(Comparator<? super T> comparator);
 977 
 978     /**
 979      * Returns the maximum element of this stream according to the provided
 980      * {@code Comparator}.  This is a special case of a
 981      * <a href="package-summary.html#Reduction">reduction</a>.
 982      *
 983      * <p>This is a <a href="package-summary.html#StreamOps">terminal
 984      * operation</a>.
 985      *
 986      * @param comparator a <a href="package-summary.html#NonInterference">non-interfering</a>,
 987      *                   <a href="package-summary.html#Statelessness">stateless</a>
 988      *                   {@code Comparator} to compare elements of this stream
 989      * @return an {@code Optional} describing the maximum element of this stream,
 990      * or an empty {@code Optional} if the stream is empty
 991      * @throws NullPointerException if the maximum element is null
 992      */
 993     Optional<T> max(Comparator<? super T> comparator);
 994 
 995     /**
 996      * Returns the count of elements in this stream.  This is a special case of
 997      * a <a href="package-summary.html#Reduction">reduction</a> and is
 998      * equivalent to:
 999      * <pre>{@code
1000      *     return mapToLong(e -> 1L).sum();
1001      * }</pre>
1002      *
1003      * <p>This is a <a href="package-summary.html#StreamOps">terminal operation</a>.
1004      *
1005      * @apiNote
1006      * An implementation may choose to not execute the stream pipeline (either
1007      * sequentially or in parallel) if it is capable of computing the count
1008      * directly from the stream source.  In such cases no source elements will
1009      * be traversed and no intermediate operations will be evaluated.
1010      * Behavioral parameters with side-effects, which are strongly discouraged
1011      * except for harmless cases such as debugging, may be affected.  For
1012      * example, consider the following stream:
1013      * <pre>{@code
1014      *     List<String> l = Arrays.asList("A", "B", "C", "D");
1015      *     long count = l.stream().peek(System.out::println).count();
1016      * }</pre>
1017      * The number of elements covered by the stream source, a {@code List}, is
1018      * known and the intermediate operation, {@code peek}, does not inject into
1019      * or remove elements from the stream (as may be the case for
1020      * {@code flatMap} or {@code filter} operations).  Thus the count is the
1021      * size of the {@code List} and there is no need to execute the pipeline
1022      * and, as a side-effect, print out the list elements.
1023      *
1024      * @return the count of elements in this stream
1025      */
1026     long count();
1027 
1028     /**
1029      * Returns whether any elements of this stream match the provided
1030      * predicate.  May not evaluate the predicate on all elements if not
1031      * necessary for determining the result.  If the stream is empty then
1032      * {@code false} is returned and the predicate is not evaluated.
1033      *
1034      * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
1035      * terminal operation</a>.
1036      *
1037      * @apiNote
1038      * This method evaluates the <em>existential quantification</em> of the
1039      * predicate over the elements of the stream (for some x P(x)).
1040      *
1041      * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>,
1042      *                  <a href="package-summary.html#Statelessness">stateless</a>
1043      *                  predicate to apply to elements of this stream
1044      * @return {@code true} if any elements of the stream match the provided
1045      * predicate, otherwise {@code false}
1046      */
1047     boolean anyMatch(Predicate<? super T> predicate);
1048 
1049     /**
1050      * Returns whether all elements of this stream match the provided predicate.
1051      * May not evaluate the predicate on all elements if not necessary for
1052      * determining the result.  If the stream is empty then {@code true} is
1053      * returned and the predicate is not evaluated.
1054      *
1055      * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
1056      * terminal operation</a>.
1057      *
1058      * @apiNote
1059      * This method evaluates the <em>universal quantification</em> of the
1060      * predicate over the elements of the stream (for all x P(x)).  If the
1061      * stream is empty, the quantification is said to be <em>vacuously
1062      * satisfied</em> and is always {@code true} (regardless of P(x)).
1063      *
1064      * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>,
1065      *                  <a href="package-summary.html#Statelessness">stateless</a>
1066      *                  predicate to apply to elements of this stream
1067      * @return {@code true} if either all elements of the stream match the
1068      * provided predicate or the stream is empty, otherwise {@code false}
1069      */
1070     boolean allMatch(Predicate<? super T> predicate);
1071 
1072     /**
1073      * Returns whether no elements of this stream match the provided predicate.
1074      * May not evaluate the predicate on all elements if not necessary for
1075      * determining the result.  If the stream is empty then {@code true} is
1076      * returned and the predicate is not evaluated.
1077      *
1078      * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
1079      * terminal operation</a>.
1080      *
1081      * @apiNote
1082      * This method evaluates the <em>universal quantification</em> of the
1083      * negated predicate over the elements of the stream (for all x ~P(x)).  If
1084      * the stream is empty, the quantification is said to be vacuously satisfied
1085      * and is always {@code true}, regardless of P(x).
1086      *
1087      * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>,
1088      *                  <a href="package-summary.html#Statelessness">stateless</a>
1089      *                  predicate to apply to elements of this stream
1090      * @return {@code true} if either no elements of the stream match the
1091      * provided predicate or the stream is empty, otherwise {@code false}
1092      */
1093     boolean noneMatch(Predicate<? super T> predicate);
1094 
1095     /**
1096      * Returns an {@link Optional} describing the first element of this stream,
1097      * or an empty {@code Optional} if the stream is empty.  If the stream has
1098      * no encounter order, then any element may be returned.
1099      *
1100      * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
1101      * terminal operation</a>.
1102      *
1103      * @return an {@code Optional} describing the first element of this stream,
1104      * or an empty {@code Optional} if the stream is empty
1105      * @throws NullPointerException if the element selected is null
1106      */
1107     Optional<T> findFirst();
1108 
1109     /**
1110      * Returns an {@link Optional} describing some element of the stream, or an
1111      * empty {@code Optional} if the stream is empty.
1112      *
1113      * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
1114      * terminal operation</a>.
1115      *
1116      * <p>The behavior of this operation is explicitly nondeterministic; it is
1117      * free to select any element in the stream.  This is to allow for maximal
1118      * performance in parallel operations; the cost is that multiple invocations
1119      * on the same source may not return the same result.  (If a stable result
1120      * is desired, use {@link #findFirst()} instead.)
1121      *
1122      * @return an {@code Optional} describing some element of this stream, or an
1123      * empty {@code Optional} if the stream is empty
1124      * @throws NullPointerException if the element selected is null
1125      * @see #findFirst()
1126      */
1127     Optional<T> findAny();
1128 
1129     // Static factories
1130 
1131     /**
1132      * Returns a builder for a {@code Stream}.
1133      *
1134      * @param <T> type of elements
1135      * @return a stream builder
1136      */
1137     public static<T> Builder<T> builder() {
1138         return new Streams.StreamBuilderImpl<>();
1139     }
1140 
1141     /**
1142      * Returns an empty sequential {@code Stream}.
1143      *
1144      * @param <T> the type of stream elements
1145      * @return an empty sequential stream
1146      */
1147     public static<T> Stream<T> empty() {
1148         return StreamSupport.stream(Spliterators.<T>emptySpliterator(), false);
1149     }
1150 
1151     /**
1152      * Returns a sequential {@code Stream} containing a single element.
1153      *
1154      * @param t the single element
1155      * @param <T> the type of stream elements
1156      * @return a singleton sequential stream
1157      */
1158     public static<T> Stream<T> of(T t) {
1159         return StreamSupport.stream(new Streams.StreamBuilderImpl<>(t), false);
1160     }
1161 
1162     /**
1163      * Returns a sequential {@code Stream} containing a single element, if
1164      * non-null, otherwise returns an empty {@code Stream}.
1165      *
1166      * @param t the single element
1167      * @param <T> the type of stream elements
1168      * @return a stream with a single element if the specified element
1169      *         is non-null, otherwise an empty stream
1170      * @since 9
1171      */
1172     public static<T> Stream<T> ofNullable(T t) {
1173         return t == null ? Stream.empty()
1174                          : StreamSupport.stream(new Streams.StreamBuilderImpl<>(t), false);
1175     }
1176 
1177     /**
1178      * Returns a sequential ordered stream whose elements are the specified values.
1179      *
1180      * @param <T> the type of stream elements
1181      * @param values the elements of the new stream
1182      * @return the new stream
1183      */
1184     @SafeVarargs
1185     @SuppressWarnings("varargs") // Creating a stream from an array is safe
1186     public static<T> Stream<T> of(T... values) {
1187         return Arrays.stream(values);
1188     }
1189 
1190     /**
1191      * Returns an infinite sequential ordered {@code Stream} produced by iterative
1192      * application of a function {@code f} to an initial element {@code seed},
1193      * producing a {@code Stream} consisting of {@code seed}, {@code f(seed)},
1194      * {@code f(f(seed))}, etc.
1195      *
1196      * <p>The first element (position {@code 0}) in the {@code Stream} will be
1197      * the provided {@code seed}.  For {@code n > 0}, the element at position
1198      * {@code n}, will be the result of applying the function {@code f} to the
1199      * element at position {@code n - 1}.
1200      *
1201      * <p>The action of applying {@code f} for one element
1202      * <a href="../concurrent/package-summary.html#MemoryVisibility"><i>happens-before</i></a>
1203      * the action of applying {@code f} for subsequent elements.  For any given
1204      * element the action may be performed in whatever thread the library
1205      * chooses.
1206      *
1207      * @param <T> the type of stream elements
1208      * @param seed the initial element
1209      * @param f a function to be applied to the previous element to produce
1210      *          a new element
1211      * @return a new sequential {@code Stream}
1212      */
1213     public static<T> Stream<T> iterate(final T seed, final UnaryOperator<T> f) {
1214         Objects.requireNonNull(f);
1215         Spliterator<T> spliterator = new Spliterators.AbstractSpliterator<>(Long.MAX_VALUE,
1216                Spliterator.ORDERED | Spliterator.IMMUTABLE) {
1217             T prev;
1218             boolean started;
1219 
1220             @Override
1221             public boolean tryAdvance(Consumer<? super T> action) {
1222                 Objects.requireNonNull(action);
1223                 T t;
1224                 if (started)
1225                     t = f.apply(prev);
1226                 else {
1227                     t = seed;
1228                     started = true;
1229                 }
1230                 action.accept(prev = t);
1231                 return true;
1232             }
1233         };
1234         return StreamSupport.stream(spliterator, false);
1235     }
1236 
1237     /**
1238      * Returns a sequential ordered {@code Stream} produced by iterative
1239      * application of the given {@code next} function to an initial element,
1240      * conditioned on satisfying the given {@code hasNext} predicate.  The
1241      * stream terminates as soon as the {@code hasNext} predicate returns false.
1242      *
1243      * <p>{@code Stream.iterate} should produce the same sequence of elements as
1244      * produced by the corresponding for-loop:
1245      * <pre>{@code
1246      *     for (T index=seed; hasNext.test(index); index = next.apply(index)) {
1247      *         ...
1248      *     }
1249      * }</pre>
1250      *
1251      * <p>The resulting sequence may be empty if the {@code hasNext} predicate
1252      * does not hold on the seed value.  Otherwise the first element will be the
1253      * supplied {@code seed} value, the next element (if present) will be the
1254      * result of applying the {@code next} function to the {@code seed} value,
1255      * and so on iteratively until the {@code hasNext} predicate indicates that
1256      * the stream should terminate.
1257      *
1258      * <p>The action of applying the {@code hasNext} predicate to an element
1259      * <a href="../concurrent/package-summary.html#MemoryVisibility"><i>happens-before</i></a>
1260      * the action of applying the {@code next} function to that element.  The
1261      * action of applying the {@code next} function for one element
1262      * <i>happens-before</i> the action of applying the {@code hasNext}
1263      * predicate for subsequent elements.  For any given element an action may
1264      * be performed in whatever thread the library chooses.
1265      *
1266      * @param <T> the type of stream elements
1267      * @param seed the initial element
1268      * @param hasNext a predicate to apply to elements to determine when the
1269      *                stream must terminate.
1270      * @param next a function to be applied to the previous element to produce
1271      *             a new element
1272      * @return a new sequential {@code Stream}
1273      * @since 9
1274      */
1275     public static<T> Stream<T> iterate(T seed, Predicate<? super T> hasNext, UnaryOperator<T> next) {
1276         Objects.requireNonNull(next);
1277         Objects.requireNonNull(hasNext);
1278         Spliterator<T> spliterator = new Spliterators.AbstractSpliterator<>(Long.MAX_VALUE,
1279                Spliterator.ORDERED | Spliterator.IMMUTABLE) {
1280             T prev;
1281             boolean started, finished;
1282 
1283             @Override
1284             public boolean tryAdvance(Consumer<? super T> action) {
1285                 Objects.requireNonNull(action);
1286                 if (finished)
1287                     return false;
1288                 T t;
1289                 if (started)
1290                     t = next.apply(prev);
1291                 else {
1292                     t = seed;
1293                     started = true;
1294                 }
1295                 if (!hasNext.test(t)) {
1296                     prev = null;
1297                     finished = true;
1298                     return false;
1299                 }
1300                 action.accept(prev = t);
1301                 return true;
1302             }
1303 
1304             @Override
1305             public void forEachRemaining(Consumer<? super T> action) {
1306                 Objects.requireNonNull(action);
1307                 if (finished)
1308                     return;
1309                 finished = true;
1310                 T t = started ? next.apply(prev) : seed;
1311                 prev = null;
1312                 while (hasNext.test(t)) {
1313                     action.accept(t);
1314                     t = next.apply(t);
1315                 }
1316             }
1317         };
1318         return StreamSupport.stream(spliterator, false);
1319     }
1320 
1321     /**
1322      * Returns an infinite sequential unordered stream where each element is
1323      * generated by the provided {@code Supplier}.  This is suitable for
1324      * generating constant streams, streams of random elements, etc.
1325      *
1326      * @param <T> the type of stream elements
1327      * @param s the {@code Supplier} of generated elements
1328      * @return a new infinite sequential unordered {@code Stream}
1329      */
1330     public static<T> Stream<T> generate(Supplier<? extends T> s) {
1331         Objects.requireNonNull(s);
1332         return StreamSupport.stream(
1333                 new StreamSpliterators.InfiniteSupplyingSpliterator.OfRef<>(Long.MAX_VALUE, s), false);
1334     }
1335 
1336     /**
1337      * Creates a lazily concatenated stream whose elements are all the
1338      * elements of the first stream followed by all the elements of the
1339      * second stream.  The resulting stream is ordered if both
1340      * of the input streams are ordered, and parallel if either of the input
1341      * streams is parallel.  When the resulting stream is closed, the close
1342      * handlers for both input streams are invoked.
1343      *
1344      * @implNote
1345      * Use caution when constructing streams from repeated concatenation.
1346      * Accessing an element of a deeply concatenated stream can result in deep
1347      * call chains, or even {@code StackOverflowError}.
1348      *
1349      * <p>Subsequent changes to the sequential/parallel execution mode of the
1350      * returned stream are not guaranteed to be propagated to the input streams.
1351      *
1352      * @param <T> The type of stream elements
1353      * @param a the first stream
1354      * @param b the second stream
1355      * @return the concatenation of the two input streams
1356      */
1357     public static <T> Stream<T> concat(Stream<? extends T> a, Stream<? extends T> b) {
1358         Objects.requireNonNull(a);
1359         Objects.requireNonNull(b);
1360 
1361         @SuppressWarnings("unchecked")
1362         Spliterator<T> split = new Streams.ConcatSpliterator.OfRef<>(
1363                 (Spliterator<T>) a.spliterator(), (Spliterator<T>) b.spliterator());
1364         Stream<T> stream = StreamSupport.stream(split, a.isParallel() || b.isParallel());
1365         return stream.onClose(Streams.composedClose(a, b));
1366     }
1367 
1368     /**
1369      * A mutable builder for a {@code Stream}.  This allows the creation of a
1370      * {@code Stream} by generating elements individually and adding them to the
1371      * {@code Builder} (without the copying overhead that comes from using
1372      * an {@code ArrayList} as a temporary buffer.)
1373      *
1374      * <p>A stream builder has a lifecycle, which starts in a building
1375      * phase, during which elements can be added, and then transitions to a built
1376      * phase, after which elements may not be added.  The built phase begins
1377      * when the {@link #build()} method is called, which creates an ordered
1378      * {@code Stream} whose elements are the elements that were added to the stream
1379      * builder, in the order they were added.
1380      *
1381      * @param <T> the type of stream elements
1382      * @see Stream#builder()
1383      * @since 1.8
1384      */
1385     public interface Builder<T> extends Consumer<T> {
1386 
1387         /**
1388          * Adds an element to the stream being built.
1389          *
1390          * @throws IllegalStateException if the builder has already transitioned to
1391          * the built state
1392          */
1393         @Override
1394         void accept(T t);
1395 
1396         /**
1397          * Adds an element to the stream being built.
1398          *
1399          * @implSpec
1400          * The default implementation behaves as if:
1401          * <pre>{@code
1402          *     accept(t)
1403          *     return this;
1404          * }</pre>
1405          *
1406          * @param t the element to add
1407          * @return {@code this} builder
1408          * @throws IllegalStateException if the builder has already transitioned to
1409          * the built state
1410          */
1411         default Builder<T> add(T t) {
1412             accept(t);
1413             return this;
1414         }
1415 
1416         /**
1417          * Builds the stream, transitioning this builder to the built state.
1418          * An {@code IllegalStateException} is thrown if there are further attempts
1419          * to operate on the builder after it has entered the built state.
1420          *
1421          * @return the built stream
1422          * @throws IllegalStateException if the builder has already transitioned to
1423          * the built state
1424          */
1425         Stream<T> build();
1426 
1427     }
1428 }