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.util.Arrays;
  28 import java.util.DoubleSummaryStatistics;
  29 import java.util.Objects;
  30 import java.util.OptionalDouble;
  31 import java.util.PrimitiveIterator;
  32 import java.util.Spliterator;
  33 import java.util.Spliterators;
  34 import java.util.function.BiConsumer;
  35 import java.util.function.DoubleBinaryOperator;
  36 import java.util.function.DoubleConsumer;
  37 import java.util.function.DoubleFunction;
  38 import java.util.function.DoublePredicate;
  39 import java.util.function.DoubleSupplier;
  40 import java.util.function.DoubleToIntFunction;
  41 import java.util.function.DoubleToLongFunction;
  42 import java.util.function.DoubleUnaryOperator;
  43 import java.util.function.Function;
  44 import java.util.function.ObjDoubleConsumer;
  45 import java.util.function.Supplier;
  46 
  47 /**
  48  * A sequence of primitive double-valued elements supporting sequential and parallel
  49  * aggregate operations.  This is the {@code double} primitive specialization of
  50  * {@link Stream}.
  51  *
  52  * <p>The following example illustrates an aggregate operation using
  53  * {@link Stream} and {@link DoubleStream}, computing the sum of the weights of the
  54  * red widgets:
  55  *
  56  * <pre>{@code
  57  *     double sum = widgets.stream()
  58  *                         .filter(w -> w.getColor() == RED)
  59  *                         .mapToDouble(w -> w.getWeight())
  60  *                         .sum();
  61  * }</pre>
  62  *
  63  * See the class documentation for {@link Stream} and the package documentation
  64  * for <a href="package-summary.html">java.util.stream</a> for additional
  65  * specification of streams, stream operations, stream pipelines, and
  66  * parallelism.
  67  *
  68  * @since 1.8
  69  * @see Stream
  70  * @see <a href="package-summary.html">java.util.stream</a>
  71  */
  72 public interface DoubleStream extends BaseStream<Double, DoubleStream> {
  73 
  74     /**
  75      * Returns a stream consisting of the elements of this stream that match
  76      * the given predicate.
  77      *
  78      * <p>This is an <a href="package-summary.html#StreamOps">intermediate
  79      * operation</a>.
  80      *
  81      * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>,
  82      *                  <a href="package-summary.html#Statelessness">stateless</a>
  83      *                  predicate to apply to each element to determine if it
  84      *                  should be included
  85      * @return the new stream
  86      */
  87     DoubleStream filter(DoublePredicate predicate);
  88 
  89     /**
  90      * Returns a stream consisting of the results of applying the given
  91      * function to the elements of this stream.
  92      *
  93      * <p>This is an <a href="package-summary.html#StreamOps">intermediate
  94      * operation</a>.
  95      *
  96      * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
  97      *               <a href="package-summary.html#Statelessness">stateless</a>
  98      *               function to apply to each element
  99      * @return the new stream
 100      */
 101     DoubleStream map(DoubleUnaryOperator mapper);
 102 
 103     /**
 104      * Returns an object-valued {@code Stream} consisting of the results of
 105      * applying the given function to the elements of this stream.
 106      *
 107      * <p>This is an <a href="package-summary.html#StreamOps">
 108      *     intermediate operation</a>.
 109      *
 110      * @param <U> the element type of the new stream
 111      * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
 112      *               <a href="package-summary.html#Statelessness">stateless</a>
 113      *               function to apply to each element
 114      * @return the new stream
 115      */
 116     <U> Stream<U> mapToObj(DoubleFunction<? extends U> mapper);
 117 
 118     /**
 119      * Returns an {@code IntStream} consisting of the results of applying the
 120      * given function to the elements of this stream.
 121      *
 122      * <p>This is an <a href="package-summary.html#StreamOps">intermediate
 123      * operation</a>.
 124      *
 125      * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
 126      *               <a href="package-summary.html#Statelessness">stateless</a>
 127      *               function to apply to each element
 128      * @return the new stream
 129      */
 130     IntStream mapToInt(DoubleToIntFunction mapper);
 131 
 132     /**
 133      * Returns a {@code LongStream} consisting of the results of applying the
 134      * given function to the elements of this stream.
 135      *
 136      * <p>This is an <a href="package-summary.html#StreamOps">intermediate
 137      * operation</a>.
 138      *
 139      * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
 140      *               <a href="package-summary.html#Statelessness">stateless</a>
 141      *               function to apply to each element
 142      * @return the new stream
 143      */
 144     LongStream mapToLong(DoubleToLongFunction mapper);
 145 
 146     /**
 147      * Returns a stream consisting of the results of replacing each element of
 148      * this stream with the contents of a mapped stream produced by applying
 149      * the provided mapping function to each element.  Each mapped stream is
 150      * {@link java.util.stream.BaseStream#close() closed} after its contents
 151      * have been placed into this stream.  (If a mapped stream is {@code null}
 152      * an empty stream is used, instead.)
 153      *
 154      * <p>This is an <a href="package-summary.html#StreamOps">intermediate
 155      * operation</a>.
 156      *
 157      * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
 158      *               <a href="package-summary.html#Statelessness">stateless</a>
 159      *               function to apply to each element which produces a
 160      *               {@code DoubleStream} of new values
 161      * @return the new stream
 162      * @see Stream#flatMap(Function)
 163      */
 164     DoubleStream flatMap(DoubleFunction<? extends DoubleStream> mapper);
 165 
 166     /**
 167      * Returns a stream consisting of the distinct elements of this stream. The
 168      * elements are compared for equality according to
 169      * {@link java.lang.Double#compare(double, double)}.
 170      *
 171      * <p>This is a <a href="package-summary.html#StreamOps">stateful
 172      * intermediate operation</a>.
 173      *
 174      * @return the result stream
 175      */
 176     DoubleStream distinct();
 177 
 178     /**
 179      * Returns a stream consisting of the elements of this stream in sorted
 180      * order. The elements are compared for equality according to
 181      * {@link java.lang.Double#compare(double, double)}.
 182      *
 183      * <p>This is a <a href="package-summary.html#StreamOps">stateful
 184      * intermediate operation</a>.
 185      *
 186      * @return the result stream
 187      */
 188     DoubleStream sorted();
 189 
 190     /**
 191      * Returns a stream consisting of the elements of this stream, additionally
 192      * performing the provided action on each element as elements are consumed
 193      * from the resulting stream.
 194      *
 195      * <p>This is an <a href="package-summary.html#StreamOps">intermediate
 196      * operation</a>.
 197      *
 198      * <p>For parallel stream pipelines, the action may be called at
 199      * whatever time and in whatever thread the element is made available by the
 200      * upstream operation.  If the action modifies shared state,
 201      * it is responsible for providing the required synchronization.
 202      *
 203      * @apiNote This method exists mainly to support debugging, where you want
 204      * to see the elements as they flow past a certain point in a pipeline:
 205      * <pre>{@code
 206      *     DoubleStream.of(1, 2, 3, 4)
 207      *         .filter(e -> e > 2)
 208      *         .peek(e -> System.out.println("Filtered value: " + e))
 209      *         .map(e -> e * e)
 210      *         .peek(e -> System.out.println("Mapped value: " + e))
 211      *         .sum();
 212      * }</pre>
 213      *
 214      * @param action a <a href="package-summary.html#NonInterference">
 215      *               non-interfering</a> action to perform on the elements as
 216      *               they are consumed from the stream
 217      * @return the new stream
 218      */
 219     DoubleStream peek(DoubleConsumer action);
 220 
 221     /**
 222      * Returns a stream consisting of the elements of this stream, truncated
 223      * to be no longer than {@code maxSize} in length.
 224      *
 225      * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
 226      * stateful intermediate operation</a>.
 227      *
 228      * @apiNote
 229      * While {@code limit()} is generally a cheap operation on sequential
 230      * stream pipelines, it can be quite expensive on ordered parallel pipelines,
 231      * especially for large values of {@code maxSize}, since {@code limit(n)}
 232      * is constrained to return not just any <em>n</em> elements, but the
 233      * <em>first n</em> elements in the encounter order.  Using an unordered
 234      * stream source (such as {@link #generate(DoubleSupplier)}) or removing the
 235      * ordering constraint with {@link #unordered()} may result in significant
 236      * speedups of {@code limit()} in parallel pipelines, if the semantics of
 237      * your situation permit.  If consistency with encounter order is required,
 238      * and you are experiencing poor performance or memory utilization with
 239      * {@code limit()} in parallel pipelines, switching to sequential execution
 240      * with {@link #sequential()} may improve performance.
 241      *
 242      * @param maxSize the number of elements the stream should be limited to
 243      * @return the new stream
 244      * @throws IllegalArgumentException if {@code maxSize} is negative
 245      */
 246     DoubleStream limit(long maxSize);
 247 
 248     /**
 249      * Returns a stream consisting of the remaining elements of this stream
 250      * after discarding the first {@code n} elements of the stream.
 251      * If this stream contains fewer than {@code n} elements then an
 252      * empty stream will be returned.
 253      *
 254      * <p>This is a <a href="package-summary.html#StreamOps">stateful
 255      * intermediate operation</a>.
 256      *
 257      * @apiNote
 258      * While {@code skip()} is generally a cheap operation on sequential
 259      * stream pipelines, it can be quite expensive on ordered parallel pipelines,
 260      * especially for large values of {@code n}, since {@code skip(n)}
 261      * is constrained to skip not just any <em>n</em> elements, but the
 262      * <em>first n</em> elements in the encounter order.  Using an unordered
 263      * stream source (such as {@link #generate(DoubleSupplier)}) or removing the
 264      * ordering constraint with {@link #unordered()} may result in significant
 265      * speedups of {@code skip()} in parallel pipelines, if the semantics of
 266      * your situation permit.  If consistency with encounter order is required,
 267      * and you are experiencing poor performance or memory utilization with
 268      * {@code skip()} in parallel pipelines, switching to sequential execution
 269      * with {@link #sequential()} may improve performance.
 270      *
 271      * @param n the number of leading elements to skip
 272      * @return the new stream
 273      * @throws IllegalArgumentException if {@code n} is negative
 274      */
 275     DoubleStream skip(long n);
 276 
 277     /**
 278      * Returns, if this stream is ordered, a stream consisting of the longest
 279      * prefix of elements taken from this stream that match the given predicate.
 280      * Otherwise returns, if this stream is unordered, a stream consisting of a
 281      * subset of elements taken from this stream that match the given predicate.
 282      *
 283      * <p>If this stream is ordered then the longest prefix is a contiguous
 284      * sequence of elements of this stream that match the given predicate.  The
 285      * first element of the sequence is the first element of this stream, and
 286      * the element immediately following the last element of the sequence does
 287      * not match the given predicate.
 288      *
 289      * <p>If this stream is unordered, and some (but not all) elements of this
 290      * stream match the given predicate, then the behavior of this operation is
 291      * nondeterministic; it is free to take any subset of matching elements
 292      * (which includes the empty set).
 293      *
 294      * <p>Independent of whether this stream is ordered or unordered if all
 295      * elements of this stream match the given predicate then this operation
 296      * takes all elements (the result is the same as the input), or if no
 297      * elements of the stream match the given predicate then no elements are
 298      * taken (the result is an empty stream).
 299      *
 300      * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
 301      * stateful intermediate operation</a>.
 302      *
 303      * @implSpec
 304      * The default implementation obtains the {@link #spliterator() spliterator}
 305      * of this stream, wraps that spliterator so as to support the semantics
 306      * of this operation on traversal, and returns a new stream associated with
 307      * the wrapped spliterator.  The returned stream preserves the execution
 308      * characteristics of this stream (namely parallel or sequential execution
 309      * as per {@link #isParallel()}) but the wrapped spliterator may choose to
 310      * not support splitting.  When the returned stream is closed, the close
 311      * handlers for both the returned and this stream are invoked.
 312      *
 313      * @apiNote
 314      * While {@code takeWhile()} is generally a cheap operation on sequential
 315      * stream pipelines, it can be quite expensive on ordered parallel
 316      * pipelines, since the operation is constrained to return not just any
 317      * valid prefix, but the longest prefix of elements in the encounter order.
 318      * Using an unordered stream source (such as
 319      * {@link #generate(DoubleSupplier)}) or removing the ordering constraint
 320      * with {@link #unordered()} may result in significant speedups of
 321      * {@code takeWhile()} in parallel pipelines, if the semantics of your
 322      * situation permit.  If consistency with encounter order is required, and
 323      * you are experiencing poor performance or memory utilization with
 324      * {@code takeWhile()} in parallel pipelines, switching to sequential
 325      * execution with {@link #sequential()} may improve performance.
 326      *
 327      * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>,
 328      *                  <a href="package-summary.html#Statelessness">stateless</a>
 329      *                  predicate to apply to elements to determine the longest
 330      *                  prefix of elements.
 331      * @return the new stream
 332      * @since 9
 333      */
 334     default DoubleStream takeWhile(DoublePredicate predicate) {
 335         Objects.requireNonNull(predicate);
 336         // Reuses the unordered spliterator, which, when encounter is present,
 337         // is safe to use as long as it configured not to split
 338         return StreamSupport.doubleStream(
 339                 new WhileOps.UnorderedWhileSpliterator.OfDouble.Taking(spliterator(), true, predicate),
 340                 isParallel()).onClose(this::close);
 341     }
 342 
 343     /**
 344      * Returns, if this stream is ordered, a stream consisting of the remaining
 345      * elements of this stream after dropping the longest prefix of elements
 346      * that match the given predicate.  Otherwise returns, if this stream is
 347      * unordered, a stream consisting of the remaining elements of this stream
 348      * after dropping a subset of elements that match the given predicate.
 349      *
 350      * <p>If this stream is ordered then the longest prefix is a contiguous
 351      * sequence of elements of this stream that match the given predicate.  The
 352      * first element of the sequence is the first element of this stream, and
 353      * the element immediately following the last element of the sequence does
 354      * not match the given predicate.
 355      *
 356      * <p>If this stream is unordered, and some (but not all) elements of this
 357      * stream match the given predicate, then the behavior of this operation is
 358      * nondeterministic; it is free to drop any subset of matching elements
 359      * (which includes the empty set).
 360      *
 361      * <p>Independent of whether this stream is ordered or unordered if all
 362      * elements of this stream match the given predicate then this operation
 363      * drops all elements (the result is an empty stream), or if no elements of
 364      * the stream match the given predicate then no elements are dropped (the
 365      * result is the same as the input).
 366      *
 367      * <p>This is a <a href="package-summary.html#StreamOps">stateful
 368      * intermediate operation</a>.
 369      *
 370      * @implSpec
 371      * The default implementation obtains the {@link #spliterator() spliterator}
 372      * of this stream, wraps that spliterator so as to support the semantics
 373      * of this operation on traversal, and returns a new stream associated with
 374      * the wrapped spliterator.  The returned stream preserves the execution
 375      * characteristics of this stream (namely parallel or sequential execution
 376      * as per {@link #isParallel()}) but the wrapped spliterator may choose to
 377      * not support splitting.  When the returned stream is closed, the close
 378      * handlers for both the returned and this stream are invoked.
 379      *
 380      * @apiNote
 381      * While {@code dropWhile()} is generally a cheap operation on sequential
 382      * stream pipelines, it can be quite expensive on ordered parallel
 383      * pipelines, since the operation is constrained to return not just any
 384      * valid prefix, but the longest prefix of elements in the encounter order.
 385      * Using an unordered stream source (such as
 386      * {@link #generate(DoubleSupplier)}) or removing the ordering constraint
 387      * with {@link #unordered()} may result in significant speedups of
 388      * {@code dropWhile()} in parallel pipelines, if the semantics of your
 389      * situation permit.  If consistency with encounter order is required, and
 390      * you are experiencing poor performance or memory utilization with
 391      * {@code dropWhile()} in parallel pipelines, switching to sequential
 392      * execution with {@link #sequential()} may improve performance.
 393      *
 394      * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>,
 395      *                  <a href="package-summary.html#Statelessness">stateless</a>
 396      *                  predicate to apply to elements to determine the longest
 397      *                  prefix of elements.
 398      * @return the new stream
 399      * @since 9
 400      */
 401     default DoubleStream dropWhile(DoublePredicate predicate) {
 402         Objects.requireNonNull(predicate);
 403         // Reuses the unordered spliterator, which, when encounter is present,
 404         // is safe to use as long as it configured not to split
 405         return StreamSupport.doubleStream(
 406                 new WhileOps.UnorderedWhileSpliterator.OfDouble.Dropping(spliterator(), true, predicate),
 407                 isParallel()).onClose(this::close);
 408     }
 409 
 410     /**
 411      * Performs an action for each element of this stream.
 412      *
 413      * <p>This is a <a href="package-summary.html#StreamOps">terminal
 414      * operation</a>.
 415      *
 416      * <p>For parallel stream pipelines, this operation does <em>not</em>
 417      * guarantee to respect the encounter order of the stream, as doing so
 418      * would sacrifice the benefit of parallelism.  For any given element, the
 419      * action may be performed at whatever time and in whatever thread the
 420      * library chooses.  If the action accesses shared state, it is
 421      * responsible for providing the required synchronization.
 422      *
 423      * @param action a <a href="package-summary.html#NonInterference">
 424      *               non-interfering</a> action to perform on the elements
 425      */
 426     void forEach(DoubleConsumer action);
 427 
 428     /**
 429      * Performs an action for each element of this stream, guaranteeing that
 430      * each element is processed in encounter order for streams that have a
 431      * defined encounter order.
 432      *
 433      * <p>This is a <a href="package-summary.html#StreamOps">terminal
 434      * operation</a>.
 435      *
 436      * @param action a <a href="package-summary.html#NonInterference">
 437      *               non-interfering</a> action to perform on the elements
 438      * @see #forEach(DoubleConsumer)
 439      */
 440     void forEachOrdered(DoubleConsumer action);
 441 
 442     /**
 443      * Returns an array containing the elements of this stream.
 444      *
 445      * <p>This is a <a href="package-summary.html#StreamOps">terminal
 446      * operation</a>.
 447      *
 448      * @return an array containing the elements of this stream
 449      */
 450     double[] toArray();
 451 
 452     /**
 453      * Performs a <a href="package-summary.html#Reduction">reduction</a> on the
 454      * elements of this stream, using the provided identity value and an
 455      * <a href="package-summary.html#Associativity">associative</a>
 456      * accumulation function, and returns the reduced value.  This is equivalent
 457      * to:
 458      * <pre>{@code
 459      *     double result = identity;
 460      *     for (double element : this stream)
 461      *         result = accumulator.applyAsDouble(result, element)
 462      *     return result;
 463      * }</pre>
 464      *
 465      * but is not constrained to execute sequentially.
 466      *
 467      * <p>The {@code identity} value must be an identity for the accumulator
 468      * function. This means that for all {@code x},
 469      * {@code accumulator.apply(identity, x)} is equal to {@code x}.
 470      * The {@code accumulator} function must be an
 471      * <a href="package-summary.html#Associativity">associative</a> function.
 472      *
 473      * <p>This is a <a href="package-summary.html#StreamOps">terminal
 474      * operation</a>.
 475      *
 476      * @apiNote Sum, min, max, and average are all special cases of reduction.
 477      * Summing a stream of numbers can be expressed as:
 478 
 479      * <pre>{@code
 480      *     double sum = numbers.reduce(0, (a, b) -> a+b);
 481      * }</pre>
 482      *
 483      * or more compactly:
 484      *
 485      * <pre>{@code
 486      *     double sum = numbers.reduce(0, Double::sum);
 487      * }</pre>
 488      *
 489      * <p>While this may seem a more roundabout way to perform an aggregation
 490      * compared to simply mutating a running total in a loop, reduction
 491      * operations parallelize more gracefully, without needing additional
 492      * synchronization and with greatly reduced risk of data races.
 493      *
 494      * @param identity the identity value for the accumulating function
 495      * @param op an <a href="package-summary.html#Associativity">associative</a>,
 496      *           <a href="package-summary.html#NonInterference">non-interfering</a>,
 497      *           <a href="package-summary.html#Statelessness">stateless</a>
 498      *           function for combining two values
 499      * @return the result of the reduction
 500      * @see #sum()
 501      * @see #min()
 502      * @see #max()
 503      * @see #average()
 504      */
 505     double reduce(double identity, DoubleBinaryOperator op);
 506 
 507     /**
 508      * Performs a <a href="package-summary.html#Reduction">reduction</a> on the
 509      * elements of this stream, using an
 510      * <a href="package-summary.html#Associativity">associative</a> accumulation
 511      * function, and returns an {@code OptionalDouble} describing the reduced
 512      * value, if any. This is equivalent to:
 513      * <pre>{@code
 514      *     boolean foundAny = false;
 515      *     double result = null;
 516      *     for (double element : this stream) {
 517      *         if (!foundAny) {
 518      *             foundAny = true;
 519      *             result = element;
 520      *         }
 521      *         else
 522      *             result = accumulator.applyAsDouble(result, element);
 523      *     }
 524      *     return foundAny ? OptionalDouble.of(result) : OptionalDouble.empty();
 525      * }</pre>
 526      *
 527      * but is not constrained to execute sequentially.
 528      *
 529      * <p>The {@code accumulator} function must be an
 530      * <a href="package-summary.html#Associativity">associative</a> function.
 531      *
 532      * <p>This is a <a href="package-summary.html#StreamOps">terminal
 533      * operation</a>.
 534      *
 535      * @param op an <a href="package-summary.html#Associativity">associative</a>,
 536      *           <a href="package-summary.html#NonInterference">non-interfering</a>,
 537      *           <a href="package-summary.html#Statelessness">stateless</a>
 538      *           function for combining two values
 539      * @return the result of the reduction
 540      * @see #reduce(double, DoubleBinaryOperator)
 541      */
 542     OptionalDouble reduce(DoubleBinaryOperator op);
 543 
 544     /**
 545      * Performs a <a href="package-summary.html#MutableReduction">mutable
 546      * reduction</a> operation on the elements of this stream.  A mutable
 547      * reduction is one in which the reduced value is a mutable result container,
 548      * such as an {@code ArrayList}, and elements are incorporated by updating
 549      * the state of the result rather than by replacing the result.  This
 550      * produces a result equivalent to:
 551      * <pre>{@code
 552      *     R result = supplier.get();
 553      *     for (double element : this stream)
 554      *         accumulator.accept(result, element);
 555      *     return result;
 556      * }</pre>
 557      *
 558      * <p>Like {@link #reduce(double, DoubleBinaryOperator)}, {@code collect}
 559      * operations can be parallelized without requiring additional
 560      * synchronization.
 561      *
 562      * <p>This is a <a href="package-summary.html#StreamOps">terminal
 563      * operation</a>.
 564      *
 565      * @param <R> type of the result
 566      * @param supplier a function that creates a new result container. For a
 567      *                 parallel execution, this function may be called
 568      *                 multiple times and must return a fresh value each time.
 569      * @param accumulator an <a href="package-summary.html#Associativity">associative</a>,
 570      *                    <a href="package-summary.html#NonInterference">non-interfering</a>,
 571      *                    <a href="package-summary.html#Statelessness">stateless</a>
 572      *                    function for incorporating an additional element into a result
 573      * @param combiner an <a href="package-summary.html#Associativity">associative</a>,
 574      *                    <a href="package-summary.html#NonInterference">non-interfering</a>,
 575      *                    <a href="package-summary.html#Statelessness">stateless</a>
 576      *                    function for combining two values, which must be
 577      *                    compatible with the accumulator function
 578      * @return the result of the reduction
 579      * @see Stream#collect(Supplier, BiConsumer, BiConsumer)
 580      */
 581     <R> R collect(Supplier<R> supplier,
 582                   ObjDoubleConsumer<R> accumulator,
 583                   BiConsumer<R, R> combiner);
 584 
 585     /**
 586      * Returns the sum of elements in this stream.
 587      *
 588      * Summation is a special case of a <a
 589      * href="package-summary.html#Reduction">reduction</a>. If
 590      * floating-point summation were exact, this method would be
 591      * equivalent to:
 592      *
 593      * <pre>{@code
 594      *     return reduce(0, Double::sum);
 595      * }</pre>
 596      *
 597      * However, since floating-point summation is not exact, the above
 598      * code is not necessarily equivalent to the summation computation
 599      * done by this method.
 600      *
 601      * <p>The value of a floating-point sum is a function both
 602      * of the input values as well as the order of addition
 603      * operations. The order of addition operations of this method is
 604      * intentionally not defined to allow for implementation
 605      * flexibility to improve the speed and accuracy of the computed
 606      * result.
 607      *
 608      * In particular, this method may be implemented using compensated
 609      * summation or other technique to reduce the error bound in the
 610      * numerical sum compared to a simple summation of {@code double}
 611      * values.
 612      *
 613      * Because of the unspecified order of operations and the
 614      * possibility of using differing summation schemes, the output of
 615      * this method may vary on the same input elements.
 616      *
 617      * <p>Various conditions can result in a non-finite sum being
 618      * computed. This can occur even if the all the elements
 619      * being summed are finite. If any element is non-finite,
 620      * the sum will be non-finite:
 621      *
 622      * <ul>
 623      *
 624      * <li>If any element is a NaN, then the final sum will be
 625      * NaN.
 626      *
 627      * <li>If the elements contain one or more infinities, the
 628      * sum will be infinite or NaN.
 629      *
 630      * <ul>
 631      *
 632      * <li>If the elements contain infinities of opposite sign,
 633      * the sum will be NaN.
 634      *
 635      * <li>If the elements contain infinities of one sign and
 636      * an intermediate sum overflows to an infinity of the opposite
 637      * sign, the sum may be NaN.
 638      *
 639      * </ul>
 640      *
 641      * </ul>
 642      *
 643      * It is possible for intermediate sums of finite values to
 644      * overflow into opposite-signed infinities; if that occurs, the
 645      * final sum will be NaN even if the elements are all
 646      * finite.
 647      *
 648      * If all the elements are zero, the sign of zero is
 649      * <em>not</em> guaranteed to be preserved in the final sum.
 650      *
 651      * <p>This is a <a href="package-summary.html#StreamOps">terminal
 652      * operation</a>.
 653      *
 654      * @apiNote Elements sorted by increasing absolute magnitude tend
 655      * to yield more accurate results.
 656      *
 657      * @return the sum of elements in this stream
 658      */
 659     double sum();
 660 
 661     /**
 662      * Returns an {@code OptionalDouble} describing the minimum element of this
 663      * stream, or an empty OptionalDouble if this stream is empty.  The minimum
 664      * element will be {@code Double.NaN} if any stream element was NaN. Unlike
 665      * the numerical comparison operators, this method considers negative zero
 666      * to be strictly smaller than positive zero. This is a special case of a
 667      * <a href="package-summary.html#Reduction">reduction</a> and is
 668      * equivalent to:
 669      * <pre>{@code
 670      *     return reduce(Double::min);
 671      * }</pre>
 672      *
 673      * <p>This is a <a href="package-summary.html#StreamOps">terminal
 674      * operation</a>.
 675      *
 676      * @return an {@code OptionalDouble} containing the minimum element of this
 677      * stream, or an empty optional if the stream is empty
 678      */
 679     OptionalDouble min();
 680 
 681     /**
 682      * Returns an {@code OptionalDouble} describing the maximum element of this
 683      * stream, or an empty OptionalDouble if this stream is empty.  The maximum
 684      * element will be {@code Double.NaN} if any stream element was NaN. Unlike
 685      * the numerical comparison operators, this method considers negative zero
 686      * to be strictly smaller than positive zero. This is a
 687      * special case of a
 688      * <a href="package-summary.html#Reduction">reduction</a> and is
 689      * equivalent to:
 690      * <pre>{@code
 691      *     return reduce(Double::max);
 692      * }</pre>
 693      *
 694      * <p>This is a <a href="package-summary.html#StreamOps">terminal
 695      * operation</a>.
 696      *
 697      * @return an {@code OptionalDouble} containing the maximum element of this
 698      * stream, or an empty optional if the stream is empty
 699      */
 700     OptionalDouble max();
 701 
 702     /**
 703      * Returns the count of elements in this stream.  This is a special case of
 704      * a <a href="package-summary.html#Reduction">reduction</a> and is
 705      * equivalent to:
 706      * <pre>{@code
 707      *     return mapToLong(e -> 1L).sum();
 708      * }</pre>
 709      *
 710      * <p>This is a <a href="package-summary.html#StreamOps">terminal operation</a>.
 711      *
 712      * @apiNote
 713      * An implementation may choose to not execute the stream pipeline (either
 714      * sequentially or in parallel) if it is capable of computing the count
 715      * directly from the stream source.  In such cases no source elements will
 716      * be traversed and no intermediate operations will be evaluated.
 717      * Behavioral parameters with side-effects, which are strongly discouraged
 718      * except for harmless cases such as debugging, may be affected.  For
 719      * example, consider the following stream:
 720      * <pre>{@code
 721      *     DoubleStream s = DoubleStream.of(1, 2, 3, 4);
 722      *     long count = s.peek(System.out::println).count();
 723      * }</pre>
 724      * The number of elements covered by the stream source is known and the
 725      * intermediate operation, {@code peek}, does not inject into or remove
 726      * elements from the stream (as may be the case for {@code flatMap} or
 727      * {@code filter} operations).  Thus the count is 4 and there is no need to
 728      * execute the pipeline and, as a side-effect, print out the elements.
 729      *
 730      * @return the count of elements in this stream
 731      */
 732     long count();
 733 
 734     /**
 735      * Returns an {@code OptionalDouble} describing the arithmetic
 736      * mean of elements of this stream, or an empty optional if this
 737      * stream is empty.
 738      *
 739      * <p>The computed average can vary numerically and have the
 740      * special case behavior as computing the sum; see {@link #sum}
 741      * for details.
 742      *
 743      *  <p>The average is a special case of a <a
 744      *  href="package-summary.html#Reduction">reduction</a>.
 745      *
 746      * <p>This is a <a href="package-summary.html#StreamOps">terminal
 747      * operation</a>.
 748      *
 749      * @apiNote Elements sorted by increasing absolute magnitude tend
 750      * to yield more accurate results.
 751      *
 752      * @return an {@code OptionalDouble} containing the average element of this
 753      * stream, or an empty optional if the stream is empty
 754      */
 755     OptionalDouble average();
 756 
 757     /**
 758      * Returns a {@code DoubleSummaryStatistics} describing various summary data
 759      * about the elements of this stream.  This is a special
 760      * case of a <a href="package-summary.html#Reduction">reduction</a>.
 761      *
 762      * <p>This is a <a href="package-summary.html#StreamOps">terminal
 763      * operation</a>.
 764      *
 765      * @return a {@code DoubleSummaryStatistics} describing various summary data
 766      * about the elements of this stream
 767      */
 768     DoubleSummaryStatistics summaryStatistics();
 769 
 770     /**
 771      * Returns whether any elements of this stream match the provided
 772      * predicate.  May not evaluate the predicate on all elements if not
 773      * necessary for determining the result.  If the stream is empty then
 774      * {@code false} is returned and the predicate is not evaluated.
 775      *
 776      * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
 777      * terminal operation</a>.
 778      *
 779      * @apiNote
 780      * This method evaluates the <em>existential quantification</em> of the
 781      * predicate over the elements of the stream (for some x P(x)).
 782      *
 783      * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>,
 784      *                  <a href="package-summary.html#Statelessness">stateless</a>
 785      *                  predicate to apply to elements of this stream
 786      * @return {@code true} if any elements of the stream match the provided
 787      * predicate, otherwise {@code false}
 788      */
 789     boolean anyMatch(DoublePredicate predicate);
 790 
 791     /**
 792      * Returns whether all elements of this stream match the provided predicate.
 793      * May not evaluate the predicate on all elements if not necessary for
 794      * determining the result.  If the stream is empty then {@code true} is
 795      * returned and the predicate is not evaluated.
 796      *
 797      * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
 798      * terminal operation</a>.
 799      *
 800      * @apiNote
 801      * This method evaluates the <em>universal quantification</em> of the
 802      * predicate over the elements of the stream (for all x P(x)).  If the
 803      * stream is empty, the quantification is said to be <em>vacuously
 804      * satisfied</em> and is always {@code true} (regardless of P(x)).
 805      *
 806      * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>,
 807      *                  <a href="package-summary.html#Statelessness">stateless</a>
 808      *                  predicate to apply to elements of this stream
 809      * @return {@code true} if either all elements of the stream match the
 810      * provided predicate or the stream is empty, otherwise {@code false}
 811      */
 812     boolean allMatch(DoublePredicate predicate);
 813 
 814     /**
 815      * Returns whether no elements of this stream match the provided predicate.
 816      * May not evaluate the predicate on all elements if not necessary for
 817      * determining the result.  If the stream is empty then {@code true} is
 818      * returned and the predicate is not evaluated.
 819      *
 820      * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
 821      * terminal operation</a>.
 822      *
 823      * @apiNote
 824      * This method evaluates the <em>universal quantification</em> of the
 825      * negated predicate over the elements of the stream (for all x ~P(x)).  If
 826      * the stream is empty, the quantification is said to be vacuously satisfied
 827      * and is always {@code true}, regardless of P(x).
 828      *
 829      * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>,
 830      *                  <a href="package-summary.html#Statelessness">stateless</a>
 831      *                  predicate to apply to elements of this stream
 832      * @return {@code true} if either no elements of the stream match the
 833      * provided predicate or the stream is empty, otherwise {@code false}
 834      */
 835     boolean noneMatch(DoublePredicate predicate);
 836 
 837     /**
 838      * Returns an {@link OptionalDouble} describing the first element of this
 839      * stream, or an empty {@code OptionalDouble} if the stream is empty.  If
 840      * the stream has no encounter order, then any element may be returned.
 841      *
 842      * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
 843      * terminal operation</a>.
 844      *
 845      * @return an {@code OptionalDouble} describing the first element of this
 846      * stream, or an empty {@code OptionalDouble} if the stream is empty
 847      */
 848     OptionalDouble findFirst();
 849 
 850     /**
 851      * Returns an {@link OptionalDouble} describing some element of the stream,
 852      * or an empty {@code OptionalDouble} if the stream is empty.
 853      *
 854      * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
 855      * terminal operation</a>.
 856      *
 857      * <p>The behavior of this operation is explicitly nondeterministic; it is
 858      * free to select any element in the stream.  This is to allow for maximal
 859      * performance in parallel operations; the cost is that multiple invocations
 860      * on the same source may not return the same result.  (If a stable result
 861      * is desired, use {@link #findFirst()} instead.)
 862      *
 863      * @return an {@code OptionalDouble} describing some element of this stream,
 864      * or an empty {@code OptionalDouble} if the stream is empty
 865      * @see #findFirst()
 866      */
 867     OptionalDouble findAny();
 868 
 869     /**
 870      * Returns a {@code Stream} consisting of the elements of this stream,
 871      * boxed to {@code Double}.
 872      *
 873      * <p>This is an <a href="package-summary.html#StreamOps">intermediate
 874      * operation</a>.
 875      *
 876      * @return a {@code Stream} consistent of the elements of this stream,
 877      * each boxed to a {@code Double}
 878      */
 879     Stream<Double> boxed();
 880 
 881     @Override
 882     DoubleStream sequential();
 883 
 884     @Override
 885     DoubleStream parallel();
 886 
 887     @Override
 888     PrimitiveIterator.OfDouble iterator();
 889 
 890     @Override
 891     Spliterator.OfDouble spliterator();
 892 
 893 
 894     // Static factories
 895 
 896     /**
 897      * Returns a builder for a {@code DoubleStream}.
 898      *
 899      * @return a stream builder
 900      */
 901     public static Builder builder() {
 902         return new Streams.DoubleStreamBuilderImpl();
 903     }
 904 
 905     /**
 906      * Returns an empty sequential {@code DoubleStream}.
 907      *
 908      * @return an empty sequential stream
 909      */
 910     public static DoubleStream empty() {
 911         return StreamSupport.doubleStream(Spliterators.emptyDoubleSpliterator(), false);
 912     }
 913 
 914     /**
 915      * Returns a sequential {@code DoubleStream} containing a single element.
 916      *
 917      * @param t the single element
 918      * @return a singleton sequential stream
 919      */
 920     public static DoubleStream of(double t) {
 921         return StreamSupport.doubleStream(new Streams.DoubleStreamBuilderImpl(t), false);
 922     }
 923 
 924     /**
 925      * Returns a sequential ordered stream whose elements are the specified values.
 926      *
 927      * @param values the elements of the new stream
 928      * @return the new stream
 929      */
 930     public static DoubleStream of(double... values) {
 931         return Arrays.stream(values);
 932     }
 933 
 934     /**
 935      * Returns an infinite sequential ordered {@code DoubleStream} produced by iterative
 936      * application of a function {@code f} to an initial element {@code seed},
 937      * producing a {@code Stream} consisting of {@code seed}, {@code f(seed)},
 938      * {@code f(f(seed))}, etc.
 939      *
 940      * <p>The first element (position {@code 0}) in the {@code DoubleStream}
 941      * will be the provided {@code seed}.  For {@code n > 0}, the element at
 942      * position {@code n}, will be the result of applying the function {@code f}
 943      *  to the element at position {@code n - 1}.
 944      *
 945      * @param seed the initial element
 946      * @param f a function to be applied to the previous element to produce
 947      *          a new element
 948      * @return a new sequential {@code DoubleStream}
 949      */
 950     public static DoubleStream iterate(final double seed, final DoubleUnaryOperator f) {
 951         Objects.requireNonNull(f);
 952         Spliterator.OfDouble spliterator = new Spliterators.AbstractDoubleSpliterator(Long.MAX_VALUE, 
 953                Spliterator.ORDERED | Spliterator.IMMUTABLE | Spliterator.NONNULL) {
 954             double prev;
 955             boolean started;
 956 
 957             @Override
 958             public boolean tryAdvance(DoubleConsumer action) {
 959                 Objects.requireNonNull(action);
 960                 double t;
 961                 if (started)
 962                     t = f.applyAsDouble(prev);
 963                 else {
 964                     t = seed;
 965                     started = true;
 966                 }
 967                 action.accept(prev = t);
 968                 return true;
 969             }
 970         };
 971         return StreamSupport.doubleStream(spliterator, false);
 972     }
 973 
 974     /**
 975      * Returns a sequential ordered {@code DoubleStream} produced by iterative
 976      * application of a function to an initial element, conditioned on 
 977      * satisfying the supplied predicate.  The stream terminates as soon as
 978      * the predicate function returns false.
 979      *
 980      * <p>
 981      * {@code DoubleStream.iterate} should produce the same sequence of
 982      * elements as produced by the corresponding for-loop:
 983      * <pre>{@code
 984      *     for (double index=seed; predicate.test(index); index = f.apply(index)) {
 985      *         ... 
 986      *     }
 987      * }</pre>
 988      *
 989      * <p>
 990      * The resulting sequence may be empty if the predicate does not hold on 
 991      * the seed value.  Otherwise the first element will be the supplied seed
 992      * value, the next element (if present) will be the result of applying the
 993      * function f to the seed value, and so on iteratively until the predicate
 994      * indicates that the stream should terminate.
 995      *
 996      * @param seed the initial element
 997      * @param predicate a predicate to apply to elements to determine when the 
 998      *          stream must terminate.
 999      * @param f a function to be applied to the previous element to produce
1000      *          a new element
1001      * @return a new sequential {@code DoubleStream}
1002      * @since 9
1003      */
1004     public static DoubleStream iterate(double seed, DoublePredicate predicate, DoubleUnaryOperator f) {
1005         Objects.requireNonNull(f);
1006         Objects.requireNonNull(predicate);
1007         Spliterator.OfDouble spliterator = new Spliterators.AbstractDoubleSpliterator(Long.MAX_VALUE, 
1008                Spliterator.ORDERED | Spliterator.IMMUTABLE | Spliterator.NONNULL) {
1009             double prev;
1010             boolean started, finished;
1011 
1012             @Override
1013             public boolean tryAdvance(DoubleConsumer action) {
1014                 Objects.requireNonNull(action);
1015                 if (finished)
1016                     return false;
1017                 double t;
1018                 if (started)
1019                     t = f.applyAsDouble(prev);
1020                 else {
1021                     t = seed;
1022                     started = true;
1023                 }
1024                 if (!predicate.test(t)) {
1025                     finished = true;
1026                     return false;
1027                 }
1028                 action.accept(prev = t);
1029                 return true;
1030             }
1031 
1032             @Override
1033             public void forEachRemaining(DoubleConsumer action) {
1034                 Objects.requireNonNull(action);
1035                 if (finished)
1036                     return;
1037                 finished = true;
1038                 double t = started ? f.applyAsDouble(prev) : seed;
1039                 while (predicate.test(t)) {
1040                     action.accept(t);
1041                     t = f.applyAsDouble(t);
1042                 }
1043             }
1044         };
1045         return StreamSupport.doubleStream(spliterator, false);
1046     }
1047 
1048     /**
1049      * Returns an infinite sequential unordered stream where each element is
1050      * generated by the provided {@code DoubleSupplier}.  This is suitable for
1051      * generating constant streams, streams of random elements, etc.
1052      *
1053      * @param s the {@code DoubleSupplier} for generated elements
1054      * @return a new infinite sequential unordered {@code DoubleStream}
1055      */
1056     public static DoubleStream generate(DoubleSupplier s) {
1057         Objects.requireNonNull(s);
1058         return StreamSupport.doubleStream(
1059                 new StreamSpliterators.InfiniteSupplyingSpliterator.OfDouble(Long.MAX_VALUE, s), false);
1060     }
1061 
1062     /**
1063      * Creates a lazily concatenated stream whose elements are all the
1064      * elements of the first stream followed by all the elements of the
1065      * second stream.  The resulting stream is ordered if both
1066      * of the input streams are ordered, and parallel if either of the input
1067      * streams is parallel.  When the resulting stream is closed, the close
1068      * handlers for both input streams are invoked.
1069      *
1070      * @implNote
1071      * Use caution when constructing streams from repeated concatenation.
1072      * Accessing an element of a deeply concatenated stream can result in deep
1073      * call chains, or even {@code StackOverflowError}.
1074      *
1075      * @param a the first stream
1076      * @param b the second stream
1077      * @return the concatenation of the two input streams
1078      */
1079     public static DoubleStream concat(DoubleStream a, DoubleStream b) {
1080         Objects.requireNonNull(a);
1081         Objects.requireNonNull(b);
1082 
1083         Spliterator.OfDouble split = new Streams.ConcatSpliterator.OfDouble(
1084                 a.spliterator(), b.spliterator());
1085         DoubleStream stream = StreamSupport.doubleStream(split, a.isParallel() || b.isParallel());
1086         return stream.onClose(Streams.composedClose(a, b));
1087     }
1088 
1089     /**
1090      * A mutable builder for a {@code DoubleStream}.
1091      *
1092      * <p>A stream builder has a lifecycle, which starts in a building
1093      * phase, during which elements can be added, and then transitions to a built
1094      * phase, after which elements may not be added.  The built phase
1095      * begins when the {@link #build()} method is called, which creates an
1096      * ordered stream whose elements are the elements that were added to the
1097      * stream builder, in the order they were added.
1098      *
1099      * @see DoubleStream#builder()
1100      * @since 1.8
1101      */
1102     public interface Builder extends DoubleConsumer {
1103 
1104         /**
1105          * Adds an element to the stream being built.
1106          *
1107          * @throws IllegalStateException if the builder has already transitioned
1108          * to the built state
1109          */
1110         @Override
1111         void accept(double t);
1112 
1113         /**
1114          * Adds an element to the stream being built.
1115          *
1116          * @implSpec
1117          * The default implementation behaves as if:
1118          * <pre>{@code
1119          *     accept(t)
1120          *     return this;
1121          * }</pre>
1122          *
1123          * @param t the element to add
1124          * @return {@code this} builder
1125          * @throws IllegalStateException if the builder has already transitioned
1126          * to the built state
1127          */
1128         default Builder add(double t) {
1129             accept(t);
1130             return this;
1131         }
1132 
1133         /**
1134          * Builds the stream, transitioning this builder to the built state.
1135          * An {@code IllegalStateException} is thrown if there are further
1136          * attempts to operate on the builder after it has entered the built
1137          * state.
1138          *
1139          * @return the built stream
1140          * @throws IllegalStateException if the builder has already transitioned
1141          * to the built state
1142          */
1143         DoubleStream build();
1144     }
1145 }