1 /*
2 * Copyright (c) 2012, 2019, 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
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7 * published by the Free Software Foundation. Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
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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).
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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. Stream implements the
107 * {@link IterableOnce} interface, allowing a stream instance to be used
108 * in an enhanced-for ("for each") statement. Note that this statement implicitly
109 * calls the {@code iterator()} method, which consumes the stream. The stream
110 * thus cannot be used for any subsequent operations.
111 *
112 * <p>A stream pipeline, like the "widgets" example above, can be viewed as
113 * a <em>query</em> on the stream source. Unless the source was explicitly
114 * designed for concurrent modification (such as a {@link ConcurrentHashMap}),
115 * unpredictable or erroneous behavior may result from modifying the stream
116 * source while it is being queried.
117 *
118 * <p>Most stream operations accept parameters that describe user-specified
119 * behavior, such as the lambda expression {@code w -> w.getWeight()} passed to
120 * {@code mapToInt} in the example above. To preserve correct behavior,
121 * these <em>behavioral parameters</em>:
122 * <ul>
123 * <li>must be <a href="package-summary.html#NonInterference">non-interfering</a>
124 * (they do not modify the stream source); and</li>
125 * <li>in most cases must be <a href="package-summary.html#Statelessness">stateless</a>
126 * (their result should not depend on any state that might change during execution
127 * of the stream pipeline).</li>
128 * </ul>
129 *
130 * <p>Such parameters are always instances of a
131 * <a href="../function/package-summary.html">functional interface</a> such
132 * as {@link java.util.function.Function}, and are often lambda expressions or
133 * method references. Unless otherwise specified these parameters must be
134 * <em>non-null</em>.
135 *
136 * <p>A stream should be operated on (invoking an intermediate or terminal stream
137 * operation) only once. This rules out, for example, "forked" streams, where
138 * the same source feeds two or more pipelines, or multiple traversals of the
139 * same stream. A stream implementation may throw {@link IllegalStateException}
140 * if it detects that the stream is being reused. However, since some stream
141 * operations may return their receiver rather than a new stream object, it may
142 * not be possible to detect reuse in all cases.
143 *
144 * <p>Streams have a {@link #close()} method and implement {@link AutoCloseable}.
145 * Operating on a stream after it has been closed will throw {@link IllegalStateException}.
146 * Most stream instances do not actually need to be closed after use, as they
147 * are backed by collections, arrays, or generating functions, which require no
148 * special resource management. Generally, only streams whose source is an IO channel,
149 * such as those returned by {@link Files#lines(Path)}, will require closing. If a
150 * stream does require closing, it must be opened as a resource within a try-with-resources
151 * statement or similar control structure to ensure that it is closed promptly after its
152 * operations have completed.
153 *
154 * <p>Stream pipelines may execute either sequentially or in
155 * <a href="package-summary.html#Parallelism">parallel</a>. This
156 * execution mode is a property of the stream. Streams are created
157 * with an initial choice of sequential or parallel execution. (For example,
158 * {@link Collection#stream() Collection.stream()} creates a sequential stream,
159 * and {@link Collection#parallelStream() Collection.parallelStream()} creates
160 * a parallel one.) This choice of execution mode may be modified by the
161 * {@link #sequential()} or {@link #parallel()} methods, and may be queried with
162 * the {@link #isParallel()} method.
163 *
164 * @param <T> the type of the stream elements
165 * @since 1.8
166 * @see IntStream
167 * @see LongStream
168 * @see DoubleStream
169 * @see <a href="package-summary.html">java.util.stream</a>
170 */
171 public interface Stream<T> extends BaseStream<T, Stream<T>>, IterableOnce<T> {
172
173 // Need to re-abstract spliterator() to avoid inheriting the default
174 // method from Iterable.
175 /**
176 * {@inheritDoc}
177 * @return {@inheritDoc}
178 */
179 Spliterator<T> spliterator();
180
181 /**
182 * Returns a stream consisting of the elements of this stream that match
183 * the given predicate.
184 *
185 * <p>This is an <a href="package-summary.html#StreamOps">intermediate
186 * operation</a>.
187 *
188 * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>,
189 * <a href="package-summary.html#Statelessness">stateless</a>
190 * predicate to apply to each element to determine if it
191 * should be included
192 * @return the new stream
193 */
194 Stream<T> filter(Predicate<? super T> predicate);
195
196 /**
197 * Returns a stream consisting of the results of applying the given
198 * function to the elements of this stream.
199 *
200 * <p>This is an <a href="package-summary.html#StreamOps">intermediate
201 * operation</a>.
202 *
203 * @param <R> The element type of the new stream
204 * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
205 * <a href="package-summary.html#Statelessness">stateless</a>
206 * function to apply to each element
207 * @return the new stream
208 */
209 <R> Stream<R> map(Function<? super T, ? extends R> mapper);
210
211 /**
212 * Returns an {@code IntStream} consisting of the results of applying the
213 * given function to the elements of this stream.
214 *
215 * <p>This is an <a href="package-summary.html#StreamOps">
216 * intermediate operation</a>.
217 *
218 * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
219 * <a href="package-summary.html#Statelessness">stateless</a>
220 * function to apply to each element
221 * @return the new stream
222 */
223 IntStream mapToInt(ToIntFunction<? super T> mapper);
224
225 /**
226 * Returns a {@code LongStream} consisting of the results of applying the
227 * given function to the elements of this stream.
228 *
229 * <p>This is an <a href="package-summary.html#StreamOps">intermediate
230 * operation</a>.
231 *
232 * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
233 * <a href="package-summary.html#Statelessness">stateless</a>
234 * function to apply to each element
235 * @return the new stream
236 */
237 LongStream mapToLong(ToLongFunction<? super T> mapper);
238
239 /**
240 * Returns a {@code DoubleStream} consisting of the results of applying the
241 * given function to the elements of this stream.
242 *
243 * <p>This is an <a href="package-summary.html#StreamOps">intermediate
244 * operation</a>.
245 *
246 * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
247 * <a href="package-summary.html#Statelessness">stateless</a>
248 * function to apply to each element
249 * @return the new stream
250 */
251 DoubleStream mapToDouble(ToDoubleFunction<? super T> mapper);
252
253 /**
254 * Returns a stream consisting of the results of replacing each element of
255 * this stream with the contents of a mapped stream produced by applying
256 * the provided mapping function to each element. Each mapped stream is
257 * {@link java.util.stream.BaseStream#close() closed} after its contents
258 * have been placed into this stream. (If a mapped stream is {@code null}
259 * an empty stream is used, instead.)
260 *
261 * <p>This is an <a href="package-summary.html#StreamOps">intermediate
262 * operation</a>.
263 *
264 * @apiNote
265 * The {@code flatMap()} operation has the effect of applying a one-to-many
266 * transformation to the elements of the stream, and then flattening the
267 * resulting elements into a new stream.
268 *
269 * <p><b>Examples.</b>
270 *
271 * <p>If {@code orders} is a stream of purchase orders, and each purchase
272 * order contains a collection of line items, then the following produces a
273 * stream containing all the line items in all the orders:
274 * <pre>{@code
275 * orders.flatMap(order -> order.getLineItems().stream())...
276 * }</pre>
277 *
278 * <p>If {@code path} is the path to a file, then the following produces a
279 * stream of the {@code words} contained in that file:
280 * <pre>{@code
281 * Stream<String> lines = Files.lines(path, StandardCharsets.UTF_8);
282 * Stream<String> words = lines.flatMap(line -> Stream.of(line.split(" +")));
283 * }</pre>
284 * The {@code mapper} function passed to {@code flatMap} splits a line,
285 * using a simple regular expression, into an array of words, and then
286 * creates a stream of words from that array.
287 *
288 * @param <R> The element type of the new stream
289 * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
290 * <a href="package-summary.html#Statelessness">stateless</a>
291 * function to apply to each element which produces a stream
292 * of new values
293 * @return the new stream
294 */
295 <R> Stream<R> flatMap(Function<? super T, ? extends Stream<? extends R>> mapper);
296
297 /**
298 * Returns an {@code IntStream} consisting of the results of replacing each
299 * element of this stream with the contents of a mapped stream produced by
300 * applying the provided mapping function to each element. Each mapped
301 * stream is {@link java.util.stream.BaseStream#close() closed} after its
302 * contents have been placed into this stream. (If a mapped stream is
303 * {@code null} an empty stream is used, instead.)
304 *
305 * <p>This is an <a href="package-summary.html#StreamOps">intermediate
306 * operation</a>.
307 *
308 * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
309 * <a href="package-summary.html#Statelessness">stateless</a>
310 * function to apply to each element which produces a stream
311 * of new values
312 * @return the new stream
313 * @see #flatMap(Function)
314 */
315 IntStream flatMapToInt(Function<? super T, ? extends IntStream> mapper);
316
317 /**
318 * Returns an {@code LongStream} consisting of the results of replacing each
319 * element of this stream with the contents of a mapped stream produced by
320 * applying the provided mapping function to each element. Each mapped
321 * stream is {@link java.util.stream.BaseStream#close() closed} after its
322 * contents have been placed into this stream. (If a mapped stream is
323 * {@code null} an empty stream is used, instead.)
324 *
325 * <p>This is an <a href="package-summary.html#StreamOps">intermediate
326 * operation</a>.
327 *
328 * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
329 * <a href="package-summary.html#Statelessness">stateless</a>
330 * function to apply to each element which produces a stream
331 * of new values
332 * @return the new stream
333 * @see #flatMap(Function)
334 */
335 LongStream flatMapToLong(Function<? super T, ? extends LongStream> mapper);
336
337 /**
338 * Returns an {@code DoubleStream} consisting of the results of replacing
339 * each element of this stream with the contents of a mapped stream produced
340 * by applying the provided mapping function to each element. Each mapped
341 * stream is {@link java.util.stream.BaseStream#close() closed} after its
342 * contents have placed been into this stream. (If a mapped stream is
343 * {@code null} an empty stream is used, instead.)
344 *
345 * <p>This is an <a href="package-summary.html#StreamOps">intermediate
346 * operation</a>.
347 *
348 * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
349 * <a href="package-summary.html#Statelessness">stateless</a>
350 * function to apply to each element which produces a stream
351 * of new values
352 * @return the new stream
353 * @see #flatMap(Function)
354 */
355 DoubleStream flatMapToDouble(Function<? super T, ? extends DoubleStream> mapper);
356
357 /**
358 * Returns a stream consisting of the distinct elements (according to
359 * {@link Object#equals(Object)}) of this stream.
360 *
361 * <p>For ordered streams, the selection of distinct elements is stable
362 * (for duplicated elements, the element appearing first in the encounter
363 * order is preserved.) For unordered streams, no stability guarantees
364 * are made.
365 *
366 * <p>This is a <a href="package-summary.html#StreamOps">stateful
367 * intermediate operation</a>.
368 *
369 * @apiNote
370 * Preserving stability for {@code distinct()} in parallel pipelines is
371 * relatively expensive (requires that the operation act as a full barrier,
372 * with substantial buffering overhead), and stability is often not needed.
373 * Using an unordered stream source (such as {@link #generate(Supplier)})
374 * or removing the ordering constraint with {@link #unordered()} may result
375 * in significantly more efficient execution for {@code distinct()} in parallel
376 * pipelines, if the semantics of your situation permit. If consistency
377 * with encounter order is required, and you are experiencing poor performance
378 * or memory utilization with {@code distinct()} in parallel pipelines,
379 * switching to sequential execution with {@link #sequential()} may improve
380 * performance.
381 *
382 * @return the new stream
383 */
384 Stream<T> distinct();
385
386 /**
387 * Returns a stream consisting of the elements of this stream, sorted
388 * according to natural order. If the elements of this stream are not
389 * {@code Comparable}, a {@code java.lang.ClassCastException} may be thrown
390 * when the terminal operation is executed.
391 *
392 * <p>For ordered streams, the sort is stable. For unordered streams, no
393 * stability guarantees are made.
394 *
395 * <p>This is a <a href="package-summary.html#StreamOps">stateful
396 * intermediate operation</a>.
397 *
398 * @return the new stream
399 */
400 Stream<T> sorted();
401
402 /**
403 * Returns a stream consisting of the elements of this stream, sorted
404 * according to the provided {@code Comparator}.
405 *
406 * <p>For ordered streams, the sort is stable. For unordered streams, no
407 * stability guarantees are made.
408 *
409 * <p>This is a <a href="package-summary.html#StreamOps">stateful
410 * intermediate operation</a>.
411 *
412 * @param comparator a <a href="package-summary.html#NonInterference">non-interfering</a>,
413 * <a href="package-summary.html#Statelessness">stateless</a>
414 * {@code Comparator} to be used to compare stream elements
415 * @return the new stream
416 */
417 Stream<T> sorted(Comparator<? super T> comparator);
418
419 /**
420 * Returns a stream consisting of the elements of this stream, additionally
421 * performing the provided action on each element as elements are consumed
422 * from the resulting stream.
423 *
424 * <p>This is an <a href="package-summary.html#StreamOps">intermediate
425 * operation</a>.
426 *
427 * <p>For parallel stream pipelines, the action may be called at
428 * whatever time and in whatever thread the element is made available by the
429 * upstream operation. If the action modifies shared state,
430 * it is responsible for providing the required synchronization.
431 *
432 * @apiNote This method exists mainly to support debugging, where you want
433 * to see the elements as they flow past a certain point in a pipeline:
434 * <pre>{@code
435 * Stream.of("one", "two", "three", "four")
436 * .filter(e -> e.length() > 3)
437 * .peek(e -> System.out.println("Filtered value: " + e))
438 * .map(String::toUpperCase)
439 * .peek(e -> System.out.println("Mapped value: " + e))
440 * .collect(Collectors.toList());
441 * }</pre>
442 *
443 * <p>In cases where the stream implementation is able to optimize away the
444 * production of some or all the elements (such as with short-circuiting
445 * operations like {@code findFirst}, or in the example described in
446 * {@link #count}), the action will not be invoked for those elements.
447 *
448 * @param action a <a href="package-summary.html#NonInterference">
449 * non-interfering</a> action to perform on the elements as
450 * they are consumed from the stream
451 * @return the new stream
452 */
453 Stream<T> peek(Consumer<? super T> action);
454
455 /**
456 * Returns a stream consisting of the elements of this stream, truncated
457 * to be no longer than {@code maxSize} in length.
458 *
459 * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
460 * stateful intermediate operation</a>.
461 *
462 * @apiNote
463 * While {@code limit()} is generally a cheap operation on sequential
464 * stream pipelines, it can be quite expensive on ordered parallel pipelines,
465 * especially for large values of {@code maxSize}, since {@code limit(n)}
466 * is constrained to return not just any <em>n</em> elements, but the
467 * <em>first n</em> elements in the encounter order. Using an unordered
468 * stream source (such as {@link #generate(Supplier)}) or removing the
469 * ordering constraint with {@link #unordered()} may result in significant
470 * speedups of {@code limit()} in parallel pipelines, if the semantics of
471 * your situation permit. If consistency with encounter order is required,
472 * and you are experiencing poor performance or memory utilization with
473 * {@code limit()} in parallel pipelines, switching to sequential execution
474 * with {@link #sequential()} may improve performance.
475 *
476 * @param maxSize the number of elements the stream should be limited to
477 * @return the new stream
478 * @throws IllegalArgumentException if {@code maxSize} is negative
479 */
480 Stream<T> limit(long maxSize);
481
482 /**
483 * Returns a stream consisting of the remaining elements of this stream
484 * after discarding the first {@code n} elements of the stream.
485 * If this stream contains fewer than {@code n} elements then an
486 * empty stream will be returned.
487 *
488 * <p>This is a <a href="package-summary.html#StreamOps">stateful
489 * intermediate operation</a>.
490 *
491 * @apiNote
492 * While {@code skip()} is generally a cheap operation on sequential
493 * stream pipelines, it can be quite expensive on ordered parallel pipelines,
494 * especially for large values of {@code n}, since {@code skip(n)}
495 * is constrained to skip not just any <em>n</em> elements, but the
496 * <em>first n</em> elements in the encounter order. Using an unordered
497 * stream source (such as {@link #generate(Supplier)}) or removing the
498 * ordering constraint with {@link #unordered()} may result in significant
499 * speedups of {@code skip()} in parallel pipelines, if the semantics of
500 * your situation permit. If consistency with encounter order is required,
501 * and you are experiencing poor performance or memory utilization with
502 * {@code skip()} in parallel pipelines, switching to sequential execution
503 * with {@link #sequential()} may improve performance.
504 *
505 * @param n the number of leading elements to skip
506 * @return the new stream
507 * @throws IllegalArgumentException if {@code n} is negative
508 */
509 Stream<T> skip(long n);
510
511 /**
512 * Returns, if this stream is ordered, a stream consisting of the longest
513 * prefix of elements taken from this stream that match the given predicate.
514 * Otherwise returns, if this stream is unordered, a stream consisting of a
515 * subset of elements taken from this stream that match the given predicate.
516 *
517 * <p>If this stream is ordered then the longest prefix is a contiguous
518 * sequence of elements of this stream that match the given predicate. The
519 * first element of the sequence is the first element of this stream, and
520 * the element immediately following the last element of the sequence does
521 * not match the given predicate.
522 *
523 * <p>If this stream is unordered, and some (but not all) elements of this
524 * stream match the given predicate, then the behavior of this operation is
525 * nondeterministic; it is free to take any subset of matching elements
526 * (which includes the empty set).
527 *
528 * <p>Independent of whether this stream is ordered or unordered if all
529 * elements of this stream match the given predicate then this operation
530 * takes all elements (the result is the same as the input), or if no
531 * elements of the stream match the given predicate then no elements are
532 * taken (the result is an empty stream).
533 *
534 * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
535 * stateful intermediate operation</a>.
536 *
537 * @implSpec
538 * The default implementation obtains the {@link #spliterator() spliterator}
539 * of this stream, wraps that spliterator so as to support the semantics
540 * of this operation on traversal, and returns a new stream associated with
541 * the wrapped spliterator. The returned stream preserves the execution
542 * characteristics of this stream (namely parallel or sequential execution
543 * as per {@link #isParallel()}) but the wrapped spliterator may choose to
544 * not support splitting. When the returned stream is closed, the close
545 * handlers for both the returned and this stream are invoked.
546 *
547 * @apiNote
548 * While {@code takeWhile()} is generally a cheap operation on sequential
549 * stream pipelines, it can be quite expensive on ordered parallel
550 * pipelines, since the operation is constrained to return not just any
551 * valid prefix, but the longest prefix of elements in the encounter order.
552 * Using an unordered stream source (such as {@link #generate(Supplier)}) or
553 * removing the ordering constraint with {@link #unordered()} may result in
554 * significant speedups of {@code takeWhile()} in parallel pipelines, if the
555 * semantics of your situation permit. If consistency with encounter order
556 * is required, and you are experiencing poor performance or memory
557 * utilization with {@code takeWhile()} in parallel pipelines, switching to
558 * sequential execution with {@link #sequential()} may improve performance.
559 *
560 * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>,
561 * <a href="package-summary.html#Statelessness">stateless</a>
562 * predicate to apply to elements to determine the longest
563 * prefix of elements.
564 * @return the new stream
565 * @since 9
566 */
567 default Stream<T> takeWhile(Predicate<? super T> predicate) {
568 Objects.requireNonNull(predicate);
569 // Reuses the unordered spliterator, which, when encounter is present,
570 // is safe to use as long as it configured not to split
571 return StreamSupport.stream(
572 new WhileOps.UnorderedWhileSpliterator.OfRef.Taking<>(spliterator(), true, predicate),
573 isParallel()).onClose(this::close);
574 }
575
576 /**
577 * Returns, if this stream is ordered, a stream consisting of the remaining
578 * elements of this stream after dropping the longest prefix of elements
579 * that match the given predicate. Otherwise returns, if this stream is
580 * unordered, a stream consisting of the remaining elements of this stream
581 * after dropping a subset of elements that match the given predicate.
582 *
583 * <p>If this stream is ordered then the longest prefix is a contiguous
584 * sequence of elements of this stream that match the given predicate. The
585 * first element of the sequence is the first element of this stream, and
586 * the element immediately following the last element of the sequence does
587 * not match the given predicate.
588 *
589 * <p>If this stream is unordered, and some (but not all) elements of this
590 * stream match the given predicate, then the behavior of this operation is
591 * nondeterministic; it is free to drop any subset of matching elements
592 * (which includes the empty set).
593 *
594 * <p>Independent of whether this stream is ordered or unordered if all
595 * elements of this stream match the given predicate then this operation
596 * drops all elements (the result is an empty stream), or if no elements of
597 * the stream match the given predicate then no elements are dropped (the
598 * result is the same as the input).
599 *
600 * <p>This is a <a href="package-summary.html#StreamOps">stateful
601 * intermediate operation</a>.
602 *
603 * @implSpec
604 * The default implementation obtains the {@link #spliterator() spliterator}
605 * of this stream, wraps that spliterator so as to support the semantics
606 * of this operation on traversal, and returns a new stream associated with
607 * the wrapped spliterator. The returned stream preserves the execution
608 * characteristics of this stream (namely parallel or sequential execution
609 * as per {@link #isParallel()}) but the wrapped spliterator may choose to
610 * not support splitting. When the returned stream is closed, the close
611 * handlers for both the returned and this stream are invoked.
612 *
613 * @apiNote
614 * While {@code dropWhile()} is generally a cheap operation on sequential
615 * stream pipelines, it can be quite expensive on ordered parallel
616 * pipelines, since the operation is constrained to return not just any
617 * valid prefix, but the longest prefix of elements in the encounter order.
618 * Using an unordered stream source (such as {@link #generate(Supplier)}) or
619 * removing the ordering constraint with {@link #unordered()} may result in
620 * significant speedups of {@code dropWhile()} in parallel pipelines, if the
621 * semantics of your situation permit. If consistency with encounter order
622 * is required, and you are experiencing poor performance or memory
623 * utilization with {@code dropWhile()} in parallel pipelines, switching to
624 * sequential execution with {@link #sequential()} may improve performance.
625 *
626 * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>,
627 * <a href="package-summary.html#Statelessness">stateless</a>
628 * predicate to apply to elements to determine the longest
629 * prefix of elements.
630 * @return the new stream
631 * @since 9
632 */
633 default Stream<T> dropWhile(Predicate<? super T> predicate) {
634 Objects.requireNonNull(predicate);
635 // Reuses the unordered spliterator, which, when encounter is present,
636 // is safe to use as long as it configured not to split
637 return StreamSupport.stream(
638 new WhileOps.UnorderedWhileSpliterator.OfRef.Dropping<>(spliterator(), true, predicate),
639 isParallel()).onClose(this::close);
640 }
641
642 /**
643 * Performs an action for each element of this stream.
644 *
645 * <p>This is a <a href="package-summary.html#StreamOps">terminal
646 * operation</a>.
647 *
648 * <p>The behavior of this operation is explicitly nondeterministic.
649 * For parallel stream pipelines, this operation does <em>not</em>
650 * guarantee to respect the encounter order of the stream, as doing so
651 * would sacrifice the benefit of parallelism. For any given element, the
652 * action may be performed at whatever time and in whatever thread the
653 * library chooses. If the action accesses shared state, it is
654 * responsible for providing the required synchronization.
655 *
656 * @param action a <a href="package-summary.html#NonInterference">
657 * non-interfering</a> action to perform on the elements
658 */
659 void forEach(Consumer<? super T> action);
660
661 /**
662 * Performs an action for each element of this stream, in the encounter
663 * order of the stream if the stream has a defined encounter order.
664 *
665 * <p>This is a <a href="package-summary.html#StreamOps">terminal
666 * operation</a>.
667 *
668 * <p>This operation processes the elements one at a time, in encounter
669 * order if one exists. Performing the action for one element
670 * <a href="../concurrent/package-summary.html#MemoryVisibility"><i>happens-before</i></a>
671 * performing the action for subsequent elements, but for any given element,
672 * the action may be performed in whatever thread the library chooses.
673 *
674 * @param action a <a href="package-summary.html#NonInterference">
675 * non-interfering</a> action to perform on the elements
676 * @see #forEach(Consumer)
677 */
678 void forEachOrdered(Consumer<? super T> action);
679
680 /**
681 * Returns an array containing the elements of this stream.
682 *
683 * <p>This is a <a href="package-summary.html#StreamOps">terminal
684 * operation</a>.
685 *
686 * @return an array, whose {@linkplain Class#getComponentType runtime component
687 * type} is {@code Object}, containing the elements of this stream
688 */
689 Object[] toArray();
690
691 /**
692 * Returns an array containing the elements of this stream, using the
693 * provided {@code generator} function to allocate the returned array, as
694 * well as any additional arrays that might be required for a partitioned
695 * execution or for resizing.
696 *
697 * <p>This is a <a href="package-summary.html#StreamOps">terminal
698 * operation</a>.
699 *
700 * @apiNote
701 * The generator function takes an integer, which is the size of the
702 * desired array, and produces an array of the desired size. This can be
703 * concisely expressed with an array constructor reference:
704 * <pre>{@code
705 * Person[] men = people.stream()
706 * .filter(p -> p.getGender() == MALE)
707 * .toArray(Person[]::new);
708 * }</pre>
709 *
710 * @param <A> the component type of the resulting array
711 * @param generator a function which produces a new array of the desired
712 * type and the provided length
713 * @return an array containing the elements in this stream
714 * @throws ArrayStoreException if the runtime type of any element of this
715 * stream is not assignable to the {@linkplain Class#getComponentType
716 * runtime component type} of the generated array
717 */
718 <A> A[] toArray(IntFunction<A[]> generator);
719
720 /**
721 * Performs a <a href="package-summary.html#Reduction">reduction</a> on the
722 * elements of this stream, using the provided identity value and an
723 * <a href="package-summary.html#Associativity">associative</a>
724 * accumulation function, and returns the reduced value. This is equivalent
725 * to:
726 * <pre>{@code
727 * T result = identity;
728 * for (T element : this stream)
729 * result = accumulator.apply(result, element)
730 * return result;
731 * }</pre>
732 *
733 * but is not constrained to execute sequentially.
734 *
735 * <p>The {@code identity} value must be an identity for the accumulator
736 * function. This means that for all {@code t},
737 * {@code accumulator.apply(identity, t)} is equal to {@code t}.
738 * The {@code accumulator} function must be an
739 * <a href="package-summary.html#Associativity">associative</a> function.
740 *
741 * <p>This is a <a href="package-summary.html#StreamOps">terminal
742 * operation</a>.
743 *
744 * @apiNote Sum, min, max, average, and string concatenation are all special
745 * cases of reduction. Summing a stream of numbers can be expressed as:
746 *
747 * <pre>{@code
748 * Integer sum = integers.reduce(0, (a, b) -> a+b);
749 * }</pre>
750 *
751 * or:
752 *
753 * <pre>{@code
754 * Integer sum = integers.reduce(0, Integer::sum);
755 * }</pre>
756 *
757 * <p>While this may seem a more roundabout way to perform an aggregation
758 * compared to simply mutating a running total in a loop, reduction
759 * operations parallelize more gracefully, without needing additional
760 * synchronization and with greatly reduced risk of data races.
761 *
762 * @param identity the identity value for the accumulating function
763 * @param accumulator an <a href="package-summary.html#Associativity">associative</a>,
764 * <a href="package-summary.html#NonInterference">non-interfering</a>,
765 * <a href="package-summary.html#Statelessness">stateless</a>
766 * function for combining two values
767 * @return the result of the reduction
768 */
769 T reduce(T identity, BinaryOperator<T> accumulator);
770
771 /**
772 * Performs a <a href="package-summary.html#Reduction">reduction</a> on the
773 * elements of this stream, using an
774 * <a href="package-summary.html#Associativity">associative</a> accumulation
775 * function, and returns an {@code Optional} describing the reduced value,
776 * if any. This is equivalent to:
777 * <pre>{@code
778 * boolean foundAny = false;
779 * T result = null;
780 * for (T element : this stream) {
781 * if (!foundAny) {
782 * foundAny = true;
783 * result = element;
784 * }
785 * else
786 * result = accumulator.apply(result, element);
787 * }
788 * return foundAny ? Optional.of(result) : Optional.empty();
789 * }</pre>
790 *
791 * but is not constrained to execute sequentially.
792 *
793 * <p>The {@code accumulator} function must be an
794 * <a href="package-summary.html#Associativity">associative</a> function.
795 *
796 * <p>This is a <a href="package-summary.html#StreamOps">terminal
797 * operation</a>.
798 *
799 * @param accumulator an <a href="package-summary.html#Associativity">associative</a>,
800 * <a href="package-summary.html#NonInterference">non-interfering</a>,
801 * <a href="package-summary.html#Statelessness">stateless</a>
802 * function for combining two values
803 * @return an {@link Optional} describing the result of the reduction
804 * @throws NullPointerException if the result of the reduction is null
805 * @see #reduce(Object, BinaryOperator)
806 * @see #min(Comparator)
807 * @see #max(Comparator)
808 */
809 Optional<T> reduce(BinaryOperator<T> accumulator);
810
811 /**
812 * Performs a <a href="package-summary.html#Reduction">reduction</a> on the
813 * elements of this stream, using the provided identity, accumulation and
814 * combining functions. This is equivalent to:
815 * <pre>{@code
816 * U result = identity;
817 * for (T element : this stream)
818 * result = accumulator.apply(result, element)
819 * return result;
820 * }</pre>
821 *
822 * but is not constrained to execute sequentially.
823 *
824 * <p>The {@code identity} value must be an identity for the combiner
825 * function. This means that for all {@code u}, {@code combiner(identity, u)}
826 * is equal to {@code u}. Additionally, the {@code combiner} function
827 * must be compatible with the {@code accumulator} function; for all
828 * {@code u} and {@code t}, the following must hold:
829 * <pre>{@code
830 * combiner.apply(u, accumulator.apply(identity, t)) == accumulator.apply(u, t)
831 * }</pre>
832 *
833 * <p>This is a <a href="package-summary.html#StreamOps">terminal
834 * operation</a>.
835 *
836 * @apiNote Many reductions using this form can be represented more simply
837 * by an explicit combination of {@code map} and {@code reduce} operations.
838 * The {@code accumulator} function acts as a fused mapper and accumulator,
839 * which can sometimes be more efficient than separate mapping and reduction,
840 * such as when knowing the previously reduced value allows you to avoid
841 * some computation.
842 *
843 * @param <U> The type of the result
844 * @param identity the identity value for the combiner function
845 * @param accumulator an <a href="package-summary.html#Associativity">associative</a>,
846 * <a href="package-summary.html#NonInterference">non-interfering</a>,
847 * <a href="package-summary.html#Statelessness">stateless</a>
848 * function for incorporating an additional element into a result
849 * @param combiner an <a href="package-summary.html#Associativity">associative</a>,
850 * <a href="package-summary.html#NonInterference">non-interfering</a>,
851 * <a href="package-summary.html#Statelessness">stateless</a>
852 * function for combining two values, which must be
853 * compatible with the accumulator function
854 * @return the result of the reduction
855 * @see #reduce(BinaryOperator)
856 * @see #reduce(Object, BinaryOperator)
857 */
858 <U> U reduce(U identity,
859 BiFunction<U, ? super T, U> accumulator,
860 BinaryOperator<U> combiner);
861
862 /**
863 * Performs a <a href="package-summary.html#MutableReduction">mutable
864 * reduction</a> operation on the elements of this stream. A mutable
865 * reduction is one in which the reduced value is a mutable result container,
866 * such as an {@code ArrayList}, and elements are incorporated by updating
867 * the state of the result rather than by replacing the result. This
868 * produces a result equivalent to:
869 * <pre>{@code
870 * R result = supplier.get();
871 * for (T element : this stream)
872 * accumulator.accept(result, element);
873 * return result;
874 * }</pre>
875 *
876 * <p>Like {@link #reduce(Object, BinaryOperator)}, {@code collect} operations
877 * can be parallelized without requiring additional synchronization.
878 *
879 * <p>This is a <a href="package-summary.html#StreamOps">terminal
880 * operation</a>.
881 *
882 * @apiNote There are many existing classes in the JDK whose signatures are
883 * well-suited for use with method references as arguments to {@code collect()}.
884 * For example, the following will accumulate strings into an {@code ArrayList}:
885 * <pre>{@code
886 * List<String> asList = stringStream.collect(ArrayList::new, ArrayList::add,
887 * ArrayList::addAll);
888 * }</pre>
889 *
890 * <p>The following will take a stream of strings and concatenates them into a
891 * single string:
892 * <pre>{@code
893 * String concat = stringStream.collect(StringBuilder::new, StringBuilder::append,
894 * StringBuilder::append)
895 * .toString();
896 * }</pre>
897 *
898 * @param <R> the type of the mutable result container
899 * @param supplier a function that creates a new mutable result container.
900 * For a parallel execution, this function may be called
901 * multiple times and must return a fresh value each time.
902 * @param accumulator an <a href="package-summary.html#Associativity">associative</a>,
903 * <a href="package-summary.html#NonInterference">non-interfering</a>,
904 * <a href="package-summary.html#Statelessness">stateless</a>
905 * function that must fold an element into a result
906 * container.
907 * @param combiner an <a href="package-summary.html#Associativity">associative</a>,
908 * <a href="package-summary.html#NonInterference">non-interfering</a>,
909 * <a href="package-summary.html#Statelessness">stateless</a>
910 * function that accepts two partial result containers
911 * and merges them, which must be compatible with the
912 * accumulator function. The combiner function must fold
913 * the elements from the second result container into the
914 * first result container.
915 * @return the result of the reduction
916 */
917 <R> R collect(Supplier<R> supplier,
918 BiConsumer<R, ? super T> accumulator,
919 BiConsumer<R, R> combiner);
920
921 /**
922 * Performs a <a href="package-summary.html#MutableReduction">mutable
923 * reduction</a> operation on the elements of this stream using a
924 * {@code Collector}. A {@code Collector}
925 * encapsulates the functions used as arguments to
926 * {@link #collect(Supplier, BiConsumer, BiConsumer)}, allowing for reuse of
927 * collection strategies and composition of collect operations such as
928 * multiple-level grouping or partitioning.
929 *
930 * <p>If the stream is parallel, and the {@code Collector}
931 * is {@link Collector.Characteristics#CONCURRENT concurrent}, and
932 * either the stream is unordered or the collector is
933 * {@link Collector.Characteristics#UNORDERED unordered},
934 * then a concurrent reduction will be performed (see {@link Collector} for
935 * details on concurrent reduction.)
936 *
937 * <p>This is a <a href="package-summary.html#StreamOps">terminal
938 * operation</a>.
939 *
940 * <p>When executed in parallel, multiple intermediate results may be
941 * instantiated, populated, and merged so as to maintain isolation of
942 * mutable data structures. Therefore, even when executed in parallel
943 * with non-thread-safe data structures (such as {@code ArrayList}), no
944 * additional synchronization is needed for a parallel reduction.
945 *
946 * @apiNote
947 * The following will accumulate strings into an ArrayList:
948 * <pre>{@code
949 * List<String> asList = stringStream.collect(Collectors.toList());
950 * }</pre>
951 *
952 * <p>The following will classify {@code Person} objects by city:
953 * <pre>{@code
954 * Map<String, List<Person>> peopleByCity
955 * = personStream.collect(Collectors.groupingBy(Person::getCity));
956 * }</pre>
957 *
958 * <p>The following will classify {@code Person} objects by state and city,
959 * cascading two {@code Collector}s together:
960 * <pre>{@code
961 * Map<String, Map<String, List<Person>>> peopleByStateAndCity
962 * = personStream.collect(Collectors.groupingBy(Person::getState,
963 * Collectors.groupingBy(Person::getCity)));
964 * }</pre>
965 *
966 * @param <R> the type of the result
967 * @param <A> the intermediate accumulation type of the {@code Collector}
968 * @param collector the {@code Collector} describing the reduction
969 * @return the result of the reduction
970 * @see #collect(Supplier, BiConsumer, BiConsumer)
971 * @see Collectors
972 */
973 <R, A> R collect(Collector<? super T, A, R> collector);
974
975 /**
976 * Returns the minimum element of this stream according to the provided
977 * {@code Comparator}. This is a special case of a
978 * <a href="package-summary.html#Reduction">reduction</a>.
979 *
980 * <p>This is a <a href="package-summary.html#StreamOps">terminal operation</a>.
981 *
982 * @param comparator a <a href="package-summary.html#NonInterference">non-interfering</a>,
983 * <a href="package-summary.html#Statelessness">stateless</a>
984 * {@code Comparator} to compare elements of this stream
985 * @return an {@code Optional} describing the minimum element of this stream,
986 * or an empty {@code Optional} if the stream is empty
987 * @throws NullPointerException if the minimum element is null
988 */
989 Optional<T> min(Comparator<? super T> comparator);
990
991 /**
992 * Returns the maximum element of this stream according to the provided
993 * {@code Comparator}. This is a special case of a
994 * <a href="package-summary.html#Reduction">reduction</a>.
995 *
996 * <p>This is a <a href="package-summary.html#StreamOps">terminal
997 * operation</a>.
998 *
999 * @param comparator a <a href="package-summary.html#NonInterference">non-interfering</a>,
1000 * <a href="package-summary.html#Statelessness">stateless</a>
1001 * {@code Comparator} to compare elements of this stream
1002 * @return an {@code Optional} describing the maximum element of this stream,
1003 * or an empty {@code Optional} if the stream is empty
1004 * @throws NullPointerException if the maximum element is null
1005 */
1006 Optional<T> max(Comparator<? super T> comparator);
1007
1008 /**
1009 * Returns the count of elements in this stream. This is a special case of
1010 * a <a href="package-summary.html#Reduction">reduction</a> and is
1011 * equivalent to:
1012 * <pre>{@code
1013 * return mapToLong(e -> 1L).sum();
1014 * }</pre>
1015 *
1016 * <p>This is a <a href="package-summary.html#StreamOps">terminal operation</a>.
1017 *
1018 * @apiNote
1019 * An implementation may choose to not execute the stream pipeline (either
1020 * sequentially or in parallel) if it is capable of computing the count
1021 * directly from the stream source. In such cases no source elements will
1022 * be traversed and no intermediate operations will be evaluated.
1023 * Behavioral parameters with side-effects, which are strongly discouraged
1024 * except for harmless cases such as debugging, may be affected. For
1025 * example, consider the following stream:
1026 * <pre>{@code
1027 * List<String> l = Arrays.asList("A", "B", "C", "D");
1028 * long count = l.stream().peek(System.out::println).count();
1029 * }</pre>
1030 * The number of elements covered by the stream source, a {@code List}, is
1031 * known and the intermediate operation, {@code peek}, does not inject into
1032 * or remove elements from the stream (as may be the case for
1033 * {@code flatMap} or {@code filter} operations). Thus the count is the
1034 * size of the {@code List} and there is no need to execute the pipeline
1035 * and, as a side-effect, print out the list elements.
1036 *
1037 * @return the count of elements in this stream
1038 */
1039 long count();
1040
1041 /**
1042 * Returns whether any elements of this stream match the provided
1043 * predicate. May not evaluate the predicate on all elements if not
1044 * necessary for determining the result. If the stream is empty then
1045 * {@code false} is returned and the predicate is not evaluated.
1046 *
1047 * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
1048 * terminal operation</a>.
1049 *
1050 * @apiNote
1051 * This method evaluates the <em>existential quantification</em> of the
1052 * predicate over the elements of the stream (for some x P(x)).
1053 *
1054 * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>,
1055 * <a href="package-summary.html#Statelessness">stateless</a>
1056 * predicate to apply to elements of this stream
1057 * @return {@code true} if any elements of the stream match the provided
1058 * predicate, otherwise {@code false}
1059 */
1060 boolean anyMatch(Predicate<? super T> predicate);
1061
1062 /**
1063 * Returns whether all elements of this stream match the provided predicate.
1064 * May not evaluate the predicate on all elements if not necessary for
1065 * determining the result. If the stream is empty then {@code true} is
1066 * returned and the predicate is not evaluated.
1067 *
1068 * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
1069 * terminal operation</a>.
1070 *
1071 * @apiNote
1072 * This method evaluates the <em>universal quantification</em> of the
1073 * predicate over the elements of the stream (for all x P(x)). If the
1074 * stream is empty, the quantification is said to be <em>vacuously
1075 * satisfied</em> and is always {@code true} (regardless of P(x)).
1076 *
1077 * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>,
1078 * <a href="package-summary.html#Statelessness">stateless</a>
1079 * predicate to apply to elements of this stream
1080 * @return {@code true} if either all elements of the stream match the
1081 * provided predicate or the stream is empty, otherwise {@code false}
1082 */
1083 boolean allMatch(Predicate<? super T> predicate);
1084
1085 /**
1086 * Returns whether no elements of this stream match the provided predicate.
1087 * May not evaluate the predicate on all elements if not necessary for
1088 * determining the result. If the stream is empty then {@code true} is
1089 * returned and the predicate is not evaluated.
1090 *
1091 * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
1092 * terminal operation</a>.
1093 *
1094 * @apiNote
1095 * This method evaluates the <em>universal quantification</em> of the
1096 * negated predicate over the elements of the stream (for all x ~P(x)). If
1097 * the stream is empty, the quantification is said to be vacuously satisfied
1098 * and is always {@code true}, regardless of P(x).
1099 *
1100 * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>,
1101 * <a href="package-summary.html#Statelessness">stateless</a>
1102 * predicate to apply to elements of this stream
1103 * @return {@code true} if either no elements of the stream match the
1104 * provided predicate or the stream is empty, otherwise {@code false}
1105 */
1106 boolean noneMatch(Predicate<? super T> predicate);
1107
1108 /**
1109 * Returns an {@link Optional} describing the first element of this stream,
1110 * or an empty {@code Optional} if the stream is empty. If the stream has
1111 * no encounter order, then any element may be returned.
1112 *
1113 * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
1114 * terminal operation</a>.
1115 *
1116 * @return an {@code Optional} describing the first element of this stream,
1117 * or an empty {@code Optional} if the stream is empty
1118 * @throws NullPointerException if the element selected is null
1119 */
1120 Optional<T> findFirst();
1121
1122 /**
1123 * Returns an {@link Optional} describing some element of the stream, or an
1124 * empty {@code Optional} if the stream is empty.
1125 *
1126 * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
1127 * terminal operation</a>.
1128 *
1129 * <p>The behavior of this operation is explicitly nondeterministic; it is
1130 * free to select any element in the stream. This is to allow for maximal
1131 * performance in parallel operations; the cost is that multiple invocations
1132 * on the same source may not return the same result. (If a stable result
1133 * is desired, use {@link #findFirst()} instead.)
1134 *
1135 * @return an {@code Optional} describing some element of this stream, or an
1136 * empty {@code Optional} if the stream is empty
1137 * @throws NullPointerException if the element selected is null
1138 * @see #findFirst()
1139 */
1140 Optional<T> findAny();
1141
1142 // Static factories
1143
1144 /**
1145 * Returns a builder for a {@code Stream}.
1146 *
1147 * @param <T> type of elements
1148 * @return a stream builder
1149 */
1150 public static<T> Builder<T> builder() {
1151 return new Streams.StreamBuilderImpl<>();
1152 }
1153
1154 /**
1155 * Returns an empty sequential {@code Stream}.
1156 *
1157 * @param <T> the type of stream elements
1158 * @return an empty sequential stream
1159 */
1160 public static<T> Stream<T> empty() {
1161 return StreamSupport.stream(Spliterators.<T>emptySpliterator(), false);
1162 }
1163
1164 /**
1165 * Returns a sequential {@code Stream} containing a single element.
1166 *
1167 * @param t the single element
1168 * @param <T> the type of stream elements
1169 * @return a singleton sequential stream
1170 */
1171 public static<T> Stream<T> of(T t) {
1172 return StreamSupport.stream(new Streams.StreamBuilderImpl<>(t), false);
1173 }
1174
1175 /**
1176 * Returns a sequential {@code Stream} containing a single element, if
1177 * non-null, otherwise returns an empty {@code Stream}.
1178 *
1179 * @param t the single element
1180 * @param <T> the type of stream elements
1181 * @return a stream with a single element if the specified element
1182 * is non-null, otherwise an empty stream
1183 * @since 9
1184 */
1185 public static<T> Stream<T> ofNullable(T t) {
1186 return t == null ? Stream.empty()
1187 : StreamSupport.stream(new Streams.StreamBuilderImpl<>(t), false);
1188 }
1189
1190 /**
1191 * Returns a sequential ordered stream whose elements are the specified values.
1192 *
1193 * @param <T> the type of stream elements
1194 * @param values the elements of the new stream
1195 * @return the new stream
1196 */
1197 @SafeVarargs
1198 @SuppressWarnings("varargs") // Creating a stream from an array is safe
1199 public static<T> Stream<T> of(T... values) {
1200 return Arrays.stream(values);
1201 }
1202
1203 /**
1204 * Returns an infinite sequential ordered {@code Stream} produced by iterative
1205 * application of a function {@code f} to an initial element {@code seed},
1206 * producing a {@code Stream} consisting of {@code seed}, {@code f(seed)},
1207 * {@code f(f(seed))}, etc.
1208 *
1209 * <p>The first element (position {@code 0}) in the {@code Stream} will be
1210 * the provided {@code seed}. For {@code n > 0}, the element at position
1211 * {@code n}, will be the result of applying the function {@code f} to the
1212 * element at position {@code n - 1}.
1213 *
1214 * <p>The action of applying {@code f} for one element
1215 * <a href="../concurrent/package-summary.html#MemoryVisibility"><i>happens-before</i></a>
1216 * the action of applying {@code f} for subsequent elements. For any given
1217 * element the action may be performed in whatever thread the library
1218 * chooses.
1219 *
1220 * @param <T> the type of stream elements
1221 * @param seed the initial element
1222 * @param f a function to be applied to the previous element to produce
1223 * a new element
1224 * @return a new sequential {@code Stream}
1225 */
1226 public static<T> Stream<T> iterate(final T seed, final UnaryOperator<T> f) {
1227 Objects.requireNonNull(f);
1228 Spliterator<T> spliterator = new Spliterators.AbstractSpliterator<>(Long.MAX_VALUE,
1229 Spliterator.ORDERED | Spliterator.IMMUTABLE) {
1230 T prev;
1231 boolean started;
1232
1233 @Override
1234 public boolean tryAdvance(Consumer<? super T> action) {
1235 Objects.requireNonNull(action);
1236 T t;
1237 if (started)
1238 t = f.apply(prev);
1239 else {
1240 t = seed;
1241 started = true;
1242 }
1243 action.accept(prev = t);
1244 return true;
1245 }
1246 };
1247 return StreamSupport.stream(spliterator, false);
1248 }
1249
1250 /**
1251 * Returns a sequential ordered {@code Stream} produced by iterative
1252 * application of the given {@code next} function to an initial element,
1253 * conditioned on satisfying the given {@code hasNext} predicate. The
1254 * stream terminates as soon as the {@code hasNext} predicate returns false.
1255 *
1256 * <p>{@code Stream.iterate} should produce the same sequence of elements as
1257 * produced by the corresponding for-loop:
1258 * <pre>{@code
1259 * for (T index=seed; hasNext.test(index); index = next.apply(index)) {
1260 * ...
1261 * }
1262 * }</pre>
1263 *
1264 * <p>The resulting sequence may be empty if the {@code hasNext} predicate
1265 * does not hold on the seed value. Otherwise the first element will be the
1266 * supplied {@code seed} value, the next element (if present) will be the
1267 * result of applying the {@code next} function to the {@code seed} value,
1268 * and so on iteratively until the {@code hasNext} predicate indicates that
1269 * the stream should terminate.
1270 *
1271 * <p>The action of applying the {@code hasNext} predicate to an element
1272 * <a href="../concurrent/package-summary.html#MemoryVisibility"><i>happens-before</i></a>
1273 * the action of applying the {@code next} function to that element. The
1274 * action of applying the {@code next} function for one element
1275 * <i>happens-before</i> the action of applying the {@code hasNext}
1276 * predicate for subsequent elements. For any given element an action may
1277 * be performed in whatever thread the library chooses.
1278 *
1279 * @param <T> the type of stream elements
1280 * @param seed the initial element
1281 * @param hasNext a predicate to apply to elements to determine when the
1282 * stream must terminate.
1283 * @param next a function to be applied to the previous element to produce
1284 * a new element
1285 * @return a new sequential {@code Stream}
1286 * @since 9
1287 */
1288 public static<T> Stream<T> iterate(T seed, Predicate<? super T> hasNext, UnaryOperator<T> next) {
1289 Objects.requireNonNull(next);
1290 Objects.requireNonNull(hasNext);
1291 Spliterator<T> spliterator = new Spliterators.AbstractSpliterator<>(Long.MAX_VALUE,
1292 Spliterator.ORDERED | Spliterator.IMMUTABLE) {
1293 T prev;
1294 boolean started, finished;
1295
1296 @Override
1297 public boolean tryAdvance(Consumer<? super T> action) {
1298 Objects.requireNonNull(action);
1299 if (finished)
1300 return false;
1301 T t;
1302 if (started)
1303 t = next.apply(prev);
1304 else {
1305 t = seed;
1306 started = true;
1307 }
1308 if (!hasNext.test(t)) {
1309 prev = null;
1310 finished = true;
1311 return false;
1312 }
1313 action.accept(prev = t);
1314 return true;
1315 }
1316
1317 @Override
1318 public void forEachRemaining(Consumer<? super T> action) {
1319 Objects.requireNonNull(action);
1320 if (finished)
1321 return;
1322 finished = true;
1323 T t = started ? next.apply(prev) : seed;
1324 prev = null;
1325 while (hasNext.test(t)) {
1326 action.accept(t);
1327 t = next.apply(t);
1328 }
1329 }
1330 };
1331 return StreamSupport.stream(spliterator, false);
1332 }
1333
1334 /**
1335 * Returns an infinite sequential unordered stream where each element is
1336 * generated by the provided {@code Supplier}. This is suitable for
1337 * generating constant streams, streams of random elements, etc.
1338 *
1339 * @param <T> the type of stream elements
1340 * @param s the {@code Supplier} of generated elements
1341 * @return a new infinite sequential unordered {@code Stream}
1342 */
1343 public static<T> Stream<T> generate(Supplier<? extends T> s) {
1344 Objects.requireNonNull(s);
1345 return StreamSupport.stream(
1346 new StreamSpliterators.InfiniteSupplyingSpliterator.OfRef<>(Long.MAX_VALUE, s), false);
1347 }
1348
1349 /**
1350 * Creates a lazily concatenated stream whose elements are all the
1351 * elements of the first stream followed by all the elements of the
1352 * second stream. The resulting stream is ordered if both
1353 * of the input streams are ordered, and parallel if either of the input
1354 * streams is parallel. When the resulting stream is closed, the close
1355 * handlers for both input streams are invoked.
1356 *
1357 * <p>This method operates on the two input streams and binds each stream
1358 * to its source. As a result subsequent modifications to an input stream
1359 * source may not be reflected in the concatenated stream result.
1360 *
1361 * @implNote
1362 * Use caution when constructing streams from repeated concatenation.
1363 * Accessing an element of a deeply concatenated stream can result in deep
1364 * call chains, or even {@code StackOverflowError}.
1365 *
1366 * <p>Subsequent changes to the sequential/parallel execution mode of the
1367 * returned stream are not guaranteed to be propagated to the input streams.
1368 *
1369 * @apiNote
1370 * To preserve optimization opportunities this method binds each stream to
1371 * its source and accepts only two streams as parameters. For example, the
1372 * exact size of the concatenated stream source can be computed if the exact
1373 * size of each input stream source is known.
1374 * To concatenate more streams without binding, or without nested calls to
1375 * this method, try creating a stream of streams and flat-mapping with the
1376 * identity function, for example:
1377 * <pre>{@code
1378 * Stream<T> concat = Stream.of(s1, s2, s3, s4).flatMap(s -> s);
1379 * }</pre>
1380 *
1381 * @param <T> The type of stream elements
1382 * @param a the first stream
1383 * @param b the second stream
1384 * @return the concatenation of the two input streams
1385 */
1386 public static <T> Stream<T> concat(Stream<? extends T> a, Stream<? extends T> b) {
1387 Objects.requireNonNull(a);
1388 Objects.requireNonNull(b);
1389
1390 @SuppressWarnings("unchecked")
1391 Spliterator<T> split = new Streams.ConcatSpliterator.OfRef<>(
1392 (Spliterator<T>) a.spliterator(), (Spliterator<T>) b.spliterator());
1393 Stream<T> stream = StreamSupport.stream(split, a.isParallel() || b.isParallel());
1394 return stream.onClose(Streams.composedClose(a, b));
1395 }
1396
1397 /**
1398 * A mutable builder for a {@code Stream}. This allows the creation of a
1399 * {@code Stream} by generating elements individually and adding them to the
1400 * {@code Builder} (without the copying overhead that comes from using
1401 * an {@code ArrayList} as a temporary buffer.)
1402 *
1403 * <p>A stream builder has a lifecycle, which starts in a building
1404 * phase, during which elements can be added, and then transitions to a built
1405 * phase, after which elements may not be added. The built phase begins
1406 * when the {@link #build()} method is called, which creates an ordered
1407 * {@code Stream} whose elements are the elements that were added to the stream
1408 * builder, in the order they were added.
1409 *
1410 * @param <T> the type of stream elements
1411 * @see Stream#builder()
1412 * @since 1.8
1413 */
1414 public interface Builder<T> extends Consumer<T> {
1415
1416 /**
1417 * Adds an element to the stream being built.
1418 *
1419 * @throws IllegalStateException if the builder has already transitioned to
1420 * the built state
1421 */
1422 @Override
1423 void accept(T t);
1424
1425 /**
1426 * Adds an element to the stream being built.
1427 *
1428 * @implSpec
1429 * The default implementation behaves as if:
1430 * <pre>{@code
1431 * accept(t)
1432 * return this;
1433 * }</pre>
1434 *
1435 * @param t the element to add
1436 * @return {@code this} builder
1437 * @throws IllegalStateException if the builder has already transitioned to
1438 * the built state
1439 */
1440 default Builder<T> add(T t) {
1441 accept(t);
1442 return this;
1443 }
1444
1445 /**
1446 * Builds the stream, transitioning this builder to the built state.
1447 * An {@code IllegalStateException} is thrown if there are further attempts
1448 * to operate on the builder after it has entered the built state.
1449 *
1450 * @return the built stream
1451 * @throws IllegalStateException if the builder has already transitioned to
1452 * the built state
1453 */
1454 Stream<T> build();
1455
1456 }
1457 }