1 /*
2 * Copyright (c) 2012, 2015, 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 *
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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.Iterator;
33 import java.util.Objects;
34 import java.util.Optional;
35 import java.util.Spliterator;
36 import java.util.Spliterators;
37 import java.util.concurrent.ConcurrentHashMap;
38 import java.util.function.BiConsumer;
39 import java.util.function.BiFunction;
40 import java.util.function.BinaryOperator;
41 import java.util.function.Consumer;
42 import java.util.function.Function;
43 import java.util.function.IntFunction;
44 import java.util.function.Predicate;
45 import java.util.function.Supplier;
46 import java.util.function.ToDoubleFunction;
47 import java.util.function.ToIntFunction;
48 import java.util.function.ToLongFunction;
49 import java.util.function.UnaryOperator;
50
51 /**
52 * A sequence of elements supporting sequential and parallel aggregate
53 * operations. The following example illustrates an aggregate operation using
54 * {@link Stream} and {@link IntStream}:
55 *
56 * <pre>{@code
57 * int sum = widgets.stream()
58 * .filter(w -> w.getColor() == RED)
59 * .mapToInt(w -> w.getWeight())
60 * .sum();
61 * }</pre>
62 *
63 * In this example, {@code widgets} is a {@code Collection<Widget>}. We create
64 * a stream of {@code Widget} objects via {@link Collection#stream Collection.stream()},
65 * filter it to produce a stream containing only the red widgets, and then
66 * transform it into a stream of {@code int} values representing the weight of
67 * each red widget. Then this stream is summed to produce a total weight.
68 *
69 * <p>In addition to {@code Stream}, which is a stream of object references,
70 * there are primitive specializations for {@link IntStream}, {@link LongStream},
71 * and {@link DoubleStream}, all of which are referred to as "streams" and
72 * conform to the characteristics and restrictions described here.
73 *
74 * <p>To perform a computation, stream
75 * <a href="package-summary.html#StreamOps">operations</a> are composed into a
76 * <em>stream pipeline</em>. A stream pipeline consists of a source (which
77 * might be an array, a collection, a generator function, an I/O channel,
78 * etc), zero or more <em>intermediate operations</em> (which transform a
79 * stream into another stream, such as {@link Stream#filter(Predicate)}), and a
80 * <em>terminal operation</em> (which produces a result or side-effect, such
81 * as {@link Stream#count()} or {@link Stream#forEach(Consumer)}).
82 * Streams are lazy; computation on the source data is only performed when the
83 * terminal operation is initiated, and source elements are consumed only
84 * as needed.
85 *
86 * <p>Collections and streams, while bearing some superficial similarities,
87 * have different goals. Collections are primarily concerned with the efficient
88 * management of, and access to, their elements. By contrast, streams do not
89 * provide a means to directly access or manipulate their elements, and are
90 * instead concerned with declaratively describing their source and the
91 * computational operations which will be performed in aggregate on that source.
92 * However, if the provided stream operations do not offer the desired
93 * functionality, the {@link #iterator()} and {@link #spliterator()} operations
94 * can be used to perform a controlled traversal.
95 *
96 * <p>A stream pipeline, like the "widgets" example above, can be viewed as
97 * a <em>query</em> on the stream source. Unless the source was explicitly
98 * designed for concurrent modification (such as a {@link ConcurrentHashMap}),
99 * unpredictable or erroneous behavior may result from modifying the stream
100 * source while it is being queried.
101 *
102 * <p>Most stream operations accept parameters that describe user-specified
103 * behavior, such as the lambda expression {@code w -> w.getWeight()} passed to
104 * {@code mapToInt} in the example above. To preserve correct behavior,
105 * these <em>behavioral parameters</em>:
106 * <ul>
107 * <li>must be <a href="package-summary.html#NonInterference">non-interfering</a>
108 * (they do not modify the stream source); and</li>
109 * <li>in most cases must be <a href="package-summary.html#Statelessness">stateless</a>
110 * (their result should not depend on any state that might change during execution
111 * of the stream pipeline).</li>
112 * </ul>
113 *
114 * <p>Such parameters are always instances of a
115 * <a href="../function/package-summary.html">functional interface</a> such
116 * as {@link java.util.function.Function}, and are often lambda expressions or
117 * method references. Unless otherwise specified these parameters must be
118 * <em>non-null</em>.
119 *
120 * <p>A stream should be operated on (invoking an intermediate or terminal stream
121 * operation) only once. This rules out, for example, "forked" streams, where
122 * the same source feeds two or more pipelines, or multiple traversals of the
123 * same stream. A stream implementation may throw {@link IllegalStateException}
124 * if it detects that the stream is being reused. However, since some stream
125 * operations may return their receiver rather than a new stream object, it may
126 * not be possible to detect reuse in all cases.
127 *
128 * <p>Streams have a {@link #close()} method and implement {@link AutoCloseable}.
129 * Operating on a stream after it has been closed will throw {@link IllegalStateException}.
130 * Most stream instances do not actually need to be closed after use, as they
131 * are backed by collections, arrays, or generating functions, which require no
132 * special resource management. Generally, only streams whose source is an IO channel,
133 * such as those returned by {@link Files#lines(Path)}, will require closing. If a
134 * stream does require closing, it must be opened as a resource within a try-with-resources
135 * statement or similar control structure to ensure that it is closed promptly after its
136 * operations have completed.
137 *
138 * <p>Stream pipelines may execute either sequentially or in
139 * <a href="package-summary.html#Parallelism">parallel</a>. This
140 * execution mode is a property of the stream. Streams are created
141 * with an initial choice of sequential or parallel execution. (For example,
142 * {@link Collection#stream() Collection.stream()} creates a sequential stream,
143 * and {@link Collection#parallelStream() Collection.parallelStream()} creates
144 * a parallel one.) This choice of execution mode may be modified by the
145 * {@link #sequential()} or {@link #parallel()} methods, and may be queried with
146 * the {@link #isParallel()} method.
147 *
148 * @param <T> the type of the stream elements
149 * @since 1.8
150 * @see IntStream
151 * @see LongStream
152 * @see DoubleStream
153 * @see <a href="package-summary.html">java.util.stream</a>
154 */
155 public interface Stream<T> extends BaseStream<T, Stream<T>> {
156
157 /**
158 * Returns a stream consisting of the elements of this stream that match
159 * the given predicate.
160 *
161 * <p>This is an <a href="package-summary.html#StreamOps">intermediate
162 * operation</a>.
163 *
164 * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>,
165 * <a href="package-summary.html#Statelessness">stateless</a>
166 * predicate to apply to each element to determine if it
167 * should be included
168 * @return the new stream
169 */
170 Stream<T> filter(Predicate<? super T> predicate);
171
172 /**
173 * Returns a stream consisting of the results of applying the given
174 * function to the elements of this stream.
175 *
176 * <p>This is an <a href="package-summary.html#StreamOps">intermediate
177 * operation</a>.
178 *
179 * @param <R> The element type of the new stream
180 * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
181 * <a href="package-summary.html#Statelessness">stateless</a>
182 * function to apply to each element
183 * @return the new stream
184 */
185 <R> Stream<R> map(Function<? super T, ? extends R> mapper);
186
187 /**
188 * Returns an {@code IntStream} consisting of the results of applying the
189 * given function to the elements of this stream.
190 *
191 * <p>This is an <a href="package-summary.html#StreamOps">
192 * intermediate operation</a>.
193 *
194 * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
195 * <a href="package-summary.html#Statelessness">stateless</a>
196 * function to apply to each element
197 * @return the new stream
198 */
199 IntStream mapToInt(ToIntFunction<? super T> mapper);
200
201 /**
202 * Returns a {@code LongStream} consisting of the results of applying the
203 * given function to the elements of this stream.
204 *
205 * <p>This is an <a href="package-summary.html#StreamOps">intermediate
206 * operation</a>.
207 *
208 * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
209 * <a href="package-summary.html#Statelessness">stateless</a>
210 * function to apply to each element
211 * @return the new stream
212 */
213 LongStream mapToLong(ToLongFunction<? super T> mapper);
214
215 /**
216 * Returns a {@code DoubleStream} consisting of the results of applying the
217 * given function to the elements of this stream.
218 *
219 * <p>This is an <a href="package-summary.html#StreamOps">intermediate
220 * operation</a>.
221 *
222 * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
223 * <a href="package-summary.html#Statelessness">stateless</a>
224 * function to apply to each element
225 * @return the new stream
226 */
227 DoubleStream mapToDouble(ToDoubleFunction<? super T> mapper);
228
229 /**
230 * Returns a stream consisting of the results of replacing each element of
231 * this stream with the contents of a mapped stream produced by applying
232 * the provided mapping function to each element. Each mapped stream is
233 * {@link java.util.stream.BaseStream#close() closed} after its contents
234 * have been placed into this stream. (If a mapped stream is {@code null}
235 * an empty stream is used, instead.)
236 *
237 * <p>This is an <a href="package-summary.html#StreamOps">intermediate
238 * operation</a>.
239 *
240 * @apiNote
241 * The {@code flatMap()} operation has the effect of applying a one-to-many
242 * transformation to the elements of the stream, and then flattening the
243 * resulting elements into a new stream.
244 *
245 * <p><b>Examples.</b>
246 *
247 * <p>If {@code orders} is a stream of purchase orders, and each purchase
248 * order contains a collection of line items, then the following produces a
249 * stream containing all the line items in all the orders:
250 * <pre>{@code
251 * orders.flatMap(order -> order.getLineItems().stream())...
252 * }</pre>
253 *
254 * <p>If {@code path} is the path to a file, then the following produces a
255 * stream of the {@code words} contained in that file:
256 * <pre>{@code
257 * Stream<String> lines = Files.lines(path, StandardCharsets.UTF_8);
258 * Stream<String> words = lines.flatMap(line -> Stream.of(line.split(" +")));
259 * }</pre>
260 * The {@code mapper} function passed to {@code flatMap} splits a line,
261 * using a simple regular expression, into an array of words, and then
262 * creates a stream of words from that array.
263 *
264 * @param <R> The element type of the new stream
265 * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
266 * <a href="package-summary.html#Statelessness">stateless</a>
267 * function to apply to each element which produces a stream
268 * of new values
269 * @return the new stream
270 */
271 <R> Stream<R> flatMap(Function<? super T, ? extends Stream<? extends R>> mapper);
272
273 /**
274 * Returns an {@code IntStream} consisting of the results of replacing each
275 * element of this stream with the contents of a mapped stream produced by
276 * applying the provided mapping function to each element. Each mapped
277 * stream is {@link java.util.stream.BaseStream#close() closed} after its
278 * contents have been placed into this stream. (If a mapped stream is
279 * {@code null} an empty stream is used, instead.)
280 *
281 * <p>This is an <a href="package-summary.html#StreamOps">intermediate
282 * operation</a>.
283 *
284 * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
285 * <a href="package-summary.html#Statelessness">stateless</a>
286 * function to apply to each element which produces a stream
287 * of new values
288 * @return the new stream
289 * @see #flatMap(Function)
290 */
291 IntStream flatMapToInt(Function<? super T, ? extends IntStream> mapper);
292
293 /**
294 * Returns an {@code LongStream} consisting of the results of replacing each
295 * element of this stream with the contents of a mapped stream produced by
296 * applying the provided mapping function to each element. Each mapped
297 * stream is {@link java.util.stream.BaseStream#close() closed} after its
298 * contents have been placed into this stream. (If a mapped stream is
299 * {@code null} an empty stream is used, instead.)
300 *
301 * <p>This is an <a href="package-summary.html#StreamOps">intermediate
302 * operation</a>.
303 *
304 * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
305 * <a href="package-summary.html#Statelessness">stateless</a>
306 * function to apply to each element which produces a stream
307 * of new values
308 * @return the new stream
309 * @see #flatMap(Function)
310 */
311 LongStream flatMapToLong(Function<? super T, ? extends LongStream> mapper);
312
313 /**
314 * Returns an {@code DoubleStream} consisting of the results of replacing
315 * each element of this stream with the contents of a mapped stream produced
316 * by applying the provided mapping function to each element. Each mapped
317 * stream is {@link java.util.stream.BaseStream#close() closed} after its
318 * contents have placed been into this stream. (If a mapped stream is
319 * {@code null} an empty stream is used, instead.)
320 *
321 * <p>This is an <a href="package-summary.html#StreamOps">intermediate
322 * operation</a>.
323 *
324 * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
325 * <a href="package-summary.html#Statelessness">stateless</a>
326 * function to apply to each element which produces a stream
327 * of new values
328 * @return the new stream
329 * @see #flatMap(Function)
330 */
331 DoubleStream flatMapToDouble(Function<? super T, ? extends DoubleStream> mapper);
332
333 /**
334 * Returns a stream consisting of the distinct elements (according to
335 * {@link Object#equals(Object)}) of this stream.
336 *
337 * <p>For ordered streams, the selection of distinct elements is stable
338 * (for duplicated elements, the element appearing first in the encounter
339 * order is preserved.) For unordered streams, no stability guarantees
340 * are made.
341 *
342 * <p>This is a <a href="package-summary.html#StreamOps">stateful
343 * intermediate operation</a>.
344 *
345 * @apiNote
346 * Preserving stability for {@code distinct()} in parallel pipelines is
347 * relatively expensive (requires that the operation act as a full barrier,
348 * with substantial buffering overhead), and stability is often not needed.
349 * Using an unordered stream source (such as {@link #generate(Supplier)})
350 * or removing the ordering constraint with {@link #unordered()} may result
351 * in significantly more efficient execution for {@code distinct()} in parallel
352 * pipelines, if the semantics of your situation permit. If consistency
353 * with encounter order is required, and you are experiencing poor performance
354 * or memory utilization with {@code distinct()} in parallel pipelines,
355 * switching to sequential execution with {@link #sequential()} may improve
356 * performance.
357 *
358 * @return the new stream
359 */
360 Stream<T> distinct();
361
362 /**
363 * Returns a stream consisting of the elements of this stream, sorted
364 * according to natural order. If the elements of this stream are not
365 * {@code Comparable}, a {@code java.lang.ClassCastException} may be thrown
366 * when the terminal operation is executed.
367 *
368 * <p>For ordered streams, the sort is stable. For unordered streams, no
369 * stability guarantees are made.
370 *
371 * <p>This is a <a href="package-summary.html#StreamOps">stateful
372 * intermediate operation</a>.
373 *
374 * @return the new stream
375 */
376 Stream<T> sorted();
377
378 /**
379 * Returns a stream consisting of the elements of this stream, sorted
380 * according to the provided {@code Comparator}.
381 *
382 * <p>For ordered streams, the sort is stable. For unordered streams, no
383 * stability guarantees are made.
384 *
385 * <p>This is a <a href="package-summary.html#StreamOps">stateful
386 * intermediate operation</a>.
387 *
388 * @param comparator a <a href="package-summary.html#NonInterference">non-interfering</a>,
389 * <a href="package-summary.html#Statelessness">stateless</a>
390 * {@code Comparator} to be used to compare stream elements
391 * @return the new stream
392 */
393 Stream<T> sorted(Comparator<? super T> comparator);
394
395 /**
396 * Returns a stream consisting of the elements of this stream, additionally
397 * performing the provided action on each element as elements are consumed
398 * from the resulting stream.
399 *
400 * <p>This is an <a href="package-summary.html#StreamOps">intermediate
401 * operation</a>.
402 *
403 * <p>For parallel stream pipelines, the action may be called at
404 * whatever time and in whatever thread the element is made available by the
405 * upstream operation. If the action modifies shared state,
406 * it is responsible for providing the required synchronization.
407 *
408 * @apiNote This method exists mainly to support debugging, where you want
409 * to see the elements as they flow past a certain point in a pipeline:
410 * <pre>{@code
411 * Stream.of("one", "two", "three", "four")
412 * .filter(e -> e.length() > 3)
413 * .peek(e -> System.out.println("Filtered value: " + e))
414 * .map(String::toUpperCase)
415 * .peek(e -> System.out.println("Mapped value: " + e))
416 * .collect(Collectors.toList());
417 * }</pre>
418 *
419 * @param action a <a href="package-summary.html#NonInterference">
420 * non-interfering</a> action to perform on the elements as
421 * they are consumed from the stream
422 * @return the new stream
423 */
424 Stream<T> peek(Consumer<? super T> action);
425
426 /**
427 * Returns a stream consisting of the elements of this stream, truncated
428 * to be no longer than {@code maxSize} in length.
429 *
430 * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
431 * stateful intermediate operation</a>.
432 *
433 * @apiNote
434 * While {@code limit()} is generally a cheap operation on sequential
435 * stream pipelines, it can be quite expensive on ordered parallel pipelines,
436 * especially for large values of {@code maxSize}, since {@code limit(n)}
437 * is constrained to return not just any <em>n</em> elements, but the
438 * <em>first n</em> elements in the encounter order. Using an unordered
439 * stream source (such as {@link #generate(Supplier)}) or removing the
440 * ordering constraint with {@link #unordered()} may result in significant
441 * speedups of {@code limit()} in parallel pipelines, if the semantics of
442 * your situation permit. If consistency with encounter order is required,
443 * and you are experiencing poor performance or memory utilization with
444 * {@code limit()} in parallel pipelines, switching to sequential execution
445 * with {@link #sequential()} may improve performance.
446 *
447 * @param maxSize the number of elements the stream should be limited to
448 * @return the new stream
449 * @throws IllegalArgumentException if {@code maxSize} is negative
450 */
451 Stream<T> limit(long maxSize);
452
453 /**
454 * Returns a stream consisting of the remaining elements of this stream
455 * after discarding the first {@code n} elements of the stream.
456 * If this stream contains fewer than {@code n} elements then an
457 * empty stream will be returned.
458 *
459 * <p>This is a <a href="package-summary.html#StreamOps">stateful
460 * intermediate operation</a>.
461 *
462 * @apiNote
463 * While {@code skip()} 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 n}, since {@code skip(n)}
466 * is constrained to skip 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 skip()} 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 skip()} in parallel pipelines, switching to sequential execution
474 * with {@link #sequential()} may improve performance.
475 *
476 * @param n the number of leading elements to skip
477 * @return the new stream
478 * @throws IllegalArgumentException if {@code n} is negative
479 */
480 Stream<T> skip(long n);
481
482 /**
483 * Returns, if this stream is ordered, a stream consisting of the longest
484 * prefix of elements taken from this stream that match the given predicate.
485 * Otherwise returns, if this stream is unordered, a stream consisting of a
486 * subset of elements taken from this stream that match the given predicate.
487 *
488 * <p>If this stream is ordered then the longest prefix is a contiguous
489 * sequence of elements of this stream that match the given predicate. The
490 * first element of the sequence is the first element of this stream, and
491 * the element immediately following the last element of the sequence does
492 * not match the given predicate.
493 *
494 * <p>If this stream is unordered, and some (but not all) elements of this
495 * stream match the given predicate, then the behavior of this operation is
496 * nondeterministic; it is free to take any subset of matching elements
497 * (which includes the empty set).
498 *
499 * <p>Independent of whether this stream is ordered or unordered if all
500 * elements of this stream match the given predicate then this operation
501 * takes all elements (the result is the same as the input), or if no
502 * elements of the stream match the given predicate then no elements are
503 * taken (the result is an empty stream).
504 *
505 * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
506 * stateful intermediate operation</a>.
507 *
508 * @implSpec
509 * The default implementation obtains the {@link #spliterator() spliterator}
510 * of this stream, wraps that spliterator so as to support the semantics
511 * of this operation on traversal, and returns a new stream associated with
512 * the wrapped spliterator. The returned stream preserves the execution
513 * characteristics of this stream (namely parallel or sequential execution
514 * as per {@link #isParallel()}) but the wrapped spliterator may choose to
515 * not support splitting. When the returned stream is closed, the close
516 * handlers for both the returned and this stream are invoked.
517 *
518 * @apiNote
519 * While {@code takeWhile()} is generally a cheap operation on sequential
520 * stream pipelines, it can be quite expensive on ordered parallel
521 * pipelines, since the operation is constrained to return not just any
522 * valid prefix, but the longest prefix of elements in the encounter order.
523 * Using an unordered stream source (such as {@link #generate(Supplier)}) or
524 * removing the ordering constraint with {@link #unordered()} may result in
525 * significant speedups of {@code takeWhile()} in parallel pipelines, if the
526 * semantics of your situation permit. If consistency with encounter order
527 * is required, and you are experiencing poor performance or memory
528 * utilization with {@code takeWhile()} in parallel pipelines, switching to
529 * sequential execution with {@link #sequential()} may improve performance.
530 *
531 * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>,
532 * <a href="package-summary.html#Statelessness">stateless</a>
533 * predicate to apply to elements to determine the longest
534 * prefix of elements.
535 * @return the new stream
536 * @since 9
537 */
538 default Stream<T> takeWhile(Predicate<? super T> predicate) {
539 Objects.requireNonNull(predicate);
540 // Reuses the unordered spliterator, which, when encounter is present,
541 // is safe to use as long as it configured not to split
542 return StreamSupport.stream(
543 new WhileOps.UnorderedWhileSpliterator.OfRef.Taking<>(spliterator(), true, predicate),
544 isParallel()).onClose(this::close);
545 }
546
547 /**
548 * Returns, if this stream is ordered, a stream consisting of the remaining
549 * elements of this stream after dropping the longest prefix of elements
550 * that match the given predicate. Otherwise returns, if this stream is
551 * unordered, a stream consisting of the remaining elements of this stream
552 * after dropping a subset of elements that match the given predicate.
553 *
554 * <p>If this stream is ordered then the longest prefix is a contiguous
555 * sequence of elements of this stream that match the given predicate. The
556 * first element of the sequence is the first element of this stream, and
557 * the element immediately following the last element of the sequence does
558 * not match the given predicate.
559 *
560 * <p>If this stream is unordered, and some (but not all) elements of this
561 * stream match the given predicate, then the behavior of this operation is
562 * nondeterministic; it is free to drop any subset of matching elements
563 * (which includes the empty set).
564 *
565 * <p>Independent of whether this stream is ordered or unordered if all
566 * elements of this stream match the given predicate then this operation
567 * drops all elements (the result is an empty stream), or if no elements of
568 * the stream match the given predicate then no elements are dropped (the
569 * result is the same as the input).
570 *
571 * <p>This is a <a href="package-summary.html#StreamOps">stateful
572 * intermediate operation</a>.
573 *
574 * @implSpec
575 * The default implementation obtains the {@link #spliterator() spliterator}
576 * of this stream, wraps that spliterator so as to support the semantics
577 * of this operation on traversal, and returns a new stream associated with
578 * the wrapped spliterator. The returned stream preserves the execution
579 * characteristics of this stream (namely parallel or sequential execution
580 * as per {@link #isParallel()}) but the wrapped spliterator may choose to
581 * not support splitting. When the returned stream is closed, the close
582 * handlers for both the returned and this stream are invoked.
583 *
584 * @apiNote
585 * While {@code dropWhile()} is generally a cheap operation on sequential
586 * stream pipelines, it can be quite expensive on ordered parallel
587 * pipelines, since the operation is constrained to return not just any
588 * valid prefix, but the longest prefix of elements in the encounter order.
589 * Using an unordered stream source (such as {@link #generate(Supplier)}) or
590 * removing the ordering constraint with {@link #unordered()} may result in
591 * significant speedups of {@code dropWhile()} in parallel pipelines, if the
592 * semantics of your situation permit. If consistency with encounter order
593 * is required, and you are experiencing poor performance or memory
594 * utilization with {@code dropWhile()} in parallel pipelines, switching to
595 * sequential execution with {@link #sequential()} may improve performance.
596 *
597 * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>,
598 * <a href="package-summary.html#Statelessness">stateless</a>
599 * predicate to apply to elements to determine the longest
600 * prefix of elements.
601 * @return the new stream
602 * @since 9
603 */
604 default Stream<T> dropWhile(Predicate<? super T> predicate) {
605 Objects.requireNonNull(predicate);
606 // Reuses the unordered spliterator, which, when encounter is present,
607 // is safe to use as long as it configured not to split
608 return StreamSupport.stream(
609 new WhileOps.UnorderedWhileSpliterator.OfRef.Dropping<>(spliterator(), true, predicate),
610 isParallel()).onClose(this::close);
611 }
612
613 /**
614 * Performs an action for each element of this stream.
615 *
616 * <p>This is a <a href="package-summary.html#StreamOps">terminal
617 * operation</a>.
618 *
619 * <p>The behavior of this operation is explicitly nondeterministic.
620 * For parallel stream pipelines, this operation does <em>not</em>
621 * guarantee to respect the encounter order of the stream, as doing so
622 * would sacrifice the benefit of parallelism. For any given element, the
623 * action may be performed at whatever time and in whatever thread the
624 * library chooses. If the action accesses shared state, it is
625 * responsible for providing the required synchronization.
626 *
627 * @param action a <a href="package-summary.html#NonInterference">
628 * non-interfering</a> action to perform on the elements
629 */
630 void forEach(Consumer<? super T> action);
631
632 /**
633 * Performs an action for each element of this stream, in the encounter
634 * order of the stream if the stream has a defined encounter order.
635 *
636 * <p>This is a <a href="package-summary.html#StreamOps">terminal
637 * operation</a>.
638 *
639 * <p>This operation processes the elements one at a time, in encounter
640 * order if one exists. Performing the action for one element
641 * <a href="../concurrent/package-summary.html#MemoryVisibility"><i>happens-before</i></a>
642 * performing the action for subsequent elements, but for any given element,
643 * the action may be performed in whatever thread the library chooses.
644 *
645 * @param action a <a href="package-summary.html#NonInterference">
646 * non-interfering</a> action to perform on the elements
647 * @see #forEach(Consumer)
648 */
649 void forEachOrdered(Consumer<? super T> action);
650
651 /**
652 * Returns an array containing the elements of this stream.
653 *
654 * <p>This is a <a href="package-summary.html#StreamOps">terminal
655 * operation</a>.
656 *
657 * @return an array containing the elements of this stream
658 */
659 Object[] toArray();
660
661 /**
662 * Returns an array containing the elements of this stream, using the
663 * provided {@code generator} function to allocate the returned array, as
664 * well as any additional arrays that might be required for a partitioned
665 * execution or for resizing.
666 *
667 * <p>This is a <a href="package-summary.html#StreamOps">terminal
668 * operation</a>.
669 *
670 * @apiNote
671 * The generator function takes an integer, which is the size of the
672 * desired array, and produces an array of the desired size. This can be
673 * concisely expressed with an array constructor reference:
674 * <pre>{@code
675 * Person[] men = people.stream()
676 * .filter(p -> p.getGender() == MALE)
677 * .toArray(Person[]::new);
678 * }</pre>
679 *
680 * @param <A> the element type of the resulting array
681 * @param generator a function which produces a new array of the desired
682 * type and the provided length
683 * @return an array containing the elements in this stream
684 * @throws ArrayStoreException if the runtime type of the array returned
685 * from the array generator is not a supertype of the runtime type of every
686 * element in this stream
687 */
688 <A> A[] toArray(IntFunction<A[]> generator);
689
690 /**
691 * Performs a <a href="package-summary.html#Reduction">reduction</a> on the
692 * elements of this stream, using the provided identity value and an
693 * <a href="package-summary.html#Associativity">associative</a>
694 * accumulation function, and returns the reduced value. This is equivalent
695 * to:
696 * <pre>{@code
697 * T result = identity;
698 * for (T element : this stream)
699 * result = accumulator.apply(result, element)
700 * return result;
701 * }</pre>
702 *
703 * but is not constrained to execute sequentially.
704 *
705 * <p>The {@code identity} value must be an identity for the accumulator
706 * function. This means that for all {@code t},
707 * {@code accumulator.apply(identity, t)} is equal to {@code t}.
708 * The {@code accumulator} function must be an
709 * <a href="package-summary.html#Associativity">associative</a> function.
710 *
711 * <p>This is a <a href="package-summary.html#StreamOps">terminal
712 * operation</a>.
713 *
714 * @apiNote Sum, min, max, average, and string concatenation are all special
715 * cases of reduction. Summing a stream of numbers can be expressed as:
716 *
717 * <pre>{@code
718 * Integer sum = integers.reduce(0, (a, b) -> a+b);
719 * }</pre>
720 *
721 * or:
722 *
723 * <pre>{@code
724 * Integer sum = integers.reduce(0, Integer::sum);
725 * }</pre>
726 *
727 * <p>While this may seem a more roundabout way to perform an aggregation
728 * compared to simply mutating a running total in a loop, reduction
729 * operations parallelize more gracefully, without needing additional
730 * synchronization and with greatly reduced risk of data races.
731 *
732 * @param identity the identity value for the accumulating function
733 * @param accumulator an <a href="package-summary.html#Associativity">associative</a>,
734 * <a href="package-summary.html#NonInterference">non-interfering</a>,
735 * <a href="package-summary.html#Statelessness">stateless</a>
736 * function for combining two values
737 * @return the result of the reduction
738 */
739 T reduce(T identity, BinaryOperator<T> accumulator);
740
741 /**
742 * Performs a <a href="package-summary.html#Reduction">reduction</a> on the
743 * elements of this stream, using an
744 * <a href="package-summary.html#Associativity">associative</a> accumulation
745 * function, and returns an {@code Optional} describing the reduced value,
746 * if any. This is equivalent to:
747 * <pre>{@code
748 * boolean foundAny = false;
749 * T result = null;
750 * for (T element : this stream) {
751 * if (!foundAny) {
752 * foundAny = true;
753 * result = element;
754 * }
755 * else
756 * result = accumulator.apply(result, element);
757 * }
758 * return foundAny ? Optional.of(result) : Optional.empty();
759 * }</pre>
760 *
761 * but is not constrained to execute sequentially.
762 *
763 * <p>The {@code accumulator} function must be an
764 * <a href="package-summary.html#Associativity">associative</a> function.
765 *
766 * <p>This is a <a href="package-summary.html#StreamOps">terminal
767 * operation</a>.
768 *
769 * @param accumulator an <a href="package-summary.html#Associativity">associative</a>,
770 * <a href="package-summary.html#NonInterference">non-interfering</a>,
771 * <a href="package-summary.html#Statelessness">stateless</a>
772 * function for combining two values
773 * @return an {@link Optional} describing the result of the reduction
774 * @throws NullPointerException if the result of the reduction is null
775 * @see #reduce(Object, BinaryOperator)
776 * @see #min(Comparator)
777 * @see #max(Comparator)
778 */
779 Optional<T> reduce(BinaryOperator<T> accumulator);
780
781 /**
782 * Performs a <a href="package-summary.html#Reduction">reduction</a> on the
783 * elements of this stream, using the provided identity, accumulation and
784 * combining functions. This is equivalent to:
785 * <pre>{@code
786 * U result = identity;
787 * for (T element : this stream)
788 * result = accumulator.apply(result, element)
789 * return result;
790 * }</pre>
791 *
792 * but is not constrained to execute sequentially.
793 *
794 * <p>The {@code identity} value must be an identity for the combiner
795 * function. This means that for all {@code u}, {@code combiner(identity, u)}
796 * is equal to {@code u}. Additionally, the {@code combiner} function
797 * must be compatible with the {@code accumulator} function; for all
798 * {@code u} and {@code t}, the following must hold:
799 * <pre>{@code
800 * combiner.apply(u, accumulator.apply(identity, t)) == accumulator.apply(u, t)
801 * }</pre>
802 *
803 * <p>This is a <a href="package-summary.html#StreamOps">terminal
804 * operation</a>.
805 *
806 * @apiNote Many reductions using this form can be represented more simply
807 * by an explicit combination of {@code map} and {@code reduce} operations.
808 * The {@code accumulator} function acts as a fused mapper and accumulator,
809 * which can sometimes be more efficient than separate mapping and reduction,
810 * such as when knowing the previously reduced value allows you to avoid
811 * some computation.
812 *
813 * @param <U> The type of the result
814 * @param identity the identity value for the combiner function
815 * @param accumulator an <a href="package-summary.html#Associativity">associative</a>,
816 * <a href="package-summary.html#NonInterference">non-interfering</a>,
817 * <a href="package-summary.html#Statelessness">stateless</a>
818 * function for incorporating an additional element into a result
819 * @param combiner an <a href="package-summary.html#Associativity">associative</a>,
820 * <a href="package-summary.html#NonInterference">non-interfering</a>,
821 * <a href="package-summary.html#Statelessness">stateless</a>
822 * function for combining two values, which must be
823 * compatible with the accumulator function
824 * @return the result of the reduction
825 * @see #reduce(BinaryOperator)
826 * @see #reduce(Object, BinaryOperator)
827 */
828 <U> U reduce(U identity,
829 BiFunction<U, ? super T, U> accumulator,
830 BinaryOperator<U> combiner);
831
832 /**
833 * Performs a <a href="package-summary.html#MutableReduction">mutable
834 * reduction</a> operation on the elements of this stream. A mutable
835 * reduction is one in which the reduced value is a mutable result container,
836 * such as an {@code ArrayList}, and elements are incorporated by updating
837 * the state of the result rather than by replacing the result. This
838 * produces a result equivalent to:
839 * <pre>{@code
840 * R result = supplier.get();
841 * for (T element : this stream)
842 * accumulator.accept(result, element);
843 * return result;
844 * }</pre>
845 *
846 * <p>Like {@link #reduce(Object, BinaryOperator)}, {@code collect} operations
847 * can be parallelized without requiring additional synchronization.
848 *
849 * <p>This is a <a href="package-summary.html#StreamOps">terminal
850 * operation</a>.
851 *
852 * @apiNote There are many existing classes in the JDK whose signatures are
853 * well-suited for use with method references as arguments to {@code collect()}.
854 * For example, the following will accumulate strings into an {@code ArrayList}:
855 * <pre>{@code
856 * List<String> asList = stringStream.collect(ArrayList::new, ArrayList::add,
857 * ArrayList::addAll);
858 * }</pre>
859 *
860 * <p>The following will take a stream of strings and concatenates them into a
861 * single string:
862 * <pre>{@code
863 * String concat = stringStream.collect(StringBuilder::new, StringBuilder::append,
864 * StringBuilder::append)
865 * .toString();
866 * }</pre>
867 *
868 * @param <R> type of the result
869 * @param supplier a function that creates a new result container. For a
870 * parallel execution, this function may be called
871 * multiple times and must return a fresh value each time.
872 * @param accumulator an <a href="package-summary.html#Associativity">associative</a>,
873 * <a href="package-summary.html#NonInterference">non-interfering</a>,
874 * <a href="package-summary.html#Statelessness">stateless</a>
875 * function for incorporating an additional element into a result
876 * @param combiner an <a href="package-summary.html#Associativity">associative</a>,
877 * <a href="package-summary.html#NonInterference">non-interfering</a>,
878 * <a href="package-summary.html#Statelessness">stateless</a>
879 * function for combining two values, which must be
880 * compatible with the accumulator function
881 * @return the result of the reduction
882 */
883 <R> R collect(Supplier<R> supplier,
884 BiConsumer<R, ? super T> accumulator,
885 BiConsumer<R, R> combiner);
886
887 /**
888 * Performs a <a href="package-summary.html#MutableReduction">mutable
889 * reduction</a> operation on the elements of this stream using a
890 * {@code Collector}. A {@code Collector}
891 * encapsulates the functions used as arguments to
892 * {@link #collect(Supplier, BiConsumer, BiConsumer)}, allowing for reuse of
893 * collection strategies and composition of collect operations such as
894 * multiple-level grouping or partitioning.
895 *
896 * <p>If the stream is parallel, and the {@code Collector}
897 * is {@link Collector.Characteristics#CONCURRENT concurrent}, and
898 * either the stream is unordered or the collector is
899 * {@link Collector.Characteristics#UNORDERED unordered},
900 * then a concurrent reduction will be performed (see {@link Collector} for
901 * details on concurrent reduction.)
902 *
903 * <p>This is a <a href="package-summary.html#StreamOps">terminal
904 * operation</a>.
905 *
906 * <p>When executed in parallel, multiple intermediate results may be
907 * instantiated, populated, and merged so as to maintain isolation of
908 * mutable data structures. Therefore, even when executed in parallel
909 * with non-thread-safe data structures (such as {@code ArrayList}), no
910 * additional synchronization is needed for a parallel reduction.
911 *
912 * @apiNote
913 * The following will accumulate strings into an ArrayList:
914 * <pre>{@code
915 * List<String> asList = stringStream.collect(Collectors.toList());
916 * }</pre>
917 *
918 * <p>The following will classify {@code Person} objects by city:
919 * <pre>{@code
920 * Map<String, List<Person>> peopleByCity
921 * = personStream.collect(Collectors.groupingBy(Person::getCity));
922 * }</pre>
923 *
924 * <p>The following will classify {@code Person} objects by state and city,
925 * cascading two {@code Collector}s together:
926 * <pre>{@code
927 * Map<String, Map<String, List<Person>>> peopleByStateAndCity
928 * = personStream.collect(Collectors.groupingBy(Person::getState,
929 * Collectors.groupingBy(Person::getCity)));
930 * }</pre>
931 *
932 * @param <R> the type of the result
933 * @param <A> the intermediate accumulation type of the {@code Collector}
934 * @param collector the {@code Collector} describing the reduction
935 * @return the result of the reduction
936 * @see #collect(Supplier, BiConsumer, BiConsumer)
937 * @see Collectors
938 */
939 <R, A> R collect(Collector<? super T, A, R> collector);
940
941 /**
942 * Returns the minimum element of this stream according to the provided
943 * {@code Comparator}. This is a special case of a
944 * <a href="package-summary.html#Reduction">reduction</a>.
945 *
946 * <p>This is a <a href="package-summary.html#StreamOps">terminal operation</a>.
947 *
948 * @param comparator a <a href="package-summary.html#NonInterference">non-interfering</a>,
949 * <a href="package-summary.html#Statelessness">stateless</a>
950 * {@code Comparator} to compare elements of this stream
951 * @return an {@code Optional} describing the minimum element of this stream,
952 * or an empty {@code Optional} if the stream is empty
953 * @throws NullPointerException if the minimum element is null
954 */
955 Optional<T> min(Comparator<? super T> comparator);
956
957 /**
958 * Returns the maximum element of this stream according to the provided
959 * {@code Comparator}. This is a special case of a
960 * <a href="package-summary.html#Reduction">reduction</a>.
961 *
962 * <p>This is a <a href="package-summary.html#StreamOps">terminal
963 * operation</a>.
964 *
965 * @param comparator a <a href="package-summary.html#NonInterference">non-interfering</a>,
966 * <a href="package-summary.html#Statelessness">stateless</a>
967 * {@code Comparator} to compare elements of this stream
968 * @return an {@code Optional} describing the maximum element of this stream,
969 * or an empty {@code Optional} if the stream is empty
970 * @throws NullPointerException if the maximum element is null
971 */
972 Optional<T> max(Comparator<? super T> comparator);
973
974 /**
975 * Returns the count of elements in this stream. This is a special case of
976 * a <a href="package-summary.html#Reduction">reduction</a> and is
977 * equivalent to:
978 * <pre>{@code
979 * return mapToLong(e -> 1L).sum();
980 * }</pre>
981 *
982 * <p>This is a <a href="package-summary.html#StreamOps">terminal operation</a>.
983 *
984 * @apiNote
985 * An implementation may choose to not execute the stream pipeline (either
986 * sequentially or in parallel) if it is capable of computing the count
987 * directly from the stream source. In such cases no source elements will
988 * be traversed and no intermediate operations will be evaluated.
989 * Behavioral parameters with side-effects, which are strongly discouraged
990 * except for harmless cases such as debugging, may be affected. For
991 * example, consider the following stream:
992 * <pre>{@code
993 * List<String> l = Arrays.asList("A", "B", "C", "D");
994 * long count = l.stream().peek(System.out::println).count();
995 * }</pre>
996 * The number of elements covered by the stream source, a {@code List}, is
997 * known and the intermediate operation, {@code peek}, does not inject into
998 * or remove elements from the stream (as may be the case for
999 * {@code flatMap} or {@code filter} operations). Thus the count is the
1000 * size of the {@code List} and there is no need to execute the pipeline
1001 * and, as a side-effect, print out the list elements.
1002 *
1003 * @return the count of elements in this stream
1004 */
1005 long count();
1006
1007 /**
1008 * Returns whether any elements of this stream match the provided
1009 * predicate. May not evaluate the predicate on all elements if not
1010 * necessary for determining the result. If the stream is empty then
1011 * {@code false} is returned and the predicate is not evaluated.
1012 *
1013 * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
1014 * terminal operation</a>.
1015 *
1016 * @apiNote
1017 * This method evaluates the <em>existential quantification</em> of the
1018 * predicate over the elements of the stream (for some x P(x)).
1019 *
1020 * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>,
1021 * <a href="package-summary.html#Statelessness">stateless</a>
1022 * predicate to apply to elements of this stream
1023 * @return {@code true} if any elements of the stream match the provided
1024 * predicate, otherwise {@code false}
1025 */
1026 boolean anyMatch(Predicate<? super T> predicate);
1027
1028 /**
1029 * Returns whether all elements of this stream match the provided predicate.
1030 * May not evaluate the predicate on all elements if not necessary for
1031 * determining the result. If the stream is empty then {@code true} is
1032 * returned and the predicate is not evaluated.
1033 *
1034 * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
1035 * terminal operation</a>.
1036 *
1037 * @apiNote
1038 * This method evaluates the <em>universal quantification</em> of the
1039 * predicate over the elements of the stream (for all x P(x)). If the
1040 * stream is empty, the quantification is said to be <em>vacuously
1041 * satisfied</em> and is always {@code true} (regardless of P(x)).
1042 *
1043 * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>,
1044 * <a href="package-summary.html#Statelessness">stateless</a>
1045 * predicate to apply to elements of this stream
1046 * @return {@code true} if either all elements of the stream match the
1047 * provided predicate or the stream is empty, otherwise {@code false}
1048 */
1049 boolean allMatch(Predicate<? super T> predicate);
1050
1051 /**
1052 * Returns whether no elements of this stream match the provided predicate.
1053 * May not evaluate the predicate on all elements if not necessary for
1054 * determining the result. If the stream is empty then {@code true} is
1055 * returned and the predicate is not evaluated.
1056 *
1057 * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
1058 * terminal operation</a>.
1059 *
1060 * @apiNote
1061 * This method evaluates the <em>universal quantification</em> of the
1062 * negated predicate over the elements of the stream (for all x ~P(x)). If
1063 * the stream is empty, the quantification is said to be vacuously satisfied
1064 * and is always {@code true}, regardless of P(x).
1065 *
1066 * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>,
1067 * <a href="package-summary.html#Statelessness">stateless</a>
1068 * predicate to apply to elements of this stream
1069 * @return {@code true} if either no elements of the stream match the
1070 * provided predicate or the stream is empty, otherwise {@code false}
1071 */
1072 boolean noneMatch(Predicate<? super T> predicate);
1073
1074 /**
1075 * Returns an {@link Optional} describing the first element of this stream,
1076 * or an empty {@code Optional} if the stream is empty. If the stream has
1077 * no encounter order, then any element may be returned.
1078 *
1079 * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
1080 * terminal operation</a>.
1081 *
1082 * @return an {@code Optional} describing the first element of this stream,
1083 * or an empty {@code Optional} if the stream is empty
1084 * @throws NullPointerException if the element selected is null
1085 */
1086 Optional<T> findFirst();
1087
1088 /**
1089 * Returns an {@link Optional} describing some element of the stream, or an
1090 * empty {@code Optional} if the stream is empty.
1091 *
1092 * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
1093 * terminal operation</a>.
1094 *
1095 * <p>The behavior of this operation is explicitly nondeterministic; it is
1096 * free to select any element in the stream. This is to allow for maximal
1097 * performance in parallel operations; the cost is that multiple invocations
1098 * on the same source may not return the same result. (If a stable result
1099 * is desired, use {@link #findFirst()} instead.)
1100 *
1101 * @return an {@code Optional} describing some element of this stream, or an
1102 * empty {@code Optional} if the stream is empty
1103 * @throws NullPointerException if the element selected is null
1104 * @see #findFirst()
1105 */
1106 Optional<T> findAny();
1107
1108 // Static factories
1109
1110 /**
1111 * Returns a builder for a {@code Stream}.
1112 *
1113 * @param <T> type of elements
1114 * @return a stream builder
1115 */
1116 public static<T> Builder<T> builder() {
1117 return new Streams.StreamBuilderImpl<>();
1118 }
1119
1120 /**
1121 * Returns an empty sequential {@code Stream}.
1122 *
1123 * @param <T> the type of stream elements
1124 * @return an empty sequential stream
1125 */
1126 public static<T> Stream<T> empty() {
1127 return StreamSupport.stream(Spliterators.<T>emptySpliterator(), false);
1128 }
1129
1130 /**
1131 * Returns a sequential {@code Stream} containing a single element.
1132 *
1133 * @param t the single element
1134 * @param <T> the type of stream elements
1135 * @return a singleton sequential stream
1136 */
1137 public static<T> Stream<T> of(T t) {
1138 return StreamSupport.stream(new Streams.StreamBuilderImpl<>(t), false);
1139 }
1140
1141 /**
1142 * Returns a sequential {@code Stream} containing a single element, if
1143 * non-null, otherwise returns an empty {@code Stream}.
1144 *
1145 * @param t the single element
1146 * @param <T> the type of stream elements
1147 * @return a stream with a single element if the specified element
1148 * is non-null, otherwise an empty stream
1149 * @since 9
1150 */
1151 public static<T> Stream<T> ofNullable(T t) {
1152 return t == null ? Stream.empty()
1153 : StreamSupport.stream(new Streams.StreamBuilderImpl<>(t), false);
1154 }
1155
1156 /**
1157 * Returns a sequential ordered stream whose elements are the specified values.
1158 *
1159 * @param <T> the type of stream elements
1160 * @param values the elements of the new stream
1161 * @return the new stream
1162 */
1163 @SafeVarargs
1164 @SuppressWarnings("varargs") // Creating a stream from an array is safe
1165 public static<T> Stream<T> of(T... values) {
1166 return Arrays.stream(values);
1167 }
1168
1169 /**
1170 * Returns an infinite sequential ordered {@code Stream} produced by iterative
1171 * application of a function {@code f} to an initial element {@code seed},
1172 * producing a {@code Stream} consisting of {@code seed}, {@code f(seed)},
1173 * {@code f(f(seed))}, etc.
1174 *
1175 * <p>The first element (position {@code 0}) in the {@code Stream} will be
1176 * the provided {@code seed}. For {@code n > 0}, the element at position
1177 * {@code n}, will be the result of applying the function {@code f} to the
1178 * element at position {@code n - 1}.
1179 *
1180 * @param <T> the type of stream elements
1181 * @param seed the initial element
1182 * @param f a function to be applied to the previous element to produce
1183 * a new element
1184 * @return a new sequential {@code Stream}
1185 */
1186 public static<T> Stream<T> iterate(final T seed, final UnaryOperator<T> f) {
1187 Objects.requireNonNull(f);
1188 final Iterator<T> iterator = new Iterator<T>() {
1189 @SuppressWarnings("unchecked")
1190 T t = (T) Streams.NONE;
1191
1192 @Override
1193 public boolean hasNext() {
1194 return true;
1195 }
1196
1197 @Override
1198 public T next() {
1199 return t = (t == Streams.NONE) ? seed : f.apply(t);
1200 }
1201 };
1202 return StreamSupport.stream(Spliterators.spliteratorUnknownSize(
1203 iterator,
1204 Spliterator.ORDERED | Spliterator.IMMUTABLE), false);
1205 }
1206
1207 /**
1208 * Returns an infinite sequential unordered stream where each element is
1209 * generated by the provided {@code Supplier}. This is suitable for
1210 * generating constant streams, streams of random elements, etc.
1211 *
1212 * @param <T> the type of stream elements
1213 * @param s the {@code Supplier} of generated elements
1214 * @return a new infinite sequential unordered {@code Stream}
1215 */
1216 public static<T> Stream<T> generate(Supplier<T> s) {
1217 Objects.requireNonNull(s);
1218 return StreamSupport.stream(
1219 new StreamSpliterators.InfiniteSupplyingSpliterator.OfRef<>(Long.MAX_VALUE, s), false);
1220 }
1221
1222 /**
1223 * Creates a lazily concatenated stream whose elements are all the
1224 * elements of the first stream followed by all the elements of the
1225 * second stream. The resulting stream is ordered if both
1226 * of the input streams are ordered, and parallel if either of the input
1227 * streams is parallel. When the resulting stream is closed, the close
1228 * handlers for both input streams are invoked.
1229 *
1230 * @implNote
1231 * Use caution when constructing streams from repeated concatenation.
1232 * Accessing an element of a deeply concatenated stream can result in deep
1233 * call chains, or even {@code StackOverflowError}.
1234 *
1235 * <p>Subsequent changes to the sequential/parallel execution mode of the
1236 * returned stream are not guaranteed to be propagated to the input streams.
1237 *
1238 * @param <T> The type of stream elements
1239 * @param a the first stream
1240 * @param b the second stream
1241 * @return the concatenation of the two input streams
1242 */
1243 public static <T> Stream<T> concat(Stream<? extends T> a, Stream<? extends T> b) {
1244 Objects.requireNonNull(a);
1245 Objects.requireNonNull(b);
1246
1247 @SuppressWarnings("unchecked")
1248 Spliterator<T> split = new Streams.ConcatSpliterator.OfRef<>(
1249 (Spliterator<T>) a.spliterator(), (Spliterator<T>) b.spliterator());
1250 Stream<T> stream = StreamSupport.stream(split, a.isParallel() || b.isParallel());
1251 return stream.onClose(Streams.composedClose(a, b));
1252 }
1253
1254 /**
1255 * A mutable builder for a {@code Stream}. This allows the creation of a
1256 * {@code Stream} by generating elements individually and adding them to the
1257 * {@code Builder} (without the copying overhead that comes from using
1258 * an {@code ArrayList} as a temporary buffer.)
1259 *
1260 * <p>A stream builder has a lifecycle, which starts in a building
1261 * phase, during which elements can be added, and then transitions to a built
1262 * phase, after which elements may not be added. The built phase begins
1263 * when the {@link #build()} method is called, which creates an ordered
1264 * {@code Stream} whose elements are the elements that were added to the stream
1265 * builder, in the order they were added.
1266 *
1267 * @param <T> the type of stream elements
1268 * @see Stream#builder()
1269 * @since 1.8
1270 */
1271 public interface Builder<T> extends Consumer<T> {
1272
1273 /**
1274 * Adds an element to the stream being built.
1275 *
1276 * @throws IllegalStateException if the builder has already transitioned to
1277 * the built state
1278 */
1279 @Override
1280 void accept(T t);
1281
1282 /**
1283 * Adds an element to the stream being built.
1284 *
1285 * @implSpec
1286 * The default implementation behaves as if:
1287 * <pre>{@code
1288 * accept(t)
1289 * return this;
1290 * }</pre>
1291 *
1292 * @param t the element to add
1293 * @return {@code this} builder
1294 * @throws IllegalStateException if the builder has already transitioned to
1295 * the built state
1296 */
1297 default Builder<T> add(T t) {
1298 accept(t);
1299 return this;
1300 }
1301
1302 /**
1303 * Builds the stream, transitioning this builder to the built state.
1304 * An {@code IllegalStateException} is thrown if there are further attempts
1305 * to operate on the builder after it has entered the built state.
1306 *
1307 * @return the built stream
1308 * @throws IllegalStateException if the builder has already transitioned to
1309 * the built state
1310 */
1311 Stream<T> build();
1312
1313 }
1314 }