1 /*
2 * Copyright (c) 2012, 2017, 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.
107 *
108 * <p>A stream pipeline, like the "widgets" example above, can be viewed as
109 * a <em>query</em> on the stream source. Unless the source was explicitly
110 * designed for concurrent modification (such as a {@link ConcurrentHashMap}),
111 * unpredictable or erroneous behavior may result from modifying the stream
112 * source while it is being queried.
113 *
114 * <p>Most stream operations accept parameters that describe user-specified
115 * behavior, such as the lambda expression {@code w -> w.getWeight()} passed to
116 * {@code mapToInt} in the example above. To preserve correct behavior,
117 * these <em>behavioral parameters</em>:
118 * <ul>
119 * <li>must be <a href="package-summary.html#NonInterference">non-interfering</a>
120 * (they do not modify the stream source); and</li>
121 * <li>in most cases must be <a href="package-summary.html#Statelessness">stateless</a>
122 * (their result should not depend on any state that might change during execution
123 * of the stream pipeline).</li>
124 * </ul>
125 *
126 * <p>Such parameters are always instances of a
127 * <a href="../function/package-summary.html">functional interface</a> such
128 * as {@link java.util.function.Function}, and are often lambda expressions or
129 * method references. Unless otherwise specified these parameters must be
130 * <em>non-null</em>.
131 *
132 * <p>A stream should be operated on (invoking an intermediate or terminal stream
133 * operation) only once. This rules out, for example, "forked" streams, where
134 * the same source feeds two or more pipelines, or multiple traversals of the
135 * same stream. A stream implementation may throw {@link IllegalStateException}
136 * if it detects that the stream is being reused. However, since some stream
137 * operations may return their receiver rather than a new stream object, it may
138 * not be possible to detect reuse in all cases.
139 *
140 * <p>Streams have a {@link #close()} method and implement {@link AutoCloseable}.
141 * Operating on a stream after it has been closed will throw {@link IllegalStateException}.
142 * Most stream instances do not actually need to be closed after use, as they
143 * are backed by collections, arrays, or generating functions, which require no
144 * special resource management. Generally, only streams whose source is an IO channel,
145 * such as those returned by {@link Files#lines(Path)}, will require closing. If a
146 * stream does require closing, it must be opened as a resource within a try-with-resources
147 * statement or similar control structure to ensure that it is closed promptly after its
148 * operations have completed.
149 *
150 * <p>Stream pipelines may execute either sequentially or in
151 * <a href="package-summary.html#Parallelism">parallel</a>. This
152 * execution mode is a property of the stream. Streams are created
153 * with an initial choice of sequential or parallel execution. (For example,
154 * {@link Collection#stream() Collection.stream()} creates a sequential stream,
155 * and {@link Collection#parallelStream() Collection.parallelStream()} creates
156 * a parallel one.) This choice of execution mode may be modified by the
157 * {@link #sequential()} or {@link #parallel()} methods, and may be queried with
158 * the {@link #isParallel()} method.
159 *
160 * @param <T> the type of the stream elements
161 * @since 1.8
162 * @see IntStream
163 * @see LongStream
164 * @see DoubleStream
165 * @see <a href="package-summary.html">java.util.stream</a>
166 */
167 public interface Stream<T> extends BaseStream<T, Stream<T>> {
168
169 /**
170 * Returns a stream consisting of the elements of this stream that match
171 * the given predicate.
172 *
173 * <p>This is an <a href="package-summary.html#StreamOps">intermediate
174 * operation</a>.
175 *
176 * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>,
177 * <a href="package-summary.html#Statelessness">stateless</a>
178 * predicate to apply to each element to determine if it
179 * should be included
180 * @return the new stream
181 */
182 Stream<T> filter(Predicate<? super T> predicate);
183
184 /**
185 * Returns a stream consisting of the results of applying the given
186 * function to the elements of this stream.
187 *
188 * <p>This is an <a href="package-summary.html#StreamOps">intermediate
189 * operation</a>.
190 *
191 * @param <R> The element type of the new stream
192 * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
193 * <a href="package-summary.html#Statelessness">stateless</a>
194 * function to apply to each element
195 * @return the new stream
196 */
197 <R> Stream<R> map(Function<? super T, ? extends R> mapper);
198
199 /**
200 * Returns an {@code IntStream} consisting of the results of applying the
201 * given function to the elements of this stream.
202 *
203 * <p>This is an <a href="package-summary.html#StreamOps">
204 * intermediate operation</a>.
205 *
206 * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
207 * <a href="package-summary.html#Statelessness">stateless</a>
208 * function to apply to each element
209 * @return the new stream
210 */
211 IntStream mapToInt(ToIntFunction<? super T> mapper);
212
213 /**
214 * Returns a {@code LongStream} consisting of the results of applying the
215 * given function to the elements of this stream.
216 *
217 * <p>This is an <a href="package-summary.html#StreamOps">intermediate
218 * operation</a>.
219 *
220 * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
221 * <a href="package-summary.html#Statelessness">stateless</a>
222 * function to apply to each element
223 * @return the new stream
224 */
225 LongStream mapToLong(ToLongFunction<? super T> mapper);
226
227 /**
228 * Returns a {@code DoubleStream} consisting of the results of applying the
229 * given function to the elements of this stream.
230 *
231 * <p>This is an <a href="package-summary.html#StreamOps">intermediate
232 * operation</a>.
233 *
234 * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
235 * <a href="package-summary.html#Statelessness">stateless</a>
236 * function to apply to each element
237 * @return the new stream
238 */
239 DoubleStream mapToDouble(ToDoubleFunction<? super T> mapper);
240
241 /**
242 * Returns a stream consisting of the results of replacing each element of
243 * this stream with the contents of a mapped stream produced by applying
244 * the provided mapping function to each element. Each mapped stream is
245 * {@link java.util.stream.BaseStream#close() closed} after its contents
246 * have been placed into this stream. (If a mapped stream is {@code null}
247 * an empty stream is used, instead.)
248 *
249 * <p>This is an <a href="package-summary.html#StreamOps">intermediate
250 * operation</a>.
251 *
252 * @apiNote
253 * The {@code flatMap()} operation has the effect of applying a one-to-many
254 * transformation to the elements of the stream, and then flattening the
255 * resulting elements into a new stream.
256 *
257 * <p><b>Examples.</b>
258 *
259 * <p>If {@code orders} is a stream of purchase orders, and each purchase
260 * order contains a collection of line items, then the following produces a
261 * stream containing all the line items in all the orders:
262 * <pre>{@code
263 * orders.flatMap(order -> order.getLineItems().stream())...
264 * }</pre>
265 *
266 * <p>If {@code path} is the path to a file, then the following produces a
267 * stream of the {@code words} contained in that file:
268 * <pre>{@code
269 * Stream<String> lines = Files.lines(path, StandardCharsets.UTF_8);
270 * Stream<String> words = lines.flatMap(line -> Stream.of(line.split(" +")));
271 * }</pre>
272 * The {@code mapper} function passed to {@code flatMap} splits a line,
273 * using a simple regular expression, into an array of words, and then
274 * creates a stream of words from that array.
275 *
276 * @param <R> The element type of the new stream
277 * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
278 * <a href="package-summary.html#Statelessness">stateless</a>
279 * function to apply to each element which produces a stream
280 * of new values
281 * @return the new stream
282 */
283 <R> Stream<R> flatMap(Function<? super T, ? extends Stream<? extends R>> mapper);
284
285 /**
286 * Returns an {@code IntStream} consisting of the results of replacing each
287 * element of this stream with the contents of a mapped stream produced by
288 * applying the provided mapping function to each element. Each mapped
289 * stream is {@link java.util.stream.BaseStream#close() closed} after its
290 * contents have been placed into this stream. (If a mapped stream is
291 * {@code null} an empty stream is used, instead.)
292 *
293 * <p>This is an <a href="package-summary.html#StreamOps">intermediate
294 * operation</a>.
295 *
296 * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
297 * <a href="package-summary.html#Statelessness">stateless</a>
298 * function to apply to each element which produces a stream
299 * of new values
300 * @return the new stream
301 * @see #flatMap(Function)
302 */
303 IntStream flatMapToInt(Function<? super T, ? extends IntStream> mapper);
304
305 /**
306 * Returns an {@code LongStream} consisting of the results of replacing each
307 * element of this stream with the contents of a mapped stream produced by
308 * applying the provided mapping function to each element. Each mapped
309 * stream is {@link java.util.stream.BaseStream#close() closed} after its
310 * contents have been placed into this stream. (If a mapped stream is
311 * {@code null} an empty stream is used, instead.)
312 *
313 * <p>This is an <a href="package-summary.html#StreamOps">intermediate
314 * operation</a>.
315 *
316 * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
317 * <a href="package-summary.html#Statelessness">stateless</a>
318 * function to apply to each element which produces a stream
319 * of new values
320 * @return the new stream
321 * @see #flatMap(Function)
322 */
323 LongStream flatMapToLong(Function<? super T, ? extends LongStream> mapper);
324
325 /**
326 * Returns an {@code DoubleStream} consisting of the results of replacing
327 * each element of this stream with the contents of a mapped stream produced
328 * by applying the provided mapping function to each element. Each mapped
329 * stream is {@link java.util.stream.BaseStream#close() closed} after its
330 * contents have placed been into this stream. (If a mapped stream is
331 * {@code null} an empty stream is used, instead.)
332 *
333 * <p>This is an <a href="package-summary.html#StreamOps">intermediate
334 * operation</a>.
335 *
336 * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
337 * <a href="package-summary.html#Statelessness">stateless</a>
338 * function to apply to each element which produces a stream
339 * of new values
340 * @return the new stream
341 * @see #flatMap(Function)
342 */
343 DoubleStream flatMapToDouble(Function<? super T, ? extends DoubleStream> mapper);
344
345 /**
346 * Returns a stream consisting of the distinct elements (according to
347 * {@link Object#equals(Object)}) of this stream.
348 *
349 * <p>For ordered streams, the selection of distinct elements is stable
350 * (for duplicated elements, the element appearing first in the encounter
351 * order is preserved.) For unordered streams, no stability guarantees
352 * are made.
353 *
354 * <p>This is a <a href="package-summary.html#StreamOps">stateful
355 * intermediate operation</a>.
356 *
357 * @apiNote
358 * Preserving stability for {@code distinct()} in parallel pipelines is
359 * relatively expensive (requires that the operation act as a full barrier,
360 * with substantial buffering overhead), and stability is often not needed.
361 * Using an unordered stream source (such as {@link #generate(Supplier)})
362 * or removing the ordering constraint with {@link #unordered()} may result
363 * in significantly more efficient execution for {@code distinct()} in parallel
364 * pipelines, if the semantics of your situation permit. If consistency
365 * with encounter order is required, and you are experiencing poor performance
366 * or memory utilization with {@code distinct()} in parallel pipelines,
367 * switching to sequential execution with {@link #sequential()} may improve
368 * performance.
369 *
370 * @return the new stream
371 */
372 Stream<T> distinct();
373
374 /**
375 * Returns a stream consisting of the elements of this stream, sorted
376 * according to natural order. If the elements of this stream are not
377 * {@code Comparable}, a {@code java.lang.ClassCastException} may be thrown
378 * when the terminal operation is executed.
379 *
380 * <p>For ordered streams, the sort is stable. For unordered streams, no
381 * stability guarantees are made.
382 *
383 * <p>This is a <a href="package-summary.html#StreamOps">stateful
384 * intermediate operation</a>.
385 *
386 * @return the new stream
387 */
388 Stream<T> sorted();
389
390 /**
391 * Returns a stream consisting of the elements of this stream, sorted
392 * according to the provided {@code Comparator}.
393 *
394 * <p>For ordered streams, the sort is stable. For unordered streams, no
395 * stability guarantees are made.
396 *
397 * <p>This is a <a href="package-summary.html#StreamOps">stateful
398 * intermediate operation</a>.
399 *
400 * @param comparator a <a href="package-summary.html#NonInterference">non-interfering</a>,
401 * <a href="package-summary.html#Statelessness">stateless</a>
402 * {@code Comparator} to be used to compare stream elements
403 * @return the new stream
404 */
405 Stream<T> sorted(Comparator<? super T> comparator);
406
407 /**
408 * Returns a stream consisting of the elements of this stream, additionally
409 * performing the provided action on each element as elements are consumed
410 * from the resulting stream.
411 *
412 * <p>This is an <a href="package-summary.html#StreamOps">intermediate
413 * operation</a>.
414 *
415 * <p>For parallel stream pipelines, the action may be called at
416 * whatever time and in whatever thread the element is made available by the
417 * upstream operation. If the action modifies shared state,
418 * it is responsible for providing the required synchronization.
419 *
420 * @apiNote This method exists mainly to support debugging, where you want
421 * to see the elements as they flow past a certain point in a pipeline:
422 * <pre>{@code
423 * Stream.of("one", "two", "three", "four")
424 * .filter(e -> e.length() > 3)
425 * .peek(e -> System.out.println("Filtered value: " + e))
426 * .map(String::toUpperCase)
427 * .peek(e -> System.out.println("Mapped value: " + e))
428 * .collect(Collectors.toList());
429 * }</pre>
430 *
431 * <p>In cases where the stream implementation is able to optimize away the
432 * production of some or all the elements (such as with short-circuiting
433 * operations like {@code findFirst}, or in the example described in
434 * {@link #count}), the action will not be invoked for those elements.
435 *
436 * @param action a <a href="package-summary.html#NonInterference">
437 * non-interfering</a> action to perform on the elements as
438 * they are consumed from the stream
439 * @return the new stream
440 */
441 Stream<T> peek(Consumer<? super T> action);
442
443 /**
444 * Returns a stream consisting of the elements of this stream, truncated
445 * to be no longer than {@code maxSize} in length.
446 *
447 * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
448 * stateful intermediate operation</a>.
449 *
450 * @apiNote
451 * While {@code limit()} is generally a cheap operation on sequential
452 * stream pipelines, it can be quite expensive on ordered parallel pipelines,
453 * especially for large values of {@code maxSize}, since {@code limit(n)}
454 * is constrained to return not just any <em>n</em> elements, but the
455 * <em>first n</em> elements in the encounter order. Using an unordered
456 * stream source (such as {@link #generate(Supplier)}) or removing the
457 * ordering constraint with {@link #unordered()} may result in significant
458 * speedups of {@code limit()} in parallel pipelines, if the semantics of
459 * your situation permit. If consistency with encounter order is required,
460 * and you are experiencing poor performance or memory utilization with
461 * {@code limit()} in parallel pipelines, switching to sequential execution
462 * with {@link #sequential()} may improve performance.
463 *
464 * @param maxSize the number of elements the stream should be limited to
465 * @return the new stream
466 * @throws IllegalArgumentException if {@code maxSize} is negative
467 */
468 Stream<T> limit(long maxSize);
469
470 /**
471 * Returns a stream consisting of the remaining elements of this stream
472 * after discarding the first {@code n} elements of the stream.
473 * If this stream contains fewer than {@code n} elements then an
474 * empty stream will be returned.
475 *
476 * <p>This is a <a href="package-summary.html#StreamOps">stateful
477 * intermediate operation</a>.
478 *
479 * @apiNote
480 * While {@code skip()} is generally a cheap operation on sequential
481 * stream pipelines, it can be quite expensive on ordered parallel pipelines,
482 * especially for large values of {@code n}, since {@code skip(n)}
483 * is constrained to skip not just any <em>n</em> elements, but the
484 * <em>first n</em> elements in the encounter order. Using an unordered
485 * stream source (such as {@link #generate(Supplier)}) or removing the
486 * ordering constraint with {@link #unordered()} may result in significant
487 * speedups of {@code skip()} in parallel pipelines, if the semantics of
488 * your situation permit. If consistency with encounter order is required,
489 * and you are experiencing poor performance or memory utilization with
490 * {@code skip()} in parallel pipelines, switching to sequential execution
491 * with {@link #sequential()} may improve performance.
492 *
493 * @param n the number of leading elements to skip
494 * @return the new stream
495 * @throws IllegalArgumentException if {@code n} is negative
496 */
497 Stream<T> skip(long n);
498
499 /**
500 * Returns, if this stream is ordered, a stream consisting of the longest
501 * prefix of elements taken from this stream that match the given predicate.
502 * Otherwise returns, if this stream is unordered, a stream consisting of a
503 * subset of elements taken from this stream that match the given predicate.
504 *
505 * <p>If this stream is ordered then the longest prefix is a contiguous
506 * sequence of elements of this stream that match the given predicate. The
507 * first element of the sequence is the first element of this stream, and
508 * the element immediately following the last element of the sequence does
509 * not match the given predicate.
510 *
511 * <p>If this stream is unordered, and some (but not all) elements of this
512 * stream match the given predicate, then the behavior of this operation is
513 * nondeterministic; it is free to take any subset of matching elements
514 * (which includes the empty set).
515 *
516 * <p>Independent of whether this stream is ordered or unordered if all
517 * elements of this stream match the given predicate then this operation
518 * takes all elements (the result is the same as the input), or if no
519 * elements of the stream match the given predicate then no elements are
520 * taken (the result is an empty stream).
521 *
522 * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
523 * stateful intermediate operation</a>.
524 *
525 * @implSpec
526 * The default implementation obtains the {@link #spliterator() spliterator}
527 * of this stream, wraps that spliterator so as to support the semantics
528 * of this operation on traversal, and returns a new stream associated with
529 * the wrapped spliterator. The returned stream preserves the execution
530 * characteristics of this stream (namely parallel or sequential execution
531 * as per {@link #isParallel()}) but the wrapped spliterator may choose to
532 * not support splitting. When the returned stream is closed, the close
533 * handlers for both the returned and this stream are invoked.
534 *
535 * @apiNote
536 * While {@code takeWhile()} is generally a cheap operation on sequential
537 * stream pipelines, it can be quite expensive on ordered parallel
538 * pipelines, since the operation is constrained to return not just any
539 * valid prefix, but the longest prefix of elements in the encounter order.
540 * Using an unordered stream source (such as {@link #generate(Supplier)}) or
541 * removing the ordering constraint with {@link #unordered()} may result in
542 * significant speedups of {@code takeWhile()} in parallel pipelines, if the
543 * semantics of your situation permit. If consistency with encounter order
544 * is required, and you are experiencing poor performance or memory
545 * utilization with {@code takeWhile()} in parallel pipelines, switching to
546 * sequential execution with {@link #sequential()} may improve performance.
547 *
548 * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>,
549 * <a href="package-summary.html#Statelessness">stateless</a>
550 * predicate to apply to elements to determine the longest
551 * prefix of elements.
552 * @return the new stream
553 * @since 9
554 */
555 default Stream<T> takeWhile(Predicate<? super T> predicate) {
556 Objects.requireNonNull(predicate);
557 // Reuses the unordered spliterator, which, when encounter is present,
558 // is safe to use as long as it configured not to split
559 return StreamSupport.stream(
560 new WhileOps.UnorderedWhileSpliterator.OfRef.Taking<>(spliterator(), true, predicate),
561 isParallel()).onClose(this::close);
562 }
563
564 /**
565 * Returns, if this stream is ordered, a stream consisting of the remaining
566 * elements of this stream after dropping the longest prefix of elements
567 * that match the given predicate. Otherwise returns, if this stream is
568 * unordered, a stream consisting of the remaining elements of this stream
569 * after dropping a subset of elements that match the given predicate.
570 *
571 * <p>If this stream is ordered then the longest prefix is a contiguous
572 * sequence of elements of this stream that match the given predicate. The
573 * first element of the sequence is the first element of this stream, and
574 * the element immediately following the last element of the sequence does
575 * not match the given predicate.
576 *
577 * <p>If this stream is unordered, and some (but not all) elements of this
578 * stream match the given predicate, then the behavior of this operation is
579 * nondeterministic; it is free to drop any subset of matching elements
580 * (which includes the empty set).
581 *
582 * <p>Independent of whether this stream is ordered or unordered if all
583 * elements of this stream match the given predicate then this operation
584 * drops all elements (the result is an empty stream), or if no elements of
585 * the stream match the given predicate then no elements are dropped (the
586 * result is the same as the input).
587 *
588 * <p>This is a <a href="package-summary.html#StreamOps">stateful
589 * intermediate operation</a>.
590 *
591 * @implSpec
592 * The default implementation obtains the {@link #spliterator() spliterator}
593 * of this stream, wraps that spliterator so as to support the semantics
594 * of this operation on traversal, and returns a new stream associated with
595 * the wrapped spliterator. The returned stream preserves the execution
596 * characteristics of this stream (namely parallel or sequential execution
597 * as per {@link #isParallel()}) but the wrapped spliterator may choose to
598 * not support splitting. When the returned stream is closed, the close
599 * handlers for both the returned and this stream are invoked.
600 *
601 * @apiNote
602 * While {@code dropWhile()} is generally a cheap operation on sequential
603 * stream pipelines, it can be quite expensive on ordered parallel
604 * pipelines, since the operation is constrained to return not just any
605 * valid prefix, but the longest prefix of elements in the encounter order.
606 * Using an unordered stream source (such as {@link #generate(Supplier)}) or
607 * removing the ordering constraint with {@link #unordered()} may result in
608 * significant speedups of {@code dropWhile()} in parallel pipelines, if the
609 * semantics of your situation permit. If consistency with encounter order
610 * is required, and you are experiencing poor performance or memory
611 * utilization with {@code dropWhile()} in parallel pipelines, switching to
612 * sequential execution with {@link #sequential()} may improve performance.
613 *
614 * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>,
615 * <a href="package-summary.html#Statelessness">stateless</a>
616 * predicate to apply to elements to determine the longest
617 * prefix of elements.
618 * @return the new stream
619 * @since 9
620 */
621 default Stream<T> dropWhile(Predicate<? super T> predicate) {
622 Objects.requireNonNull(predicate);
623 // Reuses the unordered spliterator, which, when encounter is present,
624 // is safe to use as long as it configured not to split
625 return StreamSupport.stream(
626 new WhileOps.UnorderedWhileSpliterator.OfRef.Dropping<>(spliterator(), true, predicate),
627 isParallel()).onClose(this::close);
628 }
629
630 /**
631 * Performs an action for each element of this stream.
632 *
633 * <p>This is a <a href="package-summary.html#StreamOps">terminal
634 * operation</a>.
635 *
636 * <p>The behavior of this operation is explicitly nondeterministic.
637 * For parallel stream pipelines, this operation does <em>not</em>
638 * guarantee to respect the encounter order of the stream, as doing so
639 * would sacrifice the benefit of parallelism. For any given element, the
640 * action may be performed at whatever time and in whatever thread the
641 * library chooses. If the action accesses shared state, it is
642 * responsible for providing the required synchronization.
643 *
644 * @param action a <a href="package-summary.html#NonInterference">
645 * non-interfering</a> action to perform on the elements
646 */
647 void forEach(Consumer<? super T> action);
648
649 /**
650 * Performs an action for each element of this stream, in the encounter
651 * order of the stream if the stream has a defined encounter order.
652 *
653 * <p>This is a <a href="package-summary.html#StreamOps">terminal
654 * operation</a>.
655 *
656 * <p>This operation processes the elements one at a time, in encounter
657 * order if one exists. Performing the action for one element
658 * <a href="../concurrent/package-summary.html#MemoryVisibility"><i>happens-before</i></a>
659 * performing the action for subsequent elements, but for any given element,
660 * the action may be performed in whatever thread the library chooses.
661 *
662 * @param action a <a href="package-summary.html#NonInterference">
663 * non-interfering</a> action to perform on the elements
664 * @see #forEach(Consumer)
665 */
666 void forEachOrdered(Consumer<? super T> action);
667
668 /**
669 * Returns an array containing the elements of this stream.
670 *
671 * <p>This is a <a href="package-summary.html#StreamOps">terminal
672 * operation</a>.
673 *
674 * @return an array, whose {@linkplain Class#getComponentType runtime component
675 * type} is {@code Object}, containing the elements of this stream
676 */
677 Object[] toArray();
678
679 /**
680 * Returns an array containing the elements of this stream, using the
681 * provided {@code generator} function to allocate the returned array, as
682 * well as any additional arrays that might be required for a partitioned
683 * execution or for resizing.
684 *
685 * <p>This is a <a href="package-summary.html#StreamOps">terminal
686 * operation</a>.
687 *
688 * @apiNote
689 * The generator function takes an integer, which is the size of the
690 * desired array, and produces an array of the desired size. This can be
691 * concisely expressed with an array constructor reference:
692 * <pre>{@code
693 * Person[] men = people.stream()
694 * .filter(p -> p.getGender() == MALE)
695 * .toArray(Person[]::new);
696 * }</pre>
697 *
698 * @param <A> the component type of the resulting array
699 * @param generator a function which produces a new array of the desired
700 * type and the provided length
701 * @return an array containing the elements in this stream
702 * @throws ArrayStoreException if the runtime type of any element of this
703 * stream is not assignable to the {@linkplain Class#getComponentType
704 * runtime component type} of the generated array
705 */
706 <A> A[] toArray(IntFunction<A[]> generator);
707
708 /**
709 * Performs a <a href="package-summary.html#Reduction">reduction</a> on the
710 * elements of this stream, using the provided identity value and an
711 * <a href="package-summary.html#Associativity">associative</a>
712 * accumulation function, and returns the reduced value. This is equivalent
713 * to:
714 * <pre>{@code
715 * T result = identity;
716 * for (T element : this stream)
717 * result = accumulator.apply(result, element)
718 * return result;
719 * }</pre>
720 *
721 * but is not constrained to execute sequentially.
722 *
723 * <p>The {@code identity} value must be an identity for the accumulator
724 * function. This means that for all {@code t},
725 * {@code accumulator.apply(identity, t)} is equal to {@code t}.
726 * The {@code accumulator} function must be an
727 * <a href="package-summary.html#Associativity">associative</a> function.
728 *
729 * <p>This is a <a href="package-summary.html#StreamOps">terminal
730 * operation</a>.
731 *
732 * @apiNote Sum, min, max, average, and string concatenation are all special
733 * cases of reduction. Summing a stream of numbers can be expressed as:
734 *
735 * <pre>{@code
736 * Integer sum = integers.reduce(0, (a, b) -> a+b);
737 * }</pre>
738 *
739 * or:
740 *
741 * <pre>{@code
742 * Integer sum = integers.reduce(0, Integer::sum);
743 * }</pre>
744 *
745 * <p>While this may seem a more roundabout way to perform an aggregation
746 * compared to simply mutating a running total in a loop, reduction
747 * operations parallelize more gracefully, without needing additional
748 * synchronization and with greatly reduced risk of data races.
749 *
750 * @param identity the identity value for the accumulating function
751 * @param accumulator an <a href="package-summary.html#Associativity">associative</a>,
752 * <a href="package-summary.html#NonInterference">non-interfering</a>,
753 * <a href="package-summary.html#Statelessness">stateless</a>
754 * function for combining two values
755 * @return the result of the reduction
756 */
757 T reduce(T identity, BinaryOperator<T> accumulator);
758
759 /**
760 * Performs a <a href="package-summary.html#Reduction">reduction</a> on the
761 * elements of this stream, using an
762 * <a href="package-summary.html#Associativity">associative</a> accumulation
763 * function, and returns an {@code Optional} describing the reduced value,
764 * if any. This is equivalent to:
765 * <pre>{@code
766 * boolean foundAny = false;
767 * T result = null;
768 * for (T element : this stream) {
769 * if (!foundAny) {
770 * foundAny = true;
771 * result = element;
772 * }
773 * else
774 * result = accumulator.apply(result, element);
775 * }
776 * return foundAny ? Optional.of(result) : Optional.empty();
777 * }</pre>
778 *
779 * but is not constrained to execute sequentially.
780 *
781 * <p>The {@code accumulator} function must be an
782 * <a href="package-summary.html#Associativity">associative</a> function.
783 *
784 * <p>This is a <a href="package-summary.html#StreamOps">terminal
785 * operation</a>.
786 *
787 * @param accumulator an <a href="package-summary.html#Associativity">associative</a>,
788 * <a href="package-summary.html#NonInterference">non-interfering</a>,
789 * <a href="package-summary.html#Statelessness">stateless</a>
790 * function for combining two values
791 * @return an {@link Optional} describing the result of the reduction
792 * @throws NullPointerException if the result of the reduction is null
793 * @see #reduce(Object, BinaryOperator)
794 * @see #min(Comparator)
795 * @see #max(Comparator)
796 */
797 Optional<T> reduce(BinaryOperator<T> accumulator);
798
799 /**
800 * Performs a <a href="package-summary.html#Reduction">reduction</a> on the
801 * elements of this stream, using the provided identity, accumulation and
802 * combining functions. This is equivalent to:
803 * <pre>{@code
804 * U result = identity;
805 * for (T element : this stream)
806 * result = accumulator.apply(result, element)
807 * return result;
808 * }</pre>
809 *
810 * but is not constrained to execute sequentially.
811 *
812 * <p>The {@code identity} value must be an identity for the combiner
813 * function. This means that for all {@code u}, {@code combiner(identity, u)}
814 * is equal to {@code u}. Additionally, the {@code combiner} function
815 * must be compatible with the {@code accumulator} function; for all
816 * {@code u} and {@code t}, the following must hold:
817 * <pre>{@code
818 * combiner.apply(u, accumulator.apply(identity, t)) == accumulator.apply(u, t)
819 * }</pre>
820 *
821 * <p>This is a <a href="package-summary.html#StreamOps">terminal
822 * operation</a>.
823 *
824 * @apiNote Many reductions using this form can be represented more simply
825 * by an explicit combination of {@code map} and {@code reduce} operations.
826 * The {@code accumulator} function acts as a fused mapper and accumulator,
827 * which can sometimes be more efficient than separate mapping and reduction,
828 * such as when knowing the previously reduced value allows you to avoid
829 * some computation.
830 *
831 * @param <U> The type of the result
832 * @param identity the identity value for the combiner function
833 * @param accumulator an <a href="package-summary.html#Associativity">associative</a>,
834 * <a href="package-summary.html#NonInterference">non-interfering</a>,
835 * <a href="package-summary.html#Statelessness">stateless</a>
836 * function for incorporating an additional element into a result
837 * @param combiner an <a href="package-summary.html#Associativity">associative</a>,
838 * <a href="package-summary.html#NonInterference">non-interfering</a>,
839 * <a href="package-summary.html#Statelessness">stateless</a>
840 * function for combining two values, which must be
841 * compatible with the accumulator function
842 * @return the result of the reduction
843 * @see #reduce(BinaryOperator)
844 * @see #reduce(Object, BinaryOperator)
845 */
846 <U> U reduce(U identity,
847 BiFunction<U, ? super T, U> accumulator,
848 BinaryOperator<U> combiner);
849
850 /**
851 * Performs a <a href="package-summary.html#MutableReduction">mutable
852 * reduction</a> operation on the elements of this stream. A mutable
853 * reduction is one in which the reduced value is a mutable result container,
854 * such as an {@code ArrayList}, and elements are incorporated by updating
855 * the state of the result rather than by replacing the result. This
856 * produces a result equivalent to:
857 * <pre>{@code
858 * R result = supplier.get();
859 * for (T element : this stream)
860 * accumulator.accept(result, element);
861 * return result;
862 * }</pre>
863 *
864 * <p>Like {@link #reduce(Object, BinaryOperator)}, {@code collect} operations
865 * can be parallelized without requiring additional synchronization.
866 *
867 * <p>This is a <a href="package-summary.html#StreamOps">terminal
868 * operation</a>.
869 *
870 * @apiNote There are many existing classes in the JDK whose signatures are
871 * well-suited for use with method references as arguments to {@code collect()}.
872 * For example, the following will accumulate strings into an {@code ArrayList}:
873 * <pre>{@code
874 * List<String> asList = stringStream.collect(ArrayList::new, ArrayList::add,
875 * ArrayList::addAll);
876 * }</pre>
877 *
878 * <p>The following will take a stream of strings and concatenates them into a
879 * single string:
880 * <pre>{@code
881 * String concat = stringStream.collect(StringBuilder::new, StringBuilder::append,
882 * StringBuilder::append)
883 * .toString();
884 * }</pre>
885 *
886 * @param <R> the type of the mutable result container
887 * @param supplier a function that creates a new mutable result container.
888 * For a parallel execution, this function may be called
889 * multiple times and must return a fresh value each time.
890 * @param accumulator an <a href="package-summary.html#Associativity">associative</a>,
891 * <a href="package-summary.html#NonInterference">non-interfering</a>,
892 * <a href="package-summary.html#Statelessness">stateless</a>
893 * function that must fold an element into a result
894 * container.
895 * @param combiner an <a href="package-summary.html#Associativity">associative</a>,
896 * <a href="package-summary.html#NonInterference">non-interfering</a>,
897 * <a href="package-summary.html#Statelessness">stateless</a>
898 * function that accepts two partial result containers
899 * and merges them, which must be compatible with the
900 * accumulator function. The combiner function must fold
901 * the elements from the second result container into the
902 * first result container.
903 * @return the result of the reduction
904 */
905 <R> R collect(Supplier<R> supplier,
906 BiConsumer<R, ? super T> accumulator,
907 BiConsumer<R, R> combiner);
908
909 /**
910 * Performs a <a href="package-summary.html#MutableReduction">mutable
911 * reduction</a> operation on the elements of this stream using a
912 * {@code Collector}. A {@code Collector}
913 * encapsulates the functions used as arguments to
914 * {@link #collect(Supplier, BiConsumer, BiConsumer)}, allowing for reuse of
915 * collection strategies and composition of collect operations such as
916 * multiple-level grouping or partitioning.
917 *
918 * <p>If the stream is parallel, and the {@code Collector}
919 * is {@link Collector.Characteristics#CONCURRENT concurrent}, and
920 * either the stream is unordered or the collector is
921 * {@link Collector.Characteristics#UNORDERED unordered},
922 * then a concurrent reduction will be performed (see {@link Collector} for
923 * details on concurrent reduction.)
924 *
925 * <p>This is a <a href="package-summary.html#StreamOps">terminal
926 * operation</a>.
927 *
928 * <p>When executed in parallel, multiple intermediate results may be
929 * instantiated, populated, and merged so as to maintain isolation of
930 * mutable data structures. Therefore, even when executed in parallel
931 * with non-thread-safe data structures (such as {@code ArrayList}), no
932 * additional synchronization is needed for a parallel reduction.
933 *
934 * @apiNote
935 * The following will accumulate strings into an ArrayList:
936 * <pre>{@code
937 * List<String> asList = stringStream.collect(Collectors.toList());
938 * }</pre>
939 *
940 * <p>The following will classify {@code Person} objects by city:
941 * <pre>{@code
942 * Map<String, List<Person>> peopleByCity
943 * = personStream.collect(Collectors.groupingBy(Person::getCity));
944 * }</pre>
945 *
946 * <p>The following will classify {@code Person} objects by state and city,
947 * cascading two {@code Collector}s together:
948 * <pre>{@code
949 * Map<String, Map<String, List<Person>>> peopleByStateAndCity
950 * = personStream.collect(Collectors.groupingBy(Person::getState,
951 * Collectors.groupingBy(Person::getCity)));
952 * }</pre>
953 *
954 * @param <R> the type of the result
955 * @param <A> the intermediate accumulation type of the {@code Collector}
956 * @param collector the {@code Collector} describing the reduction
957 * @return the result of the reduction
958 * @see #collect(Supplier, BiConsumer, BiConsumer)
959 * @see Collectors
960 */
961 <R, A> R collect(Collector<? super T, A, R> collector);
962
963 /**
964 * Returns the minimum element of this stream according to the provided
965 * {@code Comparator}. This is a special case of a
966 * <a href="package-summary.html#Reduction">reduction</a>.
967 *
968 * <p>This is a <a href="package-summary.html#StreamOps">terminal operation</a>.
969 *
970 * @param comparator a <a href="package-summary.html#NonInterference">non-interfering</a>,
971 * <a href="package-summary.html#Statelessness">stateless</a>
972 * {@code Comparator} to compare elements of this stream
973 * @return an {@code Optional} describing the minimum element of this stream,
974 * or an empty {@code Optional} if the stream is empty
975 * @throws NullPointerException if the minimum element is null
976 */
977 Optional<T> min(Comparator<? super T> comparator);
978
979 /**
980 * Returns the maximum element of this stream according to the provided
981 * {@code Comparator}. This is a special case of a
982 * <a href="package-summary.html#Reduction">reduction</a>.
983 *
984 * <p>This is a <a href="package-summary.html#StreamOps">terminal
985 * operation</a>.
986 *
987 * @param comparator a <a href="package-summary.html#NonInterference">non-interfering</a>,
988 * <a href="package-summary.html#Statelessness">stateless</a>
989 * {@code Comparator} to compare elements of this stream
990 * @return an {@code Optional} describing the maximum element of this stream,
991 * or an empty {@code Optional} if the stream is empty
992 * @throws NullPointerException if the maximum element is null
993 */
994 Optional<T> max(Comparator<? super T> comparator);
995
996 /**
997 * Returns the count of elements in this stream. This is a special case of
998 * a <a href="package-summary.html#Reduction">reduction</a> and is
999 * equivalent to:
1000 * <pre>{@code
1001 * return mapToLong(e -> 1L).sum();
1002 * }</pre>
1003 *
1004 * <p>This is a <a href="package-summary.html#StreamOps">terminal operation</a>.
1005 *
1006 * @apiNote
1007 * An implementation may choose to not execute the stream pipeline (either
1008 * sequentially or in parallel) if it is capable of computing the count
1009 * directly from the stream source. In such cases no source elements will
1010 * be traversed and no intermediate operations will be evaluated.
1011 * Behavioral parameters with side-effects, which are strongly discouraged
1012 * except for harmless cases such as debugging, may be affected. For
1013 * example, consider the following stream:
1014 * <pre>{@code
1015 * List<String> l = Arrays.asList("A", "B", "C", "D");
1016 * long count = l.stream().peek(System.out::println).count();
1017 * }</pre>
1018 * The number of elements covered by the stream source, a {@code List}, is
1019 * known and the intermediate operation, {@code peek}, does not inject into
1020 * or remove elements from the stream (as may be the case for
1021 * {@code flatMap} or {@code filter} operations). Thus the count is the
1022 * size of the {@code List} and there is no need to execute the pipeline
1023 * and, as a side-effect, print out the list elements.
1024 *
1025 * @return the count of elements in this stream
1026 */
1027 long count();
1028
1029 /**
1030 * Returns whether any elements of this stream match the provided
1031 * predicate. May not evaluate the predicate on all elements if not
1032 * necessary for determining the result. If the stream is empty then
1033 * {@code false} is returned and the predicate is not evaluated.
1034 *
1035 * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
1036 * terminal operation</a>.
1037 *
1038 * @apiNote
1039 * This method evaluates the <em>existential quantification</em> of the
1040 * predicate over the elements of the stream (for some x P(x)).
1041 *
1042 * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>,
1043 * <a href="package-summary.html#Statelessness">stateless</a>
1044 * predicate to apply to elements of this stream
1045 * @return {@code true} if any elements of the stream match the provided
1046 * predicate, otherwise {@code false}
1047 */
1048 boolean anyMatch(Predicate<? super T> predicate);
1049
1050 /**
1051 * Returns whether all elements of this stream match the provided predicate.
1052 * May not evaluate the predicate on all elements if not necessary for
1053 * determining the result. If the stream is empty then {@code true} is
1054 * returned and the predicate is not evaluated.
1055 *
1056 * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
1057 * terminal operation</a>.
1058 *
1059 * @apiNote
1060 * This method evaluates the <em>universal quantification</em> of the
1061 * predicate over the elements of the stream (for all x P(x)). If the
1062 * stream is empty, the quantification is said to be <em>vacuously
1063 * satisfied</em> and is always {@code true} (regardless of P(x)).
1064 *
1065 * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>,
1066 * <a href="package-summary.html#Statelessness">stateless</a>
1067 * predicate to apply to elements of this stream
1068 * @return {@code true} if either all elements of the stream match the
1069 * provided predicate or the stream is empty, otherwise {@code false}
1070 */
1071 boolean allMatch(Predicate<? super T> predicate);
1072
1073 /**
1074 * Returns whether no elements of this stream match the provided predicate.
1075 * May not evaluate the predicate on all elements if not necessary for
1076 * determining the result. If the stream is empty then {@code true} is
1077 * returned and the predicate is not evaluated.
1078 *
1079 * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
1080 * terminal operation</a>.
1081 *
1082 * @apiNote
1083 * This method evaluates the <em>universal quantification</em> of the
1084 * negated predicate over the elements of the stream (for all x ~P(x)). If
1085 * the stream is empty, the quantification is said to be vacuously satisfied
1086 * and is always {@code true}, regardless of P(x).
1087 *
1088 * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>,
1089 * <a href="package-summary.html#Statelessness">stateless</a>
1090 * predicate to apply to elements of this stream
1091 * @return {@code true} if either no elements of the stream match the
1092 * provided predicate or the stream is empty, otherwise {@code false}
1093 */
1094 boolean noneMatch(Predicate<? super T> predicate);
1095
1096 /**
1097 * Returns an {@link Optional} describing the first element of this stream,
1098 * or an empty {@code Optional} if the stream is empty. If the stream has
1099 * no encounter order, then any element may be returned.
1100 *
1101 * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
1102 * terminal operation</a>.
1103 *
1104 * @return an {@code Optional} describing the first element of this stream,
1105 * or an empty {@code Optional} if the stream is empty
1106 * @throws NullPointerException if the element selected is null
1107 */
1108 Optional<T> findFirst();
1109
1110 /**
1111 * Returns an {@link Optional} describing some element of the stream, or an
1112 * empty {@code Optional} if the stream is empty.
1113 *
1114 * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
1115 * terminal operation</a>.
1116 *
1117 * <p>The behavior of this operation is explicitly nondeterministic; it is
1118 * free to select any element in the stream. This is to allow for maximal
1119 * performance in parallel operations; the cost is that multiple invocations
1120 * on the same source may not return the same result. (If a stable result
1121 * is desired, use {@link #findFirst()} instead.)
1122 *
1123 * @return an {@code Optional} describing some element of this stream, or an
1124 * empty {@code Optional} if the stream is empty
1125 * @throws NullPointerException if the element selected is null
1126 * @see #findFirst()
1127 */
1128 Optional<T> findAny();
1129
1130 // Static factories
1131
1132 /**
1133 * Returns a builder for a {@code Stream}.
1134 *
1135 * @param <T> type of elements
1136 * @return a stream builder
1137 */
1138 public static<T> Builder<T> builder() {
1139 return new Streams.StreamBuilderImpl<>();
1140 }
1141
1142 /**
1143 * Returns an empty sequential {@code Stream}.
1144 *
1145 * @param <T> the type of stream elements
1146 * @return an empty sequential stream
1147 */
1148 public static<T> Stream<T> empty() {
1149 return StreamSupport.stream(Spliterators.<T>emptySpliterator(), false);
1150 }
1151
1152 /**
1153 * Returns a sequential {@code Stream} containing a single element.
1154 *
1155 * @param t the single element
1156 * @param <T> the type of stream elements
1157 * @return a singleton sequential stream
1158 */
1159 public static<T> Stream<T> of(T t) {
1160 return StreamSupport.stream(new Streams.StreamBuilderImpl<>(t), false);
1161 }
1162
1163 /**
1164 * Returns a sequential {@code Stream} containing a single element, if
1165 * non-null, otherwise returns an empty {@code Stream}.
1166 *
1167 * @param t the single element
1168 * @param <T> the type of stream elements
1169 * @return a stream with a single element if the specified element
1170 * is non-null, otherwise an empty stream
1171 * @since 9
1172 */
1173 public static<T> Stream<T> ofNullable(T t) {
1174 return t == null ? Stream.empty()
1175 : StreamSupport.stream(new Streams.StreamBuilderImpl<>(t), false);
1176 }
1177
1178 /**
1179 * Returns a sequential ordered stream whose elements are the specified values.
1180 *
1181 * @param <T> the type of stream elements
1182 * @param values the elements of the new stream
1183 * @return the new stream
1184 */
1185 @SafeVarargs
1186 @SuppressWarnings("varargs") // Creating a stream from an array is safe
1187 public static<T> Stream<T> of(T... values) {
1188 return Arrays.stream(values);
1189 }
1190
1191 /**
1192 * Returns an infinite sequential ordered {@code Stream} produced by iterative
1193 * application of a function {@code f} to an initial element {@code seed},
1194 * producing a {@code Stream} consisting of {@code seed}, {@code f(seed)},
1195 * {@code f(f(seed))}, etc.
1196 *
1197 * <p>The first element (position {@code 0}) in the {@code Stream} will be
1198 * the provided {@code seed}. For {@code n > 0}, the element at position
1199 * {@code n}, will be the result of applying the function {@code f} to the
1200 * element at position {@code n - 1}.
1201 *
1202 * <p>The action of applying {@code f} for one element
1203 * <a href="../concurrent/package-summary.html#MemoryVisibility"><i>happens-before</i></a>
1204 * the action of applying {@code f} for subsequent elements. For any given
1205 * element the action may be performed in whatever thread the library
1206 * chooses.
1207 *
1208 * @param <T> the type of stream elements
1209 * @param seed the initial element
1210 * @param f a function to be applied to the previous element to produce
1211 * a new element
1212 * @return a new sequential {@code Stream}
1213 */
1214 public static<T> Stream<T> iterate(final T seed, final UnaryOperator<T> f) {
1215 Objects.requireNonNull(f);
1216 Spliterator<T> spliterator = new Spliterators.AbstractSpliterator<>(Long.MAX_VALUE,
1217 Spliterator.ORDERED | Spliterator.IMMUTABLE) {
1218 T prev;
1219 boolean started;
1220
1221 @Override
1222 public boolean tryAdvance(Consumer<? super T> action) {
1223 Objects.requireNonNull(action);
1224 T t;
1225 if (started)
1226 t = f.apply(prev);
1227 else {
1228 t = seed;
1229 started = true;
1230 }
1231 action.accept(prev = t);
1232 return true;
1233 }
1234 };
1235 return StreamSupport.stream(spliterator, false);
1236 }
1237
1238 /**
1239 * Returns a sequential ordered {@code Stream} produced by iterative
1240 * application of the given {@code next} function to an initial element,
1241 * conditioned on satisfying the given {@code hasNext} predicate. The
1242 * stream terminates as soon as the {@code hasNext} predicate returns false.
1243 *
1244 * <p>{@code Stream.iterate} should produce the same sequence of elements as
1245 * produced by the corresponding for-loop:
1246 * <pre>{@code
1247 * for (T index=seed; hasNext.test(index); index = next.apply(index)) {
1248 * ...
1249 * }
1250 * }</pre>
1251 *
1252 * <p>The resulting sequence may be empty if the {@code hasNext} predicate
1253 * does not hold on the seed value. Otherwise the first element will be the
1254 * supplied {@code seed} value, the next element (if present) will be the
1255 * result of applying the {@code next} function to the {@code seed} value,
1256 * and so on iteratively until the {@code hasNext} predicate indicates that
1257 * the stream should terminate.
1258 *
1259 * <p>The action of applying the {@code hasNext} predicate to an element
1260 * <a href="../concurrent/package-summary.html#MemoryVisibility"><i>happens-before</i></a>
1261 * the action of applying the {@code next} function to that element. The
1262 * action of applying the {@code next} function for one element
1263 * <i>happens-before</i> the action of applying the {@code hasNext}
1264 * predicate for subsequent elements. For any given element an action may
1265 * be performed in whatever thread the library chooses.
1266 *
1267 * @param <T> the type of stream elements
1268 * @param seed the initial element
1269 * @param hasNext a predicate to apply to elements to determine when the
1270 * stream must terminate.
1271 * @param next a function to be applied to the previous element to produce
1272 * a new element
1273 * @return a new sequential {@code Stream}
1274 * @since 9
1275 */
1276 public static<T> Stream<T> iterate(T seed, Predicate<? super T> hasNext, UnaryOperator<T> next) {
1277 Objects.requireNonNull(next);
1278 Objects.requireNonNull(hasNext);
1279 Spliterator<T> spliterator = new Spliterators.AbstractSpliterator<>(Long.MAX_VALUE,
1280 Spliterator.ORDERED | Spliterator.IMMUTABLE) {
1281 T prev;
1282 boolean started, finished;
1283
1284 @Override
1285 public boolean tryAdvance(Consumer<? super T> action) {
1286 Objects.requireNonNull(action);
1287 if (finished)
1288 return false;
1289 T t;
1290 if (started)
1291 t = next.apply(prev);
1292 else {
1293 t = seed;
1294 started = true;
1295 }
1296 if (!hasNext.test(t)) {
1297 prev = null;
1298 finished = true;
1299 return false;
1300 }
1301 action.accept(prev = t);
1302 return true;
1303 }
1304
1305 @Override
1306 public void forEachRemaining(Consumer<? super T> action) {
1307 Objects.requireNonNull(action);
1308 if (finished)
1309 return;
1310 finished = true;
1311 T t = started ? next.apply(prev) : seed;
1312 prev = null;
1313 while (hasNext.test(t)) {
1314 action.accept(t);
1315 t = next.apply(t);
1316 }
1317 }
1318 };
1319 return StreamSupport.stream(spliterator, false);
1320 }
1321
1322 /**
1323 * Returns an infinite sequential unordered stream where each element is
1324 * generated by the provided {@code Supplier}. This is suitable for
1325 * generating constant streams, streams of random elements, etc.
1326 *
1327 * @param <T> the type of stream elements
1328 * @param s the {@code Supplier} of generated elements
1329 * @return a new infinite sequential unordered {@code Stream}
1330 */
1331 public static<T> Stream<T> generate(Supplier<? extends T> s) {
1332 Objects.requireNonNull(s);
1333 return StreamSupport.stream(
1334 new StreamSpliterators.InfiniteSupplyingSpliterator.OfRef<>(Long.MAX_VALUE, s), false);
1335 }
1336
1337 /**
1338 * Creates a lazily concatenated stream whose elements are all the
1339 * elements of the first stream followed by all the elements of the
1340 * second stream. The resulting stream is ordered if both
1341 * of the input streams are ordered, and parallel if either of the input
1342 * streams is parallel. When the resulting stream is closed, the close
1343 * handlers for both input streams are invoked.
1344 *
1345 * <p>This method operates on the two input streams and binds each stream
1346 * to its source. As a result subsequent modifications to an input stream
1347 * source may not be reflected in the concatenated stream result.
1348 *
1349 * @implNote
1350 * Use caution when constructing streams from repeated concatenation.
1351 * Accessing an element of a deeply concatenated stream can result in deep
1352 * call chains, or even {@code StackOverflowError}.
1353 *
1354 * <p>Subsequent changes to the sequential/parallel execution mode of the
1355 * returned stream are not guaranteed to be propagated to the input streams.
1356 *
1357 * @apiNote
1358 * To preserve optimization opportunities this method binds each stream to
1359 * its source and accepts only two streams as parameters. For example, the
1360 * exact size of the concatenated stream source can be computed if the exact
1361 * size of each input stream source is known.
1362 * To concatenate more streams without binding, or without nested calls to
1363 * this method, try creating a stream of streams and flat-mapping with the
1364 * identity function, for example:
1365 * <pre>{@code
1366 * Stream<T> concat = Stream.of(s1, s2, s3, s4).flatMap(s -> s);
1367 * }</pre>
1368 *
1369 * @param <T> The type of stream elements
1370 * @param a the first stream
1371 * @param b the second stream
1372 * @return the concatenation of the two input streams
1373 */
1374 public static <T> Stream<T> concat(Stream<? extends T> a, Stream<? extends T> b) {
1375 Objects.requireNonNull(a);
1376 Objects.requireNonNull(b);
1377
1378 @SuppressWarnings("unchecked")
1379 Spliterator<T> split = new Streams.ConcatSpliterator.OfRef<>(
1380 (Spliterator<T>) a.spliterator(), (Spliterator<T>) b.spliterator());
1381 Stream<T> stream = StreamSupport.stream(split, a.isParallel() || b.isParallel());
1382 return stream.onClose(Streams.composedClose(a, b));
1383 }
1384
1385 /**
1386 * A mutable builder for a {@code Stream}. This allows the creation of a
1387 * {@code Stream} by generating elements individually and adding them to the
1388 * {@code Builder} (without the copying overhead that comes from using
1389 * an {@code ArrayList} as a temporary buffer.)
1390 *
1391 * <p>A stream builder has a lifecycle, which starts in a building
1392 * phase, during which elements can be added, and then transitions to a built
1393 * phase, after which elements may not be added. The built phase begins
1394 * when the {@link #build()} method is called, which creates an ordered
1395 * {@code Stream} whose elements are the elements that were added to the stream
1396 * builder, in the order they were added.
1397 *
1398 * @param <T> the type of stream elements
1399 * @see Stream#builder()
1400 * @since 1.8
1401 */
1402 public interface Builder<T> extends Consumer<T> {
1403
1404 /**
1405 * Adds an element to the stream being built.
1406 *
1407 * @throws IllegalStateException if the builder has already transitioned to
1408 * the built state
1409 */
1410 @Override
1411 void accept(T t);
1412
1413 /**
1414 * Adds an element to the stream being built.
1415 *
1416 * @implSpec
1417 * The default implementation behaves as if:
1418 * <pre>{@code
1419 * accept(t)
1420 * return this;
1421 * }</pre>
1422 *
1423 * @param t the element to add
1424 * @return {@code this} builder
1425 * @throws IllegalStateException if the builder has already transitioned to
1426 * the built state
1427 */
1428 default Builder<T> add(T t) {
1429 accept(t);
1430 return this;
1431 }
1432
1433 /**
1434 * Builds the stream, transitioning this builder to the built state.
1435 * An {@code IllegalStateException} is thrown if there are further attempts
1436 * to operate on the builder after it has entered the built state.
1437 *
1438 * @return the built stream
1439 * @throws IllegalStateException if the builder has already transitioned to
1440 * the built state
1441 */
1442 Stream<T> build();
1443
1444 }
1445 }