1 /*
2 * Copyright (c) 2012, 2013, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation. Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
17 * You should have received a copy of the GNU General Public License version
18 * 2 along with this work; if not, write to the Free Software Foundation,
19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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23 * questions.
24 */
25 package java.util.stream;
26
27 import java.nio.charset.Charset;
28 import java.nio.file.Files;
29 import java.nio.file.Path;
30 import java.util.Arrays;
31 import java.util.Collection;
32 import java.util.Comparator;
33 import java.util.Iterator;
34 import java.util.Objects;
35 import java.util.Optional;
36 import java.util.Spliterator;
37 import java.util.Spliterators;
38 import java.util.concurrent.ConcurrentHashMap;
39 import java.util.function.BiConsumer;
40 import java.util.function.BiFunction;
41 import java.util.function.BinaryOperator;
42 import java.util.function.Consumer;
43 import java.util.function.Function;
44 import java.util.function.IntFunction;
45 import java.util.function.Predicate;
46 import java.util.function.Supplier;
47 import java.util.function.ToDoubleFunction;
48 import java.util.function.ToIntFunction;
49 import java.util.function.ToLongFunction;
50 import java.util.function.UnaryOperator;
51
52 /**
53 * A sequence of elements supporting sequential and parallel aggregate
54 * operations. The following example illustrates an aggregate operation using
55 * {@link Stream} and {@link IntStream}:
56 *
57 * <pre>{@code
58 * int sum = widgets.stream()
59 * .filter(w -> w.getColor() == RED)
60 * .mapToInt(w -> w.getWeight())
61 * .sum();
62 * }</pre>
63 *
64 * In this example, {@code widgets} is a {@code Collection<Widget>}. We create
65 * a stream of {@code Widget} objects via {@link Collection#stream Collection.stream()},
66 * filter it to produce a stream containing only the red widgets, and then
67 * transform it into a stream of {@code int} values representing the weight of
68 * each red widget. Then this stream is summed to produce a total weight.
69 *
70 * <p>To perform a computation, stream
71 * <a href="package-summary.html#StreamOps">operations</a> are composed into a
72 * <em>stream pipeline</em>. A stream pipeline consists of a source (which
73 * might be an array, a collection, a generator function, an I/O channel,
74 * etc), zero or more <em>intermediate operations</em> (which transform a
75 * stream into another stream, such as {@link Stream#filter(Predicate)}), and a
76 * <em>terminal operation</em> (which produces a result or side-effect, such
77 * as {@link Stream#count()} or {@link Stream#forEach(Consumer)}).
78 * Streams are lazy; computation on the source data is only performed when the
79 * terminal operation is initiated, and source elements are consumed only
80 * as needed.
81 *
82 * <p>Collections and streams, while bearing some superficial similarities,
83 * have different goals. Collections are primarily concerned with the efficient
84 * management of, and access to, their elements. By contrast, streams do not
85 * provide a means to directly access or manipulate their elements, and are
86 * instead concerned with declaratively describing their source and the
87 * computational operations which will be performed in aggregate on that source.
88 * However, if the provided stream operations do not offer the desired
89 * functionality, the {@link #iterator()} and {@link #spliterator()} operations
90 * can be used to perform a controlled traversal.
91 *
92 * <p>A stream pipeline, like the "widgets" example above, can be viewed as
93 * a <em>query</em> on the stream source. Unless the source was explicitly
94 * designed for concurrent modification (such as a {@link ConcurrentHashMap}),
95 * unpredictable or erroneous behavior may result from modifying the stream
96 * source while it is being queried.
97 *
98 * <p>Most stream operations accept parameters that describe user-specified
99 * behavior, such as the lambda expression {@code w -> w.getWeight()} passed to
100 * {@code mapToInt} in the example above. Such parameters are always instances
101 * of a <a href="../function/package-summary.html">functional interface</a> such
102 * as {@link java.util.function.Function}, and are often lambda expressions or
103 * method references. These parameters can never be null, should not modify the
104 * stream source, and should be
105 * <a href="package-summary.html#NonInterference">effectively stateless</a>
106 * (their result should not depend on any state that might change during
107 * execution of the stream pipeline.)
108 *
109 * <p>A stream should be operated on (invoking an intermediate or terminal stream
110 * operation) only once. This rules out, for example, "forked" streams, where
111 * the same source feeds two or more pipelines, or multiple traversals of the
112 * same stream. A stream implementation may throw {@link IllegalStateException}
113 * if it detects that the stream is being reused. However, since some stream
114 * operations may return their receiver rather than a new stream object, it may
115 * not be possible to detect reuse in all cases.
116 *
117 * <p>Streams have a {@link #close()} method and implement {@link AutoCloseable},
118 * but nearly all stream instances do not actually need to be closed after use.
119 * Generally, only streams whose source is an IO channel (such as those returned
120 * by {@link Files#lines(Path, Charset)}) will require closing. Most streams
121 * are backed by collections, arrays, or generating functions, which require no
122 * special resource management. (If a stream does require closing, it can be
123 * declared as a resource in a {@code try}-with-resources statement.)
124 *
125 * <p>Stream pipelines may execute either sequentially or in
126 * <a href="package-summary.html#Parallelism">parallel</a>. This
127 * execution mode is a property of the stream. Streams are created
128 * with an initial choice of sequential or parallel execution. (For example,
129 * {@link Collection#stream() Collection.stream()} creates a sequential stream,
130 * and {@link Collection#parallelStream() Collection.parallelStream()} creates
131 * a parallel one.) This choice of execution mode may be modified by the
132 * {@link #sequential()} or {@link #parallel()} methods, and may be queried with
133 * the {@link #isParallel()} method.
134 *
135 * @param <T> the type of the stream elements
136 * @since 1.8
137 * @see <a href="package-summary.html">java.util.stream</a>
138 */
139 public interface Stream<T> extends BaseStream<T, Stream<T>> {
140
141 /**
142 * Returns a stream consisting of the elements of this stream that match
143 * the given predicate.
144 *
145 * <p>This is an <a href="package-summary.html#StreamOps">intermediate
146 * operation</a>.
147 *
148 * @param predicate a <a href="package-summary.html#NonInterference">
149 * non-interfering, stateless</a> predicate to apply to
150 * each element to determine if it should be included
151 * @return the new stream
152 */
153 Stream<T> filter(Predicate<? super T> predicate);
154
155 /**
156 * Returns a stream consisting of the results of applying the given
157 * function to the elements of this stream.
158 *
159 * <p>This is an <a href="package-summary.html#StreamOps">intermediate
160 * operation</a>.
161 *
162 * @param <R> The element type of the new stream
163 * @param mapper a <a href="package-summary.html#NonInterference">
164 * non-interfering, stateless</a> function to apply to each
165 * element
166 * @return the new stream
167 */
168 <R> Stream<R> map(Function<? super T, ? extends R> mapper);
169
170 /**
171 * Returns an {@code IntStream} consisting of the results of applying the
172 * given function to the elements of this stream.
173 *
174 * <p>This is an <a href="package-summary.html#StreamOps">
175 * intermediate operation</a>.
176 *
177 * @param mapper a <a href="package-summary.html#NonInterference">
178 * non-interfering, stateless</a> function to apply to each
179 * element
180 * @return the new stream
181 */
182 IntStream mapToInt(ToIntFunction<? super T> mapper);
183
184 /**
185 * Returns a {@code LongStream} consisting of the results of applying the
186 * given 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 mapper a <a href="package-summary.html#NonInterference">
192 * non-interfering, stateless</a> function to apply to each
193 * element
194 * @return the new stream
195 */
196 LongStream mapToLong(ToLongFunction<? super T> mapper);
197
198 /**
199 * Returns a {@code DoubleStream} consisting of the results of applying the
200 * given function to the elements of this stream.
201 *
202 * <p>This is an <a href="package-summary.html#StreamOps">intermediate
203 * operation</a>.
204 *
205 * @param mapper a <a href="package-summary.html#NonInterference">
206 * non-interfering, stateless</a> function to apply to each
207 * element
208 * @return the new stream
209 */
210 DoubleStream mapToDouble(ToDoubleFunction<? super T> mapper);
211
212 /**
213 * Returns a stream consisting of the results of replacing each element of
214 * this stream with the contents of the stream produced by applying the
215 * provided mapping function to each element. (If the result of the mapping
216 * function is {@code null}, this is treated as if the result was an empty
217 * stream.)
218 *
219 * <p>This is an <a href="package-summary.html#StreamOps">intermediate
220 * operation</a>.
221 *
222 * @apiNote
223 * The {@code flatMap()} operation has the effect of applying a one-to-many
224 * tranformation to the elements of the stream, and then flattening the
225 * resulting elements into a new stream. For example, if {@code orders}
226 * is a stream of purchase orders, and each purchase order contains a
227 * collection of line items, then the following produces a stream of line
228 * items:
229 * <pre>{@code
230 * orderStream.flatMap(order -> order.getLineItems().stream())...
231 * }</pre>
232 *
233 * @param <R> The element type of the new stream
234 * @param mapper a <a href="package-summary.html#NonInterference">
235 * non-interfering, stateless</a> function to apply to each
236 * element which produces a stream of new values
237 * @return the new stream
238 */
239 <R> Stream<R> flatMap(Function<? super T, ? extends Stream<? extends R>> mapper);
240
241 /**
242 * Returns an {@code IntStream} consisting of the results of replacing each
243 * element of this stream with the contents of the stream produced by
244 * applying the provided mapping function to each element. (If the result
245 * of the mapping function is {@code null}, this is treated as if the result
246 * was an empty stream.)
247 *
248 * <p>This is an <a href="package-summary.html#StreamOps">intermediate
249 * operation</a>.
250 *
251 * @param mapper a <a href="package-summary.html#NonInterference">
252 * non-interfering, stateless</a> function to apply to each
253 * element which produces a stream of new values
254 * @return the new stream
255 */
256 IntStream flatMapToInt(Function<? super T, ? extends IntStream> mapper);
257
258 /**
259 * Returns a {@code LongStream} consisting of the results of replacing each
260 * element of this stream with the contents of the stream produced
261 * by applying the provided mapping function to each element. (If the result
262 * of the mapping function is {@code null}, this is treated as if the result
263 * was an empty stream.)
264 *
265 * <p>This is an <a href="package-summary.html#StreamOps">intermediate
266 * operation</a>.
267 *
268 * @param mapper a <a href="package-summary.html#NonInterference">
269 * non-interfering, stateless</a> function to apply to
270 * each element which produces a stream of new values
271 * @return the new stream
272 */
273 LongStream flatMapToLong(Function<? super T, ? extends LongStream> mapper);
274
275 /**
276 * Returns a {@code DoubleStream} consisting of the results of replacing each
277 * element of this stream with the contents of the stream produced
278 * by applying the provided mapping function to each element. (If the result
279 * of the mapping function is {@code null}, this is treated as if the result
280 * was an empty stream.)
281 *
282 * <p>This is an <a href="package-summary.html#StreamOps">intermediate
283 * operation</a>.
284 *
285 * @param mapper a <a href="package-summary.html#NonInterference">
286 * non-interfering, stateless</a> function to apply to each
287 * element which produces a stream of new values
288 * @return the new stream
289 */
290 DoubleStream flatMapToDouble(Function<? super T, ? extends DoubleStream> mapper);
291
292 /**
293 * Returns a stream consisting of the distinct elements (according to
294 * {@link Object#equals(Object)}) of this stream.
295 *
296 * <p>This is a <a href="package-summary.html#StreamOps">stateful
297 * intermediate operation</a>.
298 *
299 * @return the new stream
300 */
301 Stream<T> distinct();
302
303 /**
304 * Returns a stream consisting of the elements of this stream, sorted
305 * according to natural order. If the elements of this stream are not
306 * {@code Comparable}, a {@code java.lang.ClassCastException} may be thrown
307 * when the terminal operation is executed.
308 *
309 * <p>This is a <a href="package-summary.html#StreamOps">stateful
310 * intermediate operation</a>.
311 *
312 * @return the new stream
313 */
314 Stream<T> sorted();
315
316 /**
317 * Returns a stream consisting of the elements of this stream, sorted
318 * according to the provided {@code Comparator}.
319 *
320 * <p>This is a <a href="package-summary.html#StreamOps">stateful
321 * intermediate operation</a>.
322 *
323 * @param comparator a <a href="package-summary.html#NonInterference">
324 * non-interfering, stateless</a> {@code Comparator} to
325 * be used to compare stream elements
326 * @return the new stream
327 */
328 Stream<T> sorted(Comparator<? super T> comparator);
329
330 /**
331 * Returns a stream consisting of the elements of this stream, additionally
332 * performing the provided action on each element as elements are consumed
333 * from the resulting stream.
334 *
335 * <p>This is an <a href="package-summary.html#StreamOps">intermediate
336 * operation</a>.
337 *
338 * <p>For parallel stream pipelines, the action may be called at
339 * whatever time and in whatever thread the element is made available by the
340 * upstream operation. If the action modifies shared state,
341 * it is responsible for providing the required synchronization.
342 *
343 * @apiNote This method exists mainly to support debugging, where you want
344 * to see the elements as they flow past a certain point in a pipeline:
345 * <pre>{@code
346 * list.stream()
347 * .filter(filteringFunction)
348 * .peek(e -> System.out.println("Filtered value: " + e));
349 * .map(mappingFunction)
350 * .peek(e -> System.out.println("Mapped value: " + e));
351 * .collect(Collectors.intoList());
352 * }</pre>
353 *
354 * @param action a <a href="package-summary.html#NonInterference">
355 * non-interfering</a> action to perform on the elements as
356 * they are consumed from the stream
357 * @return the new stream
358 */
359 Stream<T> peek(Consumer<? super T> action);
360
361 /**
362 * Returns a stream consisting of the elements of this stream, truncated
363 * to be no longer than {@code maxSize} in length.
364 *
365 * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
366 * stateful intermediate operation</a>.
367 *
368 * @param maxSize the number of elements the stream should be limited to
369 * @return the new stream
370 * @throws IllegalArgumentException if {@code maxSize} is negative
371 */
372 Stream<T> limit(long maxSize);
373
374 /**
375 * Returns a stream consisting of the remaining elements of this stream
376 * after discarding the first {@code startInclusive} elements of the stream.
377 * If this stream contains fewer than {@code startInclusive} elements then an
378 * empty stream will be returned.
379 *
380 * <p>This is a <a href="package-summary.html#StreamOps">stateful
381 * intermediate operation</a>.
382 *
383 * @param startInclusive the number of leading elements to skip
384 * @return the new stream
385 * @throws IllegalArgumentException if {@code startInclusive} is negative
386 */
387 Stream<T> substream(long startInclusive);
388
389 /**
390 * Returns a stream consisting of the remaining elements of this stream
391 * after discarding the first {@code startInclusive} elements and truncating
392 * the result to be no longer than {@code endExclusive - startInclusive}
393 * elements in length. If this stream contains fewer than
394 * {@code startInclusive} elements then an empty stream will be returned.
395 *
396 * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
397 * stateful intermediate operation</a>.
398 *
399 * @param startInclusive the starting position of the substream, inclusive
400 * @param endExclusive the ending position of the substream, exclusive
401 * @return the new stream
402 * @throws IllegalArgumentException if {@code startInclusive} or
403 * {@code endExclusive} is negative or {@code startInclusive} is greater
404 * than {@code endExclusive}
405 */
406 Stream<T> substream(long startInclusive, long endExclusive);
407
408 /**
409 * Performs an action for each element of this stream.
410 *
411 * <p>This is a <a href="package-summary.html#StreamOps">terminal
412 * operation</a>.
413 *
414 * <p>For parallel stream pipelines, this operation does <em>not</em>
415 * guarantee to respect the encounter order of the stream, as doing so
416 * would sacrifice the benefit of parallelism. For any given element, the
417 * action may be performed at whatever time and in whatever thread the
418 * library chooses. If the action accesses shared state, it is
419 * responsible for providing the required synchronization.
420 *
421 * @param action a <a href="package-summary.html#NonInterference">
422 * non-interfering</a> action to perform on the elements
423 */
424 void forEach(Consumer<? super T> action);
425
426 /**
427 * Performs an action for each element of this stream, guaranteeing that
428 * each element is processed in encounter order for streams that have a
429 * defined encounter order.
430 *
431 * <p>This is a <a href="package-summary.html#StreamOps">terminal
432 * operation</a>.
433 *
434 * @param action a <a href="package-summary.html#NonInterference">
435 * non-interfering</a> action to perform on the elements
436 * @see #forEach(Consumer)
437 */
438 void forEachOrdered(Consumer<? super T> action);
439
440 /**
441 * Returns an array containing the elements of this stream.
442 *
443 * <p>This is a <a href="package-summary.html#StreamOps">terminal
444 * operation</a>.
445 *
446 * @return an array containing the elements of this stream
447 */
448 Object[] toArray();
449
450 /**
451 * Returns an array containing the elements of this stream, using the
452 * provided {@code generator} function to allocate the returned array, as
453 * well as any additional arrays that might be required for a partitioned
454 * execution or for resizing.
455 *
456 * <p>This is a <a href="package-summary.html#StreamOps">terminal
457 * operation</a>.
458 *
459 * @apiNote
460 * The generator function takes an integer, which is the size of the
461 * desired array, and produces an array of the desired size. This can be
462 * concisely expressed with an array constructor reference:
463 * <pre>{@code
464 * Person[] men = people.stream()
465 * .filter(p -> p.getGender() == MALE)
466 * .toArray(Person[]::new);
467 * }</pre>
468 *
469 * @param <A> the element type of the resulting array
470 * @param generator a function which produces a new array of the desired
471 * type and the provided length
472 * @return an array containing the elements in this stream
473 * @throws ArrayStoreException if the runtime type of the array returned
474 * from the array generator is not a supertype of the runtime type of every
475 * element in this stream
476 */
477 <A> A[] toArray(IntFunction<A[]> generator);
478
479 /**
480 * Performs a <a href="package-summary.html#Reduction">reduction</a> on the
481 * elements of this stream, using the provided identity value and an
482 * <a href="package-summary.html#Associativity">associative</a>
483 * accumulation function, and returns the reduced value. This is equivalent
484 * to:
485 * <pre>{@code
486 * T result = identity;
487 * for (T element : this stream)
488 * result = accumulator.apply(result, element)
489 * return result;
490 * }</pre>
491 *
492 * but is not constrained to execute sequentially.
493 *
494 * <p>The {@code identity} value must be an identity for the accumulator
495 * function. This means that for all {@code t},
496 * {@code accumulator.apply(identity, t)} is equal to {@code t}.
497 * The {@code accumulator} function must be an
498 * <a href="package-summary.html#Associativity">associative</a> function.
499 *
500 * <p>This is a <a href="package-summary.html#StreamOps">terminal
501 * operation</a>.
502 *
503 * @apiNote Sum, min, max, average, and string concatenation are all special
504 * cases of reduction. Summing a stream of numbers can be expressed as:
505 *
506 * <pre>{@code
507 * Integer sum = integers.reduce(0, (a, b) -> a+b);
508 * }</pre>
509 *
510 * or:
511 *
512 * <pre>{@code
513 * Integer sum = integers.reduce(0, Integer::sum);
514 * }</pre>
515 *
516 * <p>While this may seem a more roundabout way to perform an aggregation
517 * compared to simply mutating a running total in a loop, reduction
518 * operations parallelize more gracefully, without needing additional
519 * synchronization and with greatly reduced risk of data races.
520 *
521 * @param identity the identity value for the accumulating function
522 * @param accumulator an <a href="package-summary.html#Associativity">associative</a>
523 * <a href="package-summary.html#NonInterference">non-interfering,
524 * stateless</a> function for combining two values
525 * @return the result of the reduction
526 */
527 T reduce(T identity, BinaryOperator<T> accumulator);
528
529 /**
530 * Performs a <a href="package-summary.html#Reduction">reduction</a> on the
531 * elements of this stream, using an
532 * <a href="package-summary.html#Associativity">associative</a> accumulation
533 * function, and returns an {@code Optional} describing the reduced value,
534 * if any. This is equivalent to:
535 * <pre>{@code
536 * boolean foundAny = false;
537 * T result = null;
538 * for (T element : this stream) {
539 * if (!foundAny) {
540 * foundAny = true;
541 * result = element;
542 * }
543 * else
544 * result = accumulator.apply(result, element);
545 * }
546 * return foundAny ? Optional.of(result) : Optional.empty();
547 * }</pre>
548 *
549 * but is not constrained to execute sequentially.
550 *
551 * <p>The {@code accumulator} function must be an
552 * <a href="package-summary.html#Associativity">associative</a> function.
553 *
554 * <p>This is a <a href="package-summary.html#StreamOps">terminal
555 * operation</a>.
556 *
557 * @param accumulator an <a href="package-summary.html#Associativity">associative</a>
558 * <a href="package-summary.html#NonInterference">non-interfering,
559 * stateless</a> function for combining two values
560 * @return an {@link Optional} describing the result of the reduction
561 * @throws NullPointerException if the result of the reduction is null
562 * @see #reduce(Object, BinaryOperator)
563 * @see #min(java.util.Comparator)
564 * @see #max(java.util.Comparator)
565 */
566 Optional<T> reduce(BinaryOperator<T> accumulator);
567
568 /**
569 * Performs a <a href="package-summary.html#Reduction">reduction</a> on the
570 * elements of this stream, using the provided identity, accumulation and
571 * combining functions. This is equivalent to:
572 * <pre>{@code
573 * U result = identity;
574 * for (T element : this stream)
575 * result = accumulator.apply(result, element)
576 * return result;
577 * }</pre>
578 *
579 * but is not constrained to execute sequentially.
580 *
581 * <p>The {@code identity} value must be an identity for the combiner
582 * function. This means that for all {@code u}, {@code combiner(identity, u)}
583 * is equal to {@code u}. Additionally, the {@code combiner} function
584 * must be compatible with the {@code accumulator} function; for all
585 * {@code u} and {@code t}, the following must hold:
586 * <pre>{@code
587 * combiner.apply(u, accumulator.apply(identity, t)) == accumulator.apply(u, t)
588 * }</pre>
589 *
590 * <p>This is a <a href="package-summary.html#StreamOps">terminal
591 * operation</a>.
592 *
593 * @apiNote Many reductions using this form can be represented more simply
594 * by an explicit combination of {@code map} and {@code reduce} operations.
595 * The {@code accumulator} function acts as a fused mapper and accumulator,
596 * which can sometimes be more efficient than separate mapping and reduction,
597 * such as when knowing the previously reduced value allows you to avoid
598 * some computation.
599 *
600 * @param <U> The type of the result
601 * @param identity the identity value for the combiner function
602 * @param accumulator an <a href="package-summary.html#Associativity">associative</a>
603 * <a href="package-summary.html#NonInterference">non-interfering,
604 * stateless</a> function for incorporating an additional
605 * element into a result
606 * @param combiner an <a href="package-summary.html#Associativity">associative</a>
607 * <a href="package-summary.html#NonInterference">non-interfering,
608 * stateless</a> function for combining two values, which
609 * must be compatible with the accumulator function
610 * @return the result of the reduction
611 * @see #reduce(BinaryOperator)
612 * @see #reduce(Object, BinaryOperator)
613 */
614 <U> U reduce(U identity,
615 BiFunction<U, ? super T, U> accumulator,
616 BinaryOperator<U> combiner);
617
618 /**
619 * Performs a <a href="package-summary.html#MutableReduction">mutable
620 * reduction</a> operation on the elements of this stream. A mutable
621 * reduction is one in which the reduced value is a mutable result container,
622 * such as an {@code ArrayList}, and elements are incorporated by updating
623 * the state of the result rather than by replacing the result. This
624 * produces a result equivalent to:
625 * <pre>{@code
626 * R result = supplier.get();
627 * for (T element : this stream)
628 * accumulator.accept(result, element);
629 * return result;
630 * }</pre>
631 *
632 * <p>Like {@link #reduce(Object, BinaryOperator)}, {@code collect} operations
633 * can be parallelized without requiring additional synchronization.
634 *
635 * <p>This is a <a href="package-summary.html#StreamOps">terminal
636 * operation</a>.
637 *
638 * @apiNote There are many existing classes in the JDK whose signatures are
639 * well-suited for use with method references as arguments to {@code collect()}.
640 * For example, the following will accumulate strings into an {@code ArrayList}:
641 * <pre>{@code
642 * List<String> asList = stringStream.collect(ArrayList::new, ArrayList::add,
643 * ArrayList::addAll);
644 * }</pre>
645 *
646 * <p>The following will take a stream of strings and concatenates them into a
647 * single string:
648 * <pre>{@code
649 * String concat = stringStream.collect(StringBuilder::new, StringBuilder::append,
650 * StringBuilder::append)
651 * .toString();
652 * }</pre>
653 *
654 * @param <R> type of the result
655 * @param supplier a function that creates a new result container. For a
656 * parallel execution, this function may be called
657 * multiple times and must return a fresh value each time.
658 * @param accumulator an <a href="package-summary.html#Associativity">associative</a>
659 * <a href="package-summary.html#NonInterference">non-interfering,
660 * stateless</a> function for incorporating an additional
661 * element into a result
662 * @param combiner an <a href="package-summary.html#Associativity">associative</a>
663 * <a href="package-summary.html#NonInterference">non-interfering,
664 * stateless</a> function for combining two values, which
665 * must be compatible with the accumulator function
666 * @return the result of the reduction
667 */
668 <R> R collect(Supplier<R> supplier,
669 BiConsumer<R, ? super T> accumulator,
670 BiConsumer<R, R> combiner);
671
672 /**
673 * Performs a <a href="package-summary.html#MutableReduction">mutable
674 * reduction</a> operation on the elements of this stream using a
675 * {@code Collector}. A {@code Collector}
676 * encapsulates the functions used as arguments to
677 * {@link #collect(Supplier, BiConsumer, BiConsumer)}, allowing for reuse of
678 * collection strategies and composition of collect operations such as
679 * multiple-level grouping or partitioning.
680 *
681 * <p>This is a <a href="package-summary.html#StreamOps">terminal
682 * operation</a>.
683 *
684 * <p>When executed in parallel, multiple intermediate results may be
685 * instantiated, populated, and merged so as to maintain isolation of
686 * mutable data structures. Therefore, even when executed in parallel
687 * with non-thread-safe data structures (such as {@code ArrayList}), no
688 * additional synchronization is needed for a parallel reduction.
689 *
690 * @apiNote
691 * The following will accumulate strings into an ArrayList:
692 * <pre>{@code
693 * List<String> asList = stringStream.collect(Collectors.toList());
694 * }</pre>
695 *
696 * <p>The following will classify {@code Person} objects by city:
697 * <pre>{@code
698 * Map<String, List<Person>> peopleByCity
699 * = personStream.collect(Collectors.groupingBy(Person::getCity));
700 * }</pre>
701 *
702 * <p>The following will classify {@code Person} objects by state and city,
703 * cascading two {@code Collector}s together:
704 * <pre>{@code
705 * Map<String, Map<String, List<Person>>> peopleByStateAndCity
706 * = personStream.collect(Collectors.groupingBy(Person::getState,
707 * Collectors.groupingBy(Person::getCity)));
708 * }</pre>
709 *
710 * @param <R> the type of the result
711 * @param <A> the intermediate accumulation type of the {@code Collector}
712 * @param collector the {@code Collector} describing the reduction
713 * @return the result of the reduction
714 * @see #collect(Supplier, BiConsumer, BiConsumer)
715 * @see Collectors
716 */
717 <R, A> R collect(Collector<? super T, A, R> collector);
718
719 /**
720 * Returns the minimum element of this stream according to the provided
721 * {@code Comparator}. This is a special case of a
722 * <a href="package-summary.html#Reduction">reduction</a>.
723 *
724 * <p>This is a <a href="package-summary.html#StreamOps">terminal operation</a>.
725 *
726 * @param comparator a <a href="package-summary.html#NonInterference">non-interfering,
727 * stateless</a> {@code Comparator} to use to compare
728 * elements of this stream
729 * @return an {@code Optional} describing the minimum element of this stream,
730 * or an empty {@code Optional} if the stream is empty
731 * @throws NullPointerException if the minimum element is null
732 */
733 Optional<T> min(Comparator<? super T> comparator);
734
735 /**
736 * Returns the maximum element of this stream according to the provided
737 * {@code Comparator}. This is a special case of a
738 * <a href="package-summary.html#Reduction">reduction</a>.
739 *
740 * <p>This is a <a href="package-summary.html#StreamOps">terminal
741 * operation</a>.
742 *
743 * @param comparator a <a href="package-summary.html#NonInterference">non-interfering,
744 * stateless</a> {@code Comparator} to use to compare
745 * elements of this stream
746 * @return an {@code Optional} describing the maximum element of this stream,
747 * or an empty {@code Optional} if the stream is empty
748 * @throws NullPointerException if the maximum element is null
749 */
750 Optional<T> max(Comparator<? super T> comparator);
751
752 /**
753 * Returns the count of elements in this stream. This is a special case of
754 * a <a href="package-summary.html#Reduction">reduction</a> and is
755 * equivalent to:
756 * <pre>{@code
757 * return mapToLong(e -> 1L).sum();
758 * }</pre>
759 *
760 * <p>This is a <a href="package-summary.html#StreamOps">terminal operation</a>.
761 *
762 * @return the count of elements in this stream
763 */
764 long count();
765
766 /**
767 * Returns whether any elements of this stream match the provided
768 * predicate. May not evaluate the predicate on all elements if not
769 * necessary for determining the result.
770 *
771 * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
772 * terminal operation</a>.
773 *
774 * @param predicate a <a href="package-summary.html#NonInterference">non-interfering,
775 * stateless</a> predicate to apply to elements of this
776 * stream
777 * @return {@code true} if any elements of the stream match the provided
778 * predicate otherwise {@code false}
779 */
780 boolean anyMatch(Predicate<? super T> predicate);
781
782 /**
783 * Returns whether all elements of this stream match the provided predicate.
784 * May not evaluate the predicate on all elements if not necessary for
785 * determining the result.
786 *
787 * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
788 * terminal operation</a>.
789 *
790 * @param predicate a <a href="package-summary.html#NonInterference">non-interfering,
791 * stateless</a> predicate to apply to elements of this
792 * stream
793 * @return {@code true} if all elements of the stream match the provided
794 * predicate otherwise {@code false}
795 */
796 boolean allMatch(Predicate<? super T> predicate);
797
798 /**
799 * Returns whether no elements of this stream match the provided predicate.
800 * May not evaluate the predicate on all elements if not necessary for
801 * determining the result.
802 *
803 * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
804 * terminal operation</a>.
805 *
806 * @param predicate a <a href="package-summary.html#NonInterference">non-interfering,
807 * stateless</a> predicate to apply to elements of this
808 * stream
809 * @return {@code true} if no elements of the stream match the provided
810 * predicate otherwise {@code false}
811 */
812 boolean noneMatch(Predicate<? super T> predicate);
813
814 /**
815 * Returns an {@link Optional} describing the first element of this stream,
816 * or an empty {@code Optional} if the stream is empty. If the stream has
817 * no encounter order, then any element may be returned.
818 *
819 * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
820 * terminal operation</a>.
821 *
822 * @return an {@code Optional} describing the first element of this stream,
823 * or an empty {@code Optional} if the stream is empty
824 * @throws NullPointerException if the element selected is null
825 */
826 Optional<T> findFirst();
827
828 /**
829 * Returns an {@link Optional} describing some element of the stream, or an
830 * empty {@code Optional} if the stream is empty.
831 *
832 * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
833 * terminal operation</a>.
834 *
835 * <p>The behavior of this operation is explicitly nondeterministic; it is
836 * free to select any element in the stream. This is to allow for maximal
837 * performance in parallel operations; the cost is that multiple invocations
838 * on the same source may not return the same result. (If a stable result
839 * is desired, use {@link #findFirst()} instead.)
840 *
841 * @return an {@code Optional} describing some element of this stream, or an
842 * empty {@code Optional} if the stream is empty
843 * @throws NullPointerException if the element selected is null
844 * @see #findFirst()
845 */
846 Optional<T> findAny();
847
848 // Static factories
849
850 /**
851 * Returns a builder for a {@code Stream}.
852 *
853 * @param <T> type of elements
854 * @return a stream builder
855 */
856 public static<T> Builder<T> builder() {
857 return new Streams.StreamBuilderImpl<>();
858 }
859
860 /**
861 * Returns an empty sequential {@code Stream}.
862 *
863 * @param <T> the type of stream elements
864 * @return an empty sequential stream
865 */
866 public static<T> Stream<T> empty() {
867 return StreamSupport.stream(Spliterators.<T>emptySpliterator(), false);
868 }
869
870 /**
871 * Returns a sequential {@code Stream} containing a single element.
872 *
873 * @param t the single element
874 * @param <T> the type of stream elements
875 * @return a singleton sequential stream
876 */
877 public static<T> Stream<T> of(T t) {
878 return StreamSupport.stream(new Streams.StreamBuilderImpl<>(t), false);
879 }
880
881 /**
882 * Returns a sequential ordered stream whose elements are the specified values.
883 *
884 * @param <T> the type of stream elements
885 * @param values the elements of the new stream
886 * @return the new stream
887 */
888 @SafeVarargs
889 @SuppressWarnings("varargs") // Creating a stream from an array is safe
890 public static<T> Stream<T> of(T... values) {
891 return Arrays.stream(values);
892 }
893
894 /**
895 * Returns an infinite sequential ordered {@code Stream} produced by iterative
896 * application of a function {@code f} to an initial element {@code seed},
897 * producing a {@code Stream} consisting of {@code seed}, {@code f(seed)},
898 * {@code f(f(seed))}, etc.
899 *
900 * <p>The first element (position {@code 0}) in the {@code Stream} will be
901 * the provided {@code seed}. For {@code n > 0}, the element at position
902 * {@code n}, will be the result of applying the function {@code f} to the
903 * element at position {@code n - 1}.
904 *
905 * @param <T> the type of stream elements
906 * @param seed the initial element
907 * @param f a function to be applied to to the previous element to produce
908 * a new element
909 * @return a new sequential {@code Stream}
910 */
911 public static<T> Stream<T> iterate(final T seed, final UnaryOperator<T> f) {
912 Objects.requireNonNull(f);
913 final Iterator<T> iterator = new Iterator<T>() {
914 @SuppressWarnings("unchecked")
915 T t = (T) Streams.NONE;
916
917 @Override
918 public boolean hasNext() {
919 return true;
920 }
921
922 @Override
923 public T next() {
924 return t = (t == Streams.NONE) ? seed : f.apply(t);
925 }
926 };
927 return StreamSupport.stream(Spliterators.spliteratorUnknownSize(
928 iterator,
929 Spliterator.ORDERED | Spliterator.IMMUTABLE), false);
930 }
931
932 /**
933 * Returns a sequential stream where each element is generated by
934 * the provided {@code Supplier}. This is suitable for generating
935 * constant streams, streams of random elements, etc.
936 *
937 * @param <T> the type of stream elements
938 * @param s the {@code Supplier} of generated elements
939 * @return a new sequential {@code Stream}
940 */
941 public static<T> Stream<T> generate(Supplier<T> s) {
942 Objects.requireNonNull(s);
943 return StreamSupport.stream(
944 new StreamSpliterators.InfiniteSupplyingSpliterator.OfRef<>(Long.MAX_VALUE, s), false);
945 }
946
947 /**
948 * Creates a lazily concatenated stream whose elements are all the
949 * elements of the first stream followed by all the elements of the
950 * second stream. The resulting stream is ordered if both
951 * of the input streams are ordered, and parallel if either of the input
952 * streams is parallel. When the resulting stream is closed, the close
953 * handlers for both input streams are invoked.
954 *
955 * @param <T> The type of stream elements
956 * @param a the first stream
957 * @param b the second stream
958 * @return the concatenation of the two input streams
959 */
960 public static <T> Stream<T> concat(Stream<? extends T> a, Stream<? extends T> b) {
961 Objects.requireNonNull(a);
962 Objects.requireNonNull(b);
963
964 @SuppressWarnings("unchecked")
965 Spliterator<T> split = new Streams.ConcatSpliterator.OfRef<>(
966 (Spliterator<T>) a.spliterator(), (Spliterator<T>) b.spliterator());
967 Stream<T> stream = StreamSupport.stream(split, a.isParallel() || b.isParallel());
968 return stream.onClose(Streams.composedClose(a, b));
969 }
970
971 /**
972 * A mutable builder for a {@code Stream}. This allows the creation of a
973 * {@code Stream} by generating elements individually and adding them to the
974 * {@code Builder} (without the copying overhead that comes from using
975 * an {@code ArrayList} as a temporary buffer.)
976 *
977 * <p>A stream builder has a lifecycle, which starts in a building
978 * phase, during which elements can be added, and then transitions to a built
979 * phase, after which elements may not be added. The built phase begins
980 * when the {@link #build()} method is called, which creates an ordered
981 * {@code Stream} whose elements are the elements that were added to the stream
982 * builder, in the order they were added.
983 *
984 * @param <T> the type of stream elements
985 * @see Stream#builder()
986 * @since 1.8
987 */
988 public interface Builder<T> extends Consumer<T> {
989
990 /**
991 * Adds an element to the stream being built.
992 *
993 * @throws IllegalStateException if the builder has already transitioned to
994 * the built state
995 */
996 @Override
997 void accept(T t);
998
999 /**
1000 * Adds an element to the stream being built.
1001 *
1002 * @implSpec
1003 * The default implementation behaves as if:
1004 * <pre>{@code
1005 * accept(t)
1006 * return this;
1007 * }</pre>
1008 *
1009 * @param t the element to add
1010 * @return {@code this} builder
1011 * @throws IllegalStateException if the builder has already transitioned to
1012 * the built state
1013 */
1014 default Builder<T> add(T t) {
1015 accept(t);
1016 return this;
1017 }
1018
1019 /**
1020 * Builds the stream, transitioning this builder to the built state.
1021 * An {@code IllegalStateException} is thrown if there are further attempts
1022 * to operate on the builder after it has entered the built state.
1023 *
1024 * @return the built stream
1025 * @throws IllegalStateException if the builder has already transitioned to
1026 * the built state
1027 */
1028 Stream<T> build();
1029
1030 }
1031 }