| /* |
| * Copyright (c) 2012, 2013, Oracle and/or its affiliates. All rights reserved. |
| * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. |
| * |
| * This code is free software; you can redistribute it and/or modify it |
| * under the terms of the GNU General Public License version 2 only, as |
| * published by the Free Software Foundation. Oracle designates this |
| * particular file as subject to the "Classpath" exception as provided |
| * by Oracle in the LICENSE file that accompanied this code. |
| * |
| * This code is distributed in the hope that it will be useful, but WITHOUT |
| * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or |
| * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License |
| * version 2 for more details (a copy is included in the LICENSE file that |
| * accompanied this code). |
| * |
| * You should have received a copy of the GNU General Public License version |
| * 2 along with this work; if not, write to the Free Software Foundation, |
| * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. |
| * |
| * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA |
| * or visit www.oracle.com if you need additional information or have any |
| * questions. |
| */ |
| package java.util.stream; |
| |
| import java.nio.charset.Charset; |
| import java.nio.file.Files; |
| import java.nio.file.Path; |
| import java.util.Arrays; |
| import java.util.Collection; |
| import java.util.Comparator; |
| import java.util.Iterator; |
| import java.util.Objects; |
| import java.util.Optional; |
| import java.util.Spliterator; |
| import java.util.Spliterators; |
| import java.util.concurrent.ConcurrentHashMap; |
| import java.util.function.BiConsumer; |
| import java.util.function.BiFunction; |
| import java.util.function.BinaryOperator; |
| import java.util.function.Consumer; |
| import java.util.function.Function; |
| import java.util.function.IntFunction; |
| import java.util.function.Predicate; |
| import java.util.function.Supplier; |
| import java.util.function.ToDoubleFunction; |
| import java.util.function.ToIntFunction; |
| import java.util.function.ToLongFunction; |
| import java.util.function.UnaryOperator; |
| |
| /** |
| * A sequence of elements supporting sequential and parallel aggregate |
| * operations. The following example illustrates an aggregate operation using |
| * {@link Stream} and {@link IntStream}: |
| * |
| * <pre>{@code |
| * int sum = widgets.stream() |
| * .filter(w -> w.getColor() == RED) |
| * .mapToInt(w -> w.getWeight()) |
| * .sum(); |
| * }</pre> |
| * |
| * In this example, {@code widgets} is a {@code Collection<Widget>}. We create |
| * a stream of {@code Widget} objects via {@link Collection#stream Collection.stream()}, |
| * filter it to produce a stream containing only the red widgets, and then |
| * transform it into a stream of {@code int} values representing the weight of |
| * each red widget. Then this stream is summed to produce a total weight. |
| * |
| * <p>In addition to {@code Stream}, which is a stream of object references, |
| * there are primitive specializations for {@link IntStream}, {@link LongStream}, |
| * and {@link DoubleStream}, all of which are referred to as "streams" and |
| * conform to the characteristics and restrictions described here. |
| * |
| * <p>To perform a computation, stream |
| * <a href="package-summary.html#StreamOps">operations</a> are composed into a |
| * <em>stream pipeline</em>. A stream pipeline consists of a source (which |
| * might be an array, a collection, a generator function, an I/O channel, |
| * etc), zero or more <em>intermediate operations</em> (which transform a |
| * stream into another stream, such as {@link Stream#filter(Predicate)}), and a |
| * <em>terminal operation</em> (which produces a result or side-effect, such |
| * as {@link Stream#count()} or {@link Stream#forEach(Consumer)}). |
| * Streams are lazy; computation on the source data is only performed when the |
| * terminal operation is initiated, and source elements are consumed only |
| * as needed. |
| * |
| * <p>Collections and streams, while bearing some superficial similarities, |
| * have different goals. Collections are primarily concerned with the efficient |
| * management of, and access to, their elements. By contrast, streams do not |
| * provide a means to directly access or manipulate their elements, and are |
| * instead concerned with declaratively describing their source and the |
| * computational operations which will be performed in aggregate on that source. |
| * However, if the provided stream operations do not offer the desired |
| * functionality, the {@link #iterator()} and {@link #spliterator()} operations |
| * can be used to perform a controlled traversal. |
| * |
| * <p>A stream pipeline, like the "widgets" example above, can be viewed as |
| * a <em>query</em> on the stream source. Unless the source was explicitly |
| * designed for concurrent modification (such as a {@link ConcurrentHashMap}), |
| * unpredictable or erroneous behavior may result from modifying the stream |
| * source while it is being queried. |
| * |
| * <p>Most stream operations accept parameters that describe user-specified |
| * behavior, such as the lambda expression {@code w -> w.getWeight()} passed to |
| * {@code mapToInt} in the example above. To preserve correct behavior, |
| * these <em>behavioral parameters</em>: |
| * <ul> |
| * <li>must be <a href="package-summary.html#NonInterference">non-interfering</a> |
| * (they do not modify the stream source); and</li> |
| * <li>in most cases must be <a href="package-summary.html#Statelessness">stateless</a> |
| * (their result should not depend on any state that might change during execution |
| * of the stream pipeline).</li> |
| * </ul> |
| * |
| * <p>Such parameters are always instances of a |
| * <a href="../function/package-summary.html">functional interface</a> such |
| * as {@link java.util.function.Function}, and are often lambda expressions or |
| * method references. Unless otherwise specified these parameters must be |
| * <em>non-null</em>. |
| * |
| * <p>A stream should be operated on (invoking an intermediate or terminal stream |
| * operation) only once. This rules out, for example, "forked" streams, where |
| * the same source feeds two or more pipelines, or multiple traversals of the |
| * same stream. A stream implementation may throw {@link IllegalStateException} |
| * if it detects that the stream is being reused. However, since some stream |
| * operations may return their receiver rather than a new stream object, it may |
| * not be possible to detect reuse in all cases. |
| * |
| * <p>Streams have a {@link #close()} method and implement {@link AutoCloseable}, |
| * but nearly all stream instances do not actually need to be closed after use. |
| * Generally, only streams whose source is an IO channel (such as those returned |
| * by {@link Files#lines(Path, Charset)}) will require closing. Most streams |
| * are backed by collections, arrays, or generating functions, which require no |
| * special resource management. (If a stream does require closing, it can be |
| * declared as a resource in a {@code try}-with-resources statement.) |
| * |
| * <p>Stream pipelines may execute either sequentially or in |
| * <a href="package-summary.html#Parallelism">parallel</a>. This |
| * execution mode is a property of the stream. Streams are created |
| * with an initial choice of sequential or parallel execution. (For example, |
| * {@link Collection#stream() Collection.stream()} creates a sequential stream, |
| * and {@link Collection#parallelStream() Collection.parallelStream()} creates |
| * a parallel one.) This choice of execution mode may be modified by the |
| * {@link #sequential()} or {@link #parallel()} methods, and may be queried with |
| * the {@link #isParallel()} method. |
| * |
| * @param <T> the type of the stream elements |
| * @since 1.8 |
| * @see IntStream |
| * @see LongStream |
| * @see DoubleStream |
| * @see <a href="package-summary.html">java.util.stream</a> |
| */ |
| public interface Stream<T> extends BaseStream<T, Stream<T>> { |
| |
| /** |
| * Returns a stream consisting of the elements of this stream that match |
| * the given predicate. |
| * |
| * <p>This is an <a href="package-summary.html#StreamOps">intermediate |
| * operation</a>. |
| * |
| * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>, |
| * <a href="package-summary.html#Statelessness">stateless</a> |
| * predicate to apply to each element to determine if it |
| * should be included |
| * @return the new stream |
| */ |
| Stream<T> filter(Predicate<? super T> predicate); |
| |
| /** |
| * Returns a stream consisting of the results of applying the given |
| * function to the elements of this stream. |
| * |
| * <p>This is an <a href="package-summary.html#StreamOps">intermediate |
| * operation</a>. |
| * |
| * @param <R> The element type of the new stream |
| * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>, |
| * <a href="package-summary.html#Statelessness">stateless</a> |
| * function to apply to each element |
| * @return the new stream |
| */ |
| <R> Stream<R> map(Function<? super T, ? extends R> mapper); |
| |
| /** |
| * Returns an {@code IntStream} consisting of the results of applying the |
| * given function to the elements of this stream. |
| * |
| * <p>This is an <a href="package-summary.html#StreamOps"> |
| * intermediate operation</a>. |
| * |
| * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>, |
| * <a href="package-summary.html#Statelessness">stateless</a> |
| * function to apply to each element |
| * @return the new stream |
| */ |
| IntStream mapToInt(ToIntFunction<? super T> mapper); |
| |
| /** |
| * Returns a {@code LongStream} consisting of the results of applying the |
| * given function to the elements of this stream. |
| * |
| * <p>This is an <a href="package-summary.html#StreamOps">intermediate |
| * operation</a>. |
| * |
| * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>, |
| * <a href="package-summary.html#Statelessness">stateless</a> |
| * function to apply to each element |
| * @return the new stream |
| */ |
| LongStream mapToLong(ToLongFunction<? super T> mapper); |
| |
| /** |
| * Returns a {@code DoubleStream} consisting of the results of applying the |
| * given function to the elements of this stream. |
| * |
| * <p>This is an <a href="package-summary.html#StreamOps">intermediate |
| * operation</a>. |
| * |
| * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>, |
| * <a href="package-summary.html#Statelessness">stateless</a> |
| * function to apply to each element |
| * @return the new stream |
| */ |
| DoubleStream mapToDouble(ToDoubleFunction<? super T> mapper); |
| |
| /** |
| * Returns a stream consisting of the results of replacing each element of |
| * this stream with the contents of a mapped stream produced by applying |
| * the provided mapping function to each element. Each mapped stream is |
| * {@link java.util.stream.BaseStream#close() closed} after its contents |
| * have been placed into this stream. (If a mapped stream is {@code null} |
| * an empty stream is used, instead.) |
| * |
| * <p>This is an <a href="package-summary.html#StreamOps">intermediate |
| * operation</a>. |
| * |
| * @apiNote |
| * The {@code flatMap()} operation has the effect of applying a one-to-many |
| * transformation to the elements of the stream, and then flattening the |
| * resulting elements into a new stream. |
| * |
| * <p><b>Examples.</b> |
| * |
| * <p>If {@code orders} is a stream of purchase orders, and each purchase |
| * order contains a collection of line items, then the following produces a |
| * stream containing all the line items in all the orders: |
| * <pre>{@code |
| * orders.flatMap(order -> order.getLineItems().stream())... |
| * }</pre> |
| * |
| * <p>If {@code path} is the path to a file, then the following produces a |
| * stream of the {@code words} contained in that file: |
| * <pre>{@code |
| * Stream<String> lines = Files.lines(path, StandardCharsets.UTF_8); |
| * Stream<String> words = lines.flatMap(line -> Stream.of(line.split(" +"))); |
| * }</pre> |
| * The {@code mapper} function passed to {@code flatMap} splits a line, |
| * using a simple regular expression, into an array of words, and then |
| * creates a stream of words from that array. |
| * |
| * @param <R> The element type of the new stream |
| * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>, |
| * <a href="package-summary.html#Statelessness">stateless</a> |
| * function to apply to each element which produces a stream |
| * of new values |
| * @return the new stream |
| */ |
| <R> Stream<R> flatMap(Function<? super T, ? extends Stream<? extends R>> mapper); |
| |
| /** |
| * Returns an {@code IntStream} consisting of the results of replacing each |
| * element of this stream with the contents of a mapped stream produced by |
| * applying the provided mapping function to each element. Each mapped |
| * stream is {@link java.util.stream.BaseStream#close() closed} after its |
| * contents have been placed into this stream. (If a mapped stream is |
| * {@code null} an empty stream is used, instead.) |
| * |
| * <p>This is an <a href="package-summary.html#StreamOps">intermediate |
| * operation</a>. |
| * |
| * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>, |
| * <a href="package-summary.html#Statelessness">stateless</a> |
| * function to apply to each element which produces a stream |
| * of new values |
| * @return the new stream |
| * @see #flatMap(Function) |
| */ |
| IntStream flatMapToInt(Function<? super T, ? extends IntStream> mapper); |
| |
| /** |
| * Returns an {@code LongStream} consisting of the results of replacing each |
| * element of this stream with the contents of a mapped stream produced by |
| * applying the provided mapping function to each element. Each mapped |
| * stream is {@link java.util.stream.BaseStream#close() closed} after its |
| * contents have been placed into this stream. (If a mapped stream is |
| * {@code null} an empty stream is used, instead.) |
| * |
| * <p>This is an <a href="package-summary.html#StreamOps">intermediate |
| * operation</a>. |
| * |
| * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>, |
| * <a href="package-summary.html#Statelessness">stateless</a> |
| * function to apply to each element which produces a stream |
| * of new values |
| * @return the new stream |
| * @see #flatMap(Function) |
| */ |
| LongStream flatMapToLong(Function<? super T, ? extends LongStream> mapper); |
| |
| /** |
| * Returns an {@code DoubleStream} consisting of the results of replacing |
| * each element of this stream with the contents of a mapped stream produced |
| * by applying the provided mapping function to each element. Each mapped |
| * stream is {@link java.util.stream.BaseStream#close() closed} after its |
| * contents have placed been into this stream. (If a mapped stream is |
| * {@code null} an empty stream is used, instead.) |
| * |
| * <p>This is an <a href="package-summary.html#StreamOps">intermediate |
| * operation</a>. |
| * |
| * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>, |
| * <a href="package-summary.html#Statelessness">stateless</a> |
| * function to apply to each element which produces a stream |
| * of new values |
| * @return the new stream |
| * @see #flatMap(Function) |
| */ |
| DoubleStream flatMapToDouble(Function<? super T, ? extends DoubleStream> mapper); |
| |
| /** |
| * Returns a stream consisting of the distinct elements (according to |
| * {@link Object#equals(Object)}) of this stream. |
| * |
| * <p>For ordered streams, the selection of distinct elements is stable |
| * (for duplicated elements, the element appearing first in the encounter |
| * order is preserved.) For unordered streams, no stability guarantees |
| * are made. |
| * |
| * <p>This is a <a href="package-summary.html#StreamOps">stateful |
| * intermediate operation</a>. |
| * |
| * @apiNote |
| * Preserving stability for {@code distinct()} in parallel pipelines is |
| * relatively expensive (requires that the operation act as a full barrier, |
| * with substantial buffering overhead), and stability is often not needed. |
| * Using an unordered stream source (such as {@link #generate(Supplier)}) |
| * or removing the ordering constraint with {@link #unordered()} may result |
| * in significantly more efficient execution for {@code distinct()} in parallel |
| * pipelines, if the semantics of your situation permit. If consistency |
| * with encounter order is required, and you are experiencing poor performance |
| * or memory utilization with {@code distinct()} in parallel pipelines, |
| * switching to sequential execution with {@link #sequential()} may improve |
| * performance. |
| * |
| * @return the new stream |
| */ |
| Stream<T> distinct(); |
| |
| /** |
| * Returns a stream consisting of the elements of this stream, sorted |
| * according to natural order. If the elements of this stream are not |
| * {@code Comparable}, a {@code java.lang.ClassCastException} may be thrown |
| * when the terminal operation is executed. |
| * |
| * <p>For ordered streams, the sort is stable. For unordered streams, no |
| * stability guarantees are made. |
| * |
| * <p>This is a <a href="package-summary.html#StreamOps">stateful |
| * intermediate operation</a>. |
| * |
| * @return the new stream |
| */ |
| Stream<T> sorted(); |
| |
| /** |
| * Returns a stream consisting of the elements of this stream, sorted |
| * according to the provided {@code Comparator}. |
| * |
| * <p>For ordered streams, the sort is stable. For unordered streams, no |
| * stability guarantees are made. |
| * |
| * <p>This is a <a href="package-summary.html#StreamOps">stateful |
| * intermediate operation</a>. |
| * |
| * @param comparator a <a href="package-summary.html#NonInterference">non-interfering</a>, |
| * <a href="package-summary.html#Statelessness">stateless</a> |
| * {@code Comparator} to be used to compare stream elements |
| * @return the new stream |
| */ |
| Stream<T> sorted(Comparator<? super T> comparator); |
| |
| /** |
| * Returns a stream consisting of the elements of this stream, additionally |
| * performing the provided action on each element as elements are consumed |
| * from the resulting stream. |
| * |
| * <p>This is an <a href="package-summary.html#StreamOps">intermediate |
| * operation</a>. |
| * |
| * <p>For parallel stream pipelines, the action may be called at |
| * whatever time and in whatever thread the element is made available by the |
| * upstream operation. If the action modifies shared state, |
| * it is responsible for providing the required synchronization. |
| * |
| * @apiNote This method exists mainly to support debugging, where you want |
| * to see the elements as they flow past a certain point in a pipeline: |
| * <pre>{@code |
| * Stream.of("one", "two", "three", "four") |
| * .filter(e -> e.length() > 3) |
| * .peek(e -> System.out.println("Filtered value: " + e)) |
| * .map(String::toUpperCase) |
| * .peek(e -> System.out.println("Mapped value: " + e)) |
| * .collect(Collectors.toList()); |
| * }</pre> |
| * |
| * @param action a <a href="package-summary.html#NonInterference"> |
| * non-interfering</a> action to perform on the elements as |
| * they are consumed from the stream |
| * @return the new stream |
| */ |
| Stream<T> peek(Consumer<? super T> action); |
| |
| /** |
| * Returns a stream consisting of the elements of this stream, truncated |
| * to be no longer than {@code maxSize} in length. |
| * |
| * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting |
| * stateful intermediate operation</a>. |
| * |
| * @apiNote |
| * While {@code limit()} is generally a cheap operation on sequential |
| * stream pipelines, it can be quite expensive on ordered parallel pipelines, |
| * especially for large values of {@code maxSize}, since {@code limit(n)} |
| * is constrained to return not just any <em>n</em> elements, but the |
| * <em>first n</em> elements in the encounter order. Using an unordered |
| * stream source (such as {@link #generate(Supplier)}) or removing the |
| * ordering constraint with {@link #unordered()} may result in significant |
| * speedups of {@code limit()} in parallel pipelines, if the semantics of |
| * your situation permit. If consistency with encounter order is required, |
| * and you are experiencing poor performance or memory utilization with |
| * {@code limit()} in parallel pipelines, switching to sequential execution |
| * with {@link #sequential()} may improve performance. |
| * |
| * @param maxSize the number of elements the stream should be limited to |
| * @return the new stream |
| * @throws IllegalArgumentException if {@code maxSize} is negative |
| */ |
| Stream<T> limit(long maxSize); |
| |
| /** |
| * Returns a stream consisting of the remaining elements of this stream |
| * after discarding the first {@code n} elements of the stream. |
| * If this stream contains fewer than {@code n} elements then an |
| * empty stream will be returned. |
| * |
| * <p>This is a <a href="package-summary.html#StreamOps">stateful |
| * intermediate operation</a>. |
| * |
| * @apiNote |
| * While {@code skip()} is generally a cheap operation on sequential |
| * stream pipelines, it can be quite expensive on ordered parallel pipelines, |
| * especially for large values of {@code n}, since {@code skip(n)} |
| * is constrained to skip not just any <em>n</em> elements, but the |
| * <em>first n</em> elements in the encounter order. Using an unordered |
| * stream source (such as {@link #generate(Supplier)}) or removing the |
| * ordering constraint with {@link #unordered()} may result in significant |
| * speedups of {@code skip()} in parallel pipelines, if the semantics of |
| * your situation permit. If consistency with encounter order is required, |
| * and you are experiencing poor performance or memory utilization with |
| * {@code skip()} in parallel pipelines, switching to sequential execution |
| * with {@link #sequential()} may improve performance. |
| * |
| * @param n the number of leading elements to skip |
| * @return the new stream |
| * @throws IllegalArgumentException if {@code n} is negative |
| */ |
| Stream<T> skip(long n); |
| |
| /** |
| * Performs an action for each element of this stream. |
| * |
| * <p>This is a <a href="package-summary.html#StreamOps">terminal |
| * operation</a>. |
| * |
| * <p>The behavior of this operation is explicitly nondeterministic. |
| * For parallel stream pipelines, this operation does <em>not</em> |
| * guarantee to respect the encounter order of the stream, as doing so |
| * would sacrifice the benefit of parallelism. For any given element, the |
| * action may be performed at whatever time and in whatever thread the |
| * library chooses. If the action accesses shared state, it is |
| * responsible for providing the required synchronization. |
| * |
| * @param action a <a href="package-summary.html#NonInterference"> |
| * non-interfering</a> action to perform on the elements |
| */ |
| void forEach(Consumer<? super T> action); |
| |
| /** |
| * Performs an action for each element of this stream, in the encounter |
| * order of the stream if the stream has a defined encounter order. |
| * |
| * <p>This is a <a href="package-summary.html#StreamOps">terminal |
| * operation</a>. |
| * |
| * <p>This operation processes the elements one at a time, in encounter |
| * order if one exists. Performing the action for one element |
| * <a href="../concurrent/package-summary.html#MemoryVisibility"><i>happens-before</i></a> |
| * performing the action for subsequent elements, but for any given element, |
| * the action may be performed in whatever thread the library chooses. |
| * |
| * @param action a <a href="package-summary.html#NonInterference"> |
| * non-interfering</a> action to perform on the elements |
| * @see #forEach(Consumer) |
| */ |
| void forEachOrdered(Consumer<? super T> action); |
| |
| /** |
| * Returns an array containing the elements of this stream. |
| * |
| * <p>This is a <a href="package-summary.html#StreamOps">terminal |
| * operation</a>. |
| * |
| * @return an array containing the elements of this stream |
| */ |
| Object[] toArray(); |
| |
| /** |
| * Returns an array containing the elements of this stream, using the |
| * provided {@code generator} function to allocate the returned array, as |
| * well as any additional arrays that might be required for a partitioned |
| * execution or for resizing. |
| * |
| * <p>This is a <a href="package-summary.html#StreamOps">terminal |
| * operation</a>. |
| * |
| * @apiNote |
| * The generator function takes an integer, which is the size of the |
| * desired array, and produces an array of the desired size. This can be |
| * concisely expressed with an array constructor reference: |
| * <pre>{@code |
| * Person[] men = people.stream() |
| * .filter(p -> p.getGender() == MALE) |
| * .toArray(Person[]::new); |
| * }</pre> |
| * |
| * @param <A> the element type of the resulting array |
| * @param generator a function which produces a new array of the desired |
| * type and the provided length |
| * @return an array containing the elements in this stream |
| * @throws ArrayStoreException if the runtime type of the array returned |
| * from the array generator is not a supertype of the runtime type of every |
| * element in this stream |
| */ |
| <A> A[] toArray(IntFunction<A[]> generator); |
| |
| /** |
| * Performs a <a href="package-summary.html#Reduction">reduction</a> on the |
| * elements of this stream, using the provided identity value and an |
| * <a href="package-summary.html#Associativity">associative</a> |
| * accumulation function, and returns the reduced value. This is equivalent |
| * to: |
| * <pre>{@code |
| * T result = identity; |
| * for (T element : this stream) |
| * result = accumulator.apply(result, element) |
| * return result; |
| * }</pre> |
| * |
| * but is not constrained to execute sequentially. |
| * |
| * <p>The {@code identity} value must be an identity for the accumulator |
| * function. This means that for all {@code t}, |
| * {@code accumulator.apply(identity, t)} is equal to {@code t}. |
| * The {@code accumulator} function must be an |
| * <a href="package-summary.html#Associativity">associative</a> function. |
| * |
| * <p>This is a <a href="package-summary.html#StreamOps">terminal |
| * operation</a>. |
| * |
| * @apiNote Sum, min, max, average, and string concatenation are all special |
| * cases of reduction. Summing a stream of numbers can be expressed as: |
| * |
| * <pre>{@code |
| * Integer sum = integers.reduce(0, (a, b) -> a+b); |
| * }</pre> |
| * |
| * or: |
| * |
| * <pre>{@code |
| * Integer sum = integers.reduce(0, Integer::sum); |
| * }</pre> |
| * |
| * <p>While this may seem a more roundabout way to perform an aggregation |
| * compared to simply mutating a running total in a loop, reduction |
| * operations parallelize more gracefully, without needing additional |
| * synchronization and with greatly reduced risk of data races. |
| * |
| * @param identity the identity value for the accumulating function |
| * @param accumulator an <a href="package-summary.html#Associativity">associative</a>, |
| * <a href="package-summary.html#NonInterference">non-interfering</a>, |
| * <a href="package-summary.html#Statelessness">stateless</a> |
| * function for combining two values |
| * @return the result of the reduction |
| */ |
| T reduce(T identity, BinaryOperator<T> accumulator); |
| |
| /** |
| * Performs a <a href="package-summary.html#Reduction">reduction</a> on the |
| * elements of this stream, using an |
| * <a href="package-summary.html#Associativity">associative</a> accumulation |
| * function, and returns an {@code Optional} describing the reduced value, |
| * if any. This is equivalent to: |
| * <pre>{@code |
| * boolean foundAny = false; |
| * T result = null; |
| * for (T element : this stream) { |
| * if (!foundAny) { |
| * foundAny = true; |
| * result = element; |
| * } |
| * else |
| * result = accumulator.apply(result, element); |
| * } |
| * return foundAny ? Optional.of(result) : Optional.empty(); |
| * }</pre> |
| * |
| * but is not constrained to execute sequentially. |
| * |
| * <p>The {@code accumulator} function must be an |
| * <a href="package-summary.html#Associativity">associative</a> function. |
| * |
| * <p>This is a <a href="package-summary.html#StreamOps">terminal |
| * operation</a>. |
| * |
| * @param accumulator an <a href="package-summary.html#Associativity">associative</a>, |
| * <a href="package-summary.html#NonInterference">non-interfering</a>, |
| * <a href="package-summary.html#Statelessness">stateless</a> |
| * function for combining two values |
| * @return an {@link Optional} describing the result of the reduction |
| * @throws NullPointerException if the result of the reduction is null |
| * @see #reduce(Object, BinaryOperator) |
| * @see #min(Comparator) |
| * @see #max(Comparator) |
| */ |
| Optional<T> reduce(BinaryOperator<T> accumulator); |
| |
| /** |
| * Performs a <a href="package-summary.html#Reduction">reduction</a> on the |
| * elements of this stream, using the provided identity, accumulation and |
| * combining functions. This is equivalent to: |
| * <pre>{@code |
| * U result = identity; |
| * for (T element : this stream) |
| * result = accumulator.apply(result, element) |
| * return result; |
| * }</pre> |
| * |
| * but is not constrained to execute sequentially. |
| * |
| * <p>The {@code identity} value must be an identity for the combiner |
| * function. This means that for all {@code u}, {@code combiner(identity, u)} |
| * is equal to {@code u}. Additionally, the {@code combiner} function |
| * must be compatible with the {@code accumulator} function; for all |
| * {@code u} and {@code t}, the following must hold: |
| * <pre>{@code |
| * combiner.apply(u, accumulator.apply(identity, t)) == accumulator.apply(u, t) |
| * }</pre> |
| * |
| * <p>This is a <a href="package-summary.html#StreamOps">terminal |
| * operation</a>. |
| * |
| * @apiNote Many reductions using this form can be represented more simply |
| * by an explicit combination of {@code map} and {@code reduce} operations. |
| * The {@code accumulator} function acts as a fused mapper and accumulator, |
| * which can sometimes be more efficient than separate mapping and reduction, |
| * such as when knowing the previously reduced value allows you to avoid |
| * some computation. |
| * |
| * @param <U> The type of the result |
| * @param identity the identity value for the combiner function |
| * @param accumulator an <a href="package-summary.html#Associativity">associative</a>, |
| * <a href="package-summary.html#NonInterference">non-interfering</a>, |
| * <a href="package-summary.html#Statelessness">stateless</a> |
| * function for incorporating an additional element into a result |
| * @param combiner an <a href="package-summary.html#Associativity">associative</a>, |
| * <a href="package-summary.html#NonInterference">non-interfering</a>, |
| * <a href="package-summary.html#Statelessness">stateless</a> |
| * function for combining two values, which must be |
| * compatible with the accumulator function |
| * @return the result of the reduction |
| * @see #reduce(BinaryOperator) |
| * @see #reduce(Object, BinaryOperator) |
| */ |
| <U> U reduce(U identity, |
| BiFunction<U, ? super T, U> accumulator, |
| BinaryOperator<U> combiner); |
| |
| /** |
| * Performs a <a href="package-summary.html#MutableReduction">mutable |
| * reduction</a> operation on the elements of this stream. A mutable |
| * reduction is one in which the reduced value is a mutable result container, |
| * such as an {@code ArrayList}, and elements are incorporated by updating |
| * the state of the result rather than by replacing the result. This |
| * produces a result equivalent to: |
| * <pre>{@code |
| * R result = supplier.get(); |
| * for (T element : this stream) |
| * accumulator.accept(result, element); |
| * return result; |
| * }</pre> |
| * |
| * <p>Like {@link #reduce(Object, BinaryOperator)}, {@code collect} operations |
| * can be parallelized without requiring additional synchronization. |
| * |
| * <p>This is a <a href="package-summary.html#StreamOps">terminal |
| * operation</a>. |
| * |
| * @apiNote There are many existing classes in the JDK whose signatures are |
| * well-suited for use with method references as arguments to {@code collect()}. |
| * For example, the following will accumulate strings into an {@code ArrayList}: |
| * <pre>{@code |
| * List<String> asList = stringStream.collect(ArrayList::new, ArrayList::add, |
| * ArrayList::addAll); |
| * }</pre> |
| * |
| * <p>The following will take a stream of strings and concatenates them into a |
| * single string: |
| * <pre>{@code |
| * String concat = stringStream.collect(StringBuilder::new, StringBuilder::append, |
| * StringBuilder::append) |
| * .toString(); |
| * }</pre> |
| * |
| * @param <R> type of the result |
| * @param supplier a function that creates a new result container. For a |
| * parallel execution, this function may be called |
| * multiple times and must return a fresh value each time. |
| * @param accumulator an <a href="package-summary.html#Associativity">associative</a>, |
| * <a href="package-summary.html#NonInterference">non-interfering</a>, |
| * <a href="package-summary.html#Statelessness">stateless</a> |
| * function for incorporating an additional element into a result |
| * @param combiner an <a href="package-summary.html#Associativity">associative</a>, |
| * <a href="package-summary.html#NonInterference">non-interfering</a>, |
| * <a href="package-summary.html#Statelessness">stateless</a> |
| * function for combining two values, which must be |
| * compatible with the accumulator function |
| * @return the result of the reduction |
| */ |
| <R> R collect(Supplier<R> supplier, |
| BiConsumer<R, ? super T> accumulator, |
| BiConsumer<R, R> combiner); |
| |
| /** |
| * Performs a <a href="package-summary.html#MutableReduction">mutable |
| * reduction</a> operation on the elements of this stream using a |
| * {@code Collector}. A {@code Collector} |
| * encapsulates the functions used as arguments to |
| * {@link #collect(Supplier, BiConsumer, BiConsumer)}, allowing for reuse of |
| * collection strategies and composition of collect operations such as |
| * multiple-level grouping or partitioning. |
| * |
| * <p>If the stream is parallel, and the {@code Collector} |
| * is {@link Collector.Characteristics#CONCURRENT concurrent}, and |
| * either the stream is unordered or the collector is |
| * {@link Collector.Characteristics#UNORDERED unordered}, |
| * then a concurrent reduction will be performed (see {@link Collector} for |
| * details on concurrent reduction.) |
| * |
| * <p>This is a <a href="package-summary.html#StreamOps">terminal |
| * operation</a>. |
| * |
| * <p>When executed in parallel, multiple intermediate results may be |
| * instantiated, populated, and merged so as to maintain isolation of |
| * mutable data structures. Therefore, even when executed in parallel |
| * with non-thread-safe data structures (such as {@code ArrayList}), no |
| * additional synchronization is needed for a parallel reduction. |
| * |
| * @apiNote |
| * The following will accumulate strings into an ArrayList: |
| * <pre>{@code |
| * List<String> asList = stringStream.collect(Collectors.toList()); |
| * }</pre> |
| * |
| * <p>The following will classify {@code Person} objects by city: |
| * <pre>{@code |
| * Map<String, List<Person>> peopleByCity |
| * = personStream.collect(Collectors.groupingBy(Person::getCity)); |
| * }</pre> |
| * |
| * <p>The following will classify {@code Person} objects by state and city, |
| * cascading two {@code Collector}s together: |
| * <pre>{@code |
| * Map<String, Map<String, List<Person>>> peopleByStateAndCity |
| * = personStream.collect(Collectors.groupingBy(Person::getState, |
| * Collectors.groupingBy(Person::getCity))); |
| * }</pre> |
| * |
| * @param <R> the type of the result |
| * @param <A> the intermediate accumulation type of the {@code Collector} |
| * @param collector the {@code Collector} describing the reduction |
| * @return the result of the reduction |
| * @see #collect(Supplier, BiConsumer, BiConsumer) |
| * @see Collectors |
| */ |
| <R, A> R collect(Collector<? super T, A, R> collector); |
| |
| /** |
| * Returns the minimum element of this stream according to the provided |
| * {@code Comparator}. This is a special case of a |
| * <a href="package-summary.html#Reduction">reduction</a>. |
| * |
| * <p>This is a <a href="package-summary.html#StreamOps">terminal operation</a>. |
| * |
| * @param comparator a <a href="package-summary.html#NonInterference">non-interfering</a>, |
| * <a href="package-summary.html#Statelessness">stateless</a> |
| * {@code Comparator} to compare elements of this stream |
| * @return an {@code Optional} describing the minimum element of this stream, |
| * or an empty {@code Optional} if the stream is empty |
| * @throws NullPointerException if the minimum element is null |
| */ |
| Optional<T> min(Comparator<? super T> comparator); |
| |
| /** |
| * Returns the maximum element of this stream according to the provided |
| * {@code Comparator}. This is a special case of a |
| * <a href="package-summary.html#Reduction">reduction</a>. |
| * |
| * <p>This is a <a href="package-summary.html#StreamOps">terminal |
| * operation</a>. |
| * |
| * @param comparator a <a href="package-summary.html#NonInterference">non-interfering</a>, |
| * <a href="package-summary.html#Statelessness">stateless</a> |
| * {@code Comparator} to compare elements of this stream |
| * @return an {@code Optional} describing the maximum element of this stream, |
| * or an empty {@code Optional} if the stream is empty |
| * @throws NullPointerException if the maximum element is null |
| */ |
| Optional<T> max(Comparator<? super T> comparator); |
| |
| /** |
| * Returns the count of elements in this stream. This is a special case of |
| * a <a href="package-summary.html#Reduction">reduction</a> and is |
| * equivalent to: |
| * <pre>{@code |
| * return mapToLong(e -> 1L).sum(); |
| * }</pre> |
| * |
| * <p>This is a <a href="package-summary.html#StreamOps">terminal operation</a>. |
| * |
| * @return the count of elements in this stream |
| */ |
| long count(); |
| |
| /** |
| * Returns whether any elements of this stream match the provided |
| * predicate. May not evaluate the predicate on all elements if not |
| * necessary for determining the result. If the stream is empty then |
| * {@code false} is returned and the predicate is not evaluated. |
| * |
| * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting |
| * terminal operation</a>. |
| * |
| * @apiNote |
| * This method evaluates the <em>existential quantification</em> of the |
| * predicate over the elements of the stream (for some x P(x)). |
| * |
| * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>, |
| * <a href="package-summary.html#Statelessness">stateless</a> |
| * predicate to apply to elements of this stream |
| * @return {@code true} if any elements of the stream match the provided |
| * predicate, otherwise {@code false} |
| */ |
| boolean anyMatch(Predicate<? super T> predicate); |
| |
| /** |
| * Returns whether all elements of this stream match the provided predicate. |
| * May not evaluate the predicate on all elements if not necessary for |
| * determining the result. If the stream is empty then {@code true} is |
| * returned and the predicate is not evaluated. |
| * |
| * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting |
| * terminal operation</a>. |
| * |
| * @apiNote |
| * This method evaluates the <em>universal quantification</em> of the |
| * predicate over the elements of the stream (for all x P(x)). If the |
| * stream is empty, the quantification is said to be <em>vacuously |
| * satisfied</em> and is always {@code true} (regardless of P(x)). |
| * |
| * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>, |
| * <a href="package-summary.html#Statelessness">stateless</a> |
| * predicate to apply to elements of this stream |
| * @return {@code true} if either all elements of the stream match the |
| * provided predicate or the stream is empty, otherwise {@code false} |
| */ |
| boolean allMatch(Predicate<? super T> predicate); |
| |
| /** |
| * Returns whether no elements of this stream match the provided predicate. |
| * May not evaluate the predicate on all elements if not necessary for |
| * determining the result. If the stream is empty then {@code true} is |
| * returned and the predicate is not evaluated. |
| * |
| * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting |
| * terminal operation</a>. |
| * |
| * @apiNote |
| * This method evaluates the <em>universal quantification</em> of the |
| * negated predicate over the elements of the stream (for all x ~P(x)). If |
| * the stream is empty, the quantification is said to be vacuously satisfied |
| * and is always {@code true}, regardless of P(x). |
| * |
| * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>, |
| * <a href="package-summary.html#Statelessness">stateless</a> |
| * predicate to apply to elements of this stream |
| * @return {@code true} if either no elements of the stream match the |
| * provided predicate or the stream is empty, otherwise {@code false} |
| */ |
| boolean noneMatch(Predicate<? super T> predicate); |
| |
| /** |
| * Returns an {@link Optional} describing the first element of this stream, |
| * or an empty {@code Optional} if the stream is empty. If the stream has |
| * no encounter order, then any element may be returned. |
| * |
| * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting |
| * terminal operation</a>. |
| * |
| * @return an {@code Optional} describing the first element of this stream, |
| * or an empty {@code Optional} if the stream is empty |
| * @throws NullPointerException if the element selected is null |
| */ |
| Optional<T> findFirst(); |
| |
| /** |
| * Returns an {@link Optional} describing some element of the stream, or an |
| * empty {@code Optional} if the stream is empty. |
| * |
| * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting |
| * terminal operation</a>. |
| * |
| * <p>The behavior of this operation is explicitly nondeterministic; it is |
| * free to select any element in the stream. This is to allow for maximal |
| * performance in parallel operations; the cost is that multiple invocations |
| * on the same source may not return the same result. (If a stable result |
| * is desired, use {@link #findFirst()} instead.) |
| * |
| * @return an {@code Optional} describing some element of this stream, or an |
| * empty {@code Optional} if the stream is empty |
| * @throws NullPointerException if the element selected is null |
| * @see #findFirst() |
| */ |
| Optional<T> findAny(); |
| |
| // Static factories |
| |
| /** |
| * Returns a builder for a {@code Stream}. |
| * |
| * @param <T> type of elements |
| * @return a stream builder |
| */ |
| public static<T> Builder<T> builder() { |
| return new Streams.StreamBuilderImpl<>(); |
| } |
| |
| /** |
| * Returns an empty sequential {@code Stream}. |
| * |
| * @param <T> the type of stream elements |
| * @return an empty sequential stream |
| */ |
| public static<T> Stream<T> empty() { |
| return StreamSupport.stream(Spliterators.<T>emptySpliterator(), false); |
| } |
| |
| /** |
| * Returns a sequential {@code Stream} containing a single element. |
| * |
| * @param t the single element |
| * @param <T> the type of stream elements |
| * @return a singleton sequential stream |
| */ |
| public static<T> Stream<T> of(T t) { |
| return StreamSupport.stream(new Streams.StreamBuilderImpl<>(t), false); |
| } |
| |
| /** |
| * Returns a sequential ordered stream whose elements are the specified values. |
| * |
| * @param <T> the type of stream elements |
| * @param values the elements of the new stream |
| * @return the new stream |
| */ |
| @SafeVarargs |
| @SuppressWarnings("varargs") // Creating a stream from an array is safe |
| public static<T> Stream<T> of(T... values) { |
| return Arrays.stream(values); |
| } |
| |
| /** |
| * Returns an infinite sequential ordered {@code Stream} produced by iterative |
| * application of a function {@code f} to an initial element {@code seed}, |
| * producing a {@code Stream} consisting of {@code seed}, {@code f(seed)}, |
| * {@code f(f(seed))}, etc. |
| * |
| * <p>The first element (position {@code 0}) in the {@code Stream} will be |
| * the provided {@code seed}. For {@code n > 0}, the element at position |
| * {@code n}, will be the result of applying the function {@code f} to the |
| * element at position {@code n - 1}. |
| * |
| * @param <T> the type of stream elements |
| * @param seed the initial element |
| * @param f a function to be applied to to the previous element to produce |
| * a new element |
| * @return a new sequential {@code Stream} |
| */ |
| public static<T> Stream<T> iterate(final T seed, final UnaryOperator<T> f) { |
| Objects.requireNonNull(f); |
| final Iterator<T> iterator = new Iterator<T>() { |
| @SuppressWarnings("unchecked") |
| T t = (T) Streams.NONE; |
| |
| @Override |
| public boolean hasNext() { |
| return true; |
| } |
| |
| @Override |
| public T next() { |
| return t = (t == Streams.NONE) ? seed : f.apply(t); |
| } |
| }; |
| return StreamSupport.stream(Spliterators.spliteratorUnknownSize( |
| iterator, |
| Spliterator.ORDERED | Spliterator.IMMUTABLE), false); |
| } |
| |
| /** |
| * Returns an infinite sequential unordered stream where each element is |
| * generated by the provided {@code Supplier}. This is suitable for |
| * generating constant streams, streams of random elements, etc. |
| * |
| * @param <T> the type of stream elements |
| * @param s the {@code Supplier} of generated elements |
| * @return a new infinite sequential unordered {@code Stream} |
| */ |
| public static<T> Stream<T> generate(Supplier<T> s) { |
| Objects.requireNonNull(s); |
| return StreamSupport.stream( |
| new StreamSpliterators.InfiniteSupplyingSpliterator.OfRef<>(Long.MAX_VALUE, s), false); |
| } |
| |
| /** |
| * Creates a lazily concatenated stream whose elements are all the |
| * elements of the first stream followed by all the elements of the |
| * second stream. The resulting stream is ordered if both |
| * of the input streams are ordered, and parallel if either of the input |
| * streams is parallel. When the resulting stream is closed, the close |
| * handlers for both input streams are invoked. |
| * |
| * @implNote |
| * Use caution when constructing streams from repeated concatenation. |
| * Accessing an element of a deeply concatenated stream can result in deep |
| * call chains, or even {@code StackOverflowException}. |
| * |
| * @param <T> The type of stream elements |
| * @param a the first stream |
| * @param b the second stream |
| * @return the concatenation of the two input streams |
| */ |
| public static <T> Stream<T> concat(Stream<? extends T> a, Stream<? extends T> b) { |
| Objects.requireNonNull(a); |
| Objects.requireNonNull(b); |
| |
| @SuppressWarnings("unchecked") |
| Spliterator<T> split = new Streams.ConcatSpliterator.OfRef<>( |
| (Spliterator<T>) a.spliterator(), (Spliterator<T>) b.spliterator()); |
| Stream<T> stream = StreamSupport.stream(split, a.isParallel() || b.isParallel()); |
| return stream.onClose(Streams.composedClose(a, b)); |
| } |
| |
| /** |
| * A mutable builder for a {@code Stream}. This allows the creation of a |
| * {@code Stream} by generating elements individually and adding them to the |
| * {@code Builder} (without the copying overhead that comes from using |
| * an {@code ArrayList} as a temporary buffer.) |
| * |
| * <p>A stream builder has a lifecycle, which starts in a building |
| * phase, during which elements can be added, and then transitions to a built |
| * phase, after which elements may not be added. The built phase begins |
| * when the {@link #build()} method is called, which creates an ordered |
| * {@code Stream} whose elements are the elements that were added to the stream |
| * builder, in the order they were added. |
| * |
| * @param <T> the type of stream elements |
| * @see Stream#builder() |
| * @since 1.8 |
| */ |
| public interface Builder<T> extends Consumer<T> { |
| |
| /** |
| * Adds an element to the stream being built. |
| * |
| * @throws IllegalStateException if the builder has already transitioned to |
| * the built state |
| */ |
| @Override |
| void accept(T t); |
| |
| /** |
| * Adds an element to the stream being built. |
| * |
| * @implSpec |
| * The default implementation behaves as if: |
| * <pre>{@code |
| * accept(t) |
| * return this; |
| * }</pre> |
| * |
| * @param t the element to add |
| * @return {@code this} builder |
| * @throws IllegalStateException if the builder has already transitioned to |
| * the built state |
| */ |
| default Builder<T> add(T t) { |
| accept(t); |
| return this; |
| } |
| |
| /** |
| * Builds the stream, transitioning this builder to the built state. |
| * An {@code IllegalStateException} is thrown if there are further attempts |
| * to operate on the builder after it has entered the built state. |
| * |
| * @return the built stream |
| * @throws IllegalStateException if the builder has already transitioned to |
| * the built state |
| */ |
| Stream<T> build(); |
| |
| } |
| } |
