    001/*
    002 * Licensed to the Apache Software Foundation (ASF) under one or more
    003 * contributor license agreements.  See the NOTICE file distributed with
    004 * this work for additional information regarding copyright ownership.
    005 * The ASF licenses this file to You under the Apache License, Version 2.0
    006 * (the "License"); you may not use this file except in compliance with
    007 * the License.  You may obtain a copy of the License at
    008 *
    009 *      http://www.apache.org/licenses/LICENSE-2.0
    010 *
    011 * Unless required by applicable law or agreed to in writing, software
    012 * distributed under the License is distributed on an "AS IS" BASIS,
    013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    014 * See the License for the specific language governing permissions and
    015 * limitations under the License.
    016 */
    017package org.apache.commons.lang3;
    018
    019import java.lang.reflect.Array;
    020import java.util.ArrayList;
    021import java.util.Collection;
    022import java.util.Collections;
    023import java.util.List;
    024import java.util.Set;
    025import java.util.function.BiConsumer;
    026import java.util.function.BinaryOperator;
    027import java.util.function.Consumer;
    028import java.util.function.Function;
    029import java.util.function.Predicate;
    030import java.util.function.Supplier;
    031import java.util.stream.Collector;
    032import java.util.stream.Collectors;
    033import java.util.stream.Stream;
    034
    035import org.apache.commons.lang3.Functions.FailableConsumer;
    036import org.apache.commons.lang3.Functions.FailableFunction;
    037import org.apache.commons.lang3.Functions.FailablePredicate;
    038
    039/**
    040 * Provides utility functions, and classes for working with the
    041 * {@code java.util.stream} package, or more generally, with Java 8 lambdas. More
    042 * specifically, it attempts to address the fact that lambdas are supposed
    043 * not to throw Exceptions, at least not checked Exceptions, AKA instances
    044 * of {@link Exception}. This enforces the use of constructs like
    045 * <pre>
    046 *     Consumer&lt;java.lang.reflect.Method&gt; consumer = (m) -&gt; {
    047 *         try {
    048 *             m.invoke(o, args);
    049 *         } catch (Throwable t) {
    050 *             throw Functions.rethrow(t);
    051 *         }
    052 *    };
    053 *    stream.forEach(consumer);
    054 * </pre>
    055 * Using a {@link FailableStream}, this can be rewritten as follows:
    056 * <pre>
    057 *     Streams.failable(stream).forEach((m) -&gt; m.invoke(o, args));
    058 * </pre>
    059 * Obviously, the second version is much more concise and the spirit of
    060 * Lambda expressions is met better than in the first version.
    061 *
    062 * @see Stream
    063 * @see Functions
    064 * @since 3.10
    065 */
    066public class Streams {
    067
    068   /**
    069    * A reduced, and simplified version of a {@link Stream} with
    070    * failable method signatures.
    071    * @param <O> The streams element type.
    072    */
    073    public static class FailableStream<O extends Object> {
    074
    075        private Stream<O> stream;
    076        private boolean terminated;
    077
    078        /**
    079         * Constructs a new instance with the given {@code stream}.
    080         * @param stream The stream.
    081         */
    082        public FailableStream(final Stream<O> stream) {
    083            this.stream = stream;
    084        }
    085
    086        protected void assertNotTerminated() {
    087            if (terminated) {
    088                throw new IllegalStateException("This stream is already terminated.");
    089            }
    090        }
    091
    092        protected void makeTerminated() {
    093            assertNotTerminated();
    094            terminated = true;
    095        }
    096
    097        /**
    098         * Returns a FailableStream consisting of the elements of this stream that match
    099         * the given FailablePredicate.
    100         *
    101         * <p>This is an intermediate operation.
    102         *
    103         * @param predicate a non-interfering, stateless predicate to apply to each
    104         * element to determine if it should be included.
    105         * @return the new stream
    106         */
    107        public FailableStream<O> filter(final FailablePredicate<O, ?> predicate){
    108            assertNotTerminated();
    109            stream = stream.filter(Functions.asPredicate(predicate));
    110            return this;
    111        }
    112
    113        /**
    114         * Performs an action for each element of this stream.
    115         *
    116         * <p>This is a terminal operation.
    117         *
    118         * <p>The behavior of this operation is explicitly nondeterministic.
    119         * For parallel stream pipelines, this operation does <em>not</em>
    120         * guarantee to respect the encounter order of the stream, as doing so
    121         * would sacrifice the benefit of parallelism.  For any given element, the
    122         * action may be performed at whatever time and in whatever thread the
    123         * library chooses.  If the action accesses shared state, it is
    124         * responsible for providing the required synchronization.
    125         *
    126         * @param action a non-interfering action to perform on the elements
    127         */
    128        public void forEach(final FailableConsumer<O, ?> action) {
    129            makeTerminated();
    130            stream().forEach(Functions.asConsumer(action));
    131        }
    132
    133        /**
    134         * Performs a mutable reduction operation on the elements of this stream using a
    135         * {@code Collector}.  A {@code Collector}
    136         * encapsulates the functions used as arguments to
    137         * {@link #collect(Supplier, BiConsumer, BiConsumer)}, allowing for reuse of
    138         * collection strategies and composition of collect operations such as
    139         * multiple-level grouping or partitioning.
    140         *
    141         * <p>If the underlying stream is parallel, and the {@code Collector}
    142         * is concurrent, and either the stream is unordered or the collector is
    143         * unordered, then a concurrent reduction will be performed
    144         * (see {@link Collector} for details on concurrent reduction.)
    145         *
    146         * <p>This is a terminal operation.
    147         *
    148         * <p>When executed in parallel, multiple intermediate results may be
    149         * instantiated, populated, and merged so as to maintain isolation of
    150         * mutable data structures.  Therefore, even when executed in parallel
    151         * with non-thread-safe data structures (such as {@code ArrayList}), no
    152         * additional synchronization is needed for a parallel reduction.
    153         *
    154         * \@apiNote
    155         * The following will accumulate strings into an ArrayList:
    156         * <pre>{@code
    157         *     List<String> asList = stringStream.collect(Collectors.toList());
    158         * }</pre>
    159         *
    160         * <p>The following will classify {@code Person} objects by city:
    161         * <pre>{@code
    162         *     Map<String, List<Person>> peopleByCity
    163         *         = personStream.collect(Collectors.groupingBy(Person::getCity));
    164         * }</pre>
    165         *
    166         * <p>The following will classify {@code Person} objects by state and city,
    167         * cascading two {@code Collector}s together:
    168         * <pre>{@code
    169         *     Map<String, Map<String, List<Person>>> peopleByStateAndCity
    170         *         = personStream.collect(Collectors.groupingBy(Person::getState,
    171         *                                                      Collectors.groupingBy(Person::getCity)));
    172         * }</pre>
    173         *
    174         * @param <R> the type of the result
    175         * @param <A> the intermediate accumulation type of the {@code Collector}
    176         * @param collector the {@code Collector} describing the reduction
    177         * @return the result of the reduction
    178         * @see #collect(Supplier, BiConsumer, BiConsumer)
    179         * @see Collectors
    180         */
    181        public <A, R> R collect(final Collector<? super O, A, R> collector) {
    182            makeTerminated();
    183            return stream().collect(collector);
    184        }
    185
    186        /**
    187         * Performs a mutable reduction operation on the elements of this FailableStream.
    188         * A mutable reduction is one in which the reduced value is a mutable result
    189         * container, such as an {@code ArrayList}, and elements are incorporated by updating
    190         * the state of the result rather than by replacing the result. This produces a result equivalent to:
    191         * <pre>{@code
    192         *     R result = supplier.get();
    193         *     for (T element : this stream)
    194         *         accumulator.accept(result, element);
    195         *     return result;
    196         * }</pre>
    197         *
    198         * <p>Like {@link #reduce(Object, BinaryOperator)}, {@code collect} operations
    199         * can be parallelized without requiring additional synchronization.
    200         *
    201         * <p>This is a terminal operation.
    202         *
    203         * \@apiNote There are many existing classes in the JDK whose signatures are
    204         * well-suited for use with method references as arguments to {@code collect()}.
    205         * For example, the following will accumulate strings into an {@code ArrayList}:
    206         * <pre>{@code
    207         *     List<String> asList = stringStream.collect(ArrayList::new, ArrayList::add,
    208         *                                                ArrayList::addAll);
    209         * }</pre>
    210         *
    211         * <p>The following will take a stream of strings and concatenates them into a
    212         * single string:
    213         * <pre>{@code
    214         *     String concat = stringStream.collect(StringBuilder::new, StringBuilder::append,
    215         *                                          StringBuilder::append)
    216         *                                 .toString();
    217         * }</pre>
    218         *
    219         * @param <R> type of the result
    220         * @param <A> Type of the accumulator.
    221         * @param pupplier a function that creates a new result container. For a
    222         *                 parallel execution, this function may be called
    223         *                 multiple times and must return a fresh value each time.
    224         * @param accumulator An associative, non-interfering, stateless function for
    225         *   incorporating an additional element into a result
    226         * @param combiner An associative, non-interfering, stateless
    227         *   function for combining two values, which must be compatible with the
    228         *   accumulator function
    229         * @return The result of the reduction
    230         */
    231        public <A, R> R collect(final Supplier<R> pupplier, final BiConsumer<R, ? super O> accumulator, final BiConsumer<R, R> combiner) {
    232            makeTerminated();
    233            return stream().collect(pupplier, accumulator, combiner);
    234        }
    235
    236        /**
    237         * Performs a reduction on the elements of this stream, using the provided
    238         * identity value and an associative accumulation function, and returns
    239         * the reduced value.  This is equivalent to:
    240         * <pre>{@code
    241         *     T result = identity;
    242         *     for (T element : this stream)
    243         *         result = accumulator.apply(result, element)
    244         *     return result;
    245         * }</pre>
    246         *
    247         * but is not constrained to execute sequentially.
    248         *
    249         * <p>The {@code identity} value must be an identity for the accumulator
    250         * function. This means that for all {@code t},
    251         * {@code accumulator.apply(identity, t)} is equal to {@code t}.
    252         * The {@code accumulator} function must be an associative function.
    253         *
    254         * <p>This is a terminal operation.
    255         *
    256         * \@apiNote Sum, min, max, average, and string concatenation are all special
    257         * cases of reduction. Summing a stream of numbers can be expressed as:
    258         *
    259         * <pre>{@code
    260         *     Integer sum = integers.reduce(0, (a, b) -> a+b);
    261         * }</pre>
    262         *
    263         * or:
    264         *
    265         * <pre>{@code
    266         *     Integer sum = integers.reduce(0, Integer::sum);
    267         * }</pre>
    268         *
    269         * <p>While this may seem a more roundabout way to perform an aggregation
    270         * compared to simply mutating a running total in a loop, reduction
    271         * operations parallelize more gracefully, without needing additional
    272         * synchronization and with greatly reduced risk of data races.
    273         *
    274         * @param identity the identity value for the accumulating function
    275         * @param accumulator an associative, non-interfering, stateless
    276         *                    function for combining two values
    277         * @return the result of the reduction
    278         */
    279        public O reduce(final O identity, final BinaryOperator<O> accumulator) {
    280            makeTerminated();
    281            return stream().reduce(identity, accumulator);
    282        }
    283
    284        /**
    285         * Returns a stream consisting of the results of applying the given
    286         * function to the elements of this stream.
    287         *
    288         * <p>This is an intermediate operation.
    289         *
    290         * @param <R> The element type of the new stream
    291         * @param mapper A non-interfering, stateless function to apply to each element
    292         * @return the new stream
    293         */
    294        public <R> FailableStream<R> map(final FailableFunction<O, R, ?> mapper) {
    295            assertNotTerminated();
    296            return new FailableStream<>(stream.map(Functions.asFunction(mapper)));
    297        }
    298
    299        /**
    300         * Converts the FailableStream into an equivalent stream.
    301         * @return A stream, which will return the same elements, which this FailableStream would return.
    302         */
    303        public Stream<O> stream() {
    304            return stream;
    305        }
    306
    307        /**
    308         * Returns whether all elements of this stream match the provided predicate.
    309         * May not evaluate the predicate on all elements if not necessary for
    310         * determining the result.  If the stream is empty then {@code true} is
    311         * returned and the predicate is not evaluated.
    312         *
    313         * <p>This is a short-circuiting terminal operation.
    314         *
    315         * \@apiNote
    316         * This method evaluates the <em>universal quantification</em> of the
    317         * predicate over the elements of the stream (for all x P(x)).  If the
    318         * stream is empty, the quantification is said to be <em>vacuously
    319         * satisfied</em> and is always {@code true} (regardless of P(x)).
    320         *
    321         * @param predicate A non-interfering, stateless predicate to apply to
    322         * elements of this stream
    323         * @return {@code true} If either all elements of the stream match the
    324         * provided predicate or the stream is empty, otherwise {@code false}.
    325         */
    326        public boolean allMatch(final FailablePredicate<O, ?> predicate) {
    327            assertNotTerminated();
    328            return stream().allMatch(Functions.asPredicate(predicate));
    329        }
    330
    331        /**
    332         * Returns whether any elements of this stream match the provided
    333         * predicate.  May not evaluate the predicate on all elements if not
    334         * necessary for determining the result.  If the stream is empty then
    335         * {@code false} is returned and the predicate is not evaluated.
    336         *
    337         * <p>This is a short-circuiting terminal operation.
    338         *
    339         * \@apiNote
    340         * This method evaluates the <em>existential quantification</em> of the
    341         * predicate over the elements of the stream (for some x P(x)).
    342         *
    343         * @param predicate A non-interfering, stateless predicate to apply to
    344         * elements of this stream
    345         * @return {@code true} if any elements of the stream match the provided
    346         * predicate, otherwise {@code false}
    347         */
    348        public boolean anyMatch(final FailablePredicate<O, ?> predicate) {
    349            assertNotTerminated();
    350            return stream().anyMatch(Functions.asPredicate(predicate));
    351        }
    352    }
    353
    354    /**
    355     * Converts the given {@link Stream stream} into a {@link FailableStream}.
    356     * This is basically a simplified, reduced version of the {@link Stream}
    357     * class, with the same underlying element stream, except that failable
    358     * objects, like {@link FailablePredicate}, {@link FailableFunction}, or
    359     * {@link FailableConsumer} may be applied, instead of
    360     * {@link Predicate}, {@link Function}, or {@link Consumer}. The idea is
    361     * to rewrite a code snippet like this:
    362     * <pre>
    363     *     final List&lt;O&gt; list;
    364     *     final Method m;
    365     *     final Function&lt;O,String&gt; mapper = (o) -&gt; {
    366     *         try {
    367     *             return (String) m.invoke(o);
    368     *         } catch (Throwable t) {
    369     *             throw Functions.rethrow(t);
    370     *         }
    371     *     };
    372     *     final List&lt;String&gt; strList = list.stream()
    373     *         .map(mapper).collect(Collectors.toList());
    374     *  </pre>
    375     *  as follows:
    376     *  <pre>
    377     *     final List&lt;O&gt; list;
    378     *     final Method m;
    379     *     final List&lt;String&gt; strList = Functions.stream(list.stream())
    380     *         .map((o) -&gt; (String) m.invoke(o)).collect(Collectors.toList());
    381     *  </pre>
    382     *  While the second version may not be <em>quite</em> as
    383     *  efficient (because it depends on the creation of additional,
    384     *  intermediate objects, of type FailableStream), it is much more
    385     *  concise, and readable, and meets the spirit of Lambdas better
    386     *  than the first version.
    387     * @param <O> The streams element type.
    388     * @param stream The stream, which is being converted.
    389     * @return The {@link FailableStream}, which has been created by
    390     *   converting the stream.
    391     */
    392    public static <O> FailableStream<O> stream(final Stream<O> stream) {
    393        return new FailableStream<>(stream);
    394    }
    395
    396    /**
    397     * Converts the given {@link Collection} into a {@link FailableStream}.
    398     * This is basically a simplified, reduced version of the {@link Stream}
    399     * class, with the same underlying element stream, except that failable
    400     * objects, like {@link FailablePredicate}, {@link FailableFunction}, or
    401     * {@link FailableConsumer} may be applied, instead of
    402     * {@link Predicate}, {@link Function}, or {@link Consumer}. The idea is
    403     * to rewrite a code snippet like this:
    404     * <pre>
    405     *     final List&lt;O&gt; list;
    406     *     final Method m;
    407     *     final Function&lt;O,String&gt; mapper = (o) -&gt; {
    408     *         try {
    409     *             return (String) m.invoke(o);
    410     *         } catch (Throwable t) {
    411     *             throw Functions.rethrow(t);
    412     *         }
    413     *     };
    414     *     final List&lt;String&gt; strList = list.stream()
    415     *         .map(mapper).collect(Collectors.toList());
    416     *  </pre>
    417     *  as follows:
    418     *  <pre>
    419     *     final List&lt;O&gt; list;
    420     *     final Method m;
    421     *     final List&lt;String&gt; strList = Functions.stream(list.stream())
    422     *         .map((o) -&gt; (String) m.invoke(o)).collect(Collectors.toList());
    423     *  </pre>
    424     *  While the second version may not be <em>quite</em> as
    425     *  efficient (because it depends on the creation of additional,
    426     *  intermediate objects, of type FailableStream), it is much more
    427     *  concise, and readable, and meets the spirit of Lambdas better
    428     *  than the first version.
    429     * @param <O> The streams element type.
    430     * @param stream The stream, which is being converted.
    431     * @return The {@link FailableStream}, which has been created by
    432     *   converting the stream.
    433     */
    434    public static <O> FailableStream<O> stream(final Collection<O> stream) {
    435        return stream(stream.stream());
    436    }
    437
    438    public static class ArrayCollector<O> implements Collector<O, List<O>, O[]> {
    439        private static final Set<Characteristics> characteristics = Collections.emptySet();
    440        private final Class<O> elementType;
    441
    442        public ArrayCollector(final Class<O> elementType) {
    443            this.elementType = elementType;
    444        }
    445
    446        @Override
    447        public Supplier<List<O>> supplier() {
    448            return () -> new ArrayList<>();
    449        }
    450
    451        @Override
    452        public BiConsumer<List<O>, O> accumulator() {
    453            return (list, o) -> {
    454                list.add(o);
    455            };
    456        }
    457
    458        @Override
    459        public BinaryOperator<List<O>> combiner() {
    460            return (left, right) -> {
    461                left.addAll(right);
    462                return left;
    463            };
    464        }
    465
    466        @Override
    467        public Function<List<O>, O[]> finisher() {
    468            return (list) -> {
    469                @SuppressWarnings("unchecked")
    470                final O[] array = (O[]) Array.newInstance(elementType, list.size());
    471                return list.toArray(array);
    472            };
    473        }
    474
    475        @Override
    476        public Set<Characteristics> characteristics() {
    477            return characteristics;
    478        }
    479    }
    480
    481    /**
    482     * Returns a {@code Collector} that accumulates the input elements into a
    483     * new array.
    484     *
    485     * @param pElementType Type of an element in the array.
    486     * @param <O> the type of the input elements
    487     * @return a {@code Collector} which collects all the input elements into an
    488     * array, in encounter order
    489     */
    490    public static <O extends Object> Collector<O, ?, O[]> toArray(final Class<O> pElementType) {
    491        return new ArrayCollector<>(pElementType);
    492    }
    493}