    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.stream;
    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.function.Failable;
    036import org.apache.commons.lang3.function.FailableConsumer;
    037import org.apache.commons.lang3.function.FailableFunction;
    038import org.apache.commons.lang3.function.FailablePredicate;
    039
    040/**
    041 * Provides utility functions, and classes for working with the
    042 * {@code java.util.stream} package, or more generally, with Java 8 lambdas. More
    043 * specifically, it attempts to address the fact that lambdas are supposed
    044 * not to throw Exceptions, at least not checked Exceptions, AKA instances
    045 * of {@link Exception}. This enforces the use of constructs like
    046 * <pre>
    047 *     Consumer&lt;java.lang.reflect.Method&gt; consumer = (m) -&gt; {
    048 *         try {
    049 *             m.invoke(o, args);
    050 *         } catch (Throwable t) {
    051 *             throw Failable.rethrow(t);
    052 *         }
    053 *    };
    054 *    stream.forEach(consumer);
    055 * </pre>
    056 * Using a {@link FailableStream}, this can be rewritten as follows:
    057 * <pre>
    058 *     Streams.failable(stream).forEach((m) -&gt; m.invoke(o, args));
    059 * </pre>
    060 * Obviously, the second version is much more concise and the spirit of
    061 * Lambda expressions is met better than in the first version.
    062 *
    063 * @see Stream
    064 * @see Failable
    065 * @since 3.11
    066 */
    067public class Streams {
    068
    069    public static class ArrayCollector<O> implements Collector<O, List<O>, O[]> {
    070        private static final Set<Characteristics> characteristics = Collections.emptySet();
    071        private final Class<O> elementType;
    072
    073        public ArrayCollector(final Class<O> elementType) {
    074            this.elementType = elementType;
    075        }
    076
    077        @Override
    078        public BiConsumer<List<O>, O> accumulator() {
    079            return List::add;
    080        }
    081
    082        @Override
    083        public Set<Characteristics> characteristics() {
    084            return characteristics;
    085        }
    086
    087        @Override
    088        public BinaryOperator<List<O>> combiner() {
    089            return (left, right) -> {
    090                left.addAll(right);
    091                return left;
    092            };
    093        }
    094
    095        @Override
    096        public Function<List<O>, O[]> finisher() {
    097            return list -> {
    098                @SuppressWarnings("unchecked")
    099                final O[] array = (O[]) Array.newInstance(elementType, list.size());
    100                return list.toArray(array);
    101            };
    102        }
    103
    104        @Override
    105        public Supplier<List<O>> supplier() {
    106            return ArrayList::new;
    107        }
    108    }
    109
    110    /**
    111     * A reduced, and simplified version of a {@link Stream} with failable method signatures.
    112     *
    113     * @param <O> The streams element type.
    114     */
    115    public static class FailableStream<O extends Object> {
    116
    117        private Stream<O> stream;
    118        private boolean terminated;
    119
    120        /**
    121         * Constructs a new instance with the given {@code stream}.
    122         *
    123         * @param stream The stream.
    124         */
    125        public FailableStream(final Stream<O> stream) {
    126            this.stream = stream;
    127        }
    128
    129        /**
    130         * Returns whether all elements of this stream match the provided predicate. May not evaluate the predicate on
    131         * all elements if not necessary for determining the result. If the stream is empty then {@code true} is
    132         * returned and the predicate is not evaluated.
    133         *
    134         * <p>
    135         * This is a short-circuiting terminal operation.
    136         *
    137         * Note This method evaluates the <em>universal quantification</em> of the predicate over the elements of
    138         * the stream (for all x P(x)). If the stream is empty, the quantification is said to be <em>vacuously
    139         * satisfied</em> and is always {@code true} (regardless of P(x)).
    140         *
    141         * @param predicate A non-interfering, stateless predicate to apply to elements of this stream
    142         * @return {@code true} If either all elements of the stream match the provided predicate or the stream is
    143         *         empty, otherwise {@code false}.
    144         */
    145        public boolean allMatch(final FailablePredicate<O, ?> predicate) {
    146            assertNotTerminated();
    147            return stream().allMatch(Failable.asPredicate(predicate));
    148        }
    149
    150        /**
    151         * Returns whether any elements of this stream match the provided predicate. May not evaluate the predicate on
    152         * all elements if not necessary for determining the result. If the stream is empty then {@code false} is
    153         * returned and the predicate is not evaluated.
    154         *
    155         * <p>
    156         * This is a short-circuiting terminal operation.
    157         *
    158         * Note This method evaluates the <em>existential quantification</em> of the predicate over the elements of
    159         * the stream (for some x P(x)).
    160         *
    161         * @param predicate A non-interfering, stateless predicate to apply to elements of this stream
    162         * @return {@code true} if any elements of the stream match the provided predicate, otherwise {@code false}
    163         */
    164        public boolean anyMatch(final FailablePredicate<O, ?> predicate) {
    165            assertNotTerminated();
    166            return stream().anyMatch(Failable.asPredicate(predicate));
    167        }
    168
    169        protected void assertNotTerminated() {
    170            if (terminated) {
    171                throw new IllegalStateException("This stream is already terminated.");
    172            }
    173        }
    174
    175        /**
    176         * Performs a mutable reduction operation on the elements of this stream using a {@code Collector}. A
    177         * {@code Collector} encapsulates the functions used as arguments to
    178         * {@link #collect(Supplier, BiConsumer, BiConsumer)}, allowing for reuse of collection strategies and
    179         * composition of collect operations such as multiple-level grouping or partitioning.
    180         *
    181         * <p>
    182         * If the underlying stream is parallel, and the {@code Collector} is concurrent, and either the stream is
    183         * unordered or the collector is unordered, then a concurrent reduction will be performed (see {@link Collector}
    184         * for details on concurrent reduction.)
    185         *
    186         * <p>
    187         * This is a terminal operation.
    188         *
    189         * <p>
    190         * When executed in parallel, multiple intermediate results may be instantiated, populated, and merged so as to
    191         * maintain isolation of mutable data structures. Therefore, even when executed in parallel with non-thread-safe
    192         * data structures (such as {@code ArrayList}), no additional synchronization is needed for a parallel
    193         * reduction.
    194         *
    195         * Note The following will accumulate strings into an ArrayList:
    196         *
    197         * <pre>
    198         *     {@code
    199         *     List<String> asList = stringStream.collect(Collectors.toList());
    200         * }
    201         * </pre>
    202         *
    203         * <p>
    204         * The following will classify {@code Person} objects by city:
    205         *
    206         * <pre>
    207         *     {@code
    208         *     Map<String, List<Person>> peopleByCity = personStream.collect(Collectors.groupingBy(Person::getCity));
    209         * }
    210         * </pre>
    211         *
    212         * <p>
    213         * The following will classify {@code Person} objects by state and city, cascading two {@code Collector}s
    214         * together:
    215         *
    216         * <pre>
    217         *     {@code
    218         *     Map<String, Map<String, List<Person>>> peopleByStateAndCity = personStream
    219         *         .collect(Collectors.groupingBy(Person::getState, Collectors.groupingBy(Person::getCity)));
    220         * }
    221         * </pre>
    222         *
    223         * @param <R> the type of the result
    224         * @param <A> the intermediate accumulation type of the {@code Collector}
    225         * @param collector the {@code Collector} describing the reduction
    226         * @return the result of the reduction
    227         * @see #collect(Supplier, BiConsumer, BiConsumer)
    228         * @see Collectors
    229         */
    230        public <A, R> R collect(final Collector<? super O, A, R> collector) {
    231            makeTerminated();
    232            return stream().collect(collector);
    233        }
    234
    235        /**
    236         * Performs a mutable reduction operation on the elements of this FailableStream. A mutable reduction is one in
    237         * which the reduced value is a mutable result container, such as an {@code ArrayList}, and elements are
    238         * incorporated by updating the state of the result rather than by replacing the result. This produces a result
    239         * equivalent to:
    240         *
    241         * <pre>
    242         * {@code
    243         *     R result = supplier.get();
    244         *     for (T element : this stream)
    245         *         accumulator.accept(result, element);
    246         *     return result;
    247         * }
    248         * </pre>
    249         *
    250         * <p>
    251         * Like {@link #reduce(Object, BinaryOperator)}, {@code collect} operations can be parallelized without
    252         * requiring additional synchronization.
    253         *
    254         * <p>
    255         * This is a terminal operation.
    256         *
    257         * Note There are many existing classes in the JDK whose signatures are well-suited for use with method
    258         * references as arguments to {@code collect()}. For example, the following will accumulate strings into an
    259         * {@code ArrayList}:
    260         *
    261         * <pre>
    262         *     {@code
    263         *     List<String> asList = stringStream.collect(ArrayList::new, ArrayList::add, ArrayList::addAll);
    264         * }
    265         * </pre>
    266         *
    267         * <p>
    268         * The following will take a stream of strings and concatenates them into a single string:
    269         *
    270         * <pre>
    271         *     {@code
    272         *     String concat = stringStream.collect(StringBuilder::new, StringBuilder::append, StringBuilder::append)
    273         *         .toString();
    274         * }
    275         * </pre>
    276         *
    277         * @param <R> type of the result
    278         * @param <A> Type of the accumulator.
    279         * @param pupplier a function that creates a new result container. For a parallel execution, this function may
    280         *        be called multiple times and must return a fresh value each time.
    281         * @param accumulator An associative, non-interfering, stateless function for incorporating an additional
    282         *        element into a result
    283         * @param combiner An associative, non-interfering, stateless function for combining two values, which must be
    284         *        compatible with the accumulator function
    285         * @return The result of the reduction
    286         */
    287        public <A, R> R collect(final Supplier<R> pupplier, final BiConsumer<R, ? super O> accumulator,
    288            final BiConsumer<R, R> combiner) {
    289            makeTerminated();
    290            return stream().collect(pupplier, accumulator, combiner);
    291        }
    292
    293        /**
    294         * Returns a FailableStream consisting of the elements of this stream that match the given FailablePredicate.
    295         *
    296         * <p>
    297         * This is an intermediate operation.
    298         *
    299         * @param predicate a non-interfering, stateless predicate to apply to each element to determine if it should be
    300         *        included.
    301         * @return the new stream
    302         */
    303        public FailableStream<O> filter(final FailablePredicate<O, ?> predicate) {
    304            assertNotTerminated();
    305            stream = stream.filter(Failable.asPredicate(predicate));
    306            return this;
    307        }
    308
    309        /**
    310         * Performs an action for each element of this stream.
    311         *
    312         * <p>
    313         * This is a terminal operation.
    314         *
    315         * <p>
    316         * The behavior of this operation is explicitly nondeterministic. For parallel stream pipelines, this operation
    317         * does <em>not</em> guarantee to respect the encounter order of the stream, as doing so would sacrifice the
    318         * benefit of parallelism. For any given element, the action may be performed at whatever time and in whatever
    319         * thread the library chooses. If the action accesses shared state, it is responsible for providing the required
    320         * synchronization.
    321         *
    322         * @param action a non-interfering action to perform on the elements
    323         */
    324        public void forEach(final FailableConsumer<O, ?> action) {
    325            makeTerminated();
    326            stream().forEach(Failable.asConsumer(action));
    327        }
    328
    329        protected void makeTerminated() {
    330            assertNotTerminated();
    331            terminated = true;
    332        }
    333
    334        /**
    335         * Returns a stream consisting of the results of applying the given function to the elements of this stream.
    336         *
    337         * <p>
    338         * This is an intermediate operation.
    339         *
    340         * @param <R> The element type of the new stream
    341         * @param mapper A non-interfering, stateless function to apply to each element
    342         * @return the new stream
    343         */
    344        public <R> FailableStream<R> map(final FailableFunction<O, R, ?> mapper) {
    345            assertNotTerminated();
    346            return new FailableStream<>(stream.map(Failable.asFunction(mapper)));
    347        }
    348
    349        /**
    350         * Performs a reduction on the elements of this stream, using the provided identity value and an associative
    351         * accumulation function, and returns the reduced value. This is equivalent to:
    352         *
    353         * <pre>
    354         * {@code
    355         *     T result = identity;
    356         *     for (T element : this stream)
    357         *         result = accumulator.apply(result, element)
    358         *     return result;
    359         * }
    360         * </pre>
    361         *
    362         * but is not constrained to execute sequentially.
    363         *
    364         * <p>
    365         * The {@code identity} value must be an identity for the accumulator function. This means that for all
    366         * {@code t}, {@code accumulator.apply(identity, t)} is equal to {@code t}. The {@code accumulator} function
    367         * must be an associative function.
    368         *
    369         * <p>
    370         * This is a terminal operation.
    371         *
    372         * Note Sum, min, max, average, and string concatenation are all special cases of reduction. Summing a
    373         * stream of numbers can be expressed as:
    374         *
    375         * <pre>
    376         *     {@code
    377         *     Integer sum = integers.reduce(0, (a, b) -> a + b);
    378         * }
    379         * </pre>
    380         *
    381         * or:
    382         *
    383         * <pre>
    384         *     {@code
    385         *     Integer sum = integers.reduce(0, Integer::sum);
    386         * }
    387         * </pre>
    388         *
    389         * <p>
    390         * While this may seem a more roundabout way to perform an aggregation compared to simply mutating a running
    391         * total in a loop, reduction operations parallelize more gracefully, without needing additional synchronization
    392         * and with greatly reduced risk of data races.
    393         *
    394         * @param identity the identity value for the accumulating function
    395         * @param accumulator an associative, non-interfering, stateless function for combining two values
    396         * @return the result of the reduction
    397         */
    398        public O reduce(final O identity, final BinaryOperator<O> accumulator) {
    399            makeTerminated();
    400            return stream().reduce(identity, accumulator);
    401        }
    402
    403        /**
    404         * Converts the FailableStream into an equivalent stream.
    405         *
    406         * @return A stream, which will return the same elements, which this FailableStream would return.
    407         */
    408        public Stream<O> stream() {
    409            return stream;
    410        }
    411    }
    412
    413    /**
    414     * Converts the given {@link Collection} into a {@link FailableStream}. This is basically a simplified, reduced
    415     * version of the {@link Stream} class, with the same underlying element stream, except that failable objects, like
    416     * {@link FailablePredicate}, {@link FailableFunction}, or {@link FailableConsumer} may be applied, instead of
    417     * {@link Predicate}, {@link Function}, or {@link Consumer}. The idea is to rewrite a code snippet like this:
    418     *
    419     * <pre>
    420     * final List&lt;O&gt; list;
    421     * final Method m;
    422     * final Function&lt;O, String&gt; mapper = (o) -&gt; {
    423     *     try {
    424     *         return (String) m.invoke(o);
    425     *     } catch (Throwable t) {
    426     *         throw Failable.rethrow(t);
    427     *     }
    428     * };
    429     * final List&lt;String&gt; strList = list.stream().map(mapper).collect(Collectors.toList());
    430     * </pre>
    431     *
    432     * as follows:
    433     *
    434     * <pre>
    435     * final List&lt;O&gt; list;
    436     * final Method m;
    437     * final List&lt;String&gt; strList = Failable.stream(list.stream()).map((o) -&gt; (String) m.invoke(o))
    438     *     .collect(Collectors.toList());
    439     * </pre>
    440     *
    441     * While the second version may not be <em>quite</em> as efficient (because it depends on the creation of
    442     * additional, intermediate objects, of type FailableStream), it is much more concise, and readable, and meets the
    443     * spirit of Lambdas better than the first version.
    444     *
    445     * @param <O> The streams element type.
    446     * @param stream The stream, which is being converted.
    447     * @return The {@link FailableStream}, which has been created by converting the stream.
    448     */
    449    public static <O> FailableStream<O> stream(final Collection<O> stream) {
    450        return stream(stream.stream());
    451    }
    452
    453    /**
    454     * Converts the given {@link Stream stream} into a {@link FailableStream}. This is basically a simplified, reduced
    455     * version of the {@link Stream} class, with the same underlying element stream, except that failable objects, like
    456     * {@link FailablePredicate}, {@link FailableFunction}, or {@link FailableConsumer} may be applied, instead of
    457     * {@link Predicate}, {@link Function}, or {@link Consumer}. The idea is to rewrite a code snippet like this:
    458     *
    459     * <pre>
    460     * final List&lt;O&gt; list;
    461     * final Method m;
    462     * final Function&lt;O, String&gt; mapper = (o) -&gt; {
    463     *     try {
    464     *         return (String) m.invoke(o);
    465     *     } catch (Throwable t) {
    466     *         throw Failable.rethrow(t);
    467     *     }
    468     * };
    469     * final List&lt;String&gt; strList = list.stream().map(mapper).collect(Collectors.toList());
    470     * </pre>
    471     *
    472     * as follows:
    473     *
    474     * <pre>
    475     * final List&lt;O&gt; list;
    476     * final Method m;
    477     * final List&lt;String&gt; strList = Failable.stream(list.stream()).map((o) -&gt; (String) m.invoke(o))
    478     *     .collect(Collectors.toList());
    479     * </pre>
    480     *
    481     * While the second version may not be <em>quite</em> as efficient (because it depends on the creation of
    482     * additional, intermediate objects, of type FailableStream), it is much more concise, and readable, and meets the
    483     * spirit of Lambdas better than the first version.
    484     *
    485     * @param <O> The streams element type.
    486     * @param stream The stream, which is being converted.
    487     * @return The {@link FailableStream}, which has been created by converting the stream.
    488     */
    489    public static <O> FailableStream<O> stream(final Stream<O> stream) {
    490        return new FailableStream<>(stream);
    491    }
    492
    493    /**
    494     * Returns a {@code Collector} that accumulates the input elements into a new array.
    495     *
    496     * @param pElementType Type of an element in the array.
    497     * @param <O> the type of the input elements
    498     * @return a {@code Collector} which collects all the input elements into an array, in encounter order
    499     */
    500    public static <O extends Object> Collector<O, ?, O[]> toArray(final Class<O> pElementType) {
    501        return new ArrayCollector<>(pElementType);
    502    }
    503}