Source code
Description: source: package: org.apache.commons.lang3.stream, class: Streams, class: ArrayCollector

    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 *      https://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.util.ArrayList;
    020import java.util.Collection;
    021import java.util.Collections;
    022import java.util.Enumeration;
    023import java.util.Iterator;
    024import java.util.List;
    025import java.util.Objects;
    026import java.util.Set;
    027import java.util.Spliterator;
    028import java.util.Spliterators;
    029import java.util.Spliterators.AbstractSpliterator;
    030import java.util.function.BiConsumer;
    031import java.util.function.BinaryOperator;
    032import java.util.function.Consumer;
    033import java.util.function.Function;
    034import java.util.function.Predicate;
    035import java.util.function.Supplier;
    036import java.util.stream.Collector;
    037import java.util.stream.Collectors;
    038import java.util.stream.Stream;
    039import java.util.stream.StreamSupport;
    040
    041import org.apache.commons.lang3.ArrayUtils;
    042import org.apache.commons.lang3.function.Failable;
    043import org.apache.commons.lang3.function.FailableConsumer;
    044import org.apache.commons.lang3.function.FailableFunction;
    045import org.apache.commons.lang3.function.FailablePredicate;
    046
    047/**
    048 * Provides utility functions, and classes for working with the {@link java.util.stream} package, or more generally, with Java 8 lambdas. More specifically, it
    049 * attempts to address the fact that lambdas are supposed not to throw Exceptions, at least not checked Exceptions, AKA instances of {@link Exception}. This
    050 * enforces the use of constructs like:
    051 *
    052 * <pre>{@code
    053 * Consumer<java.lang.reflect.Method> consumer = m -> {
    054 *     try {
    055 *         m.invoke(o, args);
    056 *     } catch (Throwable t) {
    057 *         throw Failable.rethrow(t);
    058 *     }
    059 * };
    060 * stream.forEach(consumer);
    061 * }</pre>
    062 * <p>
    063 * Using a {@link FailableStream}, this can be rewritten as follows:
    064 * </p>
    065 *
    066 * <pre>{@code
    067 * Streams.failable(stream).forEach(m -> m.invoke(o, args));
    068 * }</pre>
    069 * <p>
    070 * Obviously, the second version is much more concise and the spirit of Lambda expressions is met better than in the first version.
    071 * </p>
    072 *
    073 * @see Stream
    074 * @see Failable
    075 * @since 3.11
    076 */
    077public class Streams {
    078
    079    /**
    080     * A Collector type for arrays.
    081     *
    082     * @param <E> The array type.
    083     */
    084    public static class ArrayCollector<E> implements Collector<E, List<E>, E[]> {
    085        private static final Set<Characteristics> characteristics = Collections.emptySet();
    086        private final Class<E> elementType;
    087
    088        /**
    089         * Constructs a new instance for the given element type.
    090         *
    091         * @param elementType The element type.
    092         */
    093        public ArrayCollector(final Class<E> elementType) {
    094            this.elementType = Objects.requireNonNull(elementType, "elementType");
    095        }
    096
    097        @Override
    098        public BiConsumer<List<E>, E> accumulator() {
    099            return List::add;
    100        }
    101
    102        @Override
    103        public Set<Characteristics> characteristics() {
    104            return characteristics;
    105        }
    106
    107        @Override
    108        public BinaryOperator<List<E>> combiner() {
    109            return (left, right) -> {
    110                left.addAll(right);
    111                return left;
    112            };
    113        }
    114
    115        @Override
    116        public Function<List<E>, E[]> finisher() {
    117            return list -> list.toArray(ArrayUtils.newInstance(elementType, list.size()));
    118        }
    119
    120        @Override
    121        public Supplier<List<E>> supplier() {
    122            return ArrayList::new;
    123        }
    124    }
    125
    126    /**
    127     * Helps implement {@link Streams#of(Enumeration)}.
    128     *
    129     * @param <T> The element type.
    130     */
    131    private static final class EnumerationSpliterator<T> extends AbstractSpliterator<T> {
    132
    133        private final Enumeration<T> enumeration;
    134
    135        /**
    136         * Creates a spliterator reporting the given estimated size and additionalCharacteristics.
    137         *
    138         * @param estimatedSize the estimated size of this spliterator if known, otherwise {@code Long.MAX_VALUE}.
    139         * @param additionalCharacteristics properties of this spliterator's source or elements. If {@code SIZED} is reported then this spliterator will
    140         *        additionally report {@code SUBSIZED}.
    141         * @param enumeration The Enumeration to wrap.
    142         */
    143        protected EnumerationSpliterator(final long estimatedSize, final int additionalCharacteristics, final Enumeration<T> enumeration) {
    144            super(estimatedSize, additionalCharacteristics);
    145            this.enumeration = Objects.requireNonNull(enumeration, "enumeration");
    146        }
    147
    148        @Override
    149        public void forEachRemaining(final Consumer<? super T> action) {
    150            while (enumeration.hasMoreElements()) {
    151                next(action);
    152            }
    153        }
    154
    155        private boolean next(final Consumer<? super T> action) {
    156            action.accept(enumeration.nextElement());
    157            return true;
    158
    159        }
    160
    161        @Override
    162        public boolean tryAdvance(final Consumer<? super T> action) {
    163            return enumeration.hasMoreElements() && next(action);
    164        }
    165    }
    166
    167    /**
    168     * A reduced, and simplified version of a {@link Stream} with failable method signatures.
    169     *
    170     * @param <T> The streams element type.
    171     */
    172    public static class FailableStream<T> {
    173
    174        private Stream<T> stream;
    175        private boolean terminated;
    176
    177        /**
    178         * Constructs a new instance with the given {@code stream}.
    179         *
    180         * @param stream The stream.
    181         */
    182        public FailableStream(final Stream<T> stream) {
    183            this.stream = stream;
    184        }
    185
    186        /**
    187         * Returns whether all elements of this stream match the provided predicate. May not evaluate the predicate on all
    188         * elements if not necessary for determining the result. If the stream is empty then {@code true} is returned and the
    189         * predicate is not evaluated.
    190         *
    191         * <p>
    192         * This is a short-circuiting terminal operation.
    193         * </p>
    194         *
    195         * Note This method evaluates the <em>universal quantification</em> of the predicate over the elements of the stream
    196         * (for all x P(x)). If the stream is empty, the quantification is said to be <em>vacuously satisfied</em> and is always
    197         * {@code true} (regardless of P(x)).
    198         *
    199         * @param predicate A non-interfering, stateless predicate to apply to elements of this stream
    200         * @return {@code true} If either all elements of the stream match the provided predicate or the stream is empty,
    201         *         otherwise {@code false}.
    202         */
    203        public boolean allMatch(final FailablePredicate<T, ?> predicate) {
    204            assertNotTerminated();
    205            return stream().allMatch(Failable.asPredicate(predicate));
    206        }
    207
    208        /**
    209         * Returns whether any elements of this stream match the provided predicate. May not evaluate the predicate on all
    210         * elements if not necessary for determining the result. If the stream is empty then {@code false} is returned and the
    211         * predicate is not evaluated.
    212         *
    213         * <p>
    214         * This is a short-circuiting terminal operation.
    215         * </p>
    216         *
    217         * Note This method evaluates the <em>existential quantification</em> of the predicate over the elements of the stream
    218         * (for some x P(x)).
    219         *
    220         * @param predicate A non-interfering, stateless predicate to apply to elements of this stream
    221         * @return {@code true} if any elements of the stream match the provided predicate, otherwise {@code false}
    222         */
    223        public boolean anyMatch(final FailablePredicate<T, ?> predicate) {
    224            assertNotTerminated();
    225            return stream().anyMatch(Failable.asPredicate(predicate));
    226        }
    227
    228        /**
    229         * Throws IllegalStateException if this stream is already terminated.
    230         *
    231         * @throws IllegalStateException if this stream is already terminated.
    232         */
    233        protected void assertNotTerminated() {
    234            if (terminated) {
    235                throw new IllegalStateException("This stream is already terminated.");
    236            }
    237        }
    238
    239        /**
    240         * Performs a mutable reduction operation on the elements of this stream using a {@link Collector}. A {@link Collector}
    241         * encapsulates the functions used as arguments to {@link #collect(Supplier, BiConsumer, BiConsumer)}, allowing for
    242         * reuse of collection strategies and composition of collect operations such as multiple-level grouping or partitioning.
    243         *
    244         * <p>
    245         * If the underlying stream is parallel, and the {@link Collector} is concurrent, and either the stream is unordered or
    246         * the collector is unordered, then a concurrent reduction will be performed (see {@link Collector} for details on
    247         * concurrent reduction.)
    248         * </p>
    249         *
    250         * <p>
    251         * This is a terminal operation.
    252         * </p>
    253         *
    254         * <p>
    255         * When executed in parallel, multiple intermediate results may be instantiated, populated, and merged so as to maintain
    256         * isolation of mutable data structures. Therefore, even when executed in parallel with non-thread-safe data structures
    257         * (such as {@link ArrayList}), no additional synchronization is needed for a parallel reduction.
    258         * </p>
    259         *
    260         * Note The following will accumulate strings into an ArrayList:
    261         *
    262         * <pre>
    263         * {@code
    264         *     List<String> asList = stringStream.collect(Collectors.toList());
    265         * }
    266         * </pre>
    267         *
    268         * <p>
    269         * The following will classify {@code Person} objects by city:
    270         * </p>
    271         *
    272         * <pre>
    273         * {@code
    274         *     Map<String, List<Person>> peopleByCity = personStream.collect(Collectors.groupingBy(Person::getCity));
    275         * }
    276         * </pre>
    277         *
    278         * <p>
    279         * The following will classify {@code Person} objects by state and city, cascading two {@link Collector}s together:
    280         * </p>
    281         *
    282         * <pre>
    283         * {@code
    284         *     Map<String, Map<String, List<Person>>> peopleByStateAndCity = personStream
    285         *         .collect(Collectors.groupingBy(Person::getState, Collectors.groupingBy(Person::getCity)));
    286         * }
    287         * </pre>
    288         *
    289         * @param <R> the type of the result
    290         * @param <A> the intermediate accumulation type of the {@link Collector}
    291         * @param collector the {@link Collector} describing the reduction
    292         * @return the result of the reduction
    293         * @see #collect(Supplier, BiConsumer, BiConsumer)
    294         * @see Collectors
    295         */
    296        public <A, R> R collect(final Collector<? super T, A, R> collector) {
    297            makeTerminated();
    298            return stream().collect(collector);
    299        }
    300
    301        /**
    302         * Performs a mutable reduction operation on the elements of this FailableStream. A mutable reduction is one in which
    303         * the reduced value is a mutable result container, such as an {@link ArrayList}, and elements are incorporated by
    304         * updating the state of the result rather than by replacing the result. This produces a result equivalent to:
    305         * <pre>
    306         * {@code
    307         *     R result = supplier.get();
    308         *     for (T element : this stream)
    309         *         accumulator.accept(result, element);
    310         *     return result;
    311         * }
    312         * </pre>
    313         * <p>
    314         * Like {@link #reduce(Object, BinaryOperator)}, {@code collect} operations can be parallelized without requiring
    315         * additional synchronization.
    316         * </p>
    317         * <p>
    318         * This is a terminal operation.
    319         * </p>
    320         * <p>
    321         * Note There are many existing classes in the JDK whose signatures are well-suited for use with method references as
    322         * arguments to {@code collect()}. For example, the following will accumulate strings into an {@link ArrayList}:
    323         * </p>
    324         * <pre>
    325         * {@code
    326         *     List<String> asList = stringStream.collect(ArrayList::new, ArrayList::add, ArrayList::addAll);
    327         * }
    328         * </pre>
    329         * <p>
    330         * The following will take a stream of strings and concatenates them into a single string:
    331         * </p>
    332         * <pre>
    333         * {@code
    334         *     String concat = stringStream.collect(StringBuilder::new, StringBuilder::append, StringBuilder::append).toString();
    335         * }
    336         * </pre>
    337         *
    338         * @param <R> type of the result
    339         * @param <A> Type of the accumulator.
    340         * @param supplier a function that creates a new result container. For a parallel execution, this function may be called
    341         *        multiple times and must return a fresh value each time.
    342         * @param accumulator An associative, non-interfering, stateless function for incorporating an additional element into a
    343         *        result
    344         * @param combiner An associative, non-interfering, stateless function for combining two values, which must be
    345         *        compatible with the accumulator function
    346         * @return The result of the reduction
    347         */
    348        public <A, R> R collect(final Supplier<R> supplier, final BiConsumer<R, ? super T> accumulator, final BiConsumer<R, R> combiner) {
    349            makeTerminated();
    350            return stream().collect(supplier, accumulator, combiner);
    351        }
    352
    353        /**
    354         * Returns a FailableStream consisting of the elements of this stream that match the given FailablePredicate.
    355         * <p>
    356         * This is an intermediate operation.
    357         * </p>
    358         *
    359         * @param predicate a non-interfering, stateless predicate to apply to each element to determine if it should be
    360         *        included.
    361         * @return the new stream
    362         */
    363        public FailableStream<T> filter(final FailablePredicate<T, ?> predicate) {
    364            assertNotTerminated();
    365            stream = stream.filter(Failable.asPredicate(predicate));
    366            return this;
    367        }
    368
    369        /**
    370         * Performs an action for each element of this stream.
    371         * <p>
    372         * This is a terminal operation.
    373         * </p>
    374         * <p>
    375         * The behavior of this operation is explicitly nondeterministic. For parallel stream pipelines, this operation does
    376         * <em>not</em> guarantee to respect the encounter order of the stream, as doing so would sacrifice the benefit of
    377         * parallelism. For any given element, the action may be performed at whatever time and in whatever thread the library
    378         * chooses. If the action accesses shared state, it is responsible for providing the required synchronization.
    379         * </p>
    380         *
    381         * @param action a non-interfering action to perform on the elements
    382         */
    383        public void forEach(final FailableConsumer<T, ?> action) {
    384            makeTerminated();
    385            stream().forEach(Failable.asConsumer(action));
    386        }
    387
    388        /**
    389         * Marks this stream as terminated.
    390         *
    391         * @throws IllegalStateException if this stream is already terminated.
    392         */
    393        protected void makeTerminated() {
    394            assertNotTerminated();
    395            terminated = true;
    396        }
    397
    398        /**
    399         * Returns a stream consisting of the results of applying the given function to the elements of this stream.
    400         *
    401         * <p>
    402         * This is an intermediate operation.
    403         * </p>
    404         *
    405         * @param <R> The element type of the new stream
    406         * @param mapper A non-interfering, stateless function to apply to each element
    407         * @return the new stream
    408         */
    409        public <R> FailableStream<R> map(final FailableFunction<T, R, ?> mapper) {
    410            assertNotTerminated();
    411            return new FailableStream<>(stream.map(Failable.asFunction(mapper)));
    412        }
    413
    414        /**
    415         * Performs a reduction on the elements of this stream, using the provided identity value and an associative
    416         * accumulation function, and returns the reduced value. This is equivalent to:
    417         *
    418         * <pre>
    419         * {@code
    420         *     T result = identity;
    421         *     for (T element : this stream)
    422         *         result = accumulator.apply(result, element)
    423         *     return result;
    424         * }
    425         * </pre>
    426         *
    427         * but is not constrained to execute sequentially.
    428         *
    429         * <p>
    430         * The {@code identity} value must be an identity for the accumulator function. This means that for all {@code t},
    431         * {@code accumulator.apply(identity, t)} is equal to {@code t}. The {@code accumulator} function must be an associative
    432         * function.
    433         * </p>
    434         *
    435         * <p>
    436         * This is a terminal operation.
    437         * </p>
    438         *
    439         * Note Sum, min, max, average, and string concatenation are all special cases of reduction. Summing a stream of numbers
    440         * can be expressed as:
    441         *
    442         * <pre>
    443         * {@code
    444         *     Integer sum = integers.reduce(0, (a, b) -> a + b);
    445         * }
    446         * </pre>
    447         *
    448         * or:
    449         *
    450         * <pre>
    451         * {@code
    452         *     Integer sum = integers.reduce(0, Integer::sum);
    453         * }
    454         * </pre>
    455         *
    456         * <p>
    457         * While this may seem a more roundabout way to perform an aggregation compared to simply mutating a running total in a
    458         * loop, reduction operations parallelize more gracefully, without needing additional synchronization and with greatly
    459         * reduced risk of data races.
    460         * </p>
    461         *
    462         * @param identity the identity value for the accumulating function
    463         * @param accumulator an associative, non-interfering, stateless function for combining two values
    464         * @return the result of the reduction
    465         */
    466        public T reduce(final T identity, final BinaryOperator<T> accumulator) {
    467            makeTerminated();
    468            return stream().reduce(identity, accumulator);
    469        }
    470
    471        /**
    472         * Converts the FailableStream into an equivalent stream.
    473         *
    474         * @return A stream, which will return the same elements, which this FailableStream would return.
    475         */
    476        public Stream<T> stream() {
    477            return stream;
    478        }
    479    }
    480
    481    /**
    482     * Converts the given {@link Collection} into a {@link FailableStream}. This is basically a simplified, reduced version
    483     * of the {@link Stream} class, with the same underlying element stream, except that failable objects, like
    484     * {@link FailablePredicate}, {@link FailableFunction}, or {@link FailableConsumer} may be applied, instead of
    485     * {@link Predicate}, {@link Function}, or {@link Consumer}. The idea is to rewrite a code snippet like this:
    486     *
    487     * <pre>
    488     * {@code
    489     * final List<O> list;
    490     * final Method m;
    491     * final Function<O, String> mapper = (o) -> {
    492     *     try {
    493     *         return (String) m.invoke(o);
    494     *     } catch (Throwable t) {
    495     *         throw Failable.rethrow(t);
    496     *     }
    497     * };
    498     * final List<String> strList = list.stream().map(mapper).collect(Collectors.toList());
    499     * }
    500     * </pre>
    501     *
    502     * as follows:
    503     *
    504     * <pre>
    505     * {@code
    506     * final List<O> list;
    507     * final Method m;
    508     * final List<String> strList = Failable.stream(list.stream()).map((o) -> (String) m.invoke(o)).collect(Collectors.toList());
    509     * }
    510     * </pre>
    511     *
    512     * While the second version may not be <em>quite</em> as efficient (because it depends on the creation of additional,
    513     * intermediate objects, of type FailableStream), it is much more concise, and readable, and meets the spirit of Lambdas
    514     * better than the first version.
    515     *
    516     * @param <T> The streams element type.
    517     * @param stream The stream, which is being converted.
    518     * @return The {@link FailableStream}, which has been created by converting the stream.
    519     * @since 3.13.0
    520     */
    521    public static <T> FailableStream<T> failableStream(final Collection<T> stream) {
    522        return failableStream(of(stream));
    523    }
    524
    525    /**
    526     * Converts the given {@link Stream stream} into a {@link FailableStream}. This is basically a simplified, reduced
    527     * version of the {@link Stream} class, with the same underlying element stream, except that failable objects, like
    528     * {@link FailablePredicate}, {@link FailableFunction}, or {@link FailableConsumer} may be applied, instead of
    529     * {@link Predicate}, {@link Function}, or {@link Consumer}. The idea is to rewrite a code snippet like this:
    530     *
    531     * <pre>
    532     * {@code
    533     * final List<O> list;
    534     * final Method m;
    535     * final Function<O, String> mapper = (o) -> {
    536     *     try {
    537     *         return (String) m.invoke(o);
    538     *     } catch (Throwable t) {
    539     *         throw Failable.rethrow(t);
    540     *     }
    541     * };
    542     * final List<String> strList = list.stream().map(mapper).collect(Collectors.toList());
    543     * }
    544     * </pre>
    545     *
    546     * as follows:
    547     *
    548     * <pre>
    549     * {@code
    550     * final List<O> list;
    551     * final Method m;
    552     * final List<String> strList = Failable.stream(list.stream()).map((o) -> (String) m.invoke(o)).collect(Collectors.toList());
    553     * }
    554     * </pre>
    555     *
    556     * While the second version may not be <em>quite</em> as efficient (because it depends on the creation of additional,
    557     * intermediate objects, of type FailableStream), it is much more concise, and readable, and meets the spirit of Lambdas
    558     * better than the first version.
    559     *
    560     * @param <T> The streams element type.
    561     * @param stream The stream, which is being converted.
    562     * @return The {@link FailableStream}, which has been created by converting the stream.
    563     * @since 3.13.0
    564     */
    565    public static <T> FailableStream<T> failableStream(final Stream<T> stream) {
    566        return new FailableStream<>(stream);
    567    }
    568
    569    /**
    570     * Shorthand for {@code Streams.failableStream(value == null ? Stream.empty() : Stream.of(value))}.
    571     *
    572     * @param <T> the type of stream elements.
    573     * @param value the single element of the new stream, may be {@code null}.
    574     * @return the new FailableStream on {@code value} or an empty stream.
    575     * @since 3.15.0
    576     */
    577    public static <T> FailableStream<T> failableStream(final T value) {
    578        return failableStream(streamOf(value));
    579    }
    580
    581    /**
    582     * Shorthand for {@code Streams.failableStream(Streams.of(arrayValues))}.
    583     *
    584     * @param <T> the type of stream elements.
    585     * @param values the elements of the new stream, may be {@code null}.
    586     * @return the new FailableStream on {@code values} or an empty stream.
    587     * @since 3.14.0
    588     */
    589    @SafeVarargs // Creating a stream from an array is safe
    590    public static <T> FailableStream<T> failableStream(final T... values) {
    591        return failableStream(of(values));
    592    }
    593
    594    /**
    595     * Streams only instances of the give Class in a collection.
    596     * <p>
    597     * This method shorthand for:
    598     * </p>
    599     * <pre>
    600     * {@code (Stream<E>) Streams.toStream(collection).filter(collection, SomeClass.class::isInstance);}
    601     * </pre>
    602     *
    603     * @param <E> the type of elements in the collection we want to stream.
    604     * @param clazz the type of elements in the collection we want to stream.
    605     * @param collection the collection to stream or null.
    606     * @return A non-null stream that only provides instances we want.
    607     * @since 3.13.0
    608     */
    609    public static <E> Stream<E> instancesOf(final Class<? super E> clazz, final Collection<? super E> collection) {
    610        return instancesOf(clazz, of(collection));
    611    }
    612
    613    @SuppressWarnings("unchecked") // After the isInstance check, we still need to type-cast.
    614    private static <E> Stream<E> instancesOf(final Class<? super E> clazz, final Stream<?> stream) {
    615        return (Stream<E>) of(stream).filter(clazz::isInstance);
    616    }
    617
    618    /**
    619     * Streams the non-null elements of a collection.
    620     *
    621     * @param <E> the type of elements in the collection.
    622     * @param collection the collection to stream or null.
    623     * @return A non-null stream that filters out null elements.
    624     * @since 3.13.0
    625     */
    626    public static <E> Stream<E> nonNull(final Collection<E> collection) {
    627        return of(collection).filter(Objects::nonNull);
    628    }
    629
    630    /**
    631     * Streams the non-null element.
    632     *
    633     * @param <E> the type of elements in the collection.
    634     * @param array the element to stream or null.
    635     * @return A non-null stream that filters out a null element.
    636     * @since 3.15.0
    637     */
    638    public static <E> Stream<E> nonNull(final E array) {
    639        return nonNull(streamOf(array));
    640    }
    641
    642    /**
    643     * Streams the non-null elements of an array.
    644     *
    645     * @param <E> the type of elements in the collection.
    646     * @param array the array to stream or null.
    647     * @return A non-null stream that filters out null elements.
    648     * @since 3.13.0
    649     */
    650    @SafeVarargs
    651    public static <E> Stream<E> nonNull(final E... array) {
    652        return nonNull(of(array));
    653    }
    654
    655    /**
    656     * Streams the non-null elements of a stream.
    657     *
    658     * @param <E> the type of elements in the collection.
    659     * @param stream the stream to stream or null.
    660     * @return A non-null stream that filters out null elements.
    661     * @since 3.13.0
    662     */
    663    public static <E> Stream<E> nonNull(final Stream<E> stream) {
    664        return of(stream).filter(Objects::nonNull);
    665    }
    666
    667    /**
    668     * Delegates to {@link Collection#stream()} or returns {@link Stream#empty()} if the collection is null.
    669     *
    670     * @param <E> the type of elements in the collection.
    671     * @param collection the collection to stream or null.
    672     * @return {@link Collection#stream()} or {@link Stream#empty()} if the collection is null.
    673     * @since 3.13.0
    674     */
    675    public static <E> Stream<E> of(final Collection<E> collection) {
    676        return collection == null ? Stream.empty() : collection.stream();
    677    }
    678
    679    /**
    680     * Streams the elements of the given enumeration in order.
    681     *
    682     * @param <E> The enumeration element type.
    683     * @param enumeration The enumeration to stream.
    684     * @return a new stream.
    685     * @since 3.13.0
    686     */
    687    public static <E> Stream<E> of(final Enumeration<E> enumeration) {
    688        return StreamSupport.stream(new EnumerationSpliterator<>(Long.MAX_VALUE, Spliterator.ORDERED, enumeration), false);
    689    }
    690
    691    /**
    692     * Creates a sequential stream on the given Iterable.
    693     *
    694     * @param <E> the type of elements in the Iterable.
    695     * @param iterable the Iterable to stream or null.
    696     * @return a new Stream or {@link Stream#empty()} if the Iterable is null.
    697     * @since 3.13.0
    698     */
    699    public static <E> Stream<E> of(final Iterable<E> iterable) {
    700        return iterable == null ? Stream.empty() : StreamSupport.stream(iterable.spliterator(), false);
    701    }
    702
    703    /**
    704     * Creates a sequential stream on the given Iterator.
    705     *
    706     * @param <E> the type of elements in the Iterator.
    707     * @param iterator the Iterator to stream or null.
    708     * @return a new Stream or {@link Stream#empty()} if the Iterator is null.
    709     * @since 3.13.0
    710     */
    711    public static <E> Stream<E> of(final Iterator<E> iterator) {
    712        return iterator == null ? Stream.empty() : StreamSupport.stream(Spliterators.spliteratorUnknownSize(iterator, Spliterator.ORDERED), false);
    713    }
    714
    715    /**
    716     * Returns the stream or {@link Stream#empty()} if the stream is null.
    717     *
    718     * @param <E> the type of elements in the collection.
    719     * @param stream the stream to stream or null.
    720     * @return the stream or {@link Stream#empty()} if the stream is null.
    721     * @since 3.13.0
    722     */
    723    private static <E> Stream<E> of(final Stream<E> stream) {
    724        return stream == null ? Stream.empty() : stream;
    725    }
    726
    727    /**
    728     * Null-safe version of {@link Stream#of(Object[])}.
    729     *
    730     * @param <T> the type of stream elements.
    731     * @param values the elements of the new stream, may be {@code null}.
    732     * @return the new stream on {@code values} or {@link Stream#empty()}.
    733     * @since 3.13.0
    734     */
    735    @SafeVarargs // Creating a stream from an array is safe
    736    public static <T> Stream<T> of(final T... values) {
    737        return values == null ? Stream.empty() : Stream.of(values);
    738    }
    739
    740    /**
    741     * Converts the given {@link Collection} into a {@link FailableStream}. This is basically a simplified, reduced version
    742     * of the {@link Stream} class, with the same underlying element stream, except that failable objects, like
    743     * {@link FailablePredicate}, {@link FailableFunction}, or {@link FailableConsumer} may be applied, instead of
    744     * {@link Predicate}, {@link Function}, or {@link Consumer}. The idea is to rewrite a code snippet like this:
    745     *
    746     * <pre>
    747     * {@code
    748     * final List<O> list;
    749     * final Method m;
    750     * final Function<O, String> mapper = (o) -> {
    751     *     try {
    752     *         return (String) m.invoke(o);
    753     *     } catch (Throwable t) {
    754     *         throw Failable.rethrow(t);
    755     *     }
    756     * };
    757     * final List<String> strList = list.stream().map(mapper).collect(Collectors.toList());
    758     * }
    759     * </pre>
    760     *
    761     * as follows:
    762     *
    763     * <pre>
    764     * {@code
    765     * final List<O> list;
    766     * final Method m;
    767     * final List<String> strList = Failable.stream(list.stream()).map((o) -> (String) m.invoke(o)).collect(Collectors.toList());
    768     * }
    769     * </pre>
    770     *
    771     * While the second version may not be <em>quite</em> as efficient (because it depends on the creation of additional,
    772     * intermediate objects, of type FailableStream), it is much more concise, and readable, and meets the spirit of Lambdas
    773     * better than the first version.
    774     *
    775     * @param <E> The streams element type.
    776     * @param collection The stream, which is being converted.
    777     * @return The {@link FailableStream}, which has been created by converting the stream.
    778     * @deprecated Use {@link #failableStream(Collection)}.
    779     */
    780    @Deprecated
    781    public static <E> FailableStream<E> stream(final Collection<E> collection) {
    782        return failableStream(collection);
    783    }
    784
    785    /**
    786     * Converts the given {@link Stream stream} into a {@link FailableStream}. This is basically a simplified, reduced
    787     * version of the {@link Stream} class, with the same underlying element stream, except that failable objects, like
    788     * {@link FailablePredicate}, {@link FailableFunction}, or {@link FailableConsumer} may be applied, instead of
    789     * {@link Predicate}, {@link Function}, or {@link Consumer}. The idea is to rewrite a code snippet like this:
    790     *
    791     * <pre>
    792     * {@code
    793     * final List<O> list;
    794     * final Method m;
    795     * final Function<O, String> mapper = (o) -> {
    796     *     try {
    797     *         return (String) m.invoke(o);
    798     *     } catch (Throwable t) {
    799     *         throw Failable.rethrow(t);
    800     *     }
    801     * };
    802     * final List<String> strList = list.stream().map(mapper).collect(Collectors.toList());
    803     * }
    804     * </pre>
    805     *
    806     * as follows:
    807     *
    808     * <pre>
    809     * {@code
    810     * final List<O> list;
    811     * final Method m;
    812     * final List<String> strList = Failable.stream(list.stream()).map((o) -> (String) m.invoke(o)).collect(Collectors.toList());
    813     * }
    814     * </pre>
    815     *
    816     * While the second version may not be <em>quite</em> as efficient (because it depends on the creation of additional,
    817     * intermediate objects, of type FailableStream), it is much more concise, and readable, and meets the spirit of Lambdas
    818     * better than the first version.
    819     *
    820     * @param <T> The streams element type.
    821     * @param stream The stream, which is being converted.
    822     * @return The {@link FailableStream}, which has been created by converting the stream.
    823     * @deprecated Use {@link #failableStream(Stream)}.
    824     */
    825    @Deprecated
    826    public static <T> FailableStream<T> stream(final Stream<T> stream) {
    827        return failableStream(stream);
    828    }
    829
    830    private static <T> Stream<T> streamOf(final T value) {
    831        return value == null ? Stream.empty() : Stream.of(value);
    832    }
    833
    834    /**
    835     * Returns a {@link Collector} that accumulates the input elements into a new array.
    836     *
    837     * @param <T> the type of the input elements
    838     * @param elementType Type of an element in the array.
    839     * @return a {@link Collector} which collects all the input elements into an array, in encounter order
    840     */
    841    public static <T> Collector<T, List<T>, T[]> toArray(final Class<T> elementType) {
    842        return new ArrayCollector<>(elementType);
    843    }
    844
    845    /**
    846     * Constructs a new instance.
    847     *
    848     * @deprecated Will be private in 4.0.0.
    849     */
    850    @Deprecated
    851    public Streams() {
    852        // empty
    853    }
    854}