    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.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,
    049 * with Java 8 lambdas. More specifically, it attempts to address the fact that lambdas are supposed not to throw
    050 * Exceptions, at least not checked Exceptions, AKA instances of {@link Exception}. This enforces the use of constructs
    051 * like:
    052 *
    053 * <pre>
    054 * {@code
    055 * Consumer<java.lang.reflect.Method> consumer = m -> {
    056 *     try {
    057 *         m.invoke(o, args);
    058 *     } catch (Throwable t) {
    059 *         throw Failable.rethrow(t);
    060 *     }
    061 * };
    062 * stream.forEach(consumer);
    063 * }
    064 * </pre>
    065 * <p>
    066 * Using a {@link FailableStream}, this can be rewritten as follows:
    067 * </p>
    068 *
    069 * <pre>
    070 * {@code
    071 * Streams.failable(stream).forEach((m) -> m.invoke(o, args));
    072 * }
    073 * </pre>
    074 *
    075 * Obviously, the second version is much more concise and the spirit of Lambda expressions is met better than in the
    076 * first version.
    077 *
    078 * @see Stream
    079 * @see Failable
    080 * @since 3.11
    081 */
    082public class Streams {
    083
    084    /**
    085     * A Collector type for arrays.
    086     *
    087     * @param <E> The array type.
    088     */
    089    public static class ArrayCollector<E> implements Collector<E, List<E>, E[]> {
    090        private static final Set<Characteristics> characteristics = Collections.emptySet();
    091        private final Class<E> elementType;
    092
    093        /**
    094         * Constructs a new instance for the given element type.
    095         *
    096         * @param elementType The element type.
    097         */
    098        public ArrayCollector(final Class<E> elementType) {
    099            this.elementType = Objects.requireNonNull(elementType, "elementType");
    100        }
    101
    102        @Override
    103        public BiConsumer<List<E>, E> accumulator() {
    104            return List::add;
    105        }
    106
    107        @Override
    108        public Set<Characteristics> characteristics() {
    109            return characteristics;
    110        }
    111
    112        @Override
    113        public BinaryOperator<List<E>> combiner() {
    114            return (left, right) -> {
    115                left.addAll(right);
    116                return left;
    117            };
    118        }
    119
    120        @Override
    121        public Function<List<E>, E[]> finisher() {
    122            return list -> list.toArray(ArrayUtils.newInstance(elementType, list.size()));
    123        }
    124
    125        @Override
    126        public Supplier<List<E>> supplier() {
    127            return ArrayList::new;
    128        }
    129    }
    130
    131    /**
    132     * Helps implement {@link Streams#of(Enumeration)}.
    133     *
    134     * @param <T> The element type.
    135     */
    136    private static final class EnumerationSpliterator<T> extends AbstractSpliterator<T> {
    137
    138        private final Enumeration<T> enumeration;
    139
    140        /**
    141         * Creates a spliterator reporting the given estimated size and additionalCharacteristics.
    142         *
    143         * @param estimatedSize the estimated size of this spliterator if known, otherwise {@code Long.MAX_VALUE}.
    144         * @param additionalCharacteristics properties of this spliterator's source or elements. If {@code SIZED} is reported then this spliterator will
    145         *        additionally report {@code SUBSIZED}.
    146         * @param enumeration The Enumeration to wrap.
    147         */
    148        protected EnumerationSpliterator(final long estimatedSize, final int additionalCharacteristics, final Enumeration<T> enumeration) {
    149            super(estimatedSize, additionalCharacteristics);
    150            this.enumeration = Objects.requireNonNull(enumeration, "enumeration");
    151        }
    152
    153        @Override
    154        public void forEachRemaining(final Consumer<? super T> action) {
    155            while (enumeration.hasMoreElements()) {
    156                next(action);
    157            }
    158        }
    159
    160        private boolean next(final Consumer<? super T> action) {
    161            action.accept(enumeration.nextElement());
    162            return true;
    163
    164        }
    165
    166        @Override
    167        public boolean tryAdvance(final Consumer<? super T> action) {
    168            return enumeration.hasMoreElements() && next(action);
    169        }
    170    }
    171
    172    /**
    173     * A reduced, and simplified version of a {@link Stream} with failable method signatures.
    174     *
    175     * @param <T> The streams element type.
    176     */
    177    public static class FailableStream<T> {
    178
    179        private Stream<T> stream;
    180        private boolean terminated;
    181
    182        /**
    183         * Constructs a new instance with the given {@code stream}.
    184         *
    185         * @param stream The stream.
    186         */
    187        public FailableStream(final Stream<T> stream) {
    188            this.stream = stream;
    189        }
    190
    191        /**
    192         * Returns whether all elements of this stream match the provided predicate. May not evaluate the predicate on all
    193         * elements if not necessary for determining the result. If the stream is empty then {@code true} is returned and the
    194         * predicate is not evaluated.
    195         *
    196         * <p>
    197         * This is a short-circuiting terminal operation.
    198         * </p>
    199         *
    200         * Note This method evaluates the <em>universal quantification</em> of the predicate over the elements of the stream
    201         * (for all x P(x)). If the stream is empty, the quantification is said to be <em>vacuously satisfied</em> and is always
    202         * {@code true} (regardless of P(x)).
    203         *
    204         * @param predicate A non-interfering, stateless predicate to apply to elements of this stream
    205         * @return {@code true} If either all elements of the stream match the provided predicate or the stream is empty,
    206         *         otherwise {@code false}.
    207         */
    208        public boolean allMatch(final FailablePredicate<T, ?> predicate) {
    209            assertNotTerminated();
    210            return stream().allMatch(Failable.asPredicate(predicate));
    211        }
    212
    213        /**
    214         * Returns whether any elements of this stream match the provided predicate. May not evaluate the predicate on all
    215         * elements if not necessary for determining the result. If the stream is empty then {@code false} is returned and the
    216         * predicate is not evaluated.
    217         *
    218         * <p>
    219         * This is a short-circuiting terminal operation.
    220         * </p>
    221         *
    222         * Note This method evaluates the <em>existential quantification</em> of the predicate over the elements of the stream
    223         * (for some x P(x)).
    224         *
    225         * @param predicate A non-interfering, stateless predicate to apply to elements of this stream
    226         * @return {@code true} if any elements of the stream match the provided predicate, otherwise {@code false}
    227         */
    228        public boolean anyMatch(final FailablePredicate<T, ?> predicate) {
    229            assertNotTerminated();
    230            return stream().anyMatch(Failable.asPredicate(predicate));
    231        }
    232
    233        /**
    234         * Throws IllegalStateException if this stream is already terminated.
    235         *
    236         * @throws IllegalStateException if this stream is already terminated.
    237         */
    238        protected void assertNotTerminated() {
    239            if (terminated) {
    240                throw new IllegalStateException("This stream is already terminated.");
    241            }
    242        }
    243
    244        /**
    245         * Performs a mutable reduction operation on the elements of this stream using a {@link Collector}. A {@link Collector}
    246         * encapsulates the functions used as arguments to {@link #collect(Supplier, BiConsumer, BiConsumer)}, allowing for
    247         * reuse of collection strategies and composition of collect operations such as multiple-level grouping or partitioning.
    248         *
    249         * <p>
    250         * If the underlying stream is parallel, and the {@link Collector} is concurrent, and either the stream is unordered or
    251         * the collector is unordered, then a concurrent reduction will be performed (see {@link Collector} for details on
    252         * concurrent reduction.)
    253         * </p>
    254         *
    255         * <p>
    256         * This is a terminal operation.
    257         * </p>
    258         *
    259         * <p>
    260         * When executed in parallel, multiple intermediate results may be instantiated, populated, and merged so as to maintain
    261         * isolation of mutable data structures. Therefore, even when executed in parallel with non-thread-safe data structures
    262         * (such as {@link ArrayList}), no additional synchronization is needed for a parallel reduction.
    263         * </p>
    264         *
    265         * Note The following will accumulate strings into an ArrayList:
    266         *
    267         * <pre>
    268         * {@code
    269         *     List<String> asList = stringStream.collect(Collectors.toList());
    270         * }
    271         * </pre>
    272         *
    273         * <p>
    274         * The following will classify {@code Person} objects by city:
    275         * </p>
    276         *
    277         * <pre>
    278         * {@code
    279         *     Map<String, List<Person>> peopleByCity = personStream.collect(Collectors.groupingBy(Person::getCity));
    280         * }
    281         * </pre>
    282         *
    283         * <p>
    284         * The following will classify {@code Person} objects by state and city, cascading two {@link Collector}s together:
    285         * </p>
    286         *
    287         * <pre>
    288         * {@code
    289         *     Map<String, Map<String, List<Person>>> peopleByStateAndCity = personStream
    290         *         .collect(Collectors.groupingBy(Person::getState, Collectors.groupingBy(Person::getCity)));
    291         * }
    292         * </pre>
    293         *
    294         * @param <R> the type of the result
    295         * @param <A> the intermediate accumulation type of the {@link Collector}
    296         * @param collector the {@link Collector} describing the reduction
    297         * @return the result of the reduction
    298         * @see #collect(Supplier, BiConsumer, BiConsumer)
    299         * @see Collectors
    300         */
    301        public <A, R> R collect(final Collector<? super T, A, R> collector) {
    302            makeTerminated();
    303            return stream().collect(collector);
    304        }
    305
    306        /**
    307         * Performs a mutable reduction operation on the elements of this FailableStream. A mutable reduction is one in which
    308         * the reduced value is a mutable result container, such as an {@link ArrayList}, and elements are incorporated by
    309         * updating the state of the result rather than by replacing the result. This produces a result equivalent to:
    310         * <pre>
    311         * {@code
    312         *     R result = supplier.get();
    313         *     for (T element : this stream)
    314         *         accumulator.accept(result, element);
    315         *     return result;
    316         * }
    317         * </pre>
    318         * <p>
    319         * Like {@link #reduce(Object, BinaryOperator)}, {@code collect} operations can be parallelized without requiring
    320         * additional synchronization.
    321         * </p>
    322         * <p>
    323         * This is a terminal operation.
    324         * </p>
    325         * <p>
    326         * Note There are many existing classes in the JDK whose signatures are well-suited for use with method references as
    327         * arguments to {@code collect()}. For example, the following will accumulate strings into an {@link ArrayList}:
    328         * </p>
    329         * <pre>
    330         * {@code
    331         *     List<String> asList = stringStream.collect(ArrayList::new, ArrayList::add, ArrayList::addAll);
    332         * }
    333         * </pre>
    334         * <p>
    335         * The following will take a stream of strings and concatenates them into a single string:
    336         * </p>
    337         * <pre>
    338         * {@code
    339         *     String concat = stringStream.collect(StringBuilder::new, StringBuilder::append, StringBuilder::append).toString();
    340         * }
    341         * </pre>
    342         *
    343         * @param <R> type of the result
    344         * @param <A> Type of the accumulator.
    345         * @param supplier a function that creates a new result container. For a parallel execution, this function may be called
    346         *        multiple times and must return a fresh value each time.
    347         * @param accumulator An associative, non-interfering, stateless function for incorporating an additional element into a
    348         *        result
    349         * @param combiner An associative, non-interfering, stateless function for combining two values, which must be
    350         *        compatible with the accumulator function
    351         * @return The result of the reduction
    352         */
    353        public <A, R> R collect(final Supplier<R> supplier, final BiConsumer<R, ? super T> accumulator, final BiConsumer<R, R> combiner) {
    354            makeTerminated();
    355            return stream().collect(supplier, accumulator, combiner);
    356        }
    357
    358        /**
    359         * Returns a FailableStream consisting of the elements of this stream that match the given FailablePredicate.
    360         * <p>
    361         * This is an intermediate operation.
    362         * </p>
    363         *
    364         * @param predicate a non-interfering, stateless predicate to apply to each element to determine if it should be
    365         *        included.
    366         * @return the new stream
    367         */
    368        public FailableStream<T> filter(final FailablePredicate<T, ?> predicate) {
    369            assertNotTerminated();
    370            stream = stream.filter(Failable.asPredicate(predicate));
    371            return this;
    372        }
    373
    374        /**
    375         * Performs an action for each element of this stream.
    376         * <p>
    377         * This is a terminal operation.
    378         * </p>
    379         * <p>
    380         * The behavior of this operation is explicitly nondeterministic. For parallel stream pipelines, this operation does
    381         * <em>not</em> guarantee to respect the encounter order of the stream, as doing so would sacrifice the benefit of
    382         * parallelism. For any given element, the action may be performed at whatever time and in whatever thread the library
    383         * chooses. If the action accesses shared state, it is responsible for providing the required synchronization.
    384         * </p>
    385         *
    386         * @param action a non-interfering action to perform on the elements
    387         */
    388        public void forEach(final FailableConsumer<T, ?> action) {
    389            makeTerminated();
    390            stream().forEach(Failable.asConsumer(action));
    391        }
    392
    393        /**
    394         * Marks this stream as terminated.
    395         *
    396         * @throws IllegalStateException if this stream is already terminated.
    397         */
    398        protected void makeTerminated() {
    399            assertNotTerminated();
    400            terminated = true;
    401        }
    402
    403        /**
    404         * Returns a stream consisting of the results of applying the given function to the elements of this stream.
    405         *
    406         * <p>
    407         * This is an intermediate operation.
    408         * </p>
    409         *
    410         * @param <R> The element type of the new stream
    411         * @param mapper A non-interfering, stateless function to apply to each element
    412         * @return the new stream
    413         */
    414        public <R> FailableStream<R> map(final FailableFunction<T, R, ?> mapper) {
    415            assertNotTerminated();
    416            return new FailableStream<>(stream.map(Failable.asFunction(mapper)));
    417        }
    418
    419        /**
    420         * Performs a reduction on the elements of this stream, using the provided identity value and an associative
    421         * accumulation function, and returns the reduced value. This is equivalent to:
    422         *
    423         * <pre>
    424         * {@code
    425         *     T result = identity;
    426         *     for (T element : this stream)
    427         *         result = accumulator.apply(result, element)
    428         *     return result;
    429         * }
    430         * </pre>
    431         *
    432         * but is not constrained to execute sequentially.
    433         *
    434         * <p>
    435         * The {@code identity} value must be an identity for the accumulator function. This means that for all {@code t},
    436         * {@code accumulator.apply(identity, t)} is equal to {@code t}. The {@code accumulator} function must be an associative
    437         * function.
    438         * </p>
    439         *
    440         * <p>
    441         * This is a terminal operation.
    442         * </p>
    443         *
    444         * Note Sum, min, max, average, and string concatenation are all special cases of reduction. Summing a stream of numbers
    445         * can be expressed as:
    446         *
    447         * <pre>
    448         * {@code
    449         *     Integer sum = integers.reduce(0, (a, b) -> a + b);
    450         * }
    451         * </pre>
    452         *
    453         * or:
    454         *
    455         * <pre>
    456         * {@code
    457         *     Integer sum = integers.reduce(0, Integer::sum);
    458         * }
    459         * </pre>
    460         *
    461         * <p>
    462         * While this may seem a more roundabout way to perform an aggregation compared to simply mutating a running total in a
    463         * loop, reduction operations parallelize more gracefully, without needing additional synchronization and with greatly
    464         * reduced risk of data races.
    465         * </p>
    466         *
    467         * @param identity the identity value for the accumulating function
    468         * @param accumulator an associative, non-interfering, stateless function for combining two values
    469         * @return the result of the reduction
    470         */
    471        public T reduce(final T identity, final BinaryOperator<T> accumulator) {
    472            makeTerminated();
    473            return stream().reduce(identity, accumulator);
    474        }
    475
    476        /**
    477         * Converts the FailableStream into an equivalent stream.
    478         *
    479         * @return A stream, which will return the same elements, which this FailableStream would return.
    480         */
    481        public Stream<T> stream() {
    482            return stream;
    483        }
    484    }
    485
    486    /**
    487     * Converts the given {@link Collection} into a {@link FailableStream}. This is basically a simplified, reduced version
    488     * of the {@link Stream} class, with the same underlying element stream, except that failable objects, like
    489     * {@link FailablePredicate}, {@link FailableFunction}, or {@link FailableConsumer} may be applied, instead of
    490     * {@link Predicate}, {@link Function}, or {@link Consumer}. The idea is to rewrite a code snippet like this:
    491     *
    492     * <pre>
    493     * {@code
    494     * final List<O> list;
    495     * final Method m;
    496     * final Function<O, String> mapper = (o) -> {
    497     *     try {
    498     *         return (String) m.invoke(o);
    499     *     } catch (Throwable t) {
    500     *         throw Failable.rethrow(t);
    501     *     }
    502     * };
    503     * final List<String> strList = list.stream().map(mapper).collect(Collectors.toList());
    504     * }
    505     * </pre>
    506     *
    507     * as follows:
    508     *
    509     * <pre>
    510     * {@code
    511     * final List<O> list;
    512     * final Method m;
    513     * final List<String> strList = Failable.stream(list.stream()).map((o) -> (String) m.invoke(o)).collect(Collectors.toList());
    514     * }
    515     * </pre>
    516     *
    517     * While the second version may not be <em>quite</em> as efficient (because it depends on the creation of additional,
    518     * intermediate objects, of type FailableStream), it is much more concise, and readable, and meets the spirit of Lambdas
    519     * better than the first version.
    520     *
    521     * @param <T> The streams element type.
    522     * @param stream The stream, which is being converted.
    523     * @return The {@link FailableStream}, which has been created by converting the stream.
    524     * @since 3.13.0
    525     */
    526    public static <T> FailableStream<T> failableStream(final Collection<T> stream) {
    527        return failableStream(of(stream));
    528    }
    529
    530    /**
    531     * Converts the given {@link Stream stream} into a {@link FailableStream}. This is basically a simplified, reduced
    532     * version of the {@link Stream} class, with the same underlying element stream, except that failable objects, like
    533     * {@link FailablePredicate}, {@link FailableFunction}, or {@link FailableConsumer} may be applied, instead of
    534     * {@link Predicate}, {@link Function}, or {@link Consumer}. The idea is to rewrite a code snippet like this:
    535     *
    536     * <pre>
    537     * {@code
    538     * final List<O> list;
    539     * final Method m;
    540     * final Function<O, String> mapper = (o) -> {
    541     *     try {
    542     *         return (String) m.invoke(o);
    543     *     } catch (Throwable t) {
    544     *         throw Failable.rethrow(t);
    545     *     }
    546     * };
    547     * final List<String> strList = list.stream().map(mapper).collect(Collectors.toList());
    548     * }
    549     * </pre>
    550     *
    551     * as follows:
    552     *
    553     * <pre>
    554     * {@code
    555     * final List<O> list;
    556     * final Method m;
    557     * final List<String> strList = Failable.stream(list.stream()).map((o) -> (String) m.invoke(o)).collect(Collectors.toList());
    558     * }
    559     * </pre>
    560     *
    561     * While the second version may not be <em>quite</em> as efficient (because it depends on the creation of additional,
    562     * intermediate objects, of type FailableStream), it is much more concise, and readable, and meets the spirit of Lambdas
    563     * better than the first version.
    564     *
    565     * @param <T> The streams element type.
    566     * @param stream The stream, which is being converted.
    567     * @return The {@link FailableStream}, which has been created by converting the stream.
    568     * @since 3.13.0
    569     */
    570    public static <T> FailableStream<T> failableStream(final Stream<T> stream) {
    571        return new FailableStream<>(stream);
    572    }
    573
    574    /**
    575     * Shorthand for {@code Streams.failableStream(value == null ? Stream.empty() : Stream.of(value))}.
    576     *
    577     * @param <T> the type of stream elements.
    578     * @param value the single element of the new stream, may be {@code null}.
    579     * @return the new FailableStream on {@code value} or an empty stream.
    580     * @since 3.15.0
    581     */
    582    public static <T> FailableStream<T> failableStream(final T value) {
    583        return failableStream(streamOf(value));
    584    }
    585
    586    /**
    587     * Shorthand for {@code Streams.failableStream(Streams.of(arrayValues))}.
    588     *
    589     * @param <T> the type of stream elements.
    590     * @param values the elements of the new stream, may be {@code null}.
    591     * @return the new FailableStream on {@code values} or an empty stream.
    592     * @since 3.14.0
    593     */
    594    @SafeVarargs // Creating a stream from an array is safe
    595    public static <T> FailableStream<T> failableStream(final T... values) {
    596        return failableStream(of(values));
    597    }
    598
    599    /**
    600     * Streams only instances of the give Class in a collection.
    601     * <p>
    602     * This method shorthand for:
    603     * </p>
    604     * <pre>
    605     * {@code (Stream<E>) Streams.toStream(collection).filter(collection, SomeClass.class::isInstance);}
    606     * </pre>
    607     *
    608     * @param <E> the type of elements in the collection we want to stream.
    609     * @param clazz the type of elements in the collection we want to stream.
    610     * @param collection the collection to stream or null.
    611     * @return A non-null stream that only provides instances we want.
    612     * @since 3.13.0
    613     */
    614    public static <E> Stream<E> instancesOf(final Class<? super E> clazz, final Collection<? super E> collection) {
    615        return instancesOf(clazz, of(collection));
    616    }
    617
    618    @SuppressWarnings("unchecked") // After the isInstance check, we still need to type-cast.
    619    private static <E> Stream<E> instancesOf(final Class<? super E> clazz, final Stream<?> stream) {
    620        return (Stream<E>) of(stream).filter(clazz::isInstance);
    621    }
    622
    623    /**
    624     * Streams the non-null elements of a collection.
    625     *
    626     * @param <E> the type of elements in the collection.
    627     * @param collection the collection to stream or null.
    628     * @return A non-null stream that filters out null elements.
    629     * @since 3.13.0
    630     */
    631    public static <E> Stream<E> nonNull(final Collection<E> collection) {
    632        return of(collection).filter(Objects::nonNull);
    633    }
    634
    635    /**
    636     * Streams the non-null element.
    637     *
    638     * @param <E> the type of elements in the collection.
    639     * @param array the element to stream or null.
    640     * @return A non-null stream that filters out a null element.
    641     * @since 3.15.0
    642     */
    643    public static <E> Stream<E> nonNull(final E array) {
    644        return nonNull(streamOf(array));
    645    }
    646
    647    /**
    648     * Streams the non-null elements of an array.
    649     *
    650     * @param <E> the type of elements in the collection.
    651     * @param array the array to stream or null.
    652     * @return A non-null stream that filters out null elements.
    653     * @since 3.13.0
    654     */
    655    @SafeVarargs
    656    public static <E> Stream<E> nonNull(final E... array) {
    657        return nonNull(of(array));
    658    }
    659
    660    /**
    661     * Streams the non-null elements of a stream.
    662     *
    663     * @param <E> the type of elements in the collection.
    664     * @param stream the stream to stream or null.
    665     * @return A non-null stream that filters out null elements.
    666     * @since 3.13.0
    667     */
    668    public static <E> Stream<E> nonNull(final Stream<E> stream) {
    669        return of(stream).filter(Objects::nonNull);
    670    }
    671
    672    /**
    673     * Delegates to {@link Collection#stream()} or returns {@link Stream#empty()} if the collection is null.
    674     *
    675     * @param <E> the type of elements in the collection.
    676     * @param collection the collection to stream or null.
    677     * @return {@link Collection#stream()} or {@link Stream#empty()} if the collection is null.
    678     * @since 3.13.0
    679     */
    680    public static <E> Stream<E> of(final Collection<E> collection) {
    681        return collection == null ? Stream.empty() : collection.stream();
    682    }
    683
    684    /**
    685     * Streams the elements of the given enumeration in order.
    686     *
    687     * @param <E> The enumeration element type.
    688     * @param enumeration The enumeration to stream.
    689     * @return a new stream.
    690     * @since 3.13.0
    691     */
    692    public static <E> Stream<E> of(final Enumeration<E> enumeration) {
    693        return StreamSupport.stream(new EnumerationSpliterator<>(Long.MAX_VALUE, Spliterator.ORDERED, enumeration), false);
    694    }
    695
    696    /**
    697     * Creates a stream on the given Iterable.
    698     *
    699     * @param <E> the type of elements in the Iterable.
    700     * @param iterable the Iterable to stream or null.
    701     * @return a new Stream or {@link Stream#empty()} if the Iterable is null.
    702     * @since 3.13.0
    703     */
    704    public static <E> Stream<E> of(final Iterable<E> iterable) {
    705        return iterable == null ? Stream.empty() : StreamSupport.stream(iterable.spliterator(), false);
    706    }
    707
    708    /**
    709     * Creates a stream on the given Iterator.
    710     *
    711     * @param <E> the type of elements in the Iterator.
    712     * @param iterator the Iterator to stream or null.
    713     * @return a new Stream or {@link Stream#empty()} if the Iterator is null.
    714     * @since 3.13.0
    715     */
    716    public static <E> Stream<E> of(final Iterator<E> iterator) {
    717        return iterator == null ? Stream.empty() : StreamSupport.stream(Spliterators.spliteratorUnknownSize(iterator, Spliterator.ORDERED), false);
    718    }
    719
    720    /**
    721     * Returns the stream or {@link Stream#empty()} if the stream is null.
    722     *
    723     * @param <E> the type of elements in the collection.
    724     * @param stream the stream to stream or null.
    725     * @return the stream or {@link Stream#empty()} if the stream is null.
    726     * @since 3.13.0
    727     */
    728    private static <E> Stream<E> of(final Stream<E> stream) {
    729        return stream == null ? Stream.empty() : stream;
    730    }
    731
    732    /**
    733     * Null-safe version of {@link Stream#of(Object[])}.
    734     *
    735     * @param <T> the type of stream elements.
    736     * @param values the elements of the new stream, may be {@code null}.
    737     * @return the new stream on {@code values} or {@link Stream#empty()}.
    738     * @since 3.13.0
    739     */
    740    @SafeVarargs // Creating a stream from an array is safe
    741    public static <T> Stream<T> of(final T... values) {
    742        return values == null ? Stream.empty() : Stream.of(values);
    743    }
    744
    745    /**
    746     * Converts the given {@link Collection} into a {@link FailableStream}. This is basically a simplified, reduced version
    747     * of the {@link Stream} class, with the same underlying element stream, except that failable objects, like
    748     * {@link FailablePredicate}, {@link FailableFunction}, or {@link FailableConsumer} may be applied, instead of
    749     * {@link Predicate}, {@link Function}, or {@link Consumer}. The idea is to rewrite a code snippet like this:
    750     *
    751     * <pre>
    752     * {@code
    753     * final List<O> list;
    754     * final Method m;
    755     * final Function<O, String> mapper = (o) -> {
    756     *     try {
    757     *         return (String) m.invoke(o);
    758     *     } catch (Throwable t) {
    759     *         throw Failable.rethrow(t);
    760     *     }
    761     * };
    762     * final List<String> strList = list.stream().map(mapper).collect(Collectors.toList());
    763     * }
    764     * </pre>
    765     *
    766     * as follows:
    767     *
    768     * <pre>
    769     * {@code
    770     * final List<O> list;
    771     * final Method m;
    772     * final List<String> strList = Failable.stream(list.stream()).map((o) -> (String) m.invoke(o)).collect(Collectors.toList());
    773     * }
    774     * </pre>
    775     *
    776     * While the second version may not be <em>quite</em> as efficient (because it depends on the creation of additional,
    777     * intermediate objects, of type FailableStream), it is much more concise, and readable, and meets the spirit of Lambdas
    778     * better than the first version.
    779     *
    780     * @param <E> The streams element type.
    781     * @param collection The stream, which is being converted.
    782     * @return The {@link FailableStream}, which has been created by converting the stream.
    783     * @deprecated Use {@link #failableStream(Collection)}.
    784     */
    785    @Deprecated
    786    public static <E> FailableStream<E> stream(final Collection<E> collection) {
    787        return failableStream(collection);
    788    }
    789
    790    /**
    791     * Converts the given {@link Stream stream} into a {@link FailableStream}. This is basically a simplified, reduced
    792     * version of the {@link Stream} class, with the same underlying element stream, except that failable objects, like
    793     * {@link FailablePredicate}, {@link FailableFunction}, or {@link FailableConsumer} may be applied, instead of
    794     * {@link Predicate}, {@link Function}, or {@link Consumer}. The idea is to rewrite a code snippet like this:
    795     *
    796     * <pre>
    797     * {@code
    798     * final List<O> list;
    799     * final Method m;
    800     * final Function<O, String> mapper = (o) -> {
    801     *     try {
    802     *         return (String) m.invoke(o);
    803     *     } catch (Throwable t) {
    804     *         throw Failable.rethrow(t);
    805     *     }
    806     * };
    807     * final List<String> strList = list.stream().map(mapper).collect(Collectors.toList());
    808     * }
    809     * </pre>
    810     *
    811     * as follows:
    812     *
    813     * <pre>
    814     * {@code
    815     * final List<O> list;
    816     * final Method m;
    817     * final List<String> strList = Failable.stream(list.stream()).map((o) -> (String) m.invoke(o)).collect(Collectors.toList());
    818     * }
    819     * </pre>
    820     *
    821     * While the second version may not be <em>quite</em> as efficient (because it depends on the creation of additional,
    822     * intermediate objects, of type FailableStream), it is much more concise, and readable, and meets the spirit of Lambdas
    823     * better than the first version.
    824     *
    825     * @param <T> The streams element type.
    826     * @param stream The stream, which is being converted.
    827     * @return The {@link FailableStream}, which has been created by converting the stream.
    828     * @deprecated Use {@link #failableStream(Stream)}.
    829     */
    830    @Deprecated
    831    public static <T> FailableStream<T> stream(final Stream<T> stream) {
    832        return failableStream(stream);
    833    }
    834
    835    private static <T> Stream<T> streamOf(final T value) {
    836        return value == null ? Stream.empty() : Stream.of(value);
    837    }
    838
    839    /**
    840     * Returns a {@link Collector} that accumulates the input elements into a new array.
    841     *
    842     * @param pElementType Type of an element in the array.
    843     * @param <T> the type of the input elements
    844     * @return a {@link Collector} which collects all the input elements into an array, in encounter order
    845     */
    846    public static <T> Collector<T, ?, T[]> toArray(final Class<T> pElementType) {
    847        return new ArrayCollector<>(pElementType);
    848    }
    849
    850    /**
    851     * Make private in 4.0.
    852     *
    853     * @deprecated TODO Make private in 4.0.
    854     */
    855    @Deprecated
    856    public Streams() {
    857        // empty
    858    }
    859}