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}