Streams.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.commons.lang3.stream;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.Enumeration;
import java.util.Iterator;
import java.util.List;
import java.util.Objects;
import java.util.Set;
import java.util.Spliterator;
import java.util.Spliterators;
import java.util.Spliterators.AbstractSpliterator;
import java.util.function.BiConsumer;
import java.util.function.BinaryOperator;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.function.Supplier;
import java.util.stream.Collector;
import java.util.stream.Collectors;
import java.util.stream.Stream;
import java.util.stream.StreamSupport;

import org.apache.commons.lang3.ArrayUtils;
import org.apache.commons.lang3.function.Failable;
import org.apache.commons.lang3.function.FailableConsumer;
import org.apache.commons.lang3.function.FailableFunction;
import org.apache.commons.lang3.function.FailablePredicate;

/**
 * Provides utility functions, and classes for working with the {@link java.util.stream} package, or more generally, with Java 8 lambdas. More specifically, it
 * attempts to address the fact that lambdas are supposed not to throw Exceptions, at least not checked Exceptions, AKA instances of {@link Exception}. This
 * enforces the use of constructs like:
 *
 * <pre>{@code
 * Consumer<java.lang.reflect.Method> consumer = m -> {
 *     try {
 *         m.invoke(o, args);
 *     } catch (Throwable t) {
 *         throw Failable.rethrow(t);
 *     }
 * };
 * stream.forEach(consumer);
 * }</pre>
 * <p>
 * Using a {@link FailableStream}, this can be rewritten as follows:
 * </p>
 *
 * <pre>{@code
 * Streams.failable(stream).forEach(m -> m.invoke(o, args));
 * }</pre>
 * <p>
 * Obviously, the second version is much more concise and the spirit of Lambda expressions is met better than in the first version.
 * </p>
 *
 * @see Stream
 * @see Failable
 * @since 3.11
 */
public class Streams {

    /**
     * A Collector type for arrays.
     *
     * @param <E> The array type.
     */
    public static class ArrayCollector<E> implements Collector<E, List<E>, E[]> {
        private static final Set<Characteristics> characteristics = Collections.emptySet();
        private final Class<E> elementType;

        /**
         * Constructs a new instance for the given element type.
         *
         * @param elementType The element type.
         */
        public ArrayCollector(final Class<E> elementType) {
            this.elementType = Objects.requireNonNull(elementType, "elementType");
        }

        @Override
        public BiConsumer<List<E>, E> accumulator() {
            return List::add;
        }

        @Override
        public Set<Characteristics> characteristics() {
            return characteristics;
        }

        @Override
        public BinaryOperator<List<E>> combiner() {
            return (left, right) -> {
                left.addAll(right);
                return left;
            };
        }

        @Override
        public Function<List<E>, E[]> finisher() {
            return list -> list.toArray(ArrayUtils.newInstance(elementType, list.size()));
        }

        @Override
        public Supplier<List<E>> supplier() {
            return ArrayList::new;
        }
    }

    /**
     * Helps implement {@link Streams#of(Enumeration)}.
     *
     * @param <T> The element type.
     */
    private static final class EnumerationSpliterator<T> extends AbstractSpliterator<T> {

        private final Enumeration<T> enumeration;

        /**
         * Creates a spliterator reporting the given estimated size and additionalCharacteristics.
         *
         * @param estimatedSize the estimated size of this spliterator if known, otherwise {@code Long.MAX_VALUE}.
         * @param additionalCharacteristics properties of this spliterator's source or elements. If {@code SIZED} is reported then this spliterator will
         *        additionally report {@code SUBSIZED}.
         * @param enumeration The Enumeration to wrap.
         */
        protected EnumerationSpliterator(final long estimatedSize, final int additionalCharacteristics, final Enumeration<T> enumeration) {
            super(estimatedSize, additionalCharacteristics);
            this.enumeration = Objects.requireNonNull(enumeration, "enumeration");
        }

        @Override
        public void forEachRemaining(final Consumer<? super T> action) {
            while (enumeration.hasMoreElements()) {
                next(action);
            }
        }

        private boolean next(final Consumer<? super T> action) {
            action.accept(enumeration.nextElement());
            return true;

        }

        @Override
        public boolean tryAdvance(final Consumer<? super T> action) {
            return enumeration.hasMoreElements() && next(action);
        }
    }

    /**
     * A reduced, and simplified version of a {@link Stream} with failable method signatures.
     *
     * @param <T> The streams element type.
     */
    public static class FailableStream<T> {

        private Stream<T> stream;
        private boolean terminated;

        /**
         * Constructs a new instance with the given {@code stream}.
         *
         * @param stream The stream.
         */
        public FailableStream(final Stream<T> stream) {
            this.stream = stream;
        }

        /**
         * Returns whether all elements of this stream match the provided predicate. May not evaluate the predicate on all
         * elements if not necessary for determining the result. If the stream is empty then {@code true} is returned and the
         * predicate is not evaluated.
         *
         * <p>
         * This is a short-circuiting terminal operation.
         * </p>
         *
         * Note This method evaluates the <em>universal quantification</em> of the predicate over the elements of the stream
         * (for all x P(x)). If the stream is empty, the quantification is said to be <em>vacuously satisfied</em> and is always
         * {@code true} (regardless of P(x)).
         *
         * @param predicate A non-interfering, stateless predicate to apply to elements of this stream
         * @return {@code true} If either all elements of the stream match the provided predicate or the stream is empty,
         *         otherwise {@code false}.
         */
        public boolean allMatch(final FailablePredicate<T, ?> predicate) {
            assertNotTerminated();
            return stream().allMatch(Failable.asPredicate(predicate));
        }

        /**
         * Returns whether any elements of this stream match the provided predicate. May not evaluate the predicate on all
         * elements if not necessary for determining the result. If the stream is empty then {@code false} is returned and the
         * predicate is not evaluated.
         *
         * <p>
         * This is a short-circuiting terminal operation.
         * </p>
         *
         * Note This method evaluates the <em>existential quantification</em> of the predicate over the elements of the stream
         * (for some x P(x)).
         *
         * @param predicate A non-interfering, stateless predicate to apply to elements of this stream
         * @return {@code true} if any elements of the stream match the provided predicate, otherwise {@code false}
         */
        public boolean anyMatch(final FailablePredicate<T, ?> predicate) {
            assertNotTerminated();
            return stream().anyMatch(Failable.asPredicate(predicate));
        }

        /**
         * Throws IllegalStateException if this stream is already terminated.
         *
         * @throws IllegalStateException if this stream is already terminated.
         */
        protected void assertNotTerminated() {
            if (terminated) {
                throw new IllegalStateException("This stream is already terminated.");
            }
        }

        /**
         * Performs a mutable reduction operation on the elements of this stream using a {@link Collector}. A {@link Collector}
         * encapsulates the functions used as arguments to {@link #collect(Supplier, BiConsumer, BiConsumer)}, allowing for
         * reuse of collection strategies and composition of collect operations such as multiple-level grouping or partitioning.
         *
         * <p>
         * If the underlying stream is parallel, and the {@link Collector} is concurrent, and either the stream is unordered or
         * the collector is unordered, then a concurrent reduction will be performed (see {@link Collector} for details on
         * concurrent reduction.)
         * </p>
         *
         * <p>
         * This is a terminal operation.
         * </p>
         *
         * <p>
         * When executed in parallel, multiple intermediate results may be instantiated, populated, and merged so as to maintain
         * isolation of mutable data structures. Therefore, even when executed in parallel with non-thread-safe data structures
         * (such as {@link ArrayList}), no additional synchronization is needed for a parallel reduction.
         * </p>
         *
         * Note The following will accumulate strings into an ArrayList:
         *
         * <pre>
         * {@code
         *     List<String> asList = stringStream.collect(Collectors.toList());
         * }
         * </pre>
         *
         * <p>
         * The following will classify {@code Person} objects by city:
         * </p>
         *
         * <pre>
         * {@code
         *     Map<String, List<Person>> peopleByCity = personStream.collect(Collectors.groupingBy(Person::getCity));
         * }
         * </pre>
         *
         * <p>
         * The following will classify {@code Person} objects by state and city, cascading two {@link Collector}s together:
         * </p>
         *
         * <pre>
         * {@code
         *     Map<String, Map<String, List<Person>>> peopleByStateAndCity = personStream
         *         .collect(Collectors.groupingBy(Person::getState, Collectors.groupingBy(Person::getCity)));
         * }
         * </pre>
         *
         * @param <R> the type of the result
         * @param <A> the intermediate accumulation type of the {@link Collector}
         * @param collector the {@link Collector} describing the reduction
         * @return the result of the reduction
         * @see #collect(Supplier, BiConsumer, BiConsumer)
         * @see Collectors
         */
        public <A, R> R collect(final Collector<? super T, A, R> collector) {
            makeTerminated();
            return stream().collect(collector);
        }

        /**
         * Performs a mutable reduction operation on the elements of this FailableStream. A mutable reduction is one in which
         * the reduced value is a mutable result container, such as an {@link ArrayList}, and elements are incorporated by
         * updating the state of the result rather than by replacing the result. This produces a result equivalent to:
         * <pre>
         * {@code
         *     R result = supplier.get();
         *     for (T element : this stream)
         *         accumulator.accept(result, element);
         *     return result;
         * }
         * </pre>
         * <p>
         * Like {@link #reduce(Object, BinaryOperator)}, {@code collect} operations can be parallelized without requiring
         * additional synchronization.
         * </p>
         * <p>
         * This is a terminal operation.
         * </p>
         * <p>
         * Note There are many existing classes in the JDK whose signatures are well-suited for use with method references as
         * arguments to {@code collect()}. For example, the following will accumulate strings into an {@link ArrayList}:
         * </p>
         * <pre>
         * {@code
         *     List<String> asList = stringStream.collect(ArrayList::new, ArrayList::add, ArrayList::addAll);
         * }
         * </pre>
         * <p>
         * The following will take a stream of strings and concatenates them into a single string:
         * </p>
         * <pre>
         * {@code
         *     String concat = stringStream.collect(StringBuilder::new, StringBuilder::append, StringBuilder::append).toString();
         * }
         * </pre>
         *
         * @param <R> type of the result
         * @param <A> Type of the accumulator.
         * @param supplier a function that creates a new result container. For a parallel execution, this function may be called
         *        multiple times and must return a fresh value each time.
         * @param accumulator An associative, non-interfering, stateless function for incorporating an additional element into a
         *        result
         * @param combiner An associative, non-interfering, stateless function for combining two values, which must be
         *        compatible with the accumulator function
         * @return The result of the reduction
         */
        public <A, R> R collect(final Supplier<R> supplier, final BiConsumer<R, ? super T> accumulator, final BiConsumer<R, R> combiner) {
            makeTerminated();
            return stream().collect(supplier, accumulator, combiner);
        }

        /**
         * Returns a FailableStream consisting of the elements of this stream that match the given FailablePredicate.
         * <p>
         * This is an intermediate operation.
         * </p>
         *
         * @param predicate a non-interfering, stateless predicate to apply to each element to determine if it should be
         *        included.
         * @return the new stream
         */
        public FailableStream<T> filter(final FailablePredicate<T, ?> predicate) {
            assertNotTerminated();
            stream = stream.filter(Failable.asPredicate(predicate));
            return this;
        }

        /**
         * Performs an action for each element of this stream.
         * <p>
         * This is a terminal operation.
         * </p>
         * <p>
         * The behavior of this operation is explicitly nondeterministic. For parallel stream pipelines, this operation does
         * <em>not</em> guarantee to respect the encounter order of the stream, as doing so would sacrifice the benefit of
         * parallelism. For any given element, the action may be performed at whatever time and in whatever thread the library
         * chooses. If the action accesses shared state, it is responsible for providing the required synchronization.
         * </p>
         *
         * @param action a non-interfering action to perform on the elements
         */
        public void forEach(final FailableConsumer<T, ?> action) {
            makeTerminated();
            stream().forEach(Failable.asConsumer(action));
        }

        /**
         * Marks this stream as terminated.
         *
         * @throws IllegalStateException if this stream is already terminated.
         */
        protected void makeTerminated() {
            assertNotTerminated();
            terminated = true;
        }

        /**
         * Returns a stream consisting of the results of applying the given function to the elements of this stream.
         *
         * <p>
         * This is an intermediate operation.
         * </p>
         *
         * @param <R> The element type of the new stream
         * @param mapper A non-interfering, stateless function to apply to each element
         * @return the new stream
         */
        public <R> FailableStream<R> map(final FailableFunction<T, R, ?> mapper) {
            assertNotTerminated();
            return new FailableStream<>(stream.map(Failable.asFunction(mapper)));
        }

        /**
         * Performs a reduction on the elements of this stream, using the provided identity value and an associative
         * accumulation function, and returns the reduced value. This is equivalent to:
         *
         * <pre>
         * {@code
         *     T result = identity;
         *     for (T element : this stream)
         *         result = accumulator.apply(result, element)
         *     return result;
         * }
         * </pre>
         *
         * but is not constrained to execute sequentially.
         *
         * <p>
         * The {@code identity} value must be an identity for the accumulator function. This means that for all {@code t},
         * {@code accumulator.apply(identity, t)} is equal to {@code t}. The {@code accumulator} function must be an associative
         * function.
         * </p>
         *
         * <p>
         * This is a terminal operation.
         * </p>
         *
         * Note Sum, min, max, average, and string concatenation are all special cases of reduction. Summing a stream of numbers
         * can be expressed as:
         *
         * <pre>
         * {@code
         *     Integer sum = integers.reduce(0, (a, b) -> a + b);
         * }
         * </pre>
         *
         * or:
         *
         * <pre>
         * {@code
         *     Integer sum = integers.reduce(0, Integer::sum);
         * }
         * </pre>
         *
         * <p>
         * While this may seem a more roundabout way to perform an aggregation compared to simply mutating a running total in a
         * loop, reduction operations parallelize more gracefully, without needing additional synchronization and with greatly
         * reduced risk of data races.
         * </p>
         *
         * @param identity the identity value for the accumulating function
         * @param accumulator an associative, non-interfering, stateless function for combining two values
         * @return the result of the reduction
         */
        public T reduce(final T identity, final BinaryOperator<T> accumulator) {
            makeTerminated();
            return stream().reduce(identity, accumulator);
        }

        /**
         * Converts the FailableStream into an equivalent stream.
         *
         * @return A stream, which will return the same elements, which this FailableStream would return.
         */
        public Stream<T> stream() {
            return stream;
        }
    }

    /**
     * Converts the given {@link Collection} into a {@link FailableStream}. This is basically a simplified, reduced version
     * of the {@link Stream} class, with the same underlying element stream, except that failable objects, like
     * {@link FailablePredicate}, {@link FailableFunction}, or {@link FailableConsumer} may be applied, instead of
     * {@link Predicate}, {@link Function}, or {@link Consumer}. The idea is to rewrite a code snippet like this:
     *
     * <pre>
     * {@code
     * final List<O> list;
     * final Method m;
     * final Function<O, String> mapper = (o) -> {
     *     try {
     *         return (String) m.invoke(o);
     *     } catch (Throwable t) {
     *         throw Failable.rethrow(t);
     *     }
     * };
     * final List<String> strList = list.stream().map(mapper).collect(Collectors.toList());
     * }
     * </pre>
     *
     * as follows:
     *
     * <pre>
     * {@code
     * final List<O> list;
     * final Method m;
     * final List<String> strList = Failable.stream(list.stream()).map((o) -> (String) m.invoke(o)).collect(Collectors.toList());
     * }
     * </pre>
     *
     * While the second version may not be <em>quite</em> as efficient (because it depends on the creation of additional,
     * intermediate objects, of type FailableStream), it is much more concise, and readable, and meets the spirit of Lambdas
     * better than the first version.
     *
     * @param <T> The streams element type.
     * @param stream The stream, which is being converted.
     * @return The {@link FailableStream}, which has been created by converting the stream.
     * @since 3.13.0
     */
    public static <T> FailableStream<T> failableStream(final Collection<T> stream) {
        return failableStream(of(stream));
    }

    /**
     * Converts the given {@link Stream stream} into a {@link FailableStream}. This is basically a simplified, reduced
     * version of the {@link Stream} class, with the same underlying element stream, except that failable objects, like
     * {@link FailablePredicate}, {@link FailableFunction}, or {@link FailableConsumer} may be applied, instead of
     * {@link Predicate}, {@link Function}, or {@link Consumer}. The idea is to rewrite a code snippet like this:
     *
     * <pre>
     * {@code
     * final List<O> list;
     * final Method m;
     * final Function<O, String> mapper = (o) -> {
     *     try {
     *         return (String) m.invoke(o);
     *     } catch (Throwable t) {
     *         throw Failable.rethrow(t);
     *     }
     * };
     * final List<String> strList = list.stream().map(mapper).collect(Collectors.toList());
     * }
     * </pre>
     *
     * as follows:
     *
     * <pre>
     * {@code
     * final List<O> list;
     * final Method m;
     * final List<String> strList = Failable.stream(list.stream()).map((o) -> (String) m.invoke(o)).collect(Collectors.toList());
     * }
     * </pre>
     *
     * While the second version may not be <em>quite</em> as efficient (because it depends on the creation of additional,
     * intermediate objects, of type FailableStream), it is much more concise, and readable, and meets the spirit of Lambdas
     * better than the first version.
     *
     * @param <T> The streams element type.
     * @param stream The stream, which is being converted.
     * @return The {@link FailableStream}, which has been created by converting the stream.
     * @since 3.13.0
     */
    public static <T> FailableStream<T> failableStream(final Stream<T> stream) {
        return new FailableStream<>(stream);
    }

    /**
     * Shorthand for {@code Streams.failableStream(value == null ? Stream.empty() : Stream.of(value))}.
     *
     * @param <T> the type of stream elements.
     * @param value the single element of the new stream, may be {@code null}.
     * @return the new FailableStream on {@code value} or an empty stream.
     * @since 3.15.0
     */
    public static <T> FailableStream<T> failableStream(final T value) {
        return failableStream(streamOf(value));
    }

    /**
     * Shorthand for {@code Streams.failableStream(Streams.of(arrayValues))}.
     *
     * @param <T> the type of stream elements.
     * @param values the elements of the new stream, may be {@code null}.
     * @return the new FailableStream on {@code values} or an empty stream.
     * @since 3.14.0
     */
    @SafeVarargs // Creating a stream from an array is safe
    public static <T> FailableStream<T> failableStream(final T... values) {
        return failableStream(of(values));
    }

    /**
     * Streams only instances of the give Class in a collection.
     * <p>
     * This method shorthand for:
     * </p>
     * <pre>
     * {@code (Stream<E>) Streams.toStream(collection).filter(collection, SomeClass.class::isInstance);}
     * </pre>
     *
     * @param <E> the type of elements in the collection we want to stream.
     * @param clazz the type of elements in the collection we want to stream.
     * @param collection the collection to stream or null.
     * @return A non-null stream that only provides instances we want.
     * @since 3.13.0
     */
    public static <E> Stream<E> instancesOf(final Class<? super E> clazz, final Collection<? super E> collection) {
        return instancesOf(clazz, of(collection));
    }

    @SuppressWarnings("unchecked") // After the isInstance check, we still need to type-cast.
    private static <E> Stream<E> instancesOf(final Class<? super E> clazz, final Stream<?> stream) {
        return (Stream<E>) of(stream).filter(clazz::isInstance);
    }

    /**
     * Streams the non-null elements of a collection.
     *
     * @param <E> the type of elements in the collection.
     * @param collection the collection to stream or null.
     * @return A non-null stream that filters out null elements.
     * @since 3.13.0
     */
    public static <E> Stream<E> nonNull(final Collection<E> collection) {
        return of(collection).filter(Objects::nonNull);
    }

    /**
     * Streams the non-null element.
     *
     * @param <E> the type of elements in the collection.
     * @param array the element to stream or null.
     * @return A non-null stream that filters out a null element.
     * @since 3.15.0
     */
    public static <E> Stream<E> nonNull(final E array) {
        return nonNull(streamOf(array));
    }

    /**
     * Streams the non-null elements of an array.
     *
     * @param <E> the type of elements in the collection.
     * @param array the array to stream or null.
     * @return A non-null stream that filters out null elements.
     * @since 3.13.0
     */
    @SafeVarargs
    public static <E> Stream<E> nonNull(final E... array) {
        return nonNull(of(array));
    }

    /**
     * Streams the non-null elements of a stream.
     *
     * @param <E> the type of elements in the collection.
     * @param stream the stream to stream or null.
     * @return A non-null stream that filters out null elements.
     * @since 3.13.0
     */
    public static <E> Stream<E> nonNull(final Stream<E> stream) {
        return of(stream).filter(Objects::nonNull);
    }

    /**
     * Delegates to {@link Collection#stream()} or returns {@link Stream#empty()} if the collection is null.
     *
     * @param <E> the type of elements in the collection.
     * @param collection the collection to stream or null.
     * @return {@link Collection#stream()} or {@link Stream#empty()} if the collection is null.
     * @since 3.13.0
     */
    public static <E> Stream<E> of(final Collection<E> collection) {
        return collection == null ? Stream.empty() : collection.stream();
    }

    /**
     * Streams the elements of the given enumeration in order.
     *
     * @param <E> The enumeration element type.
     * @param enumeration The enumeration to stream.
     * @return a new stream.
     * @since 3.13.0
     */
    public static <E> Stream<E> of(final Enumeration<E> enumeration) {
        return StreamSupport.stream(new EnumerationSpliterator<>(Long.MAX_VALUE, Spliterator.ORDERED, enumeration), false);
    }

    /**
     * Creates a stream on the given Iterable.
     *
     * @param <E> the type of elements in the Iterable.
     * @param iterable the Iterable to stream or null.
     * @return a new Stream or {@link Stream#empty()} if the Iterable is null.
     * @since 3.13.0
     */
    public static <E> Stream<E> of(final Iterable<E> iterable) {
        return iterable == null ? Stream.empty() : StreamSupport.stream(iterable.spliterator(), false);
    }

    /**
     * Creates a stream on the given Iterator.
     *
     * @param <E> the type of elements in the Iterator.
     * @param iterator the Iterator to stream or null.
     * @return a new Stream or {@link Stream#empty()} if the Iterator is null.
     * @since 3.13.0
     */
    public static <E> Stream<E> of(final Iterator<E> iterator) {
        return iterator == null ? Stream.empty() : StreamSupport.stream(Spliterators.spliteratorUnknownSize(iterator, Spliterator.ORDERED), false);
    }

    /**
     * Returns the stream or {@link Stream#empty()} if the stream is null.
     *
     * @param <E> the type of elements in the collection.
     * @param stream the stream to stream or null.
     * @return the stream or {@link Stream#empty()} if the stream is null.
     * @since 3.13.0
     */
    private static <E> Stream<E> of(final Stream<E> stream) {
        return stream == null ? Stream.empty() : stream;
    }

    /**
     * Null-safe version of {@link Stream#of(Object[])}.
     *
     * @param <T> the type of stream elements.
     * @param values the elements of the new stream, may be {@code null}.
     * @return the new stream on {@code values} or {@link Stream#empty()}.
     * @since 3.13.0
     */
    @SafeVarargs // Creating a stream from an array is safe
    public static <T> Stream<T> of(final T... values) {
        return values == null ? Stream.empty() : Stream.of(values);
    }

    /**
     * Converts the given {@link Collection} into a {@link FailableStream}. This is basically a simplified, reduced version
     * of the {@link Stream} class, with the same underlying element stream, except that failable objects, like
     * {@link FailablePredicate}, {@link FailableFunction}, or {@link FailableConsumer} may be applied, instead of
     * {@link Predicate}, {@link Function}, or {@link Consumer}. The idea is to rewrite a code snippet like this:
     *
     * <pre>
     * {@code
     * final List<O> list;
     * final Method m;
     * final Function<O, String> mapper = (o) -> {
     *     try {
     *         return (String) m.invoke(o);
     *     } catch (Throwable t) {
     *         throw Failable.rethrow(t);
     *     }
     * };
     * final List<String> strList = list.stream().map(mapper).collect(Collectors.toList());
     * }
     * </pre>
     *
     * as follows:
     *
     * <pre>
     * {@code
     * final List<O> list;
     * final Method m;
     * final List<String> strList = Failable.stream(list.stream()).map((o) -> (String) m.invoke(o)).collect(Collectors.toList());
     * }
     * </pre>
     *
     * While the second version may not be <em>quite</em> as efficient (because it depends on the creation of additional,
     * intermediate objects, of type FailableStream), it is much more concise, and readable, and meets the spirit of Lambdas
     * better than the first version.
     *
     * @param <E> The streams element type.
     * @param collection The stream, which is being converted.
     * @return The {@link FailableStream}, which has been created by converting the stream.
     * @deprecated Use {@link #failableStream(Collection)}.
     */
    @Deprecated
    public static <E> FailableStream<E> stream(final Collection<E> collection) {
        return failableStream(collection);
    }

    /**
     * Converts the given {@link Stream stream} into a {@link FailableStream}. This is basically a simplified, reduced
     * version of the {@link Stream} class, with the same underlying element stream, except that failable objects, like
     * {@link FailablePredicate}, {@link FailableFunction}, or {@link FailableConsumer} may be applied, instead of
     * {@link Predicate}, {@link Function}, or {@link Consumer}. The idea is to rewrite a code snippet like this:
     *
     * <pre>
     * {@code
     * final List<O> list;
     * final Method m;
     * final Function<O, String> mapper = (o) -> {
     *     try {
     *         return (String) m.invoke(o);
     *     } catch (Throwable t) {
     *         throw Failable.rethrow(t);
     *     }
     * };
     * final List<String> strList = list.stream().map(mapper).collect(Collectors.toList());
     * }
     * </pre>
     *
     * as follows:
     *
     * <pre>
     * {@code
     * final List<O> list;
     * final Method m;
     * final List<String> strList = Failable.stream(list.stream()).map((o) -> (String) m.invoke(o)).collect(Collectors.toList());
     * }
     * </pre>
     *
     * While the second version may not be <em>quite</em> as efficient (because it depends on the creation of additional,
     * intermediate objects, of type FailableStream), it is much more concise, and readable, and meets the spirit of Lambdas
     * better than the first version.
     *
     * @param <T> The streams element type.
     * @param stream The stream, which is being converted.
     * @return The {@link FailableStream}, which has been created by converting the stream.
     * @deprecated Use {@link #failableStream(Stream)}.
     */
    @Deprecated
    public static <T> FailableStream<T> stream(final Stream<T> stream) {
        return failableStream(stream);
    }

    private static <T> Stream<T> streamOf(final T value) {
        return value == null ? Stream.empty() : Stream.of(value);
    }

    /**
     * Returns a {@link Collector} that accumulates the input elements into a new array.
     *
     * @param elementType Type of an element in the array.
     * @param <T> the type of the input elements
     * @return a {@link Collector} which collects all the input elements into an array, in encounter order
     */
    public static <T> Collector<T, ?, T[]> toArray(final Class<T> elementType) {
        return new ArrayCollector<>(elementType);
    }

    /**
     * Constructs a new instance.
     *
     * @deprecated Will be private in 4.0.0.
     */
    @Deprecated
    public Streams() {
        // empty
    }
}