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Apache Commons Lang, a package of Java utility classes for the
classes that are in java.lang's hierarchy, or are considered to be so
standard as to justify existence in java.lang.
/*
* 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 {@code 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:
*
*
* {@code
* Consumer consumer = m -> {
* try {
* m.invoke(o, args);
* } catch (Throwable t) {
* throw Failable.rethrow(t);
* }
* };
* stream.forEach(consumer);
* }
*
*
* Using a {@link FailableStream}, this can be rewritten as follows:
*
*
*
* {@code
* Streams.failable(stream).forEach((m) -> m.invoke(o, args));
* }
*
*
* Obviously, the second version is much more concise and the spirit of Lambda expressions is met better than in the
* first version.
*
* @see Stream
* @see Failable
* @since 3.11
*/
public class Streams {
/**
* A Collector type for arrays.
*
* @param The array type.
*/
public static class ArrayCollector implements Collector, E[]> {
private static final Set characteristics = Collections.emptySet();
private final Class elementType;
/**
* Constructs a new instance for the given element type.
*
* @param elementType The element type.
*/
public ArrayCollector(final Class elementType) {
this.elementType = Objects.requireNonNull(elementType, "elementType");
}
@Override
public BiConsumer, E> accumulator() {
return List::add;
}
@Override
public Set characteristics() {
return characteristics;
}
@Override
public BinaryOperator> combiner() {
return (left, right) -> {
left.addAll(right);
return left;
};
}
@Override
public Function, E[]> finisher() {
return list -> list.toArray(ArrayUtils.newInstance(elementType, list.size()));
}
@Override
public Supplier> supplier() {
return ArrayList::new;
}
}
/**
* Helps implement {@link Streams#of(Enumeration)}.
*
* @param The element type.
*/
private static class EnumerationSpliterator extends AbstractSpliterator {
private final Enumeration 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 enumeration) {
super(estimatedSize, additionalCharacteristics);
this.enumeration = Objects.requireNonNull(enumeration, "enumeration");
}
@Override
public void forEachRemaining(final Consumer action) {
while (enumeration.hasMoreElements()) {
next(action);
}
}
private boolean next(final Consumer action) {
action.accept(enumeration.nextElement());
return true;
}
@Override
public boolean tryAdvance(final Consumer action) {
return enumeration.hasMoreElements() && next(action);
}
}
/**
* A reduced, and simplified version of a {@link Stream} with failable method signatures.
*
* @param The streams element type.
*/
public static class FailableStream {
private Stream stream;
private boolean terminated;
/**
* Constructs a new instance with the given {@code stream}.
*
* @param stream The stream.
*/
public FailableStream(final Stream 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.
*
*
* This is a short-circuiting terminal operation.
*
*
* Note This method evaluates the universal quantification 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 vacuously satisfied 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 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.
*
*
* This is a short-circuiting terminal operation.
*
*
* Note This method evaluates the existential quantification 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 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.
*
*
* 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.)
*
*
*
* This is a terminal operation.
*
*
*
* 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.
*
*
* Note The following will accumulate strings into an ArrayList:
*
*
* {@code
* List asList = stringStream.collect(Collectors.toList());
* }
*
*
*
* The following will classify {@code Person} objects by city:
*
*
*
* {@code
* Map> peopleByCity = personStream.collect(Collectors.groupingBy(Person::getCity));
* }
*
*
*
* The following will classify {@code Person} objects by state and city, cascading two {@link Collector}s together:
*
*
*
* {@code
* Map>> peopleByStateAndCity = personStream
* .collect(Collectors.groupingBy(Person::getState, Collectors.groupingBy(Person::getCity)));
* }
*
*
* @param the type of the result
* @param 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 R collect(final Collector 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:
*
*
* {@code
* R result = supplier.get();
* for (T element : this stream)
* accumulator.accept(result, element);
* return result;
* }
*
*
*
* Like {@link #reduce(Object, BinaryOperator)}, {@code collect} operations can be parallelized without requiring
* additional synchronization.
*
*
*
* This is a terminal operation.
*
*
* 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}:
*
*
* {@code
* List asList = stringStream.collect(ArrayList::new, ArrayList::add, ArrayList::addAll);
* }
*
*
*
* The following will take a stream of strings and concatenates them into a single string:
*
*
*
* {@code
* String concat = stringStream.collect(StringBuilder::new, StringBuilder::append, StringBuilder::append).toString();
* }
*
*
* @param type of the result
* @param 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 R collect(final Supplier supplier, final BiConsumer accumulator, final BiConsumer combiner) {
makeTerminated();
return stream().collect(supplier, accumulator, combiner);
}
/**
* Returns a FailableStream consisting of the elements of this stream that match the given FailablePredicate.
*
*
* This is an intermediate operation.
*
*
* @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 filter(final FailablePredicate predicate) {
assertNotTerminated();
stream = stream.filter(Failable.asPredicate(predicate));
return this;
}
/**
* Performs an action for each element of this stream.
*
*
* This is a terminal operation.
*
*
*
* The behavior of this operation is explicitly nondeterministic. For parallel stream pipelines, this operation does
* not 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.
*
*
* @param action a non-interfering action to perform on the elements
*/
public void forEach(final FailableConsumer 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.
*
*
* This is an intermediate operation.
*
*
* @param The element type of the new stream
* @param mapper A non-interfering, stateless function to apply to each element
* @return the new stream
*/
public FailableStream map(final FailableFunction 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:
*
*
* {@code
* T result = identity;
* for (T element : this stream)
* result = accumulator.apply(result, element)
* return result;
* }
*
*
* but is not constrained to execute sequentially.
*
*
* 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.
*
*
*
* This is a terminal operation.
*
*
* Note Sum, min, max, average, and string concatenation are all special cases of reduction. Summing a stream of numbers
* can be expressed as:
*
*
* {@code
* Integer sum = integers.reduce(0, (a, b) -> a + b);
* }
*
*
* or:
*
*
* {@code
* Integer sum = integers.reduce(0, Integer::sum);
* }
*
*
*
* 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.
*
*
* @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 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 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:
*
*
* {@code
* final List list;
* final Method m;
* final Function mapper = (o) -> {
* try {
* return (String) m.invoke(o);
* } catch (Throwable t) {
* throw Failable.rethrow(t);
* }
* };
* final List strList = list.stream().map(mapper).collect(Collectors.toList());
* }
*
*
* as follows:
*
*
* {@code
* final List list;
* final Method m;
* final List strList = Failable.stream(list.stream()).map((o) -> (String) m.invoke(o)).collect(Collectors.toList());
* }
*
*
* While the second version may not be quite 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 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 FailableStream failableStream(final Collection 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:
*
*
* {@code
* final List list;
* final Method m;
* final Function mapper = (o) -> {
* try {
* return (String) m.invoke(o);
* } catch (Throwable t) {
* throw Failable.rethrow(t);
* }
* };
* final List strList = list.stream().map(mapper).collect(Collectors.toList());
* }
*
*
* as follows:
*
*
* {@code
* final List list;
* final Method m;
* final List strList = Failable.stream(list.stream()).map((o) -> (String) m.invoke(o)).collect(Collectors.toList());
* }
*
*
* While the second version may not be quite 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 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 FailableStream failableStream(final Stream stream) {
return new FailableStream<>(stream);
}
/**
* Streams only instances of the give Class in a collection.
*
* This method shorthand for:
*
*
* {@code (Stream) Streams.toStream(collection).filter(collection, SomeClass.class::isInstance);}
*
*
* @param 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 Stream instancesOf(final Class clazz, final Collection collection) {
return instancesOf(clazz, of(collection));
}
@SuppressWarnings("unchecked") // After the isInstance check, we still need to type-cast.
private static Stream instancesOf(final Class clazz, final Stream stream) {
return (Stream) of(stream).filter(clazz::isInstance);
}
/**
* Streams the non-null elements of a collection.
*
* @param 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 Stream nonNull(final Collection collection) {
return of(collection).filter(Objects::nonNull);
}
/**
* Streams the non-null elements of an array.
*
* @param 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 Stream nonNull(final E... array) {
return nonNull(of(array));
}
/**
* Streams the non-null elements of a stream.
*
* @param 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 Stream nonNull(final Stream stream) {
return of(stream).filter(Objects::nonNull);
}
/**
* Delegates to {@link Collection#stream()} or returns {@link Stream#empty()} if the collection is null.
*
* @param 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 Stream of(final Collection collection) {
return collection == null ? Stream.empty() : collection.stream();
}
/**
* Streams the elements of the given enumeration in order.
*
* @param The enumeration element type.
* @param enumeration The enumeration to stream.
* @return a new stream.
* @since 3.13.0
*/
public static Stream of(final Enumeration enumeration) {
return StreamSupport.stream(new EnumerationSpliterator<>(Long.MAX_VALUE, Spliterator.ORDERED, enumeration), false);
}
/**
* Creates a stream on the given Iterable.
*
* @param 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 Stream of(final Iterable iterable) {
return iterable == null ? Stream.empty() : StreamSupport.stream(iterable.spliterator(), false);
}
/**
* Creates a stream on the given Iterator.
*
* @param 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 Stream of(final Iterator 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 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 Stream of(final Stream stream) {
return stream == null ? Stream.empty() : stream;
}
/**
* Null-safe version of {@link Stream#of(Object[])}.
*
* @param 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 Stream 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:
*
*
* {@code
* final List list;
* final Method m;
* final Function mapper = (o) -> {
* try {
* return (String) m.invoke(o);
* } catch (Throwable t) {
* throw Failable.rethrow(t);
* }
* };
* final List strList = list.stream().map(mapper).collect(Collectors.toList());
* }
*
*
* as follows:
*
*
* {@code
* final List list;
* final Method m;
* final List strList = Failable.stream(list.stream()).map((o) -> (String) m.invoke(o)).collect(Collectors.toList());
* }
*
*
* While the second version may not be quite 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 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 FailableStream stream(final Collection 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:
*
*
* {@code
* final List list;
* final Method m;
* final Function mapper = (o) -> {
* try {
* return (String) m.invoke(o);
* } catch (Throwable t) {
* throw Failable.rethrow(t);
* }
* };
* final List strList = list.stream().map(mapper).collect(Collectors.toList());
* }
*
*
* as follows:
*
*
* {@code
* final List list;
* final Method m;
* final List strList = Failable.stream(list.stream()).map((o) -> (String) m.invoke(o)).collect(Collectors.toList());
* }
*
*
* While the second version may not be quite 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 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 FailableStream stream(final Stream stream) {
return failableStream(stream);
}
/**
* Returns a {@link Collector} that accumulates the input elements into a new array.
*
* @param pElementType Type of an element in the array.
* @param the type of the input elements
* @return a {@link Collector} which collects all the input elements into an array, in encounter order
*/
public static Collector toArray(final Class pElementType) {
return new ArrayCollector<>(pElementType);
}
}