org.apache.commons.lang3.Streams Maven / Gradle / Ivy
Show all versions of commons-lang3 Show documentation
/*
* 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;
import java.lang.reflect.Array;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.List;
import java.util.Set;
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 org.apache.commons.lang3.Functions.FailableConsumer;
import org.apache.commons.lang3.Functions.FailableFunction;
import org.apache.commons.lang3.Functions.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
*
* Consumer<java.lang.reflect.Method> consumer = m -> {
* try {
* m.invoke(o, args);
* } catch (Throwable t) {
* throw Functions.rethrow(t);
* }
* };
* stream.forEach(consumer);
*
* Using a {@link FailableStream}, this can be rewritten as follows:
*
* 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 Functions
* @since 3.10
* @deprecated Use {@link org.apache.commons.lang3.stream.Streams}.
*/
@Deprecated
public class Streams {
/**
* A reduced, and simplified version of a {@link Stream} with
* failable method signatures.
* @param The streams element type.
* @deprecated Use {@link org.apache.commons.lang3.stream.Streams.FailableStream}.
*/
@Deprecated
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;
}
protected void assertNotTerminated() {
if (terminated) {
throw new IllegalStateException("This stream is already terminated.");
}
}
protected void makeTerminated() {
assertNotTerminated();
terminated = true;
}
/**
* 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(Functions.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(Functions.asConsumer(action));
}
/**
* Performs a mutable reduction operation on the elements of this stream using a
* {@code Collector}. A {@code 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 {@code 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 {@code 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 {@code 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 {@code Collector}
* @param collector the {@code 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 {@code 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 {@code 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 pupplier 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 pupplier, final BiConsumer accumulator, final BiConsumer combiner) {
makeTerminated();
return stream().collect(pupplier, accumulator, combiner);
}
/**
* 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 O reduce(final O identity, final BinaryOperator accumulator) {
makeTerminated();
return stream().reduce(identity, accumulator);
}
/**
* 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(Functions.asFunction(mapper)));
}
/**
* 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;
}
/**
* 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(Functions.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(Functions.asPredicate(predicate));
}
}
/**
* 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:
*
* final List<O> list;
* final Method m;
* final Function<O,String> mapper = (o) -> {
* try {
* return (String) m.invoke(o);
* } catch (Throwable t) {
* throw Functions.rethrow(t);
* }
* };
* final List<String> strList = list.stream()
* .map(mapper).collect(Collectors.toList());
*
* as follows:
*
* final List<O> list;
* final Method m;
* final List<String> strList = Functions.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.
*/
public static FailableStream stream(final Stream stream) {
return new FailableStream<>(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:
*
* final List<O> list;
* final Method m;
* final Function<O,String> mapper = (o) -> {
* try {
* return (String) m.invoke(o);
* } catch (Throwable t) {
* throw Functions.rethrow(t);
* }
* };
* final List<String> strList = list.stream()
* .map(mapper).collect(Collectors.toList());
*
* as follows:
*
* final List<O> list;
* final Method m;
* final List<String> strList = Functions.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.
*/
public static FailableStream stream(final Collection stream) {
return stream(stream.stream());
}
/**
* A Collector type for arrays.
*
* @param The array type.
* @deprecated Use {@link org.apache.commons.lang3.stream.Streams.ArrayCollector}.
*/
@Deprecated
public static class ArrayCollector implements Collector, O[]> {
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 = elementType;
}
@Override
public Supplier> supplier() {
return ArrayList::new;
}
@Override
public BiConsumer, O> accumulator() {
return List::add;
}
@Override
public BinaryOperator> combiner() {
return (left, right) -> {
left.addAll(right);
return left;
};
}
@Override
public Function, O[]> finisher() {
return list -> {
@SuppressWarnings("unchecked")
final O[] array = (O[]) Array.newInstance(elementType, list.size());
return list.toArray(array);
};
}
@Override
public Set characteristics() {
return characteristics;
}
}
/**
* Returns a {@code 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 {@code Collector} which collects all the input elements into an
* array, in encounter order
*/
public static Collector toArray(final Class pElementType) {
return new ArrayCollector<>(pElementType);
}
}