8024178: Difference in Stream.collect(Collector) methods located in jdk8 and jdk8-lambda repos
Reviewed-by: mduigou
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package java.util.stream;
import java.util.Arrays;
import java.util.Comparator;
import java.util.Iterator;
import java.util.Objects;
import java.util.Optional;
import java.util.Spliterator;
import java.util.Spliterators;
import java.util.function.BiConsumer;
import java.util.function.BiFunction;
import java.util.function.BinaryOperator;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.IntFunction;
import java.util.function.Predicate;
import java.util.function.Supplier;
import java.util.function.ToDoubleFunction;
import java.util.function.ToIntFunction;
import java.util.function.ToLongFunction;
import java.util.function.UnaryOperator;
// @@@ Specification to-do list @@@
// - Describe the difference between sequential and parallel streams
// - More general information about reduce, better definitions for associativity, more description of
// how reduce employs parallelism, more examples
// - Role of stream flags in various operations, specifically ordering
// - Whether each op preserves encounter order
// @@@ Specification to-do list @@@
/**
* A sequence of elements supporting sequential and parallel bulk operations.
* Streams support lazy intermediate operations (transforming a stream to
* another stream) such as {@code filter} and {@code map}, and terminal
* operations (consuming the contents of a stream to produce a result or
* side-effect), such as {@code forEach}, {@code findFirst}, and {@code
* iterator}. Once an operation has been performed on a stream, it
* is considered <em>consumed</em> and no longer usable for other operations.
*
* <p>For sequential stream pipelines, all operations are performed in the
* <a href="package-summary.html#Ordering">encounter order</a> of the pipeline
* source, if the pipeline source has a defined encounter order.
*
* <p>For parallel stream pipelines, unless otherwise specified, intermediate
* stream operations preserve the <a href="package-summary.html#Ordering">
* encounter order</a> of their source, and terminal operations
* respect the encounter order of their source, if the source
* has an encounter order. Provided that and parameters to stream operations
* satisfy the <a href="package-summary.html#NonInterference">non-interference
* requirements</a>, and excepting differences arising from the absence of
* a defined encounter order, the result of a stream pipeline should be the
* stable across multiple executions of the same operations on the same source.
* However, the timing and thread in which side-effects occur (for those
* operations which are allowed to produce side-effects, such as
* {@link #forEach(Consumer)}), are explicitly nondeterministic for parallel
* execution of stream pipelines.
*
* <p>Unless otherwise noted, passing a {@code null} argument to any stream
* method may result in a {@link NullPointerException}.
*
* @apiNote
* Streams are not data structures; they do not manage the storage for their
* elements, nor do they support access to individual elements. However,
* you can use the {@link #iterator()} or {@link #spliterator()} operations to
* perform a controlled traversal.
*
* @param <T> type of elements
* @since 1.8
* @see <a href="package-summary.html">java.util.stream</a>
*/
public interface Stream<T> extends BaseStream<T, Stream<T>> {
/**
* Returns a stream consisting of the elements of this stream that match
* the given predicate.
*
* <p>This is an <a href="package-summary.html#StreamOps">intermediate
* operation</a>.
*
* @param predicate a <a href="package-summary.html#NonInterference">
* non-interfering, stateless</a> predicate to apply to
* each element to determine if it should be included
* @return the new stream
*/
Stream<T> filter(Predicate<? super T> predicate);
/**
* Returns a stream consisting of the results of applying the given
* function to the elements of this stream.
*
* <p>This is an <a href="package-summary.html#StreamOps">intermediate
* operation</a>.
*
* @param <R> The element type of the new stream
* @param mapper a <a href="package-summary.html#NonInterference">
* non-interfering, stateless</a> function to apply to each
* element
* @return the new stream
*/
<R> Stream<R> map(Function<? super T, ? extends R> mapper);
/**
* Returns an {@code IntStream} consisting of the results of applying the
* given function to the elements of this stream.
*
* <p>This is an <a href="package-summary.html#StreamOps">
* intermediate operation</a>.
*
* @param mapper a <a href="package-summary.html#NonInterference">
* non-interfering, stateless</a> function to apply to each
* element
* @return the new stream
*/
IntStream mapToInt(ToIntFunction<? super T> mapper);
/**
* Returns a {@code LongStream} consisting of the results of applying the
* given function to the elements of this stream.
*
* <p>This is an <a href="package-summary.html#StreamOps">intermediate
* operation</a>.
*
* @param mapper a <a href="package-summary.html#NonInterference">
* non-interfering, stateless</a> function to apply to each
* element
* @return the new stream
*/
LongStream mapToLong(ToLongFunction<? super T> mapper);
/**
* Returns a {@code DoubleStream} consisting of the results of applying the
* given function to the elements of this stream.
*
* <p>This is an <a href="package-summary.html#StreamOps">intermediate
* operation</a>.
*
* @param mapper a <a href="package-summary.html#NonInterference">
* non-interfering, stateless</a> function to apply to each
* element
* @return the new stream
*/
DoubleStream mapToDouble(ToDoubleFunction<? super T> mapper);
/**
* Returns a stream consisting of the results of replacing each element of
* this stream with the contents of the stream produced by applying the
* provided mapping function to each element. If the result of the mapping
* function is {@code null}, this is treated as if the result is an empty
* stream.
*
* <p>This is an <a href="package-summary.html#StreamOps">intermediate
* operation</a>.
*
* @apiNote
* The {@code flatMap()} operation has the effect of applying a one-to-many
* tranformation to the elements of the stream, and then flattening the
* resulting elements into a new stream. For example, if {@code orders}
* is a stream of purchase orders, and each purchase order contains a
* collection of line items, then the following produces a stream of line
* items:
* <pre>{@code
* orderStream.flatMap(order -> order.getLineItems().stream())...
* }</pre>
*
* @param <R> The element type of the new stream
* @param mapper a <a href="package-summary.html#NonInterference">
* non-interfering, stateless</a> function to apply to each
* element which produces a stream of new values
* @return the new stream
*/
<R> Stream<R> flatMap(Function<? super T, ? extends Stream<? extends R>> mapper);
/**
* Returns an {@code IntStream} consisting of the results of replacing each
* element of this stream with the contents of the stream produced by
* applying the provided mapping function to each element. If the result of
* the mapping function is {@code null}, this is treated as if the result is
* an empty stream.
*
* <p>This is an <a href="package-summary.html#StreamOps">intermediate
* operation</a>.
*
* @param mapper a <a href="package-summary.html#NonInterference">
* non-interfering, stateless</a> function to apply to each
* element which produces a stream of new values
* @return the new stream
*/
IntStream flatMapToInt(Function<? super T, ? extends IntStream> mapper);
/**
* Returns a {@code LongStream} consisting of the results of replacing each
* element of this stream with the contents of the stream produced
* by applying the provided mapping function to each element. If the result
* of the mapping function is {@code null}, this is treated as if the
* result is an empty stream.
*
* <p>This is an <a href="package-summary.html#StreamOps">intermediate
* operation</a>.
*
* @param mapper a <a href="package-summary.html#NonInterference">
* non-interfering, stateless</a> function to apply to
* each element which produces a stream of new values
* @return the new stream
*/
LongStream flatMapToLong(Function<? super T, ? extends LongStream> mapper);
/**
* Returns a {@code DoubleStream} consisting of the results of replacing each
* element of this stream with the contents of the stream produced
* by applying the provided mapping function to each element. If the result
* of the mapping function is {@code null}, this is treated as if the result
* is an empty stream.
*
* <p>This is an <a href="package-summary.html#StreamOps">intermediate
* operation</a>.
*
* @param mapper a <a href="package-summary.html#NonInterference">
* non-interfering, stateless</a> function to apply to each
* element which produces a stream of new values
* @return the new stream
*/
DoubleStream flatMapToDouble(Function<? super T, ? extends DoubleStream> mapper);
/**
* Returns a stream consisting of the distinct elements (according to
* {@link Object#equals(Object)}) of this stream.
*
* <p>This is a <a href="package-summary.html#StreamOps">stateful
* intermediate operation</a>.
*
* @return the new stream
*/
Stream<T> distinct();
/**
* Returns a stream consisting of the elements of this stream, sorted
* according to natural order. If the elements of this stream are not
* {@code Comparable}, a {@code java.lang.ClassCastException} may be thrown
* when the stream pipeline is executed.
*
* <p>This is a <a href="package-summary.html#StreamOps">stateful
* intermediate operation</a>.
*
* @return the new stream
*/
Stream<T> sorted();
/**
* Returns a stream consisting of the elements of this stream, sorted
* according to the provided {@code Comparator}.
*
* <p>This is a <a href="package-summary.html#StreamOps">stateful
* intermediate operation</a>.
*
* @param comparator a <a href="package-summary.html#NonInterference">
* non-interfering, stateless</a> {@code Comparator} to
* be used to compare stream elements
* @return the new stream
*/
Stream<T> sorted(Comparator<? super T> comparator);
/**
* Returns a stream consisting of the elements of this stream, additionally
* performing the provided action on each element as elements are consumed
* from the resulting stream.
*
* <p>This is an <a href="package-summary.html#StreamOps">intermediate
* operation</a>.
*
* <p>For parallel stream pipelines, the action may be called at
* whatever time and in whatever thread the element is made available by the
* upstream operation. If the action modifies shared state,
* it is responsible for providing the required synchronization.
*
* @apiNote This method exists mainly to support debugging, where you want
* to see the elements as they flow past a certain point in a pipeline:
* <pre>{@code
* list.stream()
* .filter(filteringFunction)
* .peek(e -> {System.out.println("Filtered value: " + e); });
* .map(mappingFunction)
* .peek(e -> {System.out.println("Mapped value: " + e); });
* .collect(Collectors.intoList());
* }</pre>
*
* @param consumer a <a href="package-summary.html#NonInterference">
* non-interfering</a> action to perform on the elements as
* they are consumed from the stream
* @return the new stream
*/
Stream<T> peek(Consumer<? super T> consumer);
/**
* Returns a stream consisting of the elements of this stream, truncated
* to be no longer than {@code maxSize} in length.
*
* <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
* stateful intermediate operation</a>.
*
* @param maxSize the number of elements the stream should be limited to
* @return the new stream
* @throws IllegalArgumentException if {@code maxSize} is negative
*/
Stream<T> limit(long maxSize);
/**
* Returns a stream consisting of the remaining elements of this stream
* after indexing {@code startInclusive} elements into the stream. If the
* {@code startInclusive} index lies past the end of this stream then an
* empty stream will be returned.
*
* <p>This is a <a href="package-summary.html#StreamOps">stateful
* intermediate operation</a>.
*
* @param startInclusive the number of leading elements to skip
* @return the new stream
* @throws IllegalArgumentException if {@code startInclusive} is negative
*/
Stream<T> substream(long startInclusive);
/**
* Returns a stream consisting of the remaining elements of this stream
* after indexing {@code startInclusive} elements into the stream and
* truncated to contain no more than {@code endExclusive - startInclusive}
* elements. If the {@code startInclusive} index lies past the end
* of this stream then an empty stream will be returned.
*
* <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
* stateful intermediate operation</a>.
*
* @param startInclusive the starting position of the substream, inclusive
* @param endExclusive the ending position of the substream, exclusive
* @return the new stream
* @throws IllegalArgumentException if {@code startInclusive} or
* {@code endExclusive} is negative or {@code startInclusive} is greater
* than {@code endExclusive}
*/
Stream<T> substream(long startInclusive, long endExclusive);
/**
* Performs an action for each element of this stream.
*
* <p>This is a <a href="package-summary.html#StreamOps">terminal
* operation</a>.
*
* <p>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.
*
* @param action a <a href="package-summary.html#NonInterference">
* non-interfering</a> action to perform on the elements
*/
void forEach(Consumer<? super T> action);
/**
* Performs an action for each element of this stream, guaranteeing that
* each element is processed in encounter order for streams that have a
* defined encounter order.
*
* <p>This is a <a href="package-summary.html#StreamOps">terminal
* operation</a>.
*
* @param action a <a href="package-summary.html#NonInterference">
* non-interfering</a> action to perform on the elements
* @see #forEach(Consumer)
*/
void forEachOrdered(Consumer<? super T> action);
/**
* Returns an array containing the elements of this stream.
*
* <p>This is a <a href="package-summary.html#StreamOps">terminal
* operation</a>.
*
* @return an array containing the elements of this stream
*/
Object[] toArray();
/**
* Returns an array containing the elements of this stream, using the
* provided {@code generator} function to allocate the returned array.
*
* <p>This is a <a href="package-summary.html#StreamOps">terminal
* operation</a>.
*
* @param <A> the element type of the resulting array
* @param generator a function which produces a new array of the desired
* type and the provided length
* @return an array containing the elements in this stream
* @throws ArrayStoreException if the runtime type of the array returned
* from the array generator is not a supertype of the runtime type of every
* element in this stream
*/
<A> A[] toArray(IntFunction<A[]> generator);
/**
* Performs a <a href="package-summary.html#Reduction">reduction</a> on the
* elements of this stream, using the provided identity value and an
* <a href="package-summary.html#Associativity">associative</a>
* 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
* <a href="package-summary.html#Associativity">associative</a> function.
*
* <p>This is a <a href="package-summary.html#StreamOps">terminal
* operation</a>.
*
* @apiNote 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 more compactly:
*
* <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.
*
* @param identity the identity value for the accumulating function
* @param accumulator an <a href="package-summary.html#Associativity">associative</a>
* <a href="package-summary.html#NonInterference">non-interfering,
* stateless</a> function for combining two values
* @return the result of the reduction
*/
T reduce(T identity, BinaryOperator<T> accumulator);
/**
* Performs a <a href="package-summary.html#Reduction">reduction</a> on the
* elements of this stream, using an
* <a href="package-summary.html#Associativity">associative</a> accumulation
* function, and returns an {@code Optional} describing the reduced value,
* if any. This is equivalent to:
* <pre>{@code
* boolean foundAny = false;
* T result = null;
* for (T element : this stream) {
* if (!foundAny) {
* foundAny = true;
* result = element;
* }
* else
* result = accumulator.apply(result, element);
* }
* return foundAny ? Optional.of(result) : Optional.empty();
* }</pre>
*
* but is not constrained to execute sequentially.
*
* <p>The {@code accumulator} function must be an
* <a href="package-summary.html#Associativity">associative</a> function.
*
* <p>This is a <a href="package-summary.html#StreamOps">terminal
* operation</a>.
*
* @param accumulator an <a href="package-summary.html#Associativity">associative</a>
* <a href="package-summary.html#NonInterference">non-interfering,
* stateless</a> function for combining two values
* @return the result of the reduction
* @see #reduce(Object, BinaryOperator)
* @see #min(java.util.Comparator)
* @see #max(java.util.Comparator)
*/
Optional<T> reduce(BinaryOperator<T> accumulator);
/**
* Performs a <a href="package-summary.html#Reduction">reduction</a> on the
* elements of this stream, using the provided identity, accumulation
* function, and a combining functions. This is equivalent to:
* <pre>{@code
* U 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 combiner
* function. This means that for all {@code u}, {@code combiner(identity, u)}
* is equal to {@code u}. Additionally, the {@code combiner} function
* must be compatible with the {@code accumulator} function; for all
* {@code u} and {@code t}, the following must hold:
* <pre>{@code
* combiner.apply(u, accumulator.apply(identity, t)) == accumulator.apply(u, t)
* }</pre>
*
* <p>This is a <a href="package-summary.html#StreamOps">terminal
* operation</a>.
*
* @apiNote Many reductions using this form can be represented more simply
* by an explicit combination of {@code map} and {@code reduce} operations.
* The {@code accumulator} function acts as a fused mapper and accumulator,
* which can sometimes be more efficient than separate mapping and reduction,
* such as in the case where knowing the previously reduced value allows you
* to avoid some computation.
*
* @param <U> The type of the result
* @param identity the identity value for the combiner function
* @param accumulator an <a href="package-summary.html#Associativity">associative</a>
* <a href="package-summary.html#NonInterference">non-interfering,
* stateless</a> function for incorporating an additional
* element into a result
* @param combiner an <a href="package-summary.html#Associativity">associative</a>
* <a href="package-summary.html#NonInterference">non-interfering,
* stateless</a> function for combining two values, which
* must be compatible with the accumulator function
* @return the result of the reduction
* @see #reduce(BinaryOperator)
* @see #reduce(Object, BinaryOperator)
*/
<U> U reduce(U identity,
BiFunction<U, ? super T, U> accumulator,
BinaryOperator<U> combiner);
/**
* Performs a <a href="package-summary.html#MutableReduction">mutable
* reduction</a> operation on the elements of this stream. A mutable
* reduction is one in which the reduced value is a mutable value holder,
* 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:
* <pre>{@code
* R result = resultFactory.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>This is a <a href="package-summary.html#StreamOps">terminal
* operation</a>.
*
* @apiNote There are many existing classes in the JDK whose signatures are
* a good match for use as arguments to {@code collect()}. For example,
* the following will accumulate strings into an ArrayList:
* <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:
* <pre>{@code
* String concat = stringStream.collect(StringBuilder::new, StringBuilder::append,
* StringBuilder::append)
* .toString();
* }</pre>
*
* @param <R> type of the result
* @param resultFactory 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 <a href="package-summary.html#Associativity">associative</a>
* <a href="package-summary.html#NonInterference">non-interfering,
* stateless</a> function for incorporating an additional
* element into a result
* @param combiner an <a href="package-summary.html#Associativity">associative</a>
* <a href="package-summary.html#NonInterference">non-interfering,
* stateless</a> function for combining two values, which
* must be compatible with the accumulator function
* @return the result of the reduction
*/
<R> R collect(Supplier<R> resultFactory,
BiConsumer<R, ? super T> accumulator,
BiConsumer<R, R> combiner);
/**
* Performs a <a href="package-summary.html#MutableReduction">mutable
* reduction</a> operation on the elements of this stream using a
* {@code Collector} object to describe the reduction. 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.
*
* <p>This is a <a href="package-summary.html#StreamOps">terminal
* operation</a>.
*
* <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 {@code ArrayList}), no
* additional synchronization is needed for a parallel reduction.
*
* @apiNote
* 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:
* <pre>{@code
* Map<String, Collection<Person>> peopleByCity
* = personStream.collect(Collectors.groupBy(Person::getCity));
* }</pre>
*
* <p>The following will classify {@code Person} objects by state and city,
* cascading two {@code Collector}s together:
* <pre>{@code
* Map<String, Map<String, Collection<Person>>> peopleByStateAndCity
* = personStream.collect(Collectors.groupBy(Person::getState,
* Collectors.groupBy(Person::getCity)));
* }</pre>
*
* @param <R> the type of the result
* @param <A> 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
*/
<R, A> R collect(Collector<? super T, A, R> collector);
/**
* Returns the minimum element of this stream according to the provided
* {@code Comparator}. This is a special case of a
* <a href="package-summary.html#MutableReduction">reduction</a>.
*
* <p>This is a <a href="package-summary.html#StreamOps">terminal operation</a>.
*
* @param comparator a <a href="package-summary.html#NonInterference">non-interfering,
* stateless</a> {@code Comparator} to use to compare
* elements of this stream
* @return an {@code Optional} describing the minimum element of this stream,
* or an empty {@code Optional} if the stream is empty
*/
Optional<T> min(Comparator<? super T> comparator);
/**
* Returns the maximum element of this stream according to the provided
* {@code Comparator}. This is a special case of a
* <a href="package-summary.html#MutableReduction">reduction</a>.
*
* <p>This is a <a href="package-summary.html#StreamOps">terminal
* operation</a>.
*
* @param comparator a <a href="package-summary.html#NonInterference">non-interfering,
* stateless</a> {@code Comparator} to use to compare
* elements of this stream
* @return an {@code Optional} describing the maximum element of this stream,
* or an empty {@code Optional} if the stream is empty
*/
Optional<T> max(Comparator<? super T> comparator);
/**
* Returns the count of elements in this stream. This is a special case of
* a <a href="package-summary.html#MutableReduction">reduction</a> and is
* equivalent to:
* <pre>{@code
* return mapToLong(e -> 1L).sum();
* }</pre>
*
* <p>This is a <a href="package-summary.html#StreamOps">terminal operation</a>.
*
* @return the count of elements in this stream
*/
long count();
/**
* 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.
*
* <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
* terminal operation</a>.
*
* @param predicate a <a href="package-summary.html#NonInterference">non-interfering,
* stateless</a> predicate to apply to elements of this
* stream
* @return {@code true} if any elements of the stream match the provided
* predicate otherwise {@code false}
*/
boolean anyMatch(Predicate<? super T> predicate);
/**
* 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.
*
* <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
* terminal operation</a>.
*
* @param predicate a <a href="package-summary.html#NonInterference">non-interfering,
* stateless</a> predicate to apply to elements of this
* stream
* @return {@code true} if all elements of the stream match the provided
* predicate otherwise {@code false}
*/
boolean allMatch(Predicate<? super T> predicate);
/**
* Returns whether no elements of this stream match the provided predicate.
* May not evaluate the predicate on all elements if not necessary for
* determining the result.
*
* <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
* terminal operation</a>.
*
* @param predicate a <a href="package-summary.html#NonInterference">non-interfering,
* stateless</a> predicate to apply to elements of this
* stream
* @return {@code true} if no elements of the stream match the provided
* predicate otherwise {@code false}
*/
boolean noneMatch(Predicate<? super T> predicate);
/**
* Returns an {@link Optional} describing the first element of this stream
* (in the encounter order), or an empty {@code Optional} if the stream is
* empty. If the stream has no encounter order, then any element may be
* returned.
*
* <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
* terminal operation</a>.
*
* @return an {@code Optional} describing the first element of this stream,
* or an empty {@code Optional} if the stream is empty
* @throws NullPointerException if the element selected is null
*/
Optional<T> findFirst();
/**
* Returns an {@link Optional} describing some element of the stream, or an
* empty {@code Optional} if the stream is empty.
*
* <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
* terminal operation</a>.
*
* <p>The behavior of this operation is explicitly nondeterministic; it is
* free to select any element in the stream. This is to allow for maximal
* performance in parallel operations; the cost is that multiple invocations
* on the same source may not return the same result. (If the first element
* in the encounter order is desired, use {@link #findFirst()} instead.)
*
* @return an {@code Optional} describing some element of this stream, or an
* empty {@code Optional} if the stream is empty
* @throws NullPointerException if the element selected is null
* @see #findFirst()
*/
Optional<T> findAny();
// Static factories
/**
* Returns a builder for a {@code Stream}.
*
* @param <T> type of elements
* @return a stream builder
*/
public static<T> Builder<T> builder() {
return new Streams.StreamBuilderImpl<>();
}
/**
* Returns an empty sequential {@code Stream}.
*
* @param <T> the type of stream elements
* @return an empty sequential stream
*/
public static<T> Stream<T> empty() {
return StreamSupport.stream(Spliterators.<T>emptySpliterator(), false);
}
/**
* Returns a sequential {@code Stream} containing a single element.
*
* @param t the single element
* @param <T> the type of stream elements
* @return a singleton sequential stream
*/
public static<T> Stream<T> of(T t) {
return StreamSupport.stream(new Streams.StreamBuilderImpl<>(t), false);
}
/**
* Returns a sequential stream whose elements are the specified values.
*
* @param <T> the type of stream elements
* @param values the elements of the new stream
* @return the new stream
*/
@SafeVarargs
@SuppressWarnings("varargs") // Creating a stream from an array is safe
public static<T> Stream<T> of(T... values) {
return Arrays.stream(values);
}
/**
* Returns an infinite sequential {@code Stream} produced by iterative
* application of a function {@code f} to an initial element {@code seed},
* producing a {@code Stream} consisting of {@code seed}, {@code f(seed)},
* {@code f(f(seed))}, etc.
*
* <p>The first element (position {@code 0}) in the {@code Stream} will be
* the provided {@code seed}. For {@code n > 0}, the element at position
* {@code n}, will be the result of applying the function {@code f} to the
* element at position {@code n - 1}.
*
* @param <T> the type of stream elements
* @param seed the initial element
* @param f a function to be applied to to the previous element to produce
* a new element
* @return a new sequential {@code Stream}
*/
public static<T> Stream<T> iterate(final T seed, final UnaryOperator<T> f) {
Objects.requireNonNull(f);
final Iterator<T> iterator = new Iterator<T>() {
@SuppressWarnings("unchecked")
T t = (T) Streams.NONE;
@Override
public boolean hasNext() {
return true;
}
@Override
public T next() {
return t = (t == Streams.NONE) ? seed : f.apply(t);
}
};
return StreamSupport.stream(Spliterators.spliteratorUnknownSize(
iterator,
Spliterator.ORDERED | Spliterator.IMMUTABLE), false);
}
/**
* Returns a sequential {@code Stream} where each element is
* generated by a {@code Supplier}. This is suitable for generating
* constant streams, streams of random elements, etc.
*
* @param <T> the type of stream elements
* @param s the {@code Supplier} of generated elements
* @return a new sequential {@code Stream}
*/
public static<T> Stream<T> generate(Supplier<T> s) {
Objects.requireNonNull(s);
return StreamSupport.stream(
new StreamSpliterators.InfiniteSupplyingSpliterator.OfRef<>(Long.MAX_VALUE, s), false);
}
/**
* Creates a lazy concatenated {@code Stream} whose elements are all the
* elements of a first {@code Stream} succeeded by all the elements of the
* second {@code Stream}. The resulting stream is ordered if both
* of the input streams are ordered, and parallel if either of the input
* streams is parallel.
*
* @param <T> The type of stream elements
* @param a the first stream
* @param b the second stream to concatenate on to end of the first
* stream
* @return the concatenation of the two input streams
*/
public static <T> Stream<T> concat(Stream<? extends T> a, Stream<? extends T> b) {
Objects.requireNonNull(a);
Objects.requireNonNull(b);
@SuppressWarnings("unchecked")
Spliterator<T> split = new Streams.ConcatSpliterator.OfRef<>(
(Spliterator<T>) a.spliterator(), (Spliterator<T>) b.spliterator());
return StreamSupport.stream(split, a.isParallel() || b.isParallel());
}
/**
* A mutable builder for a {@code Stream}. This allows the creation of a
* {@code Stream} by generating elements individually and adding them to the
* {@code Builder} (without the copying overhead that comes from using
* an {@code ArrayList} as a temporary buffer.)
*
* <p>A {@code Stream.Builder} has a lifecycle, where it starts in a building
* phase, during which elements can be added, and then transitions to a built
* phase, after which elements may not be added. The built phase begins
* when the {@link #build()} method is called, which creates an ordered
* {@code Stream} whose elements are the elements that were added to the stream
* builder, in the order they were added.
*
* @param <T> the type of stream elements
* @see Stream#builder()
* @since 1.8
*/
public interface Builder<T> extends Consumer<T> {
/**
* Adds an element to the stream being built.
*
* @throws IllegalStateException if the builder has already transitioned to
* the built state
*/
@Override
void accept(T t);
/**
* Adds an element to the stream being built.
*
* @implSpec
* The default implementation behaves as if:
* <pre>{@code
* accept(t)
* return this;
* }</pre>
*
* @param t the element to add
* @return {@code this} builder
* @throws IllegalStateException if the builder has already transitioned to
* the built state
*/
default Builder<T> add(T t) {
accept(t);
return this;
}
/**
* Builds the stream, transitioning this builder to the built state.
* An {@code IllegalStateException} is thrown if there are further attempts
* to operate on the builder after it has entered the built state.
*
* @return the built stream
* @throws IllegalStateException if the builder has already transitioned to
* the built state
*/
Stream<T> build();
}
}