{@code
* long sum = widgets.stream()
* .filter(w -> w.getColor() == RED)
* .mapToLong(w -> w.getWeight())
* .sum();
* }
*
* In this example, {@code widgets} is a {@code CollectionTo perform a computation, stream * operations are composed into a * stream pipeline. A stream pipeline consists of a source (which * might be an array, a collection, a generator function, an IO channel, * etc), zero or more intermediate operations (which transform a * stream into another stream, such as {@link LongStream#filter(LongPredicate)}), and a * terminal operation (which produces a result or side-effect, such * as {@link LongStream#sum()} or {@link LongStream#forEach(LongConsumer)}). * Streams are lazy; computation on the source data is only performed when the * terminal operation is initiated, and source elements are consumed only * as needed. * *
Collections and streams, while bearing some superficial similarities, * have different goals. Collections are primarily concerned with the efficient * management of, and access to, their elements. By contrast, streams do not * provide a means to directly access or manipulate their elements, and are * instead concerned with declaratively describing their source and the * computational operations which will be performed in aggregate on that source. * However, if the provided stream operations do not offer the desired * functionality, the {@link #iterator()} and {@link #spliterator()} operations * can be used to perform a controlled traversal. * *
A stream pipeline, like the "widgets" example above, can be viewed as * a query on the stream source. Unless the source was explicitly * designed for concurrent modification (such as a {@link ConcurrentHashMap}), * unpredictable or erroneous behavior may result from modifying the stream * source while it is being queried. * *
Most stream operations accept parameters that describe user-specified * behavior, such as the lambda expression {@code w -> w.getWeight()} passed to * {@code mapToLong} in the example above. Such parameters are always instances * of a functional interface such * as {@link java.util.function.Function}, and are often lambda expressions or * method references. These parameters can never be null, should not modify the * stream source, and should be * effectively stateless * (their result should not depend on any state that might change during * execution of the stream pipeline.) * *
A stream should be operated on (invoking an intermediate or terminal stream * operation) only once. This rules out, for example, "forked" streams, where * the same source feeds two or more pipelines, or multiple traversals of the * same stream. A stream implementation may throw {@link IllegalStateException} * if it detects that the stream is being reused. However, since some stream * operations may return their receiver rather than a new stream object, it may * not be possible to detect reuse in all cases. * *
Streams have a {@link #close()} method and implement {@link AutoCloseable}, * but nearly all stream instances do not actually need to be closed after use. * Generally, only streams whose source is an IO channel (such as those returned * by {@link Files#lines(Path, Charset)}) will require closing. Most streams * are backed by collections, arrays, or generating functions, which require no * special resource management. (If a stream does require closing, it can be * declared as a resource in a {@code try}-with-resources statement.) * *
Stream pipelines may execute either sequentially or in
* parallel. This
* execution mode is a property of the stream. Streams are created
* with an initial choice of sequential or parallel execution. (For example,
* {@link Collection#stream() Collection.stream()} creates a sequential stream,
* and {@link Collection#parallelStream() Collection.parallelStream()} creates
* a parallel one.) This choice of execution mode may be modified by the
* {@link #sequential()} or {@link #parallel()} methods, and may be queried with
* the {@link #isParallel()} method.
*
* @since 1.8
* @see java.util.stream
*/
public interface LongStream extends BaseStream 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
*/
LongStream filter(LongPredicate predicate);
/**
* Returns a stream consisting of the results of applying the given
* function to the elements of this stream.
*
* This is an intermediate
* operation.
*
* @param mapper a
* non-interfering, stateless function to apply to each
* element
* @return the new stream
*/
LongStream map(LongUnaryOperator mapper);
/**
* Returns an object-valued {@code 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
*/
Stream mapToObj(LongFunction mapper);
/**
* Returns an {@code IntStream} consisting of the results of applying the
* given function to the elements of this stream.
*
* This is an intermediate
* operation.
*
* @param mapper a
* non-interfering, stateless function to apply to each
* element
* @return the new stream
*/
IntStream mapToInt(LongToIntFunction mapper);
/**
* Returns a {@code DoubleStream} consisting of the results of applying the
* given function to the elements of this stream.
*
* This is an intermediate
* operation.
*
* @param mapper a
* non-interfering, stateless function to apply to each
* element
* @return the new stream
*/
DoubleStream mapToDouble(LongToDoubleFunction 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 was an empty
* stream.)
*
* This is an intermediate
* operation.
*
* @param mapper a
* non-interfering, stateless function to apply to
* each element which produces an {@code LongStream} of new
* values
* @return the new stream
* @see Stream#flatMap(Function)
*/
LongStream flatMap(LongFunction mapper);
/**
* Returns a stream consisting of the distinct elements of this stream.
*
* This is a stateful
* intermediate operation.
*
* @return the new stream
*/
LongStream distinct();
/**
* Returns a stream consisting of the elements of this stream in sorted
* order.
*
* This is a stateful
* intermediate operation.
*
* @return the new stream
*/
LongStream sorted();
/**
* 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.
*
* This is an intermediate
* operation.
*
* 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:
* This is a short-circuiting
* stateful intermediate operation.
*
* @apiNote
* While {@code limit()} is generally a cheap operation on sequential
* stream pipelines, it can be quite expensive on ordered parallel pipelines,
* especially for large values of {@code maxSize}, since {@code limit(n)}
* is constrained to return not just any n elements, but the
* first n elements in the encounter order. Using an unordered
* stream source (such as {@link #generate(LongSupplier)}) or removing the
* ordering constraint with {@link #unordered()} may result in significant
* speedups of {@code limit()} in parallel pipelines, if the semantics of
* your situation permit. If consistency with encounter order is required,
* and you are experiencing poor performance or memory utilization with
* {@code limit()} in parallel pipelines, switching to sequential execution
* with {@link #sequential()} may improve performance.
*
* @param maxSize the number of elements the stream should be limited to
* @return the new stream
* @throws IllegalArgumentException if {@code maxSize} is negative
*/
LongStream limit(long maxSize);
/**
* Returns a stream consisting of the remaining elements of this stream
* after discarding the first {@code n} elements of the stream.
* If this stream contains fewer than {@code n} elements then an
* empty stream will be returned.
*
* This is a stateful
* intermediate operation.
*
* @apiNote
* While {@code skip()} is generally a cheap operation on sequential
* stream pipelines, it can be quite expensive on ordered parallel pipelines,
* especially for large values of {@code n}, since {@code skip(n)}
* is constrained to skip not just any n elements, but the
* first n elements in the encounter order. Using an unordered
* stream source (such as {@link #generate(LongSupplier)}) or removing the
* ordering constraint with {@link #unordered()} may result in significant
* speedups of {@code skip()} in parallel pipelines, if the semantics of
* your situation permit. If consistency with encounter order is required,
* and you are experiencing poor performance or memory utilization with
* {@code skip()} in parallel pipelines, switching to sequential execution
* with {@link #sequential()} may improve performance.
*
* @param n the number of leading elements to skip
* @return the new stream
* @throws IllegalArgumentException if {@code n} is negative
*/
LongStream skip(long n);
/**
* Performs an action for each element of this stream.
*
* This is a terminal
* operation.
*
* 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
*/
void forEach(LongConsumer 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.
*
* This is a terminal
* operation.
*
* @param action a
* non-interfering action to perform on the elements
* @see #forEach(LongConsumer)
*/
void forEachOrdered(LongConsumer action);
/**
* Returns an array containing the elements of this stream.
*
* This is a terminal
* operation.
*
* @return an array containing the elements of this stream
*/
long[] toArray();
/**
* 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:
* The {@code identity} value must be an identity for the accumulator
* function. This means that for all {@code x},
* {@code accumulator.apply(identity, x)} is equal to {@code x}.
* The {@code accumulator} function must be an
* associative function.
*
* This is a terminal
* operation.
*
* @apiNote Sum, min, max, and average are all special cases of reduction.
* Summing a stream of numbers can be expressed as:
*
* 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 op an associative
* non-interfering,
* stateless function for combining two values
* @return the result of the reduction
* @see #sum()
* @see #min()
* @see #max()
* @see #average()
*/
long reduce(long identity, LongBinaryOperator op);
/**
* Performs a reduction on the
* elements of this stream, using an
* associative accumulation
* function, and returns an {@code OptionalLong} describing the reduced value,
* if any. This is equivalent to:
* The {@code accumulator} function must be an
* associative function.
*
* This is a terminal
* operation.
*
* @param op an associative
* non-interfering,
* stateless function for combining two values
* @return the result of the reduction
* @see #reduce(long, LongBinaryOperator)
*/
OptionalLong reduce(LongBinaryOperator op);
/**
* Performs a mutable
* reduction operation on the elements of this stream. 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:
* Like {@link #reduce(long, LongBinaryOperator)}, {@code collect} operations
* can be parallelized without requiring additional synchronization.
*
* This is a terminal
* operation.
*
* @param This is a terminal
* operation.
*
* @return the sum of elements in this stream
*/
long sum();
/**
* Returns an {@code OptionalLong} describing the minimum element of this
* stream, or an empty optional if this stream is empty. This is a special
* case of a reduction
* and is equivalent to:
* This is a terminal operation.
*
* @return an {@code OptionalLong} containing the minimum element of this
* stream, or an empty {@code OptionalLong} if the stream is empty
*/
OptionalLong min();
/**
* Returns an {@code OptionalLong} describing the maximum element of this
* stream, or an empty optional if this stream is empty. This is a special
* case of a reduction
* and is equivalent to:
* This is a terminal
* operation.
*
* @return an {@code OptionalLong} containing the maximum element of this
* stream, or an empty {@code OptionalLong} if the stream is empty
*/
OptionalLong max();
/**
* Returns the count of elements in this stream. This is a special case of
* a reduction and is
* equivalent to:
* This is a terminal operation.
*
* @return the count of elements in this stream
*/
long count();
/**
* Returns an {@code OptionalDouble} describing the arithmetic mean of elements of
* this stream, or an empty optional if this stream is empty. This is a
* special case of a
* reduction.
*
* This is a terminal
* operation.
*
* @return an {@code OptionalDouble} containing the average element of this
* stream, or an empty optional if the stream is empty
*/
OptionalDouble average();
/**
* Returns a {@code LongSummaryStatistics} describing various summary data
* about the elements of this stream. This is a special case of a
* reduction.
*
* This is a terminal
* operation.
*
* @return a {@code LongSummaryStatistics} describing various summary data
* about the elements of this stream
*/
LongSummaryStatistics summaryStatistics();
/**
* 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.
*
* This is a short-circuiting
* terminal operation.
*
* @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}
*/
boolean anyMatch(LongPredicate 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.
*
* This is a short-circuiting
* terminal operation.
*
* @param predicate a non-interfering,
* stateless 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(LongPredicate 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.
*
* This is a short-circuiting
* terminal operation.
*
* @param predicate a non-interfering,
* stateless 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(LongPredicate predicate);
/**
* Returns an {@link OptionalLong} describing the first element of this
* stream, or an empty {@code OptionalLong} if the stream is empty. If the
* stream has no encounter order, then any element may be returned.
*
* This is a short-circuiting
* terminal operation.
*
* @return an {@code OptionalLong} describing the first element of this
* stream, or an empty {@code OptionalLong} if the stream is empty
*/
OptionalLong findFirst();
/**
* Returns an {@link OptionalLong} describing some element of the stream, or
* an empty {@code OptionalLong} if the stream is empty.
*
* This is a short-circuiting
* terminal operation.
*
* 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 a stable result
* is desired, use {@link #findFirst()} instead.)
*
* @return an {@code OptionalLong} describing some element of this stream,
* or an empty {@code OptionalLong} if the stream is empty
* @see #findFirst()
*/
OptionalLong findAny();
/**
* Returns a {@code DoubleStream} consisting of the elements of this stream,
* converted to {@code double}.
*
* This is an intermediate
* operation.
*
* @return a {@code DoubleStream} consisting of the elements of this stream,
* converted to {@code double}
*/
DoubleStream asDoubleStream();
/**
* Returns a {@code Stream} consisting of the elements of this stream,
* each boxed to a {@code Long}.
*
* This is an intermediate
* operation.
*
* @return a {@code Stream} consistent of the elements of this stream,
* each boxed to {@code Long}
*/
Stream The first element (position {@code 0}) in the {@code LongStream} 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 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 LongStream}
*/
public static LongStream iterate(final long seed, final LongUnaryOperator f) {
Objects.requireNonNull(f);
final PrimitiveIterator.OfLong iterator = new PrimitiveIterator.OfLong() {
long t = seed;
@Override
public boolean hasNext() {
return true;
}
@Override
public long nextLong() {
long v = t;
t = f.applyAsLong(t);
return v;
}
};
return StreamSupport.longStream(Spliterators.spliteratorUnknownSize(
iterator,
Spliterator.ORDERED | Spliterator.IMMUTABLE | Spliterator.NONNULL), false);
}
/**
* Returns a sequential stream where each element is generated by
* the provided {@code LongSupplier}. This is suitable for generating
* constant streams, streams of random elements, etc.
*
* @param s the {@code LongSupplier} for generated elements
* @return a new sequential {@code LongStream}
*/
public static LongStream generate(LongSupplier s) {
Objects.requireNonNull(s);
return StreamSupport.longStream(
new StreamSpliterators.InfiniteSupplyingSpliterator.OfLong(Long.MAX_VALUE, s), false);
}
/**
* Returns a sequential ordered {@code LongStream} from {@code startInclusive}
* (inclusive) to {@code endExclusive} (exclusive) by an incremental step of
* {@code 1}.
*
* @apiNote
* An equivalent sequence of increasing values can be produced
* sequentially using a {@code for} loop as follows:
* An equivalent sequence of increasing values can be produced
* sequentially using a {@code for} loop as follows:
* A stream builder has a lifecycle, which 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
* begins when the {@link #build()} method is called, which creates an
* ordered stream whose elements are the elements that were added to the
* stream builder, in the order they were added.
*
* @see LongStream#builder()
* @since 1.8
*/
public interface Builder extends LongConsumer {
/**
* Adds an element to the stream being built.
*
* @throws IllegalStateException if the builder has already transitioned
* to the built state
*/
@Override
void accept(long t);
/**
* Adds an element to the stream being built.
*
* @implSpec
* The default implementation behaves as if:
* {@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.toLongSummaryStastistics());
* }
*
* @param action a
* non-interfering action to perform on the elements as
* they are consumed from the stream
* @return the new stream
*/
LongStream peek(LongConsumer action);
/**
* Returns a stream consisting of the elements of this stream, truncated
* to be no longer than {@code maxSize} in length.
*
* {@code
* long result = identity;
* for (long element : this stream)
* result = accumulator.apply(result, element)
* return result;
* }
*
* but is not constrained to execute sequentially.
*
* {@code
* long sum = integers.reduce(0, (a, b) -> a+b);
* }
*
* or more compactly:
*
* {@code
* long sum = integers.reduce(0, Long::sum);
* }
*
* {@code
* boolean foundAny = false;
* long result = null;
* for (long element : this stream) {
* if (!foundAny) {
* foundAny = true;
* result = element;
* }
* else
* result = accumulator.apply(result, element);
* }
* return foundAny ? OptionalLong.of(result) : OptionalLong.empty();
* }
*
* but is not constrained to execute sequentially.
*
* {@code
* R result = supplier.get();
* for (long element : this stream)
* accumulator.accept(result, element);
* return result;
* }
*
* {@code
* return reduce(0, Long::sum);
* }
*
* {@code
* return reduce(Long::min);
* }
*
* {@code
* return reduce(Long::max);
* }
*
* {@code
* return map(e -> 1L).sum();
* }
*
* {@code
* for (long i = startInclusive; i < endExclusive ; i++) { ... }
* }
*
* @param startInclusive the (inclusive) initial value
* @param endExclusive the exclusive upper bound
* @return a sequential {@code LongStream} for the range of {@code long}
* elements
*/
public static LongStream range(long startInclusive, final long endExclusive) {
if (startInclusive >= endExclusive) {
return empty();
} else if (endExclusive - startInclusive < 0) {
// Size of range > Long.MAX_VALUE
// Split the range in two and concatenate
// Note: if the range is [Long.MIN_VALUE, Long.MAX_VALUE) then
// the lower range, [Long.MIN_VALUE, 0) will be further split in two
long m = startInclusive + Long.divideUnsigned(endExclusive - startInclusive, 2) + 1;
return concat(range(startInclusive, m), range(m, endExclusive));
} else {
return StreamSupport.longStream(
new Streams.RangeLongSpliterator(startInclusive, endExclusive, false), false);
}
}
/**
* Returns a sequential ordered {@code LongStream} from {@code startInclusive}
* (inclusive) to {@code endInclusive} (inclusive) by an incremental step of
* {@code 1}.
*
* @apiNote
* {@code
* for (long i = startInclusive; i <= endInclusive ; i++) { ... }
* }
*
* @param startInclusive the (inclusive) initial value
* @param endInclusive the inclusive upper bound
* @return a sequential {@code LongStream} for the range of {@code long}
* elements
*/
public static LongStream rangeClosed(long startInclusive, final long endInclusive) {
if (startInclusive > endInclusive) {
return empty();
} else if (endInclusive - startInclusive + 1 <= 0) {
// Size of range > Long.MAX_VALUE
// Split the range in two and concatenate
// Note: if the range is [Long.MIN_VALUE, Long.MAX_VALUE] then
// the lower range, [Long.MIN_VALUE, 0), and upper range,
// [0, Long.MAX_VALUE], will both be further split in two
long m = startInclusive + Long.divideUnsigned(endInclusive - startInclusive, 2) + 1;
return concat(range(startInclusive, m), rangeClosed(m, endInclusive));
} else {
return StreamSupport.longStream(
new Streams.RangeLongSpliterator(startInclusive, endInclusive, true), false);
}
}
/**
* Creates a lazily concatenated stream whose elements are all the
* elements of the first stream followed by all the elements of the
* second stream. The resulting stream is ordered if both
* of the input streams are ordered, and parallel if either of the input
* streams is parallel. When the resulting stream is closed, the close
* handlers for both input streams are invoked.
*
* @param a the first stream
* @param b the second stream
* @return the concatenation of the two input streams
*/
public static LongStream concat(LongStream a, LongStream b) {
Objects.requireNonNull(a);
Objects.requireNonNull(b);
Spliterator.OfLong split = new Streams.ConcatSpliterator.OfLong(
a.spliterator(), b.spliterator());
LongStream stream = StreamSupport.longStream(split, a.isParallel() || b.isParallel());
return stream.onClose(Streams.composedClose(a, b));
}
/**
* A mutable builder for a {@code LongStream}.
*
* {@code
* accept(t)
* return this;
* }
*
* @param t the element to add
* @return {@code this} builder
* @throws IllegalStateException if the builder has already transitioned
* to the built state
*/
default Builder add(long 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
*/
LongStream build();
}
}