T - the type of the stream elementspublic interface Stream<T> extends BaseStream<T,Stream<T>>
Stream and IntStream:
int sum = widgets.stream()
.filter(w -> w.getColor() == RED)
.mapToInt(w -> w.getWeight())
.sum();
widgets is a Collection<Widget>. We create
a stream of Widget objects via Collection.stream(),
filter it to produce a stream containing only the red widgets, and then
transform it into a stream of int values representing the weight of
each red widget. Then this stream is summed to produce a total weight.
In addition to Stream, which is a stream of object references,
there are primitive specializations for IntStream, LongStream,
and DoubleStream, all of which are referred to as "streams" and
conform to the characteristics and restrictions described here.
To 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 I/O channel,
etc), zero or more intermediate operations (which transform a
stream into another stream, such as filter(Predicate)), and a
terminal operation (which produces a result or side-effect, such
as count() or forEach(Consumer)).
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 BaseStream.iterator() and BaseStream.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 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 w -> w.getWeight() passed to
mapToInt in the example above. To preserve correct behavior,
these behavioral parameters:
Such parameters are always instances of a
functional interface such
as Function, and are often lambda expressions or
method references. Unless otherwise specified these parameters must be
non-null.
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 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 BaseStream.close() method and implement 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 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 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,
Collection.stream() creates a sequential stream,
and Collection.parallelStream() creates
a parallel one.) This choice of execution mode may be modified by the
BaseStream.sequential() or BaseStream.parallel() methods, and may be queried with
the BaseStream.isParallel() method.
IntStream,
LongStream,
DoubleStream,
java.util.stream| Modifier and Type | Interface and Description |
|---|---|
static interface |
Stream.Builder<T>
A mutable builder for a
Stream. |
| Modifier and Type | Method and Description |
|---|---|
boolean |
allMatch(Predicate<? super T> predicate)
Returns whether all elements of this stream match the provided predicate.
|
boolean |
anyMatch(Predicate<? super T> predicate)
Returns whether any elements of this stream match the provided
predicate.
|
<R,A> R |
collect(Collector<? super T,A,R> collector)
Performs a mutable
reduction operation on the elements of this stream using a
Collector. |
<R> R |
collect(Supplier<R> supplier,
BiConsumer<R,? super T> accumulator,
BiConsumer<R,R> combiner)
Performs a mutable
reduction operation on the elements of this stream.
|
long |
count()
Returns the count of elements in this stream.
|
Stream<T> |
distinct()
Returns a stream consisting of the distinct elements (according to
Object.equals(Object)) of this stream. |
Stream<T> |
filter(Predicate<? super T> predicate)
Returns a stream consisting of the elements of this stream that match
the given predicate.
|
Optional<T> |
findAny()
Returns an
Optional describing some element of the stream, or an
empty Optional if the stream is empty. |
Optional<T> |
findFirst()
Returns an
Optional describing the first element of this stream,
or an empty Optional if the stream is empty. |
<R> Stream<R> |
flatMap(Function<? super T,? extends Stream<? extends R>> mapper)
Returns a stream consisting of the results of replacing each element of
this stream with the contents of a mapped stream produced by applying
the provided mapping function to each element.
|
DoubleStream |
flatMapToDouble(Function<? super T,? extends DoubleStream> mapper)
Returns an
DoubleStream consisting of the results of replacing
each element of this stream with the contents of a mapped stream produced
by applying the provided mapping function to each element. |
IntStream |
flatMapToInt(Function<? super T,? extends IntStream> mapper)
Returns an
IntStream consisting of the results of replacing each
element of this stream with the contents of a mapped stream produced by
applying the provided mapping function to each element. |
LongStream |
flatMapToLong(Function<? super T,? extends LongStream> mapper)
Returns an
LongStream consisting of the results of replacing each
element of this stream with the contents of a mapped stream produced by
applying the provided mapping function to each element. |
void |
forEach(Consumer<? super T> action)
Performs an action for each element of this stream.
|
void |
forEachOrdered(Consumer<? super T> action)
Performs an action for each element of this stream, in the encounter
order of the stream if the stream has a defined encounter order.
|
Stream<T> |
limit(long maxSize)
Returns a stream consisting of the elements of this stream, truncated
to be no longer than
maxSize in length. |
<R> Stream<R> |
map(Function<? super T,? extends R> mapper)
Returns a stream consisting of the results of applying the given
function to the elements of this stream.
|
DoubleStream |
mapToDouble(ToDoubleFunction<? super T> mapper)
Returns a
DoubleStream consisting of the results of applying the
given function to the elements of this stream. |
IntStream |
mapToInt(ToIntFunction<? super T> mapper)
Returns an
IntStream consisting of the results of applying the
given function to the elements of this stream. |
LongStream |
mapToLong(ToLongFunction<? super T> mapper)
Returns a
LongStream consisting of the results of applying the
given function to the elements of this stream. |
Optional<T> |
max(java.util.Comparator<? super T> comparator)
Returns the maximum element of this stream according to the provided
Comparator. |
Optional<T> |
min(java.util.Comparator<? super T> comparator)
Returns the minimum element of this stream according to the provided
Comparator. |
boolean |
noneMatch(Predicate<? super T> predicate)
Returns whether no elements of this stream match the provided predicate.
|
Stream<T> |
peek(Consumer<? super T> action)
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.
|
Optional<T> |
reduce(BinaryOperator<T> accumulator)
Performs a reduction on the
elements of this stream, using an
associative accumulation
function, and returns an
Optional describing the reduced value,
if any. |
T |
reduce(T identity,
BinaryOperator<T> accumulator)
Performs a reduction on the
elements of this stream, using the provided identity value and an
associative
accumulation function, and returns the reduced value.
|
<U> U |
reduce(U identity,
BiFunction<U,? super T,U> accumulator,
BinaryOperator<U> combiner)
Performs a reduction on the
elements of this stream, using the provided identity, accumulation and
combining functions.
|
Stream<T> |
skip(long n)
Returns a stream consisting of the remaining elements of this stream
after discarding the first
n elements of the stream. |
Stream<T> |
sorted()
Returns a stream consisting of the elements of this stream, sorted
according to natural order.
|
Stream<T> |
sorted(java.util.Comparator<? super T> comparator)
Returns a stream consisting of the elements of this stream, sorted
according to the provided
Comparator. |
java.lang.Object[] |
toArray()
Returns an array containing the elements of this stream.
|
<A> A[] |
toArray(IntFunction<A[]> generator)
Returns an array containing the elements of this stream, using the
provided
generator function to allocate the returned array, as
well as any additional arrays that might be required for a partitioned
execution or for resizing. |
close, isParallel, iterator, onClose, parallel, sequential, spliterator, unorderedStream<T> filter(Predicate<? super T> predicate)
This is an intermediate operation.
predicate - a non-interfering,
stateless
predicate to apply to each element to determine if it
should be included<R> Stream<R> map(Function<? super T,? extends R> mapper)
This is an intermediate operation.
R - The element type of the new streammapper - a non-interfering,
stateless
function to apply to each elementIntStream mapToInt(ToIntFunction<? super T> mapper)
IntStream consisting of the results of applying the
given function to the elements of this stream.
This is an intermediate operation.
mapper - a non-interfering,
stateless
function to apply to each elementLongStream mapToLong(ToLongFunction<? super T> mapper)
LongStream consisting of the results of applying the
given function to the elements of this stream.
This is an intermediate operation.
mapper - a non-interfering,
stateless
function to apply to each elementDoubleStream mapToDouble(ToDoubleFunction<? super T> mapper)
DoubleStream consisting of the results of applying the
given function to the elements of this stream.
This is an intermediate operation.
mapper - a non-interfering,
stateless
function to apply to each element<R> Stream<R> flatMap(Function<? super T,? extends Stream<? extends R>> mapper)
closed after its contents
have been placed into this stream. (If a mapped stream is null
an empty stream is used, instead.)
This is an intermediate operation.
API Note:
The flatMap() operation has the effect of applying a one-to-many
transformation to the elements of the stream, and then flattening the
resulting elements into a new stream.
Examples.
If orders is a stream of purchase orders, and each purchase
order contains a collection of line items, then the following produces a
stream containing all the line items in all the orders:
orders.flatMap(order -> order.getLineItems().stream())...
If path is the path to a file, then the following produces a
stream of the words contained in that file:
Stream<String> lines = Files.lines(path, StandardCharsets.UTF_8);
Stream<String> words = lines.flatMap(line -> Stream.of(line.split(" +")));
mapper function passed to flatMap splits a line,
using a simple regular expression, into an array of words, and then
creates a stream of words from that array.R - The element type of the new streammapper - a non-interfering,
stateless
function to apply to each element which produces a stream
of new valuesIntStream flatMapToInt(Function<? super T,? extends IntStream> mapper)
IntStream consisting of the results of replacing each
element of this stream with the contents of a mapped stream produced by
applying the provided mapping function to each element. Each mapped
stream is closed after its
contents have been placed into this stream. (If a mapped stream is
null an empty stream is used, instead.)
This is an intermediate operation.
mapper - a non-interfering,
stateless
function to apply to each element which produces a stream
of new valuesflatMap(Function)LongStream flatMapToLong(Function<? super T,? extends LongStream> mapper)
LongStream consisting of the results of replacing each
element of this stream with the contents of a mapped stream produced by
applying the provided mapping function to each element. Each mapped
stream is closed after its
contents have been placed into this stream. (If a mapped stream is
null an empty stream is used, instead.)
This is an intermediate operation.
mapper - a non-interfering,
stateless
function to apply to each element which produces a stream
of new valuesflatMap(Function)DoubleStream flatMapToDouble(Function<? super T,? extends DoubleStream> mapper)
DoubleStream consisting of the results of replacing
each element of this stream with the contents of a mapped stream produced
by applying the provided mapping function to each element. Each mapped
stream is closed after its
contents have placed been into this stream. (If a mapped stream is
null an empty stream is used, instead.)
This is an intermediate operation.
mapper - a non-interfering,
stateless
function to apply to each element which produces a stream
of new valuesflatMap(Function)Stream<T> distinct()
Object.equals(Object)) of this stream.
For ordered streams, the selection of distinct elements is stable (for duplicated elements, the element appearing first in the encounter order is preserved.) For unordered streams, no stability guarantees are made.
This is a stateful intermediate operation.
API Note:
Preserving stability for distinct() in parallel pipelines is
relatively expensive (requires that the operation act as a full barrier,
with substantial buffering overhead), and stability is often not needed.
Using an unordered stream source (such as StreamSupport.generate(Supplier))
or removing the ordering constraint with BaseStream.unordered() may result
in significantly more efficient execution for distinct() 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 distinct() in parallel pipelines,
switching to sequential execution with BaseStream.sequential() may improve
performance.
Stream<T> sorted()
Comparable, a java.lang.ClassCastException may be thrown
when the terminal operation is executed.
For ordered streams, the sort is stable. For unordered streams, no stability guarantees are made.
This is a stateful intermediate operation.
Stream<T> sorted(java.util.Comparator<? super T> comparator)
Comparator.
For ordered streams, the sort is stable. For unordered streams, no stability guarantees are made.
This is a stateful intermediate operation.
comparator - a non-interfering,
stateless
Comparator to be used to compare stream elementsStream<T> peek(Consumer<? super T> action)
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.
API Note:
This method exists mainly to support debugging, where you want
to see the elements as they flow past a certain point in a pipeline:
Stream.of("one", "two", "three", "four")
.filter(e -> e.length() > 3)
.peek(e -> System.out.println("Filtered value: " + e))
.map(String::toUpperCase)
.peek(e -> System.out.println("Mapped value: " + e))
.collect(Collectors.toList());
action - a
non-interfering action to perform on the elements as
they are consumed from the streamStream<T> limit(long maxSize)
maxSize in length.
This is a short-circuiting stateful intermediate operation.
API Note:
While limit() is generally a cheap operation on sequential
stream pipelines, it can be quite expensive on ordered parallel pipelines,
especially for large values of maxSize, since 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 StreamSupport.generate(Supplier)) or removing the
ordering constraint with BaseStream.unordered() may result in significant
speedups of 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
limit() in parallel pipelines, switching to sequential execution
with BaseStream.sequential() may improve performance.
maxSize - the number of elements the stream should be limited tojava.lang.IllegalArgumentException - if maxSize is negativeStream<T> skip(long n)
n elements of the stream.
If this stream contains fewer than n elements then an
empty stream will be returned.
This is a stateful intermediate operation.
API Note:
While skip() is generally a cheap operation on sequential
stream pipelines, it can be quite expensive on ordered parallel pipelines,
especially for large values of n, since 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 StreamSupport.generate(Supplier)) or removing the
ordering constraint with BaseStream.unordered() may result in significant
speedups of 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
skip() in parallel pipelines, switching to sequential execution
with BaseStream.sequential() may improve performance.
n - the number of leading elements to skipjava.lang.IllegalArgumentException - if n is negativevoid forEach(Consumer<? super T> action)
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.
action - a
non-interfering action to perform on the elementsvoid forEachOrdered(Consumer<? super T> action)
This is a terminal operation.
This operation processes the elements one at a time, in encounter order if one exists. Performing the action for one element happens-before performing the action for subsequent elements, but for any given element, the action may be performed in whatever thread the library chooses.
action - a
non-interfering action to perform on the elementsforEach(Consumer)java.lang.Object[] toArray()
This is a terminal operation.
<A> A[] toArray(IntFunction<A[]> generator)
generator function to allocate the returned array, as
well as any additional arrays that might be required for a partitioned
execution or for resizing.
This is a terminal operation.
API Note:
The generator function takes an integer, which is the size of the
desired array, and produces an array of the desired size. This can be
concisely expressed with an array constructor reference:
Person[] men = people.stream()
.filter(p -> p.getGender() == MALE)
.toArray(Person[]::new);
A - the element type of the resulting arraygenerator - a function which produces a new array of the desired
type and the provided lengthjava.lang.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 streamT reduce(T identity, BinaryOperator<T> accumulator)
T result = identity;
for (T element : this stream)
result = accumulator.apply(result, element)
return result;
The identity value must be an identity for the accumulator
function. This means that for all t,
accumulator.apply(identity, t) is equal to t.
The accumulator function must be an
associative function.
This is a terminal operation.
API Note:
Sum, min, max, average, and string concatenation are all special
cases of reduction. Summing a stream of numbers can be expressed as:
Integer sum = integers.reduce(0, (a, b) -> a+b);
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.
identity - the identity value for the accumulating functionaccumulator - an associative,
non-interfering,
stateless
function for combining two valuesOptional<T> reduce(BinaryOperator<T> accumulator)
Optional describing the reduced value,
if any. This is equivalent to:
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();
The accumulator function must be an
associative function.
This is a terminal operation.
accumulator - an associative,
non-interfering,
stateless
function for combining two valuesOptional describing the result of the reductionjava.lang.NullPointerException - if the result of the reduction is nullreduce(Object, BinaryOperator),
min(Comparator),
max(Comparator)<U> U reduce(U identity,
BiFunction<U,? super T,U> accumulator,
BinaryOperator<U> combiner)
U result = identity;
for (T element : this stream)
result = accumulator.apply(result, element)
return result;
The identity value must be an identity for the combiner
function. This means that for all u, combiner(identity, u)
is equal to u. Additionally, the combiner function
must be compatible with the accumulator function; for all
u and t, the following must hold:
combiner.apply(u, accumulator.apply(identity, t)) == accumulator.apply(u, t)
This is a terminal operation.
API Note:
Many reductions using this form can be represented more simply
by an explicit combination of map and reduce operations.
The accumulator function acts as a fused mapper and accumulator,
which can sometimes be more efficient than separate mapping and reduction,
such as when knowing the previously reduced value allows you to avoid
some computation.
U - The type of the resultidentity - the identity value for the combiner functionaccumulator - an associative,
non-interfering,
stateless
function for incorporating an additional element into a resultcombiner - an associative,
non-interfering,
stateless
function for combining two values, which must be
compatible with the accumulator functionreduce(BinaryOperator),
reduce(Object, BinaryOperator)<R> R collect(Supplier<R> supplier, BiConsumer<R,? super T> accumulator, BiConsumer<R,R> combiner)
ArrayList, and elements are incorporated by updating
the state of the result rather than by replacing the result. This
produces a result equivalent to:
R result = supplier.get();
for (T element : this stream)
accumulator.accept(result, element);
return result;
Like reduce(Object, BinaryOperator), collect operations
can be parallelized without requiring additional synchronization.
This is a terminal operation.
API Note:
There are many existing classes in the JDK whose signatures are
well-suited for use with method references as arguments to collect().
For example, the following will accumulate strings into an ArrayList:
List<String> asList = stringStream.collect(ArrayList::new, ArrayList::add,
ArrayList::addAll);
The following will take a stream of strings and concatenates them into a single string:
String concat = stringStream.collect(StringBuilder::new, StringBuilder::append,
StringBuilder::append)
.toString();
R - type of the resultsupplier - 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.accumulator - an associative,
non-interfering,
stateless
function for incorporating an additional element into a resultcombiner - an associative,
non-interfering,
stateless
function for combining two values, which must be
compatible with the accumulator function<R,A> R collect(Collector<? super T,A,R> collector)
Collector. A Collector
encapsulates the functions used as arguments to
collect(Supplier, BiConsumer, BiConsumer), allowing for reuse of
collection strategies and composition of collect operations such as
multiple-level grouping or partitioning.
If the stream is parallel, and the Collector
is concurrent, and
either the stream is unordered or the collector is
unordered,
then a concurrent reduction will be performed (see 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 ArrayList), no
additional synchronization is needed for a parallel reduction.
API Note:
The following will accumulate strings into an ArrayList:
List<String> asList = stringStream.collect(Collectors.toList());
The following will classify Person objects by city:
Map<String, List<Person>> peopleByCity
= personStream.collect(Collectors.groupingBy(Person::getCity));
The following will classify Person objects by state and city,
cascading two Collectors together:
Map<String, Map<String, List<Person>>> peopleByStateAndCity
= personStream.collect(Collectors.groupingBy(Person::getState,
Collectors.groupingBy(Person::getCity)));
R - the type of the resultA - the intermediate accumulation type of the Collectorcollector - the Collector describing the reductioncollect(Supplier, BiConsumer, BiConsumer),
CollectorsOptional<T> min(java.util.Comparator<? super T> comparator)
Comparator. This is a special case of a
reduction.
This is a terminal operation.
comparator - a non-interfering,
stateless
Comparator to compare elements of this streamOptional describing the minimum element of this stream,
or an empty Optional if the stream is emptyjava.lang.NullPointerException - if the minimum element is nullOptional<T> max(java.util.Comparator<? super T> comparator)
Comparator. This is a special case of a
reduction.
This is a terminal operation.
comparator - a non-interfering,
stateless
Comparator to compare elements of this streamOptional describing the maximum element of this stream,
or an empty Optional if the stream is emptyjava.lang.NullPointerException - if the maximum element is nulllong count()
return mapToLong(e -> 1L).sum();
This is a terminal operation.
boolean anyMatch(Predicate<? super T> predicate)
false is returned and the predicate is not evaluated.
This is a short-circuiting terminal operation.
API Note:
This method evaluates the existential quantification of the
predicate over the elements of the stream (for some x P(x)).
predicate - a non-interfering,
stateless
predicate to apply to elements of this streamtrue if any elements of the stream match the provided
predicate, otherwise falseboolean allMatch(Predicate<? super T> predicate)
true is
returned and the predicate is not evaluated.
This is a short-circuiting terminal operation.
API 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 true (regardless of P(x)).
predicate - a non-interfering,
stateless
predicate to apply to elements of this streamtrue if either all elements of the stream match the
provided predicate or the stream is empty, otherwise falseboolean noneMatch(Predicate<? super T> predicate)
true is
returned and the predicate is not evaluated.
This is a short-circuiting terminal operation.
API Note:
This method evaluates the universal quantification of the
negated 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 true, regardless of P(x).
predicate - a non-interfering,
stateless
predicate to apply to elements of this streamtrue if either no elements of the stream match the
provided predicate or the stream is empty, otherwise falseOptional<T> findFirst()
Optional describing the first element of this stream,
or an empty Optional 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.
Optional describing the first element of this stream,
or an empty Optional if the stream is emptyjava.lang.NullPointerException - if the element selected is nullOptional<T> findAny()
Optional describing some element of the stream, or an
empty Optional 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 findFirst() instead.)
Optional describing some element of this stream, or an
empty Optional if the stream is emptyjava.lang.NullPointerException - if the element selected is nullfindFirst()