org.apache.commons.lang3

Class Streams.FailableStream<O>

java.lang.Object

org.apache.commons.lang3.Streams.FailableStream<O>

Type Parameters:

O - The streams element type.

Enclosing class:

Streams

public static class Streams.FailableStream<O>
extends Object

A reduced, and simplified version of a Stream with failable method signatures.

Constructor Summary

Constructors

Constructor and Description
FailableStream(Stream<O> stream) Constructs a new instance with the given stream.

Method Summary

Modifier and Type	Method and Description
boolean	allMatch(Functions.FailablePredicate<O,?> predicate) Returns whether all elements of this stream match the provided predicate.
boolean	anyMatch(Functions.FailablePredicate<O,?> predicate) Returns whether any elements of this stream match the provided predicate.
protected void	assertNotTerminated()
<A,R> R	collect(Collector<? super O,A,R> collector) Performs a mutable reduction operation on the elements of this stream using a Collector.
<A,R> R	collect(Supplier<R> pupplier, BiConsumer<R,? super O> accumulator, BiConsumer<R,R> combiner) Performs a mutable reduction operation on the elements of this FailableStream.
Streams.FailableStream<O>	filter(Functions.FailablePredicate<O,?> predicate) Returns a FailableStream consisting of the elements of this stream that match the given FailablePredicate.
void	forEach(Functions.FailableConsumer<O,?> action) Performs an action for each element of this stream.
protected void	makeTerminated()
<R> Streams.FailableStream<R>	map(Functions.FailableFunction<O,R,?> mapper) Returns a stream consisting of the results of applying the given function to the elements of this stream.
O	reduce(O identity, BinaryOperator<O> 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.
Stream<O>	stream() Converts the FailableStream into an equivalent stream.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

FailableStream

public FailableStream(Stream<O> stream)

Constructs a new instance with the given stream.

Parameters:

stream - The stream.

Method Detail

assertNotTerminated

protected void assertNotTerminated()

makeTerminated

protected void makeTerminated()

filter

public Streams.FailableStream<O> filter(Functions.FailablePredicate<O,?> predicate)

Returns a FailableStream consisting of the elements of this stream that match the given FailablePredicate.

This is an intermediate operation.

Parameters:

predicate - a non-interfering, stateless predicate to apply to each element to determine if it should be included.

Returns:

the new stream

forEach

public void forEach(Functions.FailableConsumer<O,?> action)

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.

Parameters:

action - a non-interfering action to perform on the elements

collect

public <A,R> R collect(Collector<? super O,A,R> collector)

Performs a mutable reduction operation on the elements of this stream using a 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 underlying 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. \@apiNote 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)));
 

Type Parameters:

R - the type of the result

A - the intermediate accumulation type of the Collector

Parameters:

collector - the Collector describing the reduction

Returns:

the result of the reduction

See Also:

collect(Supplier, BiConsumer, BiConsumer), Collectors

collect

public <A,R> R collect(Supplier<R> pupplier,
                       BiConsumer<R,? super O> accumulator,
                       BiConsumer<R,R> combiner)

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 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. \@apiNote 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();
 

Type Parameters:

R - type of the result

A - Type of the accumulator.

Parameters:

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.

accumulator - An associative, non-interfering, stateless function for incorporating an additional element into a result

combiner - An associative, non-interfering, stateless function for combining two values, which must be compatible with the accumulator function

Returns:

The result of the reduction

reduce

public O reduce(O identity,
                BinaryOperator<O> 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. This is equivalent to:

     T result = identity;
     for (T element : this stream)
         result = accumulator.apply(result, element)
     return result;
 

but is not constrained to execute sequentially.

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. \@apiNote 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);
 

or:

     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.

Parameters:

identity - the identity value for the accumulating function

accumulator - an associative, non-interfering, stateless function for combining two values

Returns:

the result of the reduction

map

public <R> Streams.FailableStream<R> map(Functions.FailableFunction<O,R,?> mapper)

Returns a stream consisting of the results of applying the given function to the elements of this stream.

This is an intermediate operation.

Type Parameters:

R - The element type of the new stream

Parameters:

mapper - A non-interfering, stateless function to apply to each element

Returns:

the new stream

stream

public Stream<O> stream()

Converts the FailableStream into an equivalent stream.

Returns:

A stream, which will return the same elements, which this FailableStream would return.

allMatch

public boolean allMatch(Functions.FailablePredicate<O,?> 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. If the stream is empty then true is returned and the predicate is not evaluated.

This is a short-circuiting terminal operation. \@apiNote 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)).

Parameters:

predicate - A non-interfering, stateless predicate to apply to elements of this stream

Returns:

true If either all elements of the stream match the provided predicate or the stream is empty, otherwise false.

anyMatch

public boolean anyMatch(Functions.FailablePredicate<O,?> 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 false is returned and the predicate is not evaluated.

This is a short-circuiting terminal operation. \@apiNote This method evaluates the existential quantification of the predicate over the elements of the stream (for some x P(x)).

Parameters:

predicate - A non-interfering, stateless predicate to apply to elements of this stream

Returns:

true if any elements of the stream match the provided predicate, otherwise false