package jdk
The jdk package contains utilities to interact with JDK classes.
This packages offers a number of converters, that are able to wrap or copy types from the scala library to equivalent types in the JDK class library and vice versa:
- CollectionConverters, converting collections like scala.collection.Seq, scala.collection.Map, scala.collection.Set, scala.collection.mutable.Buffer, scala.collection.Iterator and scala.collection.Iterable to their JDK counterparts
- OptionConverters, converting between Option and java.util.Optional and primitive variations
- StreamConverters, to create JDK Streams from scala collections
- DurationConverters, for conversions between scala scala.concurrent.duration.FiniteDuration and java.time.Duration
- FunctionConverters, from scala Functions to java java.util.function.Function, java.util.function.UnaryOperator, java.util.function.Consumer and java.util.function.Predicate, as well as primitive variations and Bi-variations.
By convention, converters that wrap an object to provide a different interface to the same underlying data structure use .asScala and .asJava extension methods, whereas converters that copy the underlying data structure use .toScala and .toJava.
In the javaapi package, the same converters can be found with a java-friendly interface that don't rely on implicit enrichments.
Additionally, this package offers Accumulators, capable of efficiently traversing JDK Streams.
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- abstract class Accumulator[A, +CC[X] <: collection.mutable.Seq[X], +C <: collection.mutable.Seq[A]] extends collection.mutable.Seq[A] with Builder[A, C]
Accumulators are mutable sequences with two distinct features:
Accumulators are mutable sequences with two distinct features:
- An accumulator can be appended efficiently to another
- There are manually specialized Accumulators for
Int
,Long
andDouble
that don't box the elements
These two features make Accumulators a good candidate to collect the results of a parallel Java stream pipeline into a Scala collection. The scala.collection.convert.StreamExtensions.StreamHasToScala.toScala extension method on Java streams (available by importing
scala.jdk.StreamConverters._
) is specialized for Accumulators: they are built in parallel, the parts are merged efficiently.Building specialized Accumulators is handled transparently. As a user, using the Accumulator object as a factory automatically creates an IntAccumulator, LongAccumulator, DoubleAccumulator or AnyAccumulator depending on the element type.
Note: to run the example, start the Scala REPL with
scala -Yrepl-class-based
to avoid deadlocks, see https://github.com/scala/bug/issues/9076.scala> import scala.jdk.StreamConverters._ import scala.jdk.StreamConverters._ scala> def isPrime(n: Int): Boolean = !(2 +: (3 to Math.sqrt(n).toInt by 2) exists (n % _ == 0)) isPrime: (n: Int)Boolean scala> val intAcc = (1 to 10000).asJavaParStream.filter(isPrime).toScala(scala.jdk.Accumulator) intAcc: scala.jdk.IntAccumulator = IntAccumulator(1, 3, 5, 7, 11, 13, 17, 19, ... scala> val stringAcc = (1 to 100).asJavaParStream.mapToObj("<>" * _).toScala(Accumulator) stringAcc: scala.jdk.AnyAccumulator[String] = AnyAccumulator(<>, <><>, <><><>, ...
There are two possibilities to process elements of a primitive Accumulator without boxing: specialized operations of the Accumulator, or the Stepper interface. The most common collection operations are overloaded or overridden in the primitive Accumulator classes, for example IntAccumulator.map or IntAccumulator.exists. Thanks to Scala's function specialization,
intAcc.exists(x => testOn(x))
does not incur boxing.The scala.collection.Stepper interface provides iterator-like
hasStep
andnextStep
methods, and is specialized forInt
,Long
andDouble
. TheintAccumulator.stepper
method creates an scala.collection.IntStepper that yields the elements of the accumulator without boxing.Accumulators can hold more than
Int.MaxValue
elements. They have a sizeLong method that returns the size as aLong
. Note that certain operations defined in scala.collection.Seq are implemented using length, so they will not work correctly for large accumulators.The Accumulator class is a base class to share code between AnyAccumulator (for reference types) and the manual specializations IntAccumulator, LongAccumulator and DoubleAccumulator.
- final class AnyAccumulator[A] extends Accumulator[A, AnyAccumulator, AnyAccumulator[A]] with SeqOps[A, AnyAccumulator, AnyAccumulator[A]] with IterableFactoryDefaults[A, AnyAccumulator] with Serializable
An Accumulator for arbitrary element types, see Accumulator.
- final class DoubleAccumulator extends Accumulator[Double, AnyAccumulator, DoubleAccumulator] with SeqOps[Double, AnyAccumulator, DoubleAccumulator] with Serializable
A specialized Accumulator that holds
Double
s without boxing, see Accumulator. - final class IntAccumulator extends Accumulator[Int, AnyAccumulator, IntAccumulator] with SeqOps[Int, AnyAccumulator, IntAccumulator] with Serializable
A specialized Accumulator that holds
Int
s without boxing, see Accumulator. - final class LongAccumulator extends Accumulator[Long, AnyAccumulator, LongAccumulator] with SeqOps[Long, AnyAccumulator, LongAccumulator] with Serializable
A specialized Accumulator that holds
Long
s without boxing, see Accumulator. - sealed abstract class OptionShape[A, O] extends AnyRef
A type class implementing conversions from a generic Scala
Option
or JavaOptional
to a specialized Java variant (forDouble
,Int
andLong
).A type class implementing conversions from a generic Scala
Option
or JavaOptional
to a specialized Java variant (forDouble
,Int
andLong
).- A
the primitive type wrapped in an option
- O
the specialized Java
Optional
wrapping an element of typeA
- Annotations
- @implicitNotFound()
- trait Priority0FunctionExtensions extends Priority1FunctionExtensions
- trait Priority1FunctionExtensions extends Priority2FunctionExtensions
- trait Priority2FunctionExtensions extends Priority3FunctionExtensions
- trait Priority3FunctionExtensions extends AnyRef
Value Members
- object Accumulator
Contains factory methods to build Accumulators.
Contains factory methods to build Accumulators.
Note that the
Accumulator
object itself is not a factory, but it is implicitly convert to a factory according to the element type, see Accumulator.toFactory.This allows passing the
Accumulator
object as argument when a collection.Factory, and the implicit Accumulator.AccumulatorFactoryShape instance is used to build a specialized Accumulator according to the element type:scala> val intAcc = Accumulator(1,2,3) intAcc: scala.collection.convert.IntAccumulator = IntAccumulator(1, 2, 3) scala> val anyAccc = Accumulator("K") anyAccc: scala.collection.convert.AnyAccumulator[String] = AnyAccumulator(K) scala> val intAcc2 = List(1,2,3).to(Accumulator) intAcc2: scala.jdk.IntAccumulator = IntAccumulator(1, 2, 3) scala> val anyAcc2 = List("K").to(Accumulator) anyAcc2: scala.jdk.AnyAccumulator[String] = AnyAccumulator(K)
- object AnyAccumulator extends SeqFactory[AnyAccumulator]
- object CollectionConverters extends AsJavaExtensions with AsScalaExtensions
This object provides extension methods that convert between Scala and Java collections.
This object provides extension methods that convert between Scala and Java collections.
When writing Java code, use the explicit conversion methods defined in javaapi.CollectionConverters instead.
Note: to create Java Streams that operate on Scala collections (sequentially or in parallel), use StreamConverters.
import scala.jdk.CollectionConverters._ val s: java.util.Set[String] = Set("one", "two").asJava
The conversions return adapters for the corresponding API, i.e., the collections are wrapped, not converted. Changes to the original collection are reflected in the view, and vice versa:
scala> import scala.jdk.CollectionConverters._ scala> val s = collection.mutable.Set("one") s: scala.collection.mutable.Set[String] = HashSet(one) scala> val js = s.asJava js: java.util.Set[String] = [one] scala> js.add("two") scala> s res2: scala.collection.mutable.Set[String] = HashSet(two, one)
The following conversions are supported via
asScala
andasJava
:scala.collection.Iterable <=> java.lang.Iterable scala.collection.Iterator <=> java.util.Iterator scala.collection.mutable.Buffer <=> java.util.List scala.collection.mutable.Set <=> java.util.Set scala.collection.mutable.Map <=> java.util.Map scala.collection.concurrent.Map <=> java.util.concurrent.ConcurrentMap
The following conversions are supported via
asScala
and through specially-named extension methods to convert to Java collections, as shown:scala.collection.Iterable <=> java.util.Collection (via asJavaCollection) scala.collection.Iterator <=> java.util.Enumeration (via asJavaEnumeration) scala.collection.mutable.Map <=> java.util.Dictionary (via asJavaDictionary)
In addition, the following one-way conversions are provided via
asJava
:scala.collection.Seq => java.util.List scala.collection.mutable.Seq => java.util.List scala.collection.Set => java.util.Set scala.collection.Map => java.util.Map
The following one way conversion is provided via
asScala
:java.util.Properties => scala.collection.mutable.Map
In all cases, converting from a source type to a target type and back again will return the original source object. For example:
import scala.jdk.CollectionConverters._ val source = new scala.collection.mutable.ListBuffer[Int] val target: java.util.List[Int] = source.asJava val other: scala.collection.mutable.Buffer[Int] = target.asScala assert(source eq other)
- object DoubleAccumulator extends SpecificIterableFactory[Double, DoubleAccumulator] with java.io.Serializable
- object DurationConverters
This object provides extension methods that convert between Scala and Java duration types.
This object provides extension methods that convert between Scala and Java duration types.
When writing Java code, use the explicit conversion methods defined in javaapi.DurationConverters instead.
- object FunctionConverters extends Priority0FunctionExtensions
This object provides extension methods that convert between Scala and Java function types.
This object provides extension methods that convert between Scala and Java function types.
When writing Java code, use the explicit conversion methods defined in javaapi.FunctionConverters instead.
Using the
.asJava
extension method on a Scala function produces the most specific possible Java function type:scala> import scala.jdk.FunctionConverters._ scala> val f = (x: Int) => x + 1 scala> val jf1 = f.asJava jf1: java.util.function.IntUnaryOperator = ...
More generic Java function types can be created using the corresponding
asJavaXYZ
extension method:scala> val jf2 = f.asJavaFunction jf2: java.util.function.Function[Int,Int] = ... scala> val jf3 = f.asJavaUnaryOperator jf3: java.util.function.UnaryOperator[Int] = ...
Converting a Java function to Scala is done using the
asScala
extension method:scala> List(1,2,3).map(jf2.asScala) res1: List[Int] = List(2, 3, 4)
- object FunctionWrappers
- object FutureConverters
This object provides extension methods that convert between Scala scala.concurrent.Future and Java java.util.concurrent.CompletionStage
This object provides extension methods that convert between Scala scala.concurrent.Future and Java java.util.concurrent.CompletionStage
When writing Java code, use the explicit conversion methods defined in javaapi.FutureConverters instead.
Note that the bridge is implemented at the read-only side of asynchronous handles, namely scala.concurrent.Future (instead of scala.concurrent.Promise) and java.util.concurrent.CompletionStage (instead of java.util.concurrent.CompletableFuture). This is intentional, as the semantics of bridging the write-handles would be prone to race conditions; if both ends (
CompletableFuture
andPromise
) are completed independently at the same time, they may contain different values afterwards. For this reason,toCompletableFuture
is not supported on the createdCompletionStage
s. - object IntAccumulator extends SpecificIterableFactory[Int, IntAccumulator] with java.io.Serializable
- object LongAccumulator extends SpecificIterableFactory[Long, LongAccumulator] with java.io.Serializable
- object OptionConverters
This object provides extension methods that convert between Scala
Option
and JavaOptional
types.This object provides extension methods that convert between Scala
Option
and JavaOptional
types.When writing Java code, use the explicit conversion methods defined in javaapi.OptionConverters instead.
Scala
Option
is extended with atoJava
method that creates a correspondingOptional
, and atoJavaPrimitive
method that creates a specialized variant (e.g.,OptionalInt
) if applicable.Java
Optional
is extended with atoScala
method and atoJavaPrimitive
method.Finally, specialized
Optional
types are extended withtoScala
andtoJavaGeneric
methods.Example usage:
import scala.jdk.OptionConverters._ val a = Option("example").toJava // Creates java.util.Optional[String] containing "example" val b = (None: Option[String]).toJava // Creates an empty java.util.Optional[String] val c = a.toScala // Back to Option("example") val d = b.toScala // Back to None typed as Option[String] val e = Option(2.7).toJava // java.util.Optional[Double] containing boxed 2.7 val f = Option(2.7).toJavaPrimitive // java.util.OptionalDouble containing 2.7 (not boxed) val g = f.toScala // Back to Option(2.7) val h = f.toJavaGeneric // Same as e val i = e.toJavaPrimitive // Same as f
- object OptionShape
- object StreamConverters extends StreamExtensions
This object provides extension methods to create Java Streams that operate on Scala collections (sequentially or in parallel).
This object provides extension methods to create Java Streams that operate on Scala collections (sequentially or in parallel). For more information on Java streams, consult the documentation (https://docs.oracle.com/javase/8/docs/api/java/util/stream/package-summary.html).
When writing Java code, use the explicit conversion methods defined in javaapi.StreamConverters instead.
The methods
asJavaSeqStream
andasJavaParStream
convert a collection to a Java Stream:scala> import scala.jdk.StreamConverters._ scala> val s = (1 to 10).toList.asJavaSeqStream s: java.util.stream.IntStream = java.util.stream.IntPipeline$Head@7b1e5e55 scala> s.map(_ * 2).filter(_ > 5).toScala(List) res1: List[Int] = List(6, 8, 10, 12, 14, 16, 18, 20)
Note: using parallel streams in the Scala REPL causes deadlocks, see https://github.com/scala/bug/issues/9076. As a workaround, use
scala -Yrepl-class-based
.scala> def isPrime(n: Int): Boolean = !(2 +: (3 to Math.sqrt(n).toInt by 2) exists (n % _ == 0)) isPrime: (n: Int)Boolean scala> (10000 to 1000000).asJavaParStream.filter(isPrime).toScala(Vector) res6: scala.collection.immutable.Vector[Int] = Vector(10007, 10009, 10037, 10039, ...
A Java Stream provides operations on a sequence of elements. Streams are created from Spliterators, which are similar to Iterators with the additional capability to partition off some of their elements. This partitioning, if supported by the Spliterator, is used for parallelizing Stream operations.
Scala collections have a method
stepper
that returns a scala.collection.Stepper for the collection, which in turn can be converted to a Spliterator for creating a Java Stream.The
asJavaSeqStream
extension method is available on any Scala collection. TheasJavaParStream
extension method can only be invoked on collections where the return type of thestepper
method is marked with the scala.collection.Stepper.EfficientSplit marker trait. This trait is added to steppers that support partitioning, and therefore efficient parallel processing.The following extension methods are available:
Collection Type
Extension Methods
IterableOnce
asJavaSeqStream
IndexedSeq
, Arrays,BitSet
,Accumulator
,HashMap
,HashSet
,Range
,TreeMap
,TreeSet
,Vector
, StringsasJavaParStream
Map
asJavaSeqKeyStream
,asJavaSeqValueStream
HashMap
,TreeMap
asJavaParKeyStream
,asJavaParValueStream
Stepper
asJavaSeqStream
Stepper with EfficientSplit
asJavaParStream
Strings
asJavaSeqStream
,asJavaParStream
,asJavaSeqCharStream
,asJavaParCharStream
,asJavaSeqCodePointStream
,asJavaParCodePointStream
Java streams
toScala
,asJavaPrimitiveStream
The
asJavaPrimitiveStream
method converts aStream[Int]
to anIntStream
. It is the dual of theboxed
method defined on primitive streams (e.g.,IntStream.boxed
is aStream[Integer]
).The
toScala
extension methods on Java streams collects the result of a stream pipeline into a Scala collection, for examplestream.toScala(List)
,stream.toScala(Vector)
. Note that transformation operations on streams are lazy (also called "intermediate"), terminal operations such asforEach
,count
ortoScala
trigger the evaluation.Collecting a parallel stream to a collection can be performed in parallel. This is beneficial if the target collection supports efficient merging of the segments that are built in parallel. To support this use case, the Scala standard library provides the Accumulator collection. This collection supports efficient parallel construction, and it has specialized subtypes for
Int
,Long
andDouble
so that primitive Java streams can be collected to a Scala collection without boxing the elements.
This is the documentation for the Scala standard library.
Package structure
The scala package contains core types like
Int
,Float
,Array
orOption
which are accessible in all Scala compilation units without explicit qualification or imports.Notable packages include:
scala.collection
and its sub-packages contain Scala's collections frameworkscala.collection.immutable
- Immutable, sequential data-structures such asVector
,List
,Range
,HashMap
orHashSet
scala.collection.mutable
- Mutable, sequential data-structures such asArrayBuffer
,StringBuilder
,HashMap
orHashSet
scala.collection.concurrent
- Mutable, concurrent data-structures such asTrieMap
scala.concurrent
- Primitives for concurrent programming such asFutures
andPromises
scala.io
- Input and output operationsscala.math
- Basic math functions and additional numeric types likeBigInt
andBigDecimal
scala.sys
- Interaction with other processes and the operating systemscala.util.matching
- Regular expressionsOther packages exist. See the complete list on the right.
Additional parts of the standard library are shipped as separate libraries. These include:
scala.reflect
- Scala's reflection API (scala-reflect.jar)scala.xml
- XML parsing, manipulation, and serialization (scala-xml.jar)scala.collection.parallel
- Parallel collections (scala-parallel-collections.jar)scala.util.parsing
- Parser combinators (scala-parser-combinators.jar)scala.swing
- A convenient wrapper around Java's GUI framework called Swing (scala-swing.jar)Automatic imports
Identifiers in the scala package and the
scala.Predef
object are always in scope by default.Some of these identifiers are type aliases provided as shortcuts to commonly used classes. For example,
List
is an alias forscala.collection.immutable.List
.Other aliases refer to classes provided by the underlying platform. For example, on the JVM,
String
is an alias forjava.lang.String
.