scala-java8-compat

A Java 8 compatibility kit for Scala.

The API is currently still experimental: we do not yet guarantee source or binary compatibility with future releases.

Functional Interfaces for Scala functions

A set of Functional Interfaces for scala.FunctionN. These are designed for convenient construction of Scala functions using Java 8 lambda syntax.

Usage

import scala.concurrent.*;
import static scala.compat.java8.JFunction.*;

class Test {
	private static Future<Integer> futureExample(Future<String> future, ExecutionContext ec) {
	    return future.map(func(s -> s.toUpperCase()), ec).map(func(s -> s.length()), ec);
	}
}

More Examples / Documentation

Converters between scala.FunctionN and java.util.function

A set of converters that enable interconversion between Java's standard Functional Interfaces defined in java.util.function and Scala's Function0, Function1, and Function2 traits. These are intended for use when you already have an instance of a java.util.function and need a Scala function, or have a Scala function and need an instance of a java.util.function.

The .asScala extension method will convert a java.util.function to the corresponding Scala function. The .asJava extension method will convert a Scala function to the most specific corresponding Java functional interface. If you wish to obtain a less specific functional interface, there are named methods that start with asJava and continue with the name of the Java functional interface. For instance, the most specific interface corresponding to the Scala function val rev = (s: String) => s.reverse is UnaryOperator[String], and that is what rev.asJava will produce. However, asJavaFunction(rev) will return a java.util.function.Function[String, String] instead.

The asJava methods can also be called conveniently from Java. There are additional asScalaFrom methods (e.g. asScalaFromUnaryOperator) that will perform the functional-interface-to-Scala-function conversion; this is primarily of use when calling from Java since the .asScala extension method is more convenient in Scala.

Usage examples

In Scala:

import java.util.function._
import scala.compat.java8.FunctionConverters._

val foo: Int => Boolean = i => i > 7
def testBig(ip: IntPredicate) = ip.test(9)
println(testBig(foo.asJava))  // Prints true

val bar = new UnaryOperator[String]{ def apply(s: String) = s.reverse }
List("cod", "herring").map(bar.asScala)    // List("doc", "gnirrih")

def testA[A](p: Predicate[A])(a: A) = p.test(a)
println(testA(asJavaPredicate(foo))(4))  // Prints false

// println(testA(foo.asJava)(4))  <-- doesn't work
//                                    IntPredicate does not extend Predicate!

In Java:

import java.util.function.*;
import scala.compat.java8.FunctionConverters;

class Example {
  String foo(UnaryOperator<String> f) {
    return f.apply("halibut");
  }
  String bar(scala.Function1<String, String> f) {
    return foo(functionConverters.asJavaUnaryOperator(f));
  }
  String baz(Function<String, String> f) {
    return bar(functionConverters.asScalaFromFunction(f));
  }
}

Converters between scala.concurrent and java.util.concurrent

• API
• Test Cases

Converters between scala.Option and java.util classes Optional, OptionalDouble, OptionalInt, and OptionalLong.

A set of extension methods to enable explicit conversion between Scala Option and the Java 8 optional types, Optional, OptionalDouble, OptionalInt, and OptionalLong.

Note that the four Java classes have no inheritance relationship despite all encoding optional types.

Usage example

import scala.compat.java8.OptionConverters._

class Test {
  val o = Option(2.7)
  val oj = o.asJava        // Optional[Double]
  val ojd = o.asPrimitive  // OptionalDouble
  val ojds = ojd.asScala   // Option(2.7) again
}

Future work

• Converters for java.util.stream
• Spliterators for Scala collections