Richard Searle


Mixing Scala and Java lambdas

03 Oct 2015

The following code is extracted from a Scala project that parses a stream of marshalled binary messages. The code that actually unmarshals the bytes is defined by Unmarshaller.

type Unmarshaller = (Int , Adapter)=>Payload

private def payload(unmarshal:Unmarshaller)(id: Int) = new Decoder[Payload] {
   def decode(bits: BitVector): Attempt[DecodeResult[Payload]] = {
     val t = Try(Attempt.successful(DecodeResult(
         new AdapterImplementation(bits.toByteBuffer)),
     t.getOrElse(Attempt.failure(InsufficientBits(8, 0, Nil)))

The Scala code implements a test harness for a Java system. That Java (only) codebase also has to unmarshal the byte stream and would thus be convenient to make the Unmarshaller code common between the two codebases.

The Java 8 lambdas provide an equivalent representation.

type Unmarshaller = BiFunction[Integer,Adapter,Payload]

Note that is the only change required in the Scala code. The scala compiler desugars unmarshal(x,y) to unmarshal.apply(x,y), which corresponds to the BiFunction API.

Unfortunately, the Java API does not consistently use apply, so this syntactic convenience is limited. For example (Int)=>Unit corresponds to Consumer<Unit>, whose API defines an accept method.