Scala single method interface implementation

Question

Does Scala have any syntactic sugar to replace the following code:

val thread = new Thread(new Runnable {
   def run() {
     println(\"hello world\")
   }           


        

Answer 1

While doing this in a generic way is certainly complicated, if you found that you really only needed this for a few certain Java types, then a few simple implicit conversions can do the job nicely. For instance:

val thread = new Thread(() => println("hello world"))
thread.start

implicit def function0ToRunnable(f:() => Unit):Runnable = 
  new Runnable{def run() = f()}

Sometimes trying to solve a problem in a generic and completely re-useable way is the wrong approach if your actual problem is more bounded then you think.

Answer 2

SAM types are supported using invokeDynamic since scala-2.12 similar to JDK-8, Below was tested on 2.12.3 - Release notes about SAM can be found here - http://www.scala-lang.org/news/2.12.0/

object ThreadApp extends App {
  println("Main thread - begins")
  val runnable: Runnable = () => println("hello world - from first thread")

  val thread = new Thread(runnable)
  println("Main thread - spins first thread")
  thread.start()

  val thread2 = new Thread(() => println("hello world - from second thread"))
  println("Main thread - spins second thread")
  thread2.start
  thread.join()
  thread2.join()

  println("Main thread - end")
}

Answer 3

Scala has experimental support for SAMs starting with 2.11, under the flag -Xexperimental:

Welcome to Scala version 2.11.0-RC3 (OpenJDK 64-Bit Server VM, Java 1.7.0_51).
Type in expressions to have them evaluated.
Type :help for more information.

scala> :set -Xexperimental

scala> val r: Runnable = () => println("hello world")
r: Runnable = $anonfun$1@7861ff33

scala> new Thread(r).run
hello world

Edit: Since 2.11.5, this can also be done inline:

scala> new Thread(() => println("hello world")).run
hello world

The usual limitations about the expected type also apply:

• it must define a single abstract method,
• its primary constructor (if any) must be public, no-args, not overloaded,
• the abstract method must take a single argument list,
• the abstract method must be monomorphic.

According to the original commit by Adriaan, some of those restrictions may be lifted in the future, especially the last two.