Does Scala have any syntactic sugar to replace the following code:
val thread = new Thread(new Runnable { def run() { println("hello world") } })
with something more like:
val thread = new Thread(() => println("hello world"))
in cases when the trait/interface needs only one method to be implemented? If not, is there any chance to have this feature in Scala in the future? It is especially useful when one deals with Java classes.
I found a similar question asked three years ago: Generically implementing a Java Single-Abstract-Method interface with a Scala closure? The answer says we should have the feature in Scala 2.10. I've looked for Single Abstract Method keyword but I have not found anything. What's happened with the feature?
An interface with only one abstract method is called a functional interface, or a Single Abstract Method (SAM) interface. The functional interface can have several non-abstract members but only one abstract member. To declare a functional interface in Kotlin, use the fun modifier.
Here is one major difference that Scala has from Java: there are no interfaces. There is no interface keyword. Yes, even though Scala is primarily a JVM language and often touted as a "better Java", it doesn't have interfaces.
Like a class, Traits can have methods(both abstract and non-abstract), and fields as its members. Traits are just like interfaces in Java.
The interfaces with only one method are called Single Abstract Method(SAM) Interfaces. With the advent of Java8, such interfaces are also called Functional Interfaces.
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:
According to the original commit by Adriaan, some of those restrictions may be lifted in the future, especially the last two.
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") }
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