Java vs Scala Functional Interface Usage

Question

In Java I can do this:

Runnable task = () -> { System.out.println("Task is running"); };

But how come in Scala I can't do the same!

val task: Runnable = () => {println("Task is running")}

I get a compiler error! I am using Scala version 2.11.8.

type mismatch;  found   : () => Unit  required: Runnable

What compiler error? I don't see any compiler error with the code above. scala> val task: Runnable = () => {System.out.println("Task is running")} gives task: Runnable = $$Lambda$1011/1678449328@2917b6cb — Unsigned
– Unsigned, Commented Apr 27, 2017 at 17:44
Scala version is 2.11. As commented by Raman below.. I guess 2.12 solves this. Thx. — Apurva Singh
– Apurva Singh, Commented Apr 27, 2017 at 18:00

Raman · Accepted Answer · 2017-04-27 17:59:19Z

19

Scala version 2.12 supports converting lambda expressions to types with a "single abstract method" (SAM), aka "Functional Interface", like Java 8 does. See http://www.scala-lang.org/news/2.12.0#lambda-syntax-for-sam-types.

Earlier Scala versions are not capable of automatically converting the lambda expression to a Java functional interface / SAM type (such as Runnable). You are most likely using a version prior to 2.12.

The code you provided works perfectly fine in Scala 2.12.

edited Apr 27, 2017 at 17:59

answered Apr 27, 2017 at 17:53

Raman

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