CODEWITHSUNDEEP
scalaprogram-entry-point
itsraghz
itsraghz

Reputation: 1017

scala object extends app does not print anything

I am learning Scala and as part of the journey I had come across two different ways to write your scala class - one with a main method and other without the main method but by extending App (earlier Application is deprecated due to concurrency issues).

I am executing the scripts in Command Line via scala executable as scala <nameOfScript>.scala. I run Scala 2.11.7 in Windows.

I have no issues when running a scala script/class with a main method.

object ObjectWithMainMethod {
    def main(args:Array[String]) = {
            println("Object with a main() method executed..")
    }
}

It produces the following output.

Object with a main() method executed..

But, I don't get the output with its counterpart, which is extending App trait but without the main method.

object AppWithoutMainMethod extends App {
    println("AppWithout main() method executed")
}

When I run this scala script, it does not print anything. However, when I looked at the complied .class files via javap utility, I could see the PSVM (public static void main() method) inside.

Am I missing something? Any help on this will be highly appreciated.

Upvotes: 0

Views: 1406

Answers (2)

itsraghz
itsraghz

Reputation: 1017

The same thing works if I run the file without the .scala extension. I am not sure the reason behind this. 

scala AppWithoutMainMethod

It produces the following output

AppWithout main() method executed

Upvotes: -1

Tzach Zohar
Tzach Zohar

Reputation: 37832

If you run scala -help you'll see this comment:

A file argument will be run as a scala script unless it contains only self-contained compilation units (classes and objects) and exactly one runnable main method. In that case the file will be compiled and the main method invoked. This provides a bridge between scripts and standard scala source.

This explains exactly what you're seeing - the scala command is primarily meant to execute "scripts", or a series of expressions - for quick, interactive evaluation of code snippets. It only runs "applications" (i.e. objects with main method) as a "special case", because it makes sense users will try to use it this way. So:

  • When you run scala ObjectWithMainMethod.scala, the main method is identified and the command enters this "special case", figuring out that you probably meant for it to work that way
  • When you run scala AppwithoutMainMethod.scala, even though App has a main method, this isn't identified as the "special case" but just as a series of expressions, so the main method isn't invoked.

If you compile your classes into .class files and run them via java -cp <classpath> <class-name> command, the two will produce identical results.

Upvotes: 3

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