Learn Java for Web Development

APPENDIX C: Introduction to Scala 425

Note The semicolon at the end of a statement is usually optional.

   Line 1: The main() method is defined in an object, not in a class. Scala has an

object construct with which you can declare a singleton object. You will learn

more about singletons later in this appendix.

   Line 2: Scala program processing starts from the main() method, which is a

mandatory part of every Scala program. The main() method is not marked

as static. In Scala, everything is an object. The main() method is an instance

method on a singleton object that is automatically instantiated.

   Line 2: There is no return type. Actually, there is Unit, which is similar to void,

but it is inferred by the compiler. You can explicitly specify the return type by

putting a colon and the type after the parameters.

def main(args: Array[String]) : Unit = {
}

   Line 2: There is no access-level modifier in Scala. You have a public modifier in

Java in this context, but Scala does not specify the public modifier because the

default access level is public.

   Line 2: Scala uses the def keyword to tell the compiler that this is a method.

Save the code in Listing C-1 in a file called HelloWorld.scala and compile the code using the
following command:

scalac HelloWorld.scala

Now run the program using this command:

scala HelloWorld

Hello, World!

Note Java requires you to put a public class in a file named after the class. For example, you should put

class HelloWorld in file HelloWorld.java. In Scala, you can name .scala files anything you want, no

matter what Scala classes or code you put in them. However, it is recommended you name files after the

classes they contain as is done in Java so as to easily locate classes based on file names.