class can't have main() in scala? - scala

Question

I'm a beginner of Scala.
In Calulator.scala file
class Calulator {
var brand: String ="HP"
def add(m:Int, n:Int):Int = m+n
}
In CalculatorTest.scala file
object CalulatorTest {
def main(args:Array[String]) {
val cal = new Calulator
println(cal.add(1,2))
}
}
Compile two scala files and run CalculatorTest. This works.
My question is can I make them as one scala file like Java?
In Calulator.scala file
class Calulator {
var brand: String ="HP"
def add(m:Int, n:Int):Int = m+n
def main(args:Array[String]) {
val cal = new Calulator
println(cal.add(1,2))
}
}
This doesn't work !!!
I googled it and looks like main() should be in object.
Is there a way to put them in one file to run main()?

Answer 1

To put them in one file use the concept of the 'Companion Object'.
The class and companion object must be in the same file and share the same name.
class Calculator {
var brand: String = "HP"
def add(m: Int, n: Int): Int = m + n
}
object Calculator {
def main(args:Array[String]) {
val cal = new Calculator
println(cal.add(1,2))
}
}
Further reading: http://docs.scala-lang.org/tutorials/tour/singleton-objects.html

Answer 2

The simple answer is No
a deeper answer is that the scala compiler refers to an object as static and class it refers as regular.
when you want to run Java program your main must be declared as static otherwise the JVM will not recognize it as your program entry point.
regards the question about is it possible to union both of them ,the answer is not because thats how the scala compiler works.
The compiler would create MyClass$.class for scala object and MyClass.class for scala class thats why you should never ever call your class something like :
class MyClass$ {
}
because it will override the companion object for MyClass.