Understanding Methods and Functions in Scala

In Scala, a method is a member of a class, while a function is a full object that can be assigned to a variable; defining a function inside a class is effectively defining a method.

Scala uses val to define a function and def to define a method. Example:

class Test{
  def m(x: Int) = x + 3
  val f = (x: Int) => x + 3
}

Method declaration follows the format def functionName([parameter list]): [return type]. Omitting the equals sign and body makes the method abstract.

Method definition starts with the def keyword, followed by an optional parameter list, a colon, the return type, an equals sign, and the method body:

def functionName([parameter list]): [return type] = {
   function body
   return [expr]
}

The return type can be any valid Scala type. For example, a method that adds two integers:

object add{
   def addInt(a: Int, b: Int): Int = {
      var sum: Int = 0
      sum = a + b
      return sum
   }
}

If a method does not return a value, it can return Unit, which is analogous to Java's void:

object Hello{
   def printMe(): Unit = {
      println("Hello, Scala!")
   }
}

Scala provides several ways to call methods. The standard call syntax is functionName(parameter list), and when using an instance you can call instance.functionName(parameter list), similar to Java.

Example combining definition and invocation:

object Test {
   def main(args: Array[String]) {
        println("Returned Value : " + addInt(5,7))
   }
   def addInt(a: Int, b: Int): Int = {
      var sum: Int = 0
      sum = a + b
      return sum
   }
}

Running the above code produces:

$ scalac Test.scala 
$ scala Test
Returned Value : 12