Scala - object, singletons, classes and constructors

Today, a simple example by way of explanation of the basic constructs of OO in Scala.

Things to note:

• Scala does not use static methods, in this sense Scala is more purely OO than Java.
• The equivalent is achieved using object singletons and companion objects.

The following code example demonstrates:

• Use of object singleton and "main" application entry point
• Declaration of new objects
• Declaration of new class
• Class primary constructor
• Declaring fields
• Declaring alternative constructors
• Use of override keyword

Example Code:

package test

// object declares a singleton (like Java static)
object Test {
  // the usual main, takes an Array[String] of arguments
  def main(args : Array[String]) {

      var myClass1 = new MyClass(5, "Louis")  // using primary constructor
      var myClass2 = new MyClass(10);         // using alternate constructor

      // access public field value
      println("myClass1.value1 = " + myClass1.value1)

      // output using overriden toString
      println("myClass1 = " + myClass1)
      println("myClass2 = " + myClass2)
  }
}

// define MyClass and its primary constructor (Int, String)
class MyClass(v1 : Int, s1 : String) {

  // these are fields (public)
  def value1 : Int = v1
  def string1 : String = s1

  // this is an alternate constructor (calls primary constructor)
  def this(v1 : Int) = {
      this(v1, "hello")
  }

  // can override toString like in Java, note no return statement required + the override keyword
  override def toString() : String =
  {
      "MyClass: (" + value1 + ", " + string1 + ")"
  }
}