singleton object, a class that can have only one instance using the keyword object instead of class keyword

Cannot instantiate a singleton object, cannot pass parameters to the primary constructor

object Demo {
     def hello()
     {
       println("Hello");
     }

     def world()
     {
       println("World");
     }

     def main(args: Array[String]) {
       hello();
       world();
   }
}
		

The class name works as a class primary constructor

class Car(val m : String) // immutable paramters
{
  var maker : String = m; // mutable field

  def getMaker() : String = {return maker;}
}

object Demo
{
  def main(args: Array[String])
  {
    val c = new Car("Buick"); // immutable reference
    println(c.getMaker());

    c.maker = "Honda";
    println(c.maker);
  }
}
		

class Car(var maker : String)
{
  def getMaker() : String = {return maker;}
}

object Demo
{
  def main(args: Array[String])
  {
    val c = new Car("Buick"); // immutable reference
    println(c.getMaker());

    c.maker = "Honda";
    println(c.maker);
  }
}
		

class Car(var maker : String, var year : Int)
{
  // second constructor
  def this(m: String)
  {
    this(m, 0);
  }

  def getMaker() : String = {return maker;}

  override def toString : String = {return "Maker: %10s; Year: %10d".format(maker, year)}
}

object Demo
{
  def main(args: Array[String])
  {
    val c = new Car("Buick", 1998); // immutable reference
    println(c); // Buick, 1998

    val c2 = new Car("Honda");
    println(c2); // Honda, 0
  }
}
		

class Vehicle(val m : String) // immutable parameters
{
  var maker : String = m; // mutable fields

  def disp() {println("Maker: "+maker)}

  // override toString
  override def toString : String = {return "Maker: %10s;".format(maker)}
}
		

class Car(override val m : String, var year : Int) extends Vehicle(m) // override parameter in the parent
{
  // override the function defined in the parent
  override def disp()
  {
    super.disp(); // call the function defined in the parent
    println("Year: "+year)
  }

  // override toString
  override def toString : String = {return "Maker: %10s; Year: %10d".format(maker, year)}
}
		

object Demo
{
  def main(args: Array[String])
  {
    val c = new Car("Buick", 1998); // immutable reference
    println(c); // Buick, 1998

    c.disp();
  }
}